Suspension

Suspension Geometry

There are a number of different types of suspension geometries: live axel, swing axel, multi-link, double wishbone, MacPherson, etc. The most easily-adjustable suspension type is the multi-link geometry. It allows the highest amount of control over everything from kinematic roll center height to camber gain. The multi-link suspension for my car will have four main components:

  1. Upper Control Arm (UCA)
  2. Lower Control Arm (LCA)
  3. Upright
  4. Steering Link

The main variables in suspension geometry are:

  1. Upper and Lower Control Arm Vertical Angles – create the roll center height.
  2. Upright Fore-Aft Angle – creates the castor angle, which creates the mechanical trail distance.
  3. Upright Port-Starboard Angle – creates the kingpin inclination, which creates the scrub radius.
  4. Steering Link Vertical Angle – should intersect the instant center of the two control arms to avoid bump steer.
  5. Steering Link Horizontal Angle – creates the castor angle.
  6. Steering Link Length – should cause the pivot point axes to intersect at the rear tire, creating 100% Ackermann steering.

Coming up with these geometry dimensions requires setting limits and optimizing for specific design goals.

 

Track Width

Track width, often just called “track,” is the distance between the center of the tire contact patches for tires on the same axel. For 4-wheeled cars there is a front track and a rear track, for reverse three-wheelers, there is only a front track. All things being equal, a car with a wider track will have lower lateral weight transfer. Lower weight transfer results in improved traction, so obviously in order to improve performance we want to make the track width as wide as possible. A wider track also allows tires to load up more slowly as lateral weight transfers more slowly – this results in “smoother” handling. The trade off with having a wide track is it can make the vehicle hard to maneuver in narrow streets and in traffic. A wider track also adds weight unsprung weight to the vehicle. So the goal is to design the car with the widest possible track that still allows for easy maneuverability in traffic.

Here are track widths of some three-wheelers:

  • Aptera 2e: 1,953 mm
  • Elio 3-Wheeler: 1,697 mm
  • Morgan 3-Wheeler: 1,473 mm
  • Lotus Formula 1: 1,450 mm (front); 1,400 mm (rear)
  • Harley Trike: 1,397 mm
  • Bombardier Can Am Spyder: 1,308 mm

The maximum allowable vehicle width in the United States is 102 inches (2590.8 mm). The widest pickup truck available for sale is the the Dodge 3500 Dually, which is 96 inches (2,438 mm) wide. The track width on the Lotus Elise is 1,453 mm.

For my car I’m going to design it with a 1400 mm front track width.

 

Wheelbase-to-Track Ratio

The wheelbase is the distance between the tire contact patch of the front wheels and the tire contact patch of the rear wheel(s). The wheelbase-to-track ratio is the ratio between the the wheelbase length and the track width. Formula 1 cars must have a minimum wheelbase length of 3000mm and a maximum track width of 1800mm – giving them a minimum wheelbase-to-track ratio 0f 1.66:1. Coincidentally, this is almost exactly the golden ratio (1.618:1). The FIU FSAE team says that “for quick steering response an optimal ratio of 1.3 should be achieved.”

Based on my very rough estimates, I believe the Aprilia Magnet’s wheelbase is about 1700mm and its track is about 850mm – that is a wheelbase-to-track ratio of 2:1.

A 1.618:1 (golden ratio) wheelbase-to-track ratio with a 1400mm front track would put the wheelbase length at 2265mm.

 

Ride Height / Ground Clearance
The ride height is important because it helps determine the height of the vehicles center of gravity (CG). We want to keep our CG as low as possible, but lowering the ride height also lowers the amount of suspension travel that is possible, making it necessary to use harder springs, making the ride less comfortable. Lowering the ride height too much also risks bottoming out the car on bumps in the road. Thus, the ideal ride height is found by a compromise.

Here are some ride heights of various vehicles:

  • McLaren F1: 119mm
  • Ducati 848: 125mm
  • Harley Davidson Fat Boy: 130mm
  • Lotus Elise: 130mm
  • Ducati Monster: 150mm
  • Ferrari 355: 163mm
  • Ford Raptor: 241mm

I’m shooting for a ride height of  130mm.

 

Pushrod vs Pullrod
Instead of connecting the springs and dampers directly to the suspension arms, it is better to use pushrods or pullrods. Pushrods and pullrods reduce the unsprung mass of the car and improve the aerodynamics. Pullrods require the springs and dampers to be located low in the car, while pushrods require them to be located higher in the car. Since the motors and batteries will likely have a higher overall mass density than the springs and dampers, it makes more sense to use a pushrod suspension in order to keep the heavier components lower in the car, thereby lowering the center of gravity of the vehicle.

 

Bellcrank Geometry
The bellcrank is the component that connects the pushrod to the spring and damper. The bellcrank pivots around an axis in order to change the direction of force. Bellcranks can be designed with three different types of geometry: progressive, neutral or regressive. With a neutral bellcrank geometry, as the wheel moves up, the spring and damper are compressed the same amount. If the wheel moves 1 inch, the spring and damper are compressed 1 inch. If  the wheel moves two inches, the spring and damper compress by 2 inches. With a regressive bellcrank geometry, the spring and damper are only compressed at a fraction of the distance that the wheel moves, meaning the effective spring rate decreases as the suspension is compressed. So with a 0.5:1 regressive bellcrank geometry, a 2 inch wheel movement will only result in 1 inch of spring and damper compression. A progressive suspension geometry causes the spring and damper to compress at a multiple of the wheel movement, making the effective spring rate increase as the suspension is compressed. In a 2:1 progressive bellcrank geometry, a 2 inch wheel movement would result in a 4 inch spring and damper compression. The best design is a progressive bellcrank geometry. This is because under small wheel movements, the spring and damper don’t move much, but under large movements, the spring and damper provide increasing force.

A simple analysis of the Ariel Atom bellcrank shows that it has a 2.6:1 progressive bellcrank ratio:

ArielAtomBellcrank

I plan to have a simple 2:1 progressive bellcrank geometry on my car (meaning a 2 inch wheel movement would result in a 4 inch spring and damper compression).

 

Suspension Travel – bump/rebound mm
Suspension travel is important because it determines how large a bump the suspension can soak up. A car that is driven off-road needs a large suspension travel. A car that is driven on a smooth racetrack needs a smaller suspension travel. A road car needs a suspension travel somewhere in the middle.

Here are the suspension travels for some vehicles:

  • McLaren F1: 90 mm bump and droop front, 80 mm bump and droop rear
  • Lotus Elise: 50 mm droop / 60 mm bump front, 50 mm droop / 70 mm bump rear
  • 2004 Subaru WRX STI: 95 mm droop / 75 mm bump front, 100 mm droop / 100 mm bump rear
  • 2005 Subaru WRX STI: 95 mm droop / 65 mm bump front, 95 mm droop / 85 mm bump rear
  • Ducati 1199: 120 mm bump and droop front, 130mm bump and droop rear
  • Zero MMX Electric Motorcycle: 218 mm front, 227 mm rear
  • Baja 1000 Trophy Truck: 760+ mm

I’m planning to use three  Öhlins TTX36 in my suspension (the stock rear shock absorber for the Ducati 1198). This shock has a maximum stroke of 57mm. With a 2:1 progressive bellcrank geometry, the maximum bump/rebound of the car will be 28.5 mm.

If I wanted to make the car more 0ff-road capable (or simply better able to soak up big potholes) I could choose a shock absorber with a larger stroke, like the Öhlins TTX44 KT 994, which is used on the KTM 450 motorcycle. This shock has a 109mm stroke, which translates into a 54.5mm maximum suspension travel with a 2:1 bellcrank geometry.

