Suspension

Brian Coombs

The suspension on a car serves two purposes, first it contributes to good handling and stability making the car safe to drive, and second it keeps the vehicle occupants comfortable insulating them from bumps and vibrations.

These goals are generally at odds, so the tuning of suspensions involves finding the right compromise. The suspension also protects the vehicle itself from damage and wear.

Suspension for a land speed record car

As you will probably imagine the suspension you need for a land speed record car is different in many ways than that of a road car (but also have a lot of similarities). One of the many differences is driver comfort, all our effort is put into making the car stable throughout the speed range and as long as the driver can put up with the uncomfortable ride for the short time, then that is fine with me! Another is the limited steering lock, which is currently 5 degrees. We only have 5 degrees for a number of reasons; firstly we will not need to get around any tight corners, secondly the chassis would have to be too narrow between the front wheels to give a suitable structure to mount the suspension on and the bodywork would have to be wider at the front to accommodate the steered wheels, increasing the aerodynamic drag.

Jet powered land speed record cars generally do not have tyres. Our wheels will be rotating at 10000 rpm and the centrifugal forces on the outside diameter make manufacturing a strong enough tyre impossible. This means all the shocks and bumps that the wheels see when they are traveling over the ground are fed directly into the suspension, and not partially absorbed by the pneumatic tyre, so the springs and dampers need to be optimized to suit this.

Although the salt flats and dried up lake beds where we are planning to run the car on look flat, we think we need about 100mm of bump and droop travel in the suspension to absorb any undulations in the surface. Running out of suspension travel can severely upset the cars stability so it is better to design in too much suspension movement, rather than find out once you are running the car that there is not enough.

The car currently weighs about 6400kg when fully fueled, so the front suspension is designed in such a way to make the steering light enough for Andy Green to turn the steering wheel without too much effort at speed, but also not too sensitive so it would be difficult to drive in a straight line when traveling at speed.

Currently we have :-8 degrees King Pin Inclination, 6 degrees of caster, 2mm ground offset, and 100mm of trail. The trail is quit a bit higher than you would see in a road car as this would normally give a high steering load, so to counteract this load we have a steering ratio of 30:1. With only 5 degrees of steering lock, Andy can get from full lock to full lock without taking his hands off the steering wheel; this layout should also give us a manageable steering load, which is not too sensitive.

We have a problem with gyroscopic effects in the steering. The large front wheels currently weigh 137kg each and spin at 10000 rpm, creating huge forces in the steering system, so high in fact in some cases you would not be able to hold the steering wheel even with a 30:1 ratio. To fix this, we have designed the steering with a worm and wheel gear system which has no feedback, i.e. any loads in the steering system do not feed back up the steering column to the steering wheel. The down side to this is that there is no feedback to the driver through the steering wheel, so we are working on a system using a servo motor attached to the steering column to apply loads in the column giving artificial feedback. This could also be used as a powered steering system if the steering loads are too high.

When we started looking at ideas for the suspension we looked at many different layouts. Currently we have a double wishbone front suspension with a front track of 1 meter and a double wishbone rear suspension with a rear track of 2.39 meters, as a compromise between vehicle lateral stability, aerodynamic drag, the ability to get a working suspension layout packaged in the car, and camber and toe stiffness. All suspensions are a compromise, and it is the designer who needs to find a solution which best suits his needs. Our double wishbone pull rod rear suspension is a prime example; we needed a suspension which would have good stiffness, but also very low drag, as it is in the air stream at the rear of the car. We could have gone for a small diameter beam rear suspension which would have only one link in the airstream, giving low drag, but this would have given us poor camber and toe stiffness, so we have compromised on the drag, to make sure we have good control of the wheel.

The track widths chosen, the current layout of the car gave us a stable car up to 1g lateral load.

From the many options we looked at we reduced them down to 3 layouts, which were considered in more detail the pictures below show some early car layout schemes.
 

                                                               Option 1

Narrow Front Track Wide Rear Track

Double wishbone independent rear suspension, leading arm non-independent front.

This was rejected because of the drag of the wide rear suspension, and difficulties in getting the front non independent geometry to behave in a stable manner.

Option 2

Wide Front Track Narrow Rear Track

Live axle front suspension, rear trailing arm independent suspension

This was rejected because the supersonic shock waves coming off the front suspension would upset the ground that the rear wheels are running on. This was a lesson learnt from thrust SSC. The rear trailing links are also difficult to design to give good vehicle stability.

Option 3

1 Meter front Track 2.39 meter Rear Track

Double wishbone front suspension Double wishbone rear suspension with pull rod operating the spring and damper assembly.

This is the layout we have chosen as, I believe, it is currently the best compromise between all the design criteria. They are many un-defined details in our current suspension layout and as this is a constantly evolving design process, it may yet change significantly.