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:

Will Martin is an energy analyst and expert on peak oil and alternative currencies. He is an MBA graduate of Cornell University, where he was a Roy H. Park Leadership Fellow and concentrated on studying sustainability in business through the school’s Center for Sustainable Global Enterprise. Prior to his MBA, Will worked in the energy industry, living in Singapore, Houston and Dubai. Will is a recipient of the 2012 “Pioneer Award” from the Association for the Study of Peak Oil and Gas (ASPO-USA). He currently works as a carbon trading commercial adviser in the San Francisco Bay Area. Will is a bitcoin enthusiast and in 2014 published the book “Anonymous Cryptocurrencies,” which became a #1 best seller in 3 Amazon categories and was the first book to be sold on a decentralized marketplace.

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