 

Kinematic Roll Center Height (RCH)

The roll center is a theoretical point in space created by the intersection of two imaginary lines drawn from the instant centers of the suspension and the center of the tire contact patches. The roll center is the point about which the chassis rolls. By changing the angle of the suspension arms you can change the roll center and affect how the car will react to cornering forces. The the following diagram, the instant centers are represented by the blue dots while roll center height is represented by the yellow dot:

RollCenterHeight

The roll center height (RCH) is the vertical height of the roll center and is important insofar as it determines how the car will roll in a corner. The RCH must be designed in its relation to the center of gravity (CG) And to the ground plane. There are five possibilities:

  1. RCH above CG
  2. RCH=CG
  3. RCH below CG and above the ground
  4. RCH = ground level
  5. RCH below ground level

Here’s how each of the five possibilities plays out:

  1. RCH above CG: Bad. Putting the roll center height above the center of gravity height would actually cause the car to lean into corners – which sounds like it would be very desirable. However, putting the RCH above the CG also leads to extreme jacking forces, raising the sprung mass of the car. This can cause the car to flip over in a corner. Modern Formula 1 cars often have upward-sloping front a-arms that can cause the front roll center to be above the center of gravity. The purpose of this design is to create more aerodynamic downforce.  F1 teams offset this by having a normal suspension design in the rear, thereby creating a rearward-sloping roll axis. This likely causes modern F1 cars to understeer at low speeds.
  2. RCH=CG: Bad. All of the lateral force is transferred through the suspension arms and no force is transferred through the springs and dampers. The car will have no body roll, but the springs and dampers won’t be able to control anything.
  3. RCH below CG and above the ground: Good. The height as a percentage of the distance between the ground and the CG will determine what percentage of lateral forces will be transferred through the suspension arms and what percentage will be transferred through the springs and dampers. A higher roll center heights put more force through the suspension links and less force through the springs.
  4. RCH = ground level: Bad. All of the lateral force will be transferred through the springs and dampers and no lateral force will be transferred through the suspension arms. There would be no jacking or anti-jacking. The roll center will also migrate above and below the ground plane as the suspension moves, causing the forces acting through the suspension arms to suddenly change direction, making the car handle unpredictably. Parallel equal-length a-arms can create a situation where RCH = ground level.
  5. RCH below ground level: Bad. Creates extreme anti-jacking forces, lowering the sprung mass of the car, causing the car to squat in corners.

Higher roll center heights create less body roll but more jacking. Lower roll center heights create more body roll but less jacking. Since the car will be automatically tilted in towards the corner, we don’t need to worry about body roll and should instead focus on minimizing jacking forces. So for a tilting three-wheeler, the optimal RCH for this car is slightly above the ground but not too low that the RCH will fall below the ground plane as the car rolls in a corner.

The Lotus Elise has a kinematic roll center height of 30mm above the ground and a center of gravity height of 470mm. The Lotus Elise RCH is 6% the height of the CG, meaning 6% of lateral force is transferred through the suspension arms and 94% is transferred through the springs and dampers.

For this car I will aim for a front roll center 40 mm above the ground.

 

Static Camber Angle

Some camber angles of high-performance cars:

  • Ferrari F50: -0.7 degrees front, -1.0 degrees rear
  • Lotus Elise: -0.01 degrees front, -1.8 degrees rear
  • Ferrari 355: -0.5 to -0.8 degrees front, -1.8 to -2.0 degrees rear

I’m going to target a static camber angle of -2.0 degrees.

 

Scrub Radius

The scrub radius is the distance between the center of the tire contact patch and the point at which the kingpin axis intersects the ground. The scrub radius is thus determined by both the kingpin inclination and the width of the tire. The scrub radius provides driver feedback under braking by causing the steering wheel to pull towards the tire that is doing the most braking. Some driver feedback is good, but too much scrub radius can make the car difficult to handle under extreme braking. There are three possibilities for the scrub radius:

  1. Positive scrub radius – the kingpin axis intersection is on the inside side of the center of the tire contact patch. Almost all cars have a positive scrub radius. 
  2. Zero scrub radius – the kingpin axis intersects the center of the tire contact patch. Cars with zero scrub radius are described as “squirmy” because the scrub radius will actually move back and forth from positive to negative as forces change, causing the handling to become unpredictable.
  3. Negative scrub radius – the kingpin axis intersection is on the outer side of the center of the tire contact patch. The advantage of negative scrub radius is that in the event of a tire failure or brake failure on one wheel, the car will want to naturally steer itself in a straight line.

KPIandScrub

For performance cars with independent suspensions it seems to be generally accepted that the scrub radius should be positive and less than 1 inch (25.4mm). Some scrub radii of famous sports cars:

  • McLaren F1: 16.25 mm
  • Lotus Elise: 10.5 mm
  • Corvette C5: 10 mm
  • Mazda Miata: 0 mm

I am targeting a scrub radius of  15 mm.

 

Rod Ends

Rod ends are one of the most important and often most overlooked components of a car. Rod ends are spherical bearings build into threaded rod ends. They connect the suspension links to the uprights on one end and to the car’s hardpoints on the other end. The best rod ends are three-piece “self lubricating” units which use a Teflon liner instead of a grease bearing, making them nearly maintenance-free. Cheaper rod ends (called “economy” or “commercial” rod ends) are usually two-piece construction where the body is swaged around the bearing.

Key considerations when choosing a rod end are:

  1. Maximum force to be applied to the bearing – for example hitting a big pothole at 150 MPH – once multiplied by a safety factor of 3, this force should be below the operating load capacity of the rod end.
  2. Maximum misalignment angle – this is the angle when the suspension is at full compression or full release – a small amount of additional angle should be added to account for any flex in the suspension arms

I’m planning on buying from Aurora Bearing Company because they have a lot of good educational information on their website and because they provide 3D cad drawings of every part they sell.

For the front suspension I’m planning to use the same rod ends for all six points: Aurora Item # MM-M12T

For a comparison, McMaster-Carr sells a rod end with the same dimensions (#2988k151). The McMaster-Carr rod end has a static radial load capacity of 3,905 lbs of force. The Aurora rod end has a maximum static radial load capacity of 18,215 Newtons, which is approximately 4,094 lbs of force.

A very common design mistake is to put the rod end in bending.

 

Suspension Dimensions

Huw Davies has produced a nice visual suspension geometry calculator that allows you to play with these variables. My key model input parameters (in the “dimensions from vehicle” menu) are:

  • Chassis
    • Upper width: 320 mm (The tilting frame is 350 mm wide. The center of the rod end holes will be 15 mm from the sides)
    • Lower width: 480 mm (3 batteries side-by-side are 429 mm wide + 25 mm on either side for the chassis + 1 mm to make it round)
    • Height vertically: 370 mm (The tilting frame is 420 mm high. The center of the rod end holes will be 15 mm from the top of the tilting frame and 35 mm from the bottom of the frame (the rod end ball body is 30mm in diameter))
  • Wheels and Tires
    • Size: 190/55 17
    • Offset: 22 mm (measured here)
    • Camber: -2 deg

Playing around with the various parameters I can meet the goals of:

  • Ride Height: 130 mm (adjust the corners of the chassis to get this – be careful to keep your dimensions the same)
  • Track Width: 1400 mm (adjust the outermost green arrow to get this)
  • Scrub Radius: +15 mm (move the two wheel-side rod ends side-to-side to get this)
  • Kingpin Inclination: +10 degrees (move the two wheel-side rod ends side-to-side to get this)
  • Roll Center: +40 mm (this was limited by how close I could get the rod ends to the edge of the wheels without having a collision during bump and rebound)

This results in the following suspension dimensions:

  • Control Arms
    • UCA length: 435 mm
    • UCA bearing:-1.2 deg
    • LCA length: 425 mm
    • LCA bearing: +2.3 deg
    • Distance between UCA and LCA: 379 mm
  • Uprights
    • KP / spindle offset: 199 mm
    • KP / hub offset: 82 mm
    • Kingpin Length: 402 mm

The final model can be found here.

