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:
- 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.
- Ackermann Steering – good – 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.
- 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:
Ackermann Steering Geometry:
Reverse Ackermann Steering Geometry:
A 100% Ackermann steering geometry has the angle of the steering pivot points intersect each other at the rear axis:
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.
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.
Positive vs Negative Castor Angle:
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 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).
Some mechanical trail numbers of high-performance cars:
- Lotus Elise: 4mm
Formula Seven sells a carbon fiber steering rack that weighs only 810 grams. This is lightest steering rack I have found so far.
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.
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.
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.