The goal is to use an unequal length double ‘A’ arm wishbone setup. A ‘push rod‘ transfers the suspension loads to a ‘bell crank‘ mounted on the chassis. The bell crank ratio has been tuned to provide a slightly rising compression relationship between wheel movement and damper movement; this makes the car more comfortable and predictable for the driver, yet helping stability in hard turns.
Double wishbone suspension has been very popular on American cars for decades, but in Europe this practice died out in the 1970’s with the Ford Cortina, Vauxhall Viva and Triumph Herald. This is due to European cars being much smaller and thus unable to accommodate this relatively bulky type of suspension. A-arms are also more costly than McPherson struts involving many more parts to maintain.
For these reasons, very few smaller cars adopt it. Honda are one of the few manufacturers still using it.
The bell crank is actually two arms with a pivot position at an angle less than 90-degrees. The arms can differ in length giving different compression ratios; however, most arms are of the exact same length, forming a perfect 90-degree angle.
The mounting positions alter the ratio of pull and can be made to move one component a great deal with only minimal movement of a corresponding component.
The goal is to source bell cranks and coil−over shocks from a large motorbike. Motorbikes are a great source for sophisticated coil-over shock absorbers, that are adjustable for both rebound and compression. However, the problem is the spring rates. A sevenesque roadster performs well with shocks in the 300-350lbs/in range. A Hayabusa has a rear spring in excess of 735lbs/in. However, I may have found the perfect & relatively cheap solution…. Watch this space…..
The advantages of using push/pull rods and bell cranks are:
- Less un-sprung weight
- Linear or rising compression rates with wheel movement
- Compression ratio control
- Better aerodynamics on open wheeled vehicles
1. Less Un-sprung Weight
The coil-over shock absorber can be moved in-board, leaving only the much lighter push-rod as un-sprung weight.
Un-sprung weight (or the un-sprung mass) is the mass of the suspension, wheels and other components directly connected to them. It does not include the weight of items supported by this combination e.g. the chassis and engine.
With a lower un-sprung mass, the wheels respond to road bumps more effectively with less inertia. This can give more grip when tracking over uneven roads.
2. Linear or rising compression rates with wheel movement.
With a traditional outboard position, the shock absorber will compress with a more or less 1:1 ratio to any wheel movement. E.g. 1cm of wheel movement will roughly equal 1cm of coil-over compression.
In reality, due to the coil-over leaning over towards the car, the compression ratio actually drops slightly the more the wheel moves.
With a ‘Bell Cranked’ push rod system, it is possible to increase the amount of coil-over compression with wheel movement. This has a similar effect to using a anti-roll bar. Theoretically, this can give a supple initial movement (and ride), rising and stiffening as the suspension is worked harder, for better handling.
The diagram below shows the Bell crank rotating around a fixed pivot point. When the vehicle is at normal level ride height, the bell crank’s push-rod arm will be horizontal. Relatively large movements of the wheel and attached push-rod (Y direction) will result in relatively small amounts of shock absorber compression (x direction). This will give a softer ride under ‘normal’ conditions.
However, as suspension movement increases (e.g. under roll or bounce) the amount of shock absorber compression will increase logarithmically. This is true until the Bell crank becomes vertical.
As can be seen in the diagram below only a small amount of push-rod movement (Y direction) produces a large amount of shock absorber travel (X direction). This has a similar effect to fitting an anti-roll bar, as the effective spring rate under peak conditions is much greater.
3. Compression rate control.
In some cases more suspension travel is needed than the amount of travel in a shock absorber. A Bell Crank with different crank lengths could be used to give a ‘multiplied’ travel ratio:
e.g. 2:1 or 2cm of wheel movement to 1cm of shock compression.
4. Better Aerodynamics.
With open wheeled vehicles, simply moving the shock assembly out of the main air flow can have significant effects on aerodynamic disturbances.
The final possible advantage is that there are less bolts and sharp edges that will need to be covered for the IVA test on open wheeled vehicles.