SuspensionGeometry

Here you can see the suspension’s instance center:

SuspensionRollCenterandInstantCenter

 

Wishbone / A-Arm Dimensions

Now that I have the length of the upper and lower wishbones figured out, I need to figure out how wide the wishbones will be where they connect with the chassis. Wider wishbones will be stronger and will resist torsional forces better. The biggest torsional force on the wishbones will be from braking. Narrower wishbones will be lighter and will allow for a tighter turning radius. This website has the equations for how to figure out the maximum torque on your suspension a-arm. Combining two equations we get: Brake Torque (Nm) = (Total Vehicle Mas s(kg) * Deceleration (g units) * Acceleration Due to Gravity (m/sec^2)* Static Laden Radius of the Tire (M))/Speed Ratio Between the Wheel and the Brake (r)

Using these parameters:

  • Total Vehicle Mass(kg) = 500 Kilograms
  • Deceleration (g units) = 5 Gs (Equivalent to what a Formula 1 car is capable of)
  • Acceleration Due to Gravity (m/sec^2) = 9.806 m/sec^2
  • Static Laden Radius of the Tire (M) = 0.226314 Meters (The tires are sized 190/55 17 so they are actually 17.82 inches in diameter)
  • Speed Ratio Between the Wheel and the Brake (r) = 1 (assuming the tire is not slipping, the wheel will be going the same speed as the brake)

Equation: (500*5*9.806*0.226314)/1 = 5548.08771 Nm

For reference, the Lotus 7 (“locost“) uses a 222 mm wide wishbone. My tilting frame is 200 mm across and the top wishbones will bolt to the sides of this. The Aurora MM-M12T rod ends are 16 mm wide at the ball head. So assuming the rod ends will rest against the tilting frame, the wishbone with will be 216 mm wide.

Assuming the wishbone is an isosceles triangle (which it should be to allow for the maximum turning radius in both directions), we can use the Pythagorean theorem to figure out the lengths of the legs. The equation is:

Leg = Sqrt((A^2)+((B/2)^2), where A = altitude (eg the total height of the triangle) and B = base of the triangle.

For the upper control arm, A=546mm and B=200mm so Leg = Sqrt((435^2)+((216/2)^2) = 448.206425657 mm

For the lower control arm,  A=434mm and B=200mm so Leg = Sqrt((425^2)+((216/2)^2) = 438.507696626 mm

Of course those “leg” distances are the complete distance from the center of one rod end to the center of the other rod end. The rod end’s “base to center” distance is 54 mm and the thread length is 33 mm. Assuming you thread the rod 2/3rds of the way in (to allow some room for adjustment) the total distance from the center of the ball to the edge of the wishbone leg is 65 mm. Between the edge of the tube end and the rod end is a weld-in threaded “tube end” (also known as a “weld-in bung” or “threaded insert”) and a “jam nut.” A weld-in tube end for a M12 x 1.75 RH thread (used on the v) is designed to work with 23mm OD X 3.5mm Thickness tubing. The tube end has an exposed length of 24 mm. A  M12 x 1.75 RH jam nut is 6 mm thick. So combined each leg has 65 mm for each rod end, 24 mm for each tube end, and 6 mm for each jam nut: meaning each leg should be 190 mm shorter:

  • UCA Legs: 258.2 mm long
  • LCA Legs: 248.5 mm long

Cold-drawn seamless structural round steel tubing conforms to ISO 10799-2:2011; This is a great handbook showing the various standard tubing sizes (see Table 26 for ISO sizes). Parker Steel, out of Toledo, Ohio, is the largest supplier of metric-sized metals in North America. You can also buy metric steel tubing from Metric Express and World Wide Metric.

I’m planning to use 23 mm OD X 3.5 mm Thickness tubing for the wishbones.

The problem is, in order to choose the proper tubing diameter and wall thickness it is necessary to test the design using finite element analysis (FEA) and unfortunately Solidworks doesn’t have these tubing diameters as standard weldments! So I did a little work and created a two importable libraries of standard tube weldments and uploaded them to my GrabCad page. They are located here:

 

Uprights

 

 

 

Kingpin Inclination

The kingpin inclination (KPI) is the angle between vertical axis of the upright and the vertical axis through the center line of the wheel. A larger kingpin inclination shortens the scrub radius (which can be good) but causes positive camber gain while steering (which is usually bad). As the wheel is turned, the kingpin inclination causes the chassis to rise, creating self-aligning force. Both the castor angle and the kingpin inclination provide self-centering torque, but the castor angle provides self-centering torque with good camber effects, while the KPI provides self-centering torque with bad camber effects. Thus, it is better to create self-centering torque with a higher castor angle than a higher kingpin inclination. The kingpin inclination should always be positive (with the top of the kingpin closer to the body than the bottom of the kingpin).

KPIandScrub

Some kingpin inclination numbers for high-performance cars:

  • Ferrari 355: 13.16 degrees
  • Lotus Elise: 12.0 degrees
  • Mazda Miata: 11.3 degrees
  • Ford Mustang (second generation): 11 degrees
  • Lotus 7: 9 degrees
  • McLaren F1: 9 degrees
  • Corvette C5 & C6: 8.8 degrees
  • Triumph Spitfire: 7 degrees

The kingpin inclination is dependent on the scrub radius. For my car’s geometry, with a 15 mm scrub radius, the kingpin inclination is 8.7 degrees.

 

Camber Gain in Bump

Some camber gain figures for high-performance cars:

  • Lotus Elise: 0.31 degrees per inch (25.4 mm)
  • Mazda Miata: 0.91 degrees per inch front / 0.21/0.58 degrees per inch initial/final rate

Using the Racing Aspirations model, I calculate the camber gain in bump for my car’s suspension geometry to be -0.20 degrees per inch

 

Camber Gain in Roll

Camber gain in roll is the number of degrees camber changes per degree of body roll. Ideally a car should have very little camber change with roll. Chassis Engineering explains the various ways of reducing camber gain in roll:

  1. Lower the center of gravity (CG) height
  2. Lower the roll center height by adjust (RCH) by adjusting the suspension geometry
  3. Widen the track width
  4. Increase the roll stiffness of the suspension by increasing the spring rates or adding anti-roll bars:

Using the Racing Aspirations model, I calculate the camber gain in bump for my car’s suspension geometry to be 0.90 degrees of camber change per degree of body roll.

 

Anti-Roll Bars

Anti-roll bars, sometimes called anti-sway bars, sway bars or stabilizer bars, connect the two wheels together with a simple lever arm. The longer and thinner the lever arm, the less force is transferred between the two wheels. When the car is cornering hard and the body begins to roll to the outside, the outside suspension will compress more than the inside suspension and the anti-roll bar will transfer force from the outside wheel to the inside wheel, lessening the body roll. The downside to anti-roll bars is that when you are driving straight and hit a bump with one tire, some of that bump force is transferred from the wheel that hit the bump to the wheel that did not hit the bump. This can cause the car to become unsettled and makes the ride comfort worse.

 

Spring Rate
The ideal spring rate for a car is dependent on the sprung and unsprung mass of the car as well as the desired suspension frequency.

A rule of thumb for motorcycle design (taken from Carl Vogel’s book) is that the suspension should sag 30% of the full travel under the weight of the rider (150-200 lbs).

I’m planning to use a Ducati 1198 rear suspension for my car in order to minimize the number of custom-manufactured parts. The stock rear shock absorber for the Ducati 1198 is the Öhlins TTX36. These dampers have shock and rebound dials that allow for easy adjustment. The stock Ducati 1198 rear spring has a spring rate of 90 N/mm. Because I am changing both the sprung and unsprung mass that this spring must control, it is likely that I will need a different spring with a different spring rate.

<side note rant> despite being a European shock designed for European motorcycles, the the Öhlins TTX36 shock has rod ends with 0.5 INCH diameter rod end balls. I don’t know why they use a mix of the imperial system and metric system on the same part, but it makes any engineering solution very confusing and unnecessarily complicated. What don’t we all just use the metric system!? It’s better! </end rant>

This spreadsheet allows you to calculate the proper spring rate for your car.

