This is a continuation of an earlier article entitled “Auto Tuning Part 1: Fundamentals of Vehicle Dynamics” where we began an in-depth look at the dynamics and fine tuning of vehicles. The first article covered subjects such as understeer, oversteer and neutral steer as well as ride comfort and how each relatively small adjustment can have a significant impact on the performance of a car.
In this article, we look at the subject of suspension and strip it down into individual suspension elements to explain what each part does and how it impacts driving experience and performance. It is only when we strip down the elements of the modern-day suspension systems that we can begin to understand how best to grip the road while maximising performance and what compromises are made to suit different types of vehicle. As you will see below, a relatively small tweak of any of the key suspension elements will have a knock-on effect to different areas of a vehicle and so the challenges begin.
Suspension geometry and key suspension elements
Spring and Anti-Roll Bar (ARB) Selection
As I mentioned in the first article in this series, the springs should be as soft as possible to maximize tire contact. However, they should still be stiff enough to avoid grounding and to prevent excessive roll. This stiffness dictates the car’s natural frequency which is usually in the range of 1-1.5Hz.
As seen in Figure 3, the 1-1.5Hz range results in acceptable RMS acceleration for low frequencies. You will notice a slightly higher vibration in your car when you start accelerating from full stop and pass through its resonant frequency.
Normally, spring change is unwanted for a tuning operation. The regular process is to start with bush mounts, anti-roll bars and dampers before moving to a spring change. From a handling point of view, the springs and anti-roll bars affect mostly steady-state behaviour, for example in the middle of a corner.
A brief summary:
|Increase Understeer||Increase Front ARB stiffness||Decrease Rear ARB stiffness|
|Increase Oversteer||Decrease Front ARB stiffness||Increase Rear ARB stiffness|
Figure 2 – Sprung mass natural frequency RMS acceleration (Gillespie, 1992)
Also known as shock absorbers, the dampