Designing Suspension Systems in Motorsport

In this article, I will be looking at the aspects of designed a suspension system for a racing car. This will involve looking at suspension requirements, why we need suspension, motorsport related requirements, performance, tuning, reliability, making it predictable, responsiveness, geometry of the wheel with respect to the kingpin geometry, mechanical and pneumatic trail, caster angle, components that make up the suspension as so on. Please feel free to skip to the part that is most relevant to you.


Before designing any suspension system for a motorsport purpose, the objectives or requirements of the system needs to be laid out first. The suspension should:

  • Give structural strength
  • Provide isolation from high frequency vibrations such as from the tyres.
  • Control the attitude of the body to prevent roll.
  • Have a good a response to input forces such as from accelerating, braking and cornering.
  • Give a good ride and provide good handling performance. This is achieved through isolating the car from road inputs such as bumps and gradients.
  • Be able to give good steering control so the stability of the wheel when turning is good.
The main objective of a suspension system is to make sure the contact between the tyres and the road stays as constant as possible. Let's take two examples to explain this: a car without suspension and one with suspension going around a corner with a slightly bump on the surface of the track:
  • When the car without suspension goes around this corner and hits hits the bump, the wheel will raise over it which is expected. However, once the car has cleared the bump, there is no force pushing the tyre back onto the road except the force of weight produced by the car. For this reason, for a split second, the tyre is out of contact completely with the road producing no level of grip whatsoever. This will cause handling problems around the corner.
  • On the other hand, a car with suspension would hit the bump compression the spring in the suspension system. This causes the body of the car to not tilt since the extra energy due to the bump is being stored in the suspension spring. Therefore, when the tyre clears the bump, it has the force of the spring and the weight of the car that is pushing the tyre back down onto the road. This means the tyre will regain contact with the road much sooner increasing the handling performance of the car going around the corner.
As well as there being requirements for a suspension system in general, there are also race car requirements that engineers need to take into account specific for motorsport racing:
  • Performance - How well does it improve the suspension improve the handling of the car? Force and moment characteristics need to be taking into account. As well as this, the damping curves of the shock absorbers need to be analysed to see how they affect the vehicle's handling under different loads.
  • Tuneable - Is it possible to change the damping coefficient, spring rates, camber, anti roll bars, toe and tyre pressured to affect the handling of the car?
  • Predictable - If predictable, the driver will be able to throw the car more into corners knowing how the suspension will react. A predictable car will have a minimal roll centre migration. This is the theoretical point at which the car rolls on the suspension system (less roll, the better and more predictable). Programs such as Lotus Suspension analysis make it possible to visualise the roll centre as well as other helpful parameters. In essence, for whatever force you apply to the suspension system, you want it to react in a uniform constant way.
  • Reliable - A faulty suspension system will not win races. It is strongly recommended to always stick all the fasteners in double shear as this will help distribute the load much better than in single shear, minimising the risk of failure. As well as this, fasteners can be kept tight using lockwire. When it comes nuts, there are a wide range that can be used such as prevailing torque nuts, philidas nuts, stover nuts, binx nuts and nyloc nuts (making sure they are lightly lubricated to overcome friction when tightening them). Torque seal can be used to keep nuts tight too (a type of glue). Reliability can also be achieved by knowing where the load paths are going and by using quality components.
  • Responsive - The quicker the suspension reacts to external and internal input forces, the better. It should have a low yaw inertia and, of course, be as light as possible. The suspension stiffness will play a big factor too, which will depend on the natural frequencies of it, the wheel stiffness and the roll rates.
  • Steering is light - This will help to provide good feedback to the driver so he or she can feel the car better.
Double Shear Diagram

Lockwire Diagram

Geometry

The geometry of suspension systems can be considered a bit like dark magic: it is pretty difficult to understand since it is so complex!
Side View Wheel Kingpin Geometry

Wheel Kingpin Geometry

Looking at the above images:
  • The steering axis is defined by the kingpin.
  • The offset of the centre of the tyre print to the steering axis is either called the 'kingpin offset' or 'scrub radius'.
  • By measuring horizontally from the axle heigh, the distance between the wheel centre plane and kingpin offset is known as the 'spindle length'.
  • In side view, the kingpin is known as the 'caster angle'.
  • The distance measured from the the point the steering (or kingpin) axis contacts the ground to the contact patch of the tyre is known as the 'mechanical trail'.
  • If the kingpin axis does not pass through the centre of the wheel, it is known as the 'kingpin offset'.
  • The kingpin inclination, spindle length and scrub radius are usually a compromise between packing and performance. For example, by adopting a positive spindle length, the car will raise as a result of the front wheel steering and the more kingpin inclination the more the car will be raised when the front wheels are steered.
  • The kingpin inclination will affect the camber of the tyre around the corner. When a wheel is steered, a kingpin inclination will cause a positive camber (which is not ideal).
  • The kingpin inclination and spindle length cause the car to raise the front end. This helps centering the steering at low speed.
  • When driving, the accelerating/braking forces will create steer torques that will be proportional to the scrub radius.
  • If there is a zero scrub radius, there will be no steering torque effects since there will be no bending moment occurring.
  • For a front wheel drive car, by having a negative scrub radius will have critical preserving characteristics. This include when one wheel loses traction, the other wheel will toe out which will cause the car to steer in a straight line. As well as this, a negative scrub tends to keep the car in a straight line under braking: even when the forces on the tyres are different.
  • When the car is moving forward, the tyre print follows the steering kingpin axis - think of it like the shape of a shopping trolley wheel.
  • The larger the caster, the more trail there will be and the more self centering there will also be: this causes the self centering moment. Caster is the primary source of self centering moment about the kingpin axis.
  • A larger mechanical trail equals a higher steering force.
  • Positive caster will cause negative wheel camber when steered,
  • As well as mechanical trail, there is pneumatic trail. This is caused by the tyre itself, for the reason that the tyre will not behave linearly but in a non-linear fashion. As pneumatic trail tends to zero, the self centering torque reduces which is a signal that the driver is approaching the limit of cornering speed.

Components that Make Up the Suspension

  • Springs - Absorbs the energy in the suspension.
  • Anti roll bar - Enables both suspension systems on either tyre to transmit opposing forces to each other to prevent roll.
  • Shock absorbers - Dampers absorb shocks and vibrations. There are different types of dampers such as mono tube dampers, mono tube dampers with piggy back and mono tube damper with remote reservoir.
  • Joints - These are usually ball bearing joint to enable movement at the joint.
  • Fasteners - There are many different types of nuts that can be used (mentioned above) along with lockwire and torque seal.
  • Wishbones - connect the upright to the chassis.
  • Upright - Connects the wheel to the  wishbones, toe rod and push/pull rod.
  • Push/pull rod - Attaches upright to the rocker.
  • Rocker - rotates to transmits loads to the anti roll bar and spring/damper.
  • Toe rod - Attaches from the chassis to the upright and controls the amount of toe the wheel has.
  • Camber plates - used on the upright to change the angle of the wheel to vertical.

Some Tips for Designing a Good Suspension System

  • Any rod ends in behind is bad! Rods should only ever be in compression.
  • Load paths should be through nodes.
  • Make sure the wheels are always spigot mounted.
  • Surface treat components dependent on what they are made from and what they are.
  • Try not to due tapered wheel nuts on alloy rims.
  • Consider the worse case loading conditions such as a car accelerating around a fast corner and hits a bump.
  • Use lockwire, locking nuts and torque seal.
  • Make the toe base on the upright as large as possible to reduce the force on the toe rod (which will help make the wheel stable when steered).