 

Suspension Frequency

Here are some examples of suspension frequencies from well-designed cars:

  • McLaren F1: 1.43hz front, 1.80hz rear
  • Lotus Elise: 1.50hz front, 1.63hz rear

 

 

Toe Angle

A slight toe-in at the front helps keep the steering linkages under tension, making steering more immediate.

Some toe angles of high-performance cars:

  • Lotus Elise: 0.2mm toe-out front, 1.2mm toe-in rear
  • Ferrari 355: 2mm toe-in front and rear

The toe angle is easily adjusted adjusting how far the steering rack rod ends are screwed in to the tubing ends. For this car I’m going to target a 2mm toe-in at the front.

 

Anti-Squat Geometry

 

Anti-Dive Geometry

 

Manufacturing

  • Notching / Coping the tubing
  • Tapping the threads for the rod end
  • Tig Welding

 

 

 

 

 

 

 

 

 

Other miscellaneous suspension topics:

Unsprung Downforce

Ideally, downforce should be applied directly to the unsprung mass of the car. Instead of transferring force through the body and then through the springs to the tires, wings that are mounted directly to the suspension can transfer force directly to the tires. The Lotus 49 had wings that were bolted directly to the suspension, creating unsprung downforce. The FIA banned this type of wing as a “movable aerodynamic device,” but the the idea remains sound. By changing the shape of the suspension arms to mimic upside-down airplane wings, it is possible to induce downforce directly on the tires without adding much unsprung mass. One of the easiest ways to accomplish this is to use “streamline tubing” for the suspension arms and tilt their angle slightly so they are flat on the top and curved on the bottom, thus creating downforce.

streamlinetubing

 

Inerters

Inerters, also known as J-dampers, are an important development in modern suspension design. When you compare a cars suspension to an electrical circuit, dampers act like a resistor, springs act like an inducer and inerters act like a capacitor. Resistors dissipate current flow just as dampers dissipate suspension force. Inducers resist current flow just as springs resist suspension forces. Capacitors store and release current just as inerters store and release suspension force. By adding an inerter, a car’s suspension can store up the force of a bump and release it back. This allows the use of a softer spring, which allows the car to have both a smoother ride and to leave the tire in contact with the road longer, which improves handling.

As far as I can tell, the only combination inerter/damper sold on the market today is the Penske 8786. Hopefully in the future more interters in different sizes will be sold so I can add them to the car.

 

Carbon Fiber Springs

A company called Hyperco makes carbon fiber drop-in replacement springs for suspension dampers. They are supposedly up to 70% lighter than steel springs. This might be another way to decrease unsprung mass in the future, but for now I will just use standard springs.

 

Roll Axis

On 4-wheeled cars with separate front and rear suspensions, it is possible to have two different roll center heights. If you draw an imaginary line between the two roll center heights you will see the roll axis. The roll axis is the axis about which a car rolls in a turn. If you create another imaginary line between the center of gravity at the point of the front roll center and the center of gravity at the point of the rear roll center you will create what’s been dubbed the “Mike axis” after its inventor Mike Kojima. According to Sport Compact Car, “If the space between the two lines is less in the front of the car, with an upward sloping Mike axis, the car will tend to understeer due to greater weight transfer to the outside wheels at the front of the car due to a greater amount of geometric anti roll. If the distance between the lines is greater at the front and less in the rear, the car will understeer less due to the greater amount of rear geometric anti roll giving more rear outside weight transfer. Front engine RWD cars typicaly exhibit the latter trait can can oversteer a lot despite a front weight bias. Rear heavy rear and mid engine cars also exhibit this trait, partially due to a larger rear polar moment of inertia and a smaller rear than front roll couple. A front wheel drive car is typically so nose heavy that it is pretty hard to overcome the tenancy to understeer. It takes pretty high rear roll stiffness and a bit of geometric antiroll to get these cars to rotate with reasonable wheel rates.”

Luckily, on a 3-wheeled car, there is no rear roll center, so we don’t need to worry about the roll axis and its effects on understeer and oversteer.

 

3D Cad Parts Here:

http://www.mcmaster.com/#94640a285/=vfas0m

http://www.mcmaster.com/#93839a825/=vfaxdh

http://cad.aurorabearing.com/item/all-categories/am-t-hxab-t-high-misalignment-series-male-rod-ends/hxam-7t-2

http://www.ohlinsusa.com/ohlins-ttx-25-fsae

 

 

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Street Legal Requirements

Because the car will be a 3-wheeler, it will technically be classified as a motorcycle for safety purposes. This greatly reduces the amount of legally-required equipment, thereby following our main design principles of simplifying and adding lightness.

California CHP

Since I live in California and the car will technically be a “motorcycle,” I need to follow the California Highway Patrol (CHP) rules for motorcycles.

California DMV

Since I live in California and am building the car myself, I need to follow California Department of Motor Vehicles (DMV) rules for specially constructed vehicles (SPCNS).

Federal Motor Vehicle Safety Standard (FMVSS)

The US Department of Transportation (DOT) Federal Motor Vehicle Safety Standards and Regulations (FMVSS) govern what safety equipment must be install on all vehicles. The standards are here. The actual laws are here.

FMVSS 108 regulates vehicle lighting.

Horn

Motorcycles in California are required to have a horn.

Standard motorcycle horns are pretty wimpy and ineffective. I like the BansheeHorn that Peter Olt has designed.

Mirror

Motorcycles in California are required to have one rear-view mirror. FMVSS 111 regulates mirrors. The absolute minimum size for motorcycle mirrors is convex mirror with “6450 mm2 of reflective surface.” Since a circle encloses the largest amount of area in the smallest amount of space, the smallest possible mirror would be a circle. The area of a circle is pi*r^2, so the smallest possible mirror would have a reflective surface with a radius of sqrt(6450/pi)=45.3 mm. So the smallest possible mirror would be a convex mirror with a diameter of 3.57 inches. For a flat mirror, the smallest possible diameter is 3.99 inches. What’s interesting is that a lot of companies sell 3″ diameter mirrors, which would be illegal in the US.

One of the smallest mirrors I’ve found in the stock Yamaha XT 500 mirror.

416j2Ci+lUL

Headlamp

CHP and federal rules state that the headlamp must be mounted between 22 inches and 54 inches above the roadway. Whenever the motor is on the low beam must be lit.

The headlamp system consists of a high/low headlamp unit, a switch for off/on switch for the high beam, and a high beam indicator lamp for the dashboard.

I’m planning to use a single LED headlamp from Truck-Lite:

27270C-lr

This LED unit pulls just 1.8 amps under low beam, as opposed to 4.6 amps for a standard 55 watt 7″ H4 motorcycle headlamp.

Stop Lamp / Tail Lamp with Reflector

For motorcycles, the CHP requires a single stop lamp, a single tail lamp and a single rear red reflector. To save weight, these three components can be combined into a single assembly.

The lamp “shall be mounted between 15 and 72 inches above the roadway.” Really? 72 inches? Now I’m no politician, but I don’t think we should be allowing 6-foot-tall motorcycles on the road. Federal regulations also require a minimum mounting height of 15 inches.

The CHP rules state that the tail lamp “shall be visible from 1,000 feet to the rear.” This leads to all kinds of questions. In what kind of environment? Pitch darkness? Full daylight? Visible through a dense fog or the perfect vacuum of outer space? And 1,000 feet where? Straight back? So I could use a laser and and could only see it 1,000 feet back if you were standing in exactly the right spot? This kind of ambiguity would never fly in a formula racing rule book, but apparently government lawmakers aren’t engineers, they’re politicians.

I’m planning to use a simple plastic LED tail light from KC HiLights:

1002__02926_zoom

Turn Signal Lamps

Turn signals are required in California for newly-built motorcycles. The front turn signals must be spaced at least 16 inches apart. The rear turn signals must be spaced at least 9 inches apart. They all must be mounted at least 15 inches above the roadway. They must be amber lamps and they must flash simultaneously.

A turn signal system consists of four turn signal lamps, an on-off-on switch, a turn signal flasher relay, and an indicator lamp on the dashboard.

License Plate Holder and License Plate Lamp

California motorcycle license plates are 7″ x 4″ with 1/4″ bolt holes on the four corners 5-3/4″ apart from each other horizontally and 2-3/4″ apart from each other vertically. According to Section 5201, California motorcycle plates must be mounted horizontally between 12 inches and 60 inches off the ground.

Fenders

Three-wheeled motorcycles in California are required to have fenders on each wheel. The fenders must be at least as wide as the “tire thread” and must “effectively minimize the spray or splash of mud or water to the rear.” I’ve never heard of a tire “thread” before, only a tire TREAD and the CHP doesn’t define the word “thread.” This entire forum post is dedicated to figuring out what is meant by “tire thread.” I’m going to assume the worst case scenario and design the fenders to be the full width of the tire. In order to “effectively minimize the spray or splash of mud or water to the rear,” the minimum possible fender size would begin at the top of the tire and end 90 degrees later at the rear.

Passenger Seat and Footrests

Motorcycles that are capable of carrying a passenger must have a dedicated seat and footrests for the passenger.

Helmets

California has a helmet exemption for “fully enclosed three-wheeled motor vehicle that is not less than seven feet in length and not less than four feet in width, and has an unladen weight of 900 pounds or more, if the vehicle meets or exceeds all applicable FMVSS and the requirements contained in the VC.” But since I am not fully-enclosing the vehicle, both driver and passenger will be required to wear helmets.

Minimum Seat Height

Keeping the seat height as low as possible allows the vehicle to have a lower center of mass, which improves handling, and creates a lower aerodynamic wetted area, improving top speed.

While California has laws banning “ape hanger” handlebars, there doesn’t seem to be a law about minimum seat height. I also can’t seem to find any federal law governing minimum motorcycle seat heights. New York has a minimum seat height of 20 inches for 3-wheeled motorcycles and 25 inches for 2-wheeled motorcycles. Connecticut’s minimum motorcycle seat height is 26 inches. Alberta has a 650mm (25.6 inch) minimum.

Looking at seat heights of 187 production motorcycles (excel sheet here), we see that most motorcycles have a 27 inch or 33 inch seat height:

Motorcycle_Seat_Heights

Optional Equipment

There are rules for optional equipment like fog lamps, front reflectors, modulating headlampswindshields, speedometers, odometers, tachometers, , etc. Since these components are not required, I will not install them since doing so would violate my design principles of simplicity and lightness.

I plan to include a mobile phone mount on the steering wheel with a built-in charging cord. That way I can run the trapster app which will both tell me my current speed and alert me to police speed traps. In order to hear the speed trap alerts, you can buy a helmet with a built in bluetooth system.

Radar Detectors and Laser Jammers

Radar detectors are legal in California. I plan to design in a mounting system for a Valentine One radar detector with a direct-wire power adapter.

Laser jammers are illegal in California.

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Steering System

Steering Ratio

Ackermann Steering Geometry

As your car turns, the outer wheels are further from the center of the steering circle than the inner wheels. This causes your wheels to follow two separate paths with two separate circle radii. An Ackermann steering geometry allows the wheels to follow the correct geometric paths.

There are three geometry possibilities:

  1. Parallel Steering – bad – also known as 0% Ackermann Steering – each tire has the same turning circle – this causes the outside front tire and the inside rear tire to have the largest slip angles.
  2. Ackermann Steeringgood – the inside tire has a tighter turning circle than the outside tire – this increases the slip angle for the inside front tire. This improves low-speed handling.
  3. Reverse Ackermann Steering – bad – the outside tire has a tighter turning circle than the inside tire – this increases the slip angle for the outside front tire. In some instances this can be good for high-speed stability.

Parallel Steering Geometry:

ParallelSteering

Ackermann Steering Geometry:

AckermannSteering

Reverse Ackermann Steering Geometry:

ReverseAckermannSteering

A 100% Ackermann steering geometry has the angle of the steering pivot points intersect each other at the rear axis:

170px-Ackermann_simple_design.svg

Bump Steer

If the axis of the steering tie rod does not align with the instant centers of the suspension arms, you will get bump steer. This will cause the wheels to steer on their own when they hit bumps. Inside the car, bump steer will cause the steering wheel to move on its own when you hit bumps. When people lower their cars with aftermarket suspension kits, they often induce bump steer by changing the instant center of their suspension while not changing the angle of their steering linkage. Bump steer is very bad for vehicle handling and should be minimized at all costs. In order to minimize bump steer you should design the steering linkage with an axis that intersects the instant center of the suspension.

Bad Design:

BumpSteer

Good Design:

NoBumpSteer

Castor Angle / Steering Axis Angle

The castor angle, also called the steering axis angle, is the angle between the the steering axis and a line perpendicular to the ground plane. The castor angle provides self-centering torque. In a car with a large castor angle, you can let go of the steering wheel and the car will continue in a straight line. Increasing the castor angle will increase the mechanical trail distance (which can be good) and causes negative camber gain while steering (which is usually good). Both the castor angle and the kingpin inclination provide self-centering torque, but the castor angle provides self-centering torque with good camber effects, while the KPI provides self-centering torque with bad camber effects. Thus, it is better to create self-centering torque with a higher castor angle than a higher kingpin inclination.

CastorandTrail

 

Positive vs Negative Castor Angle:

castor-positiveandnegative castor-positiveandnegative

Some caster angle numbers of high-performance cars:

  • Corvette C5 & C6: 6.5 degrees
  • McLaren F1: 6 degrees
  • Ferrari F50: 5.5 to 5.7 degrees
  • Lotus 7: 5 degrees
  • Mazda Miata: 5 degrees
  • Triumph Spitfire: 4.7 degrees
  • Lotus Elise: 3.8 degrees

Mechanical Trail

Mechanical Trail distance, also known as castor distance, is the horizontal distance between the center of the tire contact patch and the theoretical point where the steering axis intersects the ground plane. The trail distance is dependent on the castor angle. Almost all high-performance cars have a positive trail distance (meaning the cars have a positive castor angle).

CastorandTrail

Some mechanical trail numbers of high-performance cars:

  • Lotus Elise: 4mm

 

 

Steering Rack

Formula Seven sells a carbon fiber steering rack that weighs only 810 grams. This is lightest steering rack I have found so far.

steering-racks-nosensor

Steering Wheel

Formula Seven sells a nice carbon fiber steering wheel that weighs only 250 grams. This is the lightest steering wheel I have found so far.

wheel

 

Mobile Phone Mount

I plan to mount my mobile phone to the center of the steering wheel to allow me use to use Waze, Google Maps, Pandora, Songza, etc. while I am driving. I plan to use a Ram “model specific cradle” to hold the phone. This will require drilling two holes through the steering wheel to permanently mount the holder. To listen to the audio I’ll use a standard bluetooth motorcycle headset. For longer-distance or wilderness trips, the Earl Backcountry Tablet is waterproof, visible in broad daylight and has a built-in weather alerts and VHF and UHF transceiver.

41Qm4GUrW4L

 

I also plan to install a waterproof USB port into the dashboard. This will allow me to keep the phone charged using the standard phone charger.

usb-5vs

 

 

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Motors & Gearing

One of my main design goals is to keep the mass of the car as low as possible. This means selecting motors that have the most power per unit of weight. I’m planning to run three motors on the car – one on each wheel – which would make the car “all wheel drive.” This will help with acceleration, weight distribution, and simplicity. It will make the system more simple by avoiding a complex mechanical differential system. It will help with weight distribution by allowing the weight of the motors to be more evenly spaced around the car. It will help with acceleration by spreading the acceleration force more evenly across the tires.

I plan to buy the motors from Electric Motorsports in Oakland, California (where I live). They offer a number of motor systems, but one clearly stands above the rest. For peak horsepower per lb, the Motenergy DLC-28 is the best motor:

PeakPerLb

For continuous horsepower per lb, the Motenergy DLC-28 is also the best motor:

ContinuousPerLb

 

Motenergy is a company that designs motors specifically for electric cars and then has them manufactured in China. By pure coincidence, their warehouse is in my home town of Mequon, Wisconsin – maybe the universe is telling me I was destined to use this motor. The DLC-28 is a pretty incredible feat of engineering. It can produce 15 kW of continuous power and 38 kW of peak power (51 horsepower) and it weighs just 35 lbs. For comparison, and old Volkswagen Beetle 1600cc engine (like the kind I had in my old dune buggy) produces about the same peak horsepower (50 hp for the dual port engine) but weighs 220 lbs – more than 6 times more! The other really great thing about these motors are they are sealed to IP66 standards. This means that there is no need to worry about dirt, dust or water getting into the motors and fouling them up over time. As a result, it means that they are basically maintenance-free.

dlc-motor-front

 

With three motors combined, the car should have a 150 peak horsepower and a 60 continuous horsepower for cruising. This should be more than adequate to allow high-speed cruising as well as fast acceleration and high top-speeds for short periods of time.

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Motor Controllers

I’m planning on putting three motors on the car, making it “all wheel drive.” This makes motor control a bit more complicated as I need to have three separate motor controllers that are all linked to the same throttle. Multiple controllers can be linked together to form a “truck drive system” using the “CAN bus.” The front two motors would need to be operated as a “dual traction motor” setup, allowing the controllers to turn each one at different speeds, thus allowing for turning. So my plan is to have all three motor controllers share the same throttle input but to have the rear wheel operate independent and have the front two motors operate as a “dual traction motor” “twin motor system.”

I’m planning on using three Sevcon Gen 4 controllers. Since the Motenergy DLC-28 motors can accept a peak current of 480 AC amps, the motor controllers must be able to accept 660 DC amps. The motors are also capable of taking 180 AC amps of continuous current, so the motor controllers must be able to accept 220 DC amps of continuous current. The Sevcon Gen 4 Size 6 motor controller has a short-term (2 minutes) rating of 650 amps and a continuous rating of 260 amps. The next size down, the Sevgon Gen 4 Size 4, can only handle 180 amps of continuous current, so I’ll have to use the Size 6 controllers.

The Sevcon Gen 4 Size 6 motor controller:

sevcon-gen4-size6

Weight

Each Sevcon Gen 4 Size 6 controller weighs 4.6 kilograms without a heatsink. Combined this is 13.8 kilograms of weight (30.4 lbs). Unfortunately, there’s not much I can do to lower this weight – these components are absolutely essential and I haven’t been able to find any lighter controllers that do as much as these do. In order to save a bit of weight, I’m planning on integrating a heatsink into the body/chassis section where these controllers will be mounted. Since I’m planning on making the chassis out of aluminum – and most heatsinks are made of aluminum – this should be fairly straightforward.

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Chassis

The chassis is the skeleton of the car and is responsible for supporting all components and transferring loads between the tires. In keeping with the design principles, the primary goals are to keep sprung mass low and reduce the total number of components.

Chassis Stiffness / Torsional Rigidity

Weight transfer between tires is dependent on having a high chassis torsional rigidity. Higher chassis stiffness allows for faster weight transfer between tires, making the car feel “sharp” when cornering. Low torsional rigidity causes the chassis to act like a spring, slowing weight transfer and making the car feel “floppy.” High torsional rigidity should be a main goal of any chassis designer. The following are some chassis stiffness numbers from a number of cars in newton meters of flex per degree (Nm/deg):

  • Koenigsegg Agera R: 65,000
  • Bugatti Veyron: 60,000
  • Koenigsegg Agera: 58,000
  • Rolls Royce Phantom: 40,500
  • Audi R8 (2014 MY): 40,000
  • Lexus LF-A: 39,130
  • BMW F10 5: 37,500
  • Volkswagen Phaeton: 37,000
  • Lamborghini Aventador: 35,000
  • Lamborghini Gallardo Super Trofeo Stradale: 35,000
  • Ferrari F50: 34,600
  • Fisker Karma: 33,000
  • Porsche 911 (997): 33,000
  • Volkswagen Passat (2006): 32,400
  • BMW Z4 Coupe: 32,000
  • Alfa 159: 31,400
  • BMW F07 5GT: 31,000
  • Mazda Rx-8: 30,000
  • Mercedes Benz W212 E: 29,920
  • Aston Martin Vanquish: 28,500
  • Koenigsegg CC8: 28,100
  • Aston Martin Rapide: 28,000
  • BMW E70 X5: 28,000
  • Land rover Freelander 2: 28,000
  • Ford GT: 27,100
  • Aston Martin DB9 Coupe: 27,000
  • Pagani Zonda F: 27,000
  • Porsche 911 Turbo 996: 27,000
  • Lotus Evora: 26,600
  • Pagani Zonda C12 S: 26,300
  • Porsche Carrera GT: 26,000
  • Audi A8: 25,000
  • Pagani Zonda C12: 25,000
  • Volkswagen Golf V GTI: 25,000
  • Mini (2003): 24,500
  • BMW E39 5: 24,000
  • BMW E60 5: 24,000
  • BMW E53 X5 (2004): 23,100
  • BMW E30 M3: 23,000
  • Lambo Gallardo: 23,000
  • BMW E90: 22,500
  • Bugatti Veyron Grand Sport: 22,000
  • Jaguar X (Sedan): 22,000
  • Mercedes Benz SL (top up): 21,000
  • Saab 9-3 Sportcombi: 21,000
  • Ford Mustang 2005: 21,000
  • Chrysler Crossfire: 20,140
  • Lamborghini Murcielago: 20,000
  • Volvo S60: 20,000
  • Ford Focus 3d: 19,600
  • Audi TT Coupe: 19,000
  • Bugatti EB110: 19,000
  • Volvo S80: 18,600
  • Bentley Azure: 18000
  • BMW E46 Sedan (w/o folding seats): 18,000
  • Maserati QP: 18,000
  • Pagani Zonda Roadster: 18,000
  • Volkswagen Fox: 17,941
  • Ford Focus 5d: 17,900
  • Chevrolet Cruze: 17,600
  • Ford GT40 MkI: 17,000
  • Mercedes Benz SL (top down): 17,000
  • Jaguar X (Estate): 16,319
  • Ford Mustang 2003: 16,000
  • Jaguar XK: 16,000
  • Aston Martin DB9 Convertible: 15,500
  • Mazda Rx-7: 15,000
  • BMW Z4 Roadster: 14,500
  • Ferrari 360: 14,455
  • BMW E46 Wagon (w/folding seats): 14,000
  • McLaren F1: 13,500
  • Porsche 911 Turbo (2000): 13,500
  • BMW E46 Sedan (w/folding seats): 13,000
  • Porsche 959: 12,900
  • BMW E46 Coupe (w/folding seats): 12,500
  • Opel Astra: 12,000
  • Audi A2: 11,900
  • Porsche 911 Turbo 996 Convertible: 11,600
  • Lotus Elise 111s: 11,000
  • BMW E36 Touring: 10,900
  • BMW E46 Convertible: 10,500
  • Lotus Elise S2 Exige (2004): 10,500
  • Lotus Elise Series 1: 10,133
  • Ferrari 355: 10,042
  • Renault Sport Spider: 10,000
  • Chevrolet Corvette C5: 9,100
  • Lotus Elan GRP body: 8,900
  • Ford Mustang Convertible (5th Gen): 8,800
  • Ferrari 360 Spider: 8,500
  • McLaren M6B: 8,100
  • Lotus Elan: 7,900
  • Dodge Viper Coupe: 7,600
  • Chrysler Durango: 6,800
  • Lotus Esprit SE Turbo: 5,850
  • BMW E36 Z3: 5,600
  • Mazda MX-5 (later w/ bracing): 5,150
  • Mazda MX-5 (1990): 4,880
  • Ultima GTR: 4,500
  • Ford Mustang Convertible (4th Gen): 4,000
  • Lamborghini Countach: 2,600
  • Porsche 906: 2,300
  • Porsche 904: 2,000

As you can see, the cars with higher torsional rigidity tend to be cars that have higher performance. A common complaint of the BMW Z3 or the old Ford Mustang convertibles is that their chassis feels “floppy.” For my car, I’m going to set a minimum torsional rigidity goal of 10,000 Nm/degree – the same as the Lotus Elise.

 

Modeling

The main problem with doing everything in metric while living in the USA is that most of the easy-to-get tubing sizes are in ANSI (imperial) rather than ISO (metric). Common imperial steel tubing OD x Thickness dimensions are:

  • 0.375 in OD x 0.058 in Thickness
  • 0.500 in OD x 0.058 in Thickness
  • 0.625 in OD x 0.058 in Thickness
  • 0.625 in OD x 0.065 in Thickness
  • 0.750 in OD x 0.058 in Thickness
  • 0.750 in OD x 0.065 in Thickness
  • 0.750 in OD x 0.125 in Thickness (used in Factory Five chassis)
  • 0.875 in OD x 0.058 in Thickness
  • 0.875 in OD x 0.065 in Thickness
  • 0.875 in OD x 0.083 in Thickness
  • 1.000 in OD x 0.058 in Thickness
  • 1.000 in OD x 0.065 in. Thickness (used in the Ariel Atom chassis. Also the tubing size required for FSAE race cars)
  • 1.000 in OD x 0.083 in Thickness
  • 1.000 in OD x 0.095 in Thickness
  • 1.125 in OD x 0.058 in Thickness
  • 1.125 in OD x 0.065 in Thickness
  • 1.125 in OD x 0.083 in Thickness
  • 1.125 in OD x 0.095 in Thickness
  • 1.250 in OD x 0.095 in Thickness
  • 1.250 in OD x 0.120 in Thickness
  • 1.250 in OD x 0.250 in Thickness
  • 1.500 in OD x 0.120 in Thickness (used in Factory Five chassis)
  • 1.750 in OD x 0.120 in Thickness (used by the Local Motors Rally Fighter chassis and meets SCORE desert racing series requirements)

Cold-drawn seamless structural round steel tubing conforms to ISO 10799-2:2011; This is a great handbook showing the various standard tubing sizes (see Table 26 for ISO sizes). Parker Steel, out of Toledo, Ohio, is the largest supplier of metric-sized metals in North America. You can also buy metric steel tubing from Metric Express and World Wide Metric. Common metric steel tubing OD x Thickness dimensions are:

  • 8 mm OD x 1 mm Thickness
  • 8 mm OD x 1.5 mm Thickness
  • 8 mm OD x 2 mm Thickness
  • 10 mm OD x 1.5 mm Thickness
  • 10 mm OD x 2 mm Thickness
  • 10 mm OD x 2.5 mm Thickness
  • 12 mm OD x 2 mm Thickness
  • 12 mm OD x 2.5 mm Thickness
  • 12 mm OD x 3 mm Thickness
  • 14 mm OD x 1 mm Thickness
  • 14 mm OD x 2 mm Thickness
  • 14 mm OD x 2.5 mm Thickness
  • 15 mm OD x 1 mm Thickness
  • 15 mm OD x 2 mm Thickness
  • 15 mm OD x 2.5 mm Thickness
  • 16 mm OD  x 1 mm Thickness
  • 16 mm OD x 2 mm Thickness
  • 16 mm OD x 3 mm Thickness
  • 18 mm OD  x 1.5 mm Thickness
  • 18 mm OD x 2 mm Thickness
  • 18 mm OD x 2.5 mm Thickness
  • 18 mm OD x 3 mm Thickness
  • 20 mm OD  x 1.5 mm Thickness
  • 20 mm OD x 2 mm Thickness
  • 20 mm OD x 2.5 mm Thickness
  • 20 mm OD x 3 mm Thickness
  • 23 mm OD X 3.5 mm Thickness (used for the wishbones)
  • 25 mm OD x 1.5 mm Thickness
  • 25 mm OD x 2 mm Thickness (The tubing size required for FSAE race cars is 25.0 mm OD x 1.75 mm Thickness but nobody sells a 1.75mm thickness 25mm OD tube!)
  • 25 mm OD x 2.5 mm Thickness
  • 25 mm OD x 3 mm Thickness
  • 30 mm OD x 1.5 mm Thickness
  • 30 mm OD x 2 mm Thickness
  • 30 mm OD x 2.5 mm Thickness
  • 30 mm OD x 3 mm Thickness

The problem is, in order to choose the proper tubing diameter and wall thickness it is necessary to test the design using finite element analysis (FEA) and unfortunately Solidworks doesn’t have these tubing diameters as standard weldments! So I did a little work and created a two importable libraries of standard tube weldments and uploaded them to my GrabCad page. They are located here:

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Battery Charging

Onboard Charger

I’m planning on having an on-board battery charger to allow me to take advantage of charging stations.

Charging Station Map

 

J1772 Combo Socket

J1772 chargers are usually called “level 1” or “level 2” chargers. These are far more common but charge cars far more slowly than CHAdeMO chargers. Level 1 chargers provide up to 16 amps at 120 volts (1.9 kW). Level 2 chargers provide up to 80 amps at 240 volts (19.2 kW). Chargers from ChargePoint are typically 6.6 kW Level 2 J1772 chargers. For comparison, a standard household outlet can provide 12 amps at 110 volts (1.4 kW).

J1772 Plug and Socket:

J1772Combo_AZPM_Wr1

 

The people at tucsonev.com sell a “level 1” J1772 socket:

AdapterControlsTop_web

 

CHAdeMO Socket

CHAdeMO chargers are usually called a “DC Quick Charge” “Supercharger” or “Level 3” chargers. They can provide up to 62.5 kW of high-voltage DC power.

CHAdeMO plug:

CHAdeMO_2_AZPM_Wr1

 

 

CHAdeMO socket (left) on a Nissan Leaf:

NissanLeaf_Plugs_AZPM_Wr1

 

Tesla Supercharger Socket

Tesla has it’s own proprietary charging socket design.

The Tesla Superchargers are capable of an output of 250 amps at 400 volts (100 kw). Tesla sells a CHAdeMO-to-Telsa adaptor, but no one makes an adapter the other way around.

Tesla Plug:

20120923-142007

Tesla Socket:

Tesla-Model-S-Charging-Port

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Batteries

Energy Density

The two main problem with electric cars today are:

  1. The low energy density of existing battery technology
  2. The slow recharge time of those batteries

For a simple comparison, let’s look at a typical electric car battery vs. a gallon of gasoline. A gallon of gasoline weighs about 2.83 kg and contains about 120 MJ of energy – that’s 42.4 MJ/KG of energy density. A typical 40 amp-hour lithium ion battery weighs about 1.4 kg, so two 40AH batteries would weigh about as much as a gallon of gas. Since the voltage of a lithium ion battery is 3.2 volts and watts = volts x amps, two 40AH batteries will hold 256 watt hours of power. Since 1 watt-hour equals 3,600 joules, two 40AH batteries would hold about 0.9216 MJ of energy. So a gallon of gasoline holds 130 more times energy than the same weight of lithium ion batteries. Or to put it another way, a pile of batteries that weigh as much as a gallon of gasoline would only provide two tablespoons worth of gasoline energy.

Of course electric motors are about 90% efficient while internal combustion engines are about 20% efficient – so you need a lot less initial energy per mile. A car that gets 30 MPG will require 4 MJ per mile of energy while a comparable electric car will only require 1 MJ per mile. Thus, while gasoline is 130 times as energy dense as batteries, on a work-accomplished basis, it is 32.5 times as energy dense. Nevertheless, this is an enormous discrepancy in energy density and still leaves electric cars at a disadvantage. This is why it is so incredibly important to choose the most energy dense battery available on the market.

Electric cars also take far longer to fill up than gasoline cars. The EPA has limited gas pumps to a maximum flow rate of 10 gallons per minute. Most fuel dispensers pump between 5 and 7 gallons per minute. If the fuel filter is clogged on the pump, it will pump far more slowly. So including time spent paying, the typical gas station full-up takes around 5 minutes. Electric cars take far longer to charge and the variability in charging times is enormous depending on the charging source. In the worst-case scenario, charging a large 60-kW Tesla Model S with a standard 110-volt house socket would take 52 hours. Using a 240-volt outlet (like your home dryer uses), you can charge a Nissan Leaf from empty in 4-5 hours. In a best-case scenario, Tesla’s superchargers can provide half a charge to a Model S in as little as 20 minutes. Even in the best-case scenario, however, it still takes far longer to charge an electric car than it takes to fill up a gasoline car. Until battery technology improves, this will simply remain a trade-off of owning an electric car.

Battery Options

There are three main types of batteries you can use in an electric car: lead acid batteries (which are used in golf carts), nickel metal hydride batteries (which are used in the Toyota Prius) or lithium ion batteries (which are used in almost every electric car today). The energy densities of the three technologies are as follows:

  • Lead Acid: 33–42 W·h/kg
  • Nickel Metal Hydride: 60–120 W·h/kg
  • Lithium Ion: 100–265 W·h/kg

So the lithium ion battery clearly is the best. There are three main companies selling lithium-ion car batteries to the general public: CALB, Thunder Sky and GBS. All three companies are Chinese.

In general, the larger batteries (those with more AH) provide more continuous wattage per kilogram of weight and per centiliter of volume:

Continuous Watts per KGContinuous Watts per CL

The larger batteries also generally provide more peak watts per kilogram of weight and centiliter of volume:

Peak Watts per KGPeak Watts per CL

So one might be tempted to simply choose the largest battery they can find. However, this ignores the fact that the voltage of most lithium ion batteries is 3.2 volts. While the torque of an electric motor is dependent on amperage, the speed of the motor is dependent on voltage. Putting 3.2 volts of power into a typical electric motor would only allow it to spin at 166 RPM. In order to get the full 5000 RPM out of the motor, you need to input 96 volts. In order to get 96 volts from 3.2 volt batteries, you must connect 30 of them in series. The motor I am planning on using has a peak amperage into the controller of 1,800 amps. So in order to optimize weight, I need to select the battery type that would provide me with 1,800 amps of power at 96 volts with the lowest weight and volume.

Best Batteries KG Best Batteries L

Best Batteries Price

Based on this analysis, the best battery is the 90 amp-hour Thunder Sky TS-LFP90AHA. Using the battery, the planned configuration for the battery pack is 2 parallel sets of 30 batteries in series. This pack of 60 batteries would have the following characteristics:

  • 96 volts
  • 3600 amps of peak current
  • 90 amps of continuous current
  • 192 kilograms of weight (423 lbs)
  • 119 miles of vehicle range at 42 mph with the motor producing 6.0 kW of continuous power
  • 5.8 second 0-60 estimate
  • $5,805 battery pack price
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Sprung Components

The sprung components are all of the components that are held up by the suspension. The heaviest sprung components are the batteries, the body, the chassis, and the motors. In keeping with the design principles, the primary goals are to keep sprung mass low and reduce the total number of components.

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Sustainable Design

One of my key design goals is to build this car as sustainably as possible. One way to build a sustainable car is to use a Cradle-to-Cradle design methodology. If I ever decided to commercialize this design, I’d want to go through the full C2C certification process, but for now, I’m just following the core principles. Cradel-to-Cradel design involves making the entire product’s lifecycle sustainable. The product should be build out of the most sustainable materials available. Both the manufacturing process and the operation of the product should create no waste, pollution or negative human health effects. And at the end-of-life, the product should be completely recycled without producing any waste or pollution or it should be “upcycled” into a higher-value product. In order to achieve this level of lifecycle sustainability, material selection is key.

 

Sustainable Materials

Materials should be:

  • Infinitely recyclable – without any degradation – no “down-cycling”
  • Reusable – Rather than recycling components, its’ even better to design them to be “upcycled” to a higher-value product after the useful life of the product runs out

Many common car materials do not fit these design criteria. Some common materials in cars:

  • Steel – Must be painted or it will corrode over time
  • Paint – Causes air and water pollution during its application and is toxic to the environment at end-of-life
  • Plastics and Adhesives – the average car has 150 kilograms of plastic in it – plastics are used in the bumpers, seat upholstery, dashboard, interior trim, exterior trim, and numerous mechanical and electrical components. The vast majority of the plastic in cars causes pollution during its manufacturing, and is not reusable or recyclable.

Some materials that might be considered “sustainable”:

  • Aluminum – aluminum is infinitely recyclable without any degradation. It never needs to be “downcycled.” Aluminum is also extremely resistant to corrosion, as aluminum oxide produces a thin protective layer, leading to a dull finish but production against further corrosion.
  • Stainless steel – stainless steel is infinitely recyclable and extremely resistant to corrosion.
  • Certified wood – wood is renewable and can be grown sustainably.
  • Bioplastics – Instead of being made from oil, bioplastics are made from plant starch. Parts can be 3D printed from bioplastics, eliminating manufacturing waste.
  • Soy Foam – As an alternative to petroleum-based polyurethane foam rubber used in seat cushions, soy-based foam looks promising, but may not be totally sustainable.
  • Cork – Natural cork is another sustainable alternative to polyurethane foam.
  • Natural Latex Rubber – An alternative to  petroleum-based polyurethane foam rubber is natural latex rubber, which comes from the rubber tree. Because it can be sustainably-harvested, this product may be preferable to soy foam.
  • Renewable Fabrics – Natural fabrics made of sustainably-grown bamboo, hemp, cotton, linen, or wool can be good alternatives to petroleum-based fabrics like polyester, nylon or vinyl. However, in order to meet FMVSS 302 safety standards, these materials must be coated with a fire retardant chemical such as PBDE, which besides being unsustainable, has some pretty nasty health effects.
  • Leather – concerns about animal welfare aside, leather can be produced in ways that are more sustainable than synthetic fabrics.

 

Non-Sustainable Materials I Need to Use

So far I have identified a number of non-sustainable materials in parts I have selected that I have not yet found a sustainable alternative for. I hope to do additional research to find alternatives to these.

  • Batteries – The batteries will be the largest component of the car by mass. They are typically constructed out of lithium, plastic (for the container), graphite (for the negative electrode), organic carbonates (for the electrolyte) and metal oxides (for the positive electrode). The lithium iron phosphate battery (LiFePO4) I’ve chosen uses lithium iron phosphate as the cathode.
  • Wheel Bearing Grease – found sustainable “food grade” grease alternatives here.
  • Teflon Rod End Liners – Teflon is polytetrafluoroethylene (PTFE), which is produced from tetrafluoroethylene (TFE), which is produced from chloroform, which is produced from methane (which is natural gas) and chlorine (which comes from salt). While it is technically possible to create plant-based PTFE, I haven’t been able to find any, let alone find rod ends with sustainable high compression strength PTFE liners. On the other hand, these liners only weigh a few grams, so they’re not a high priority for finding sustainable alternatives.
  • Motor Coolant – sustainable motor coolant here.
  • Plastic Wiring Connectors
  • Epoxy Wiring SealantMasterBond makes “food grade” epoxy sealants.

 

 

Sustainable Manufacturing

 

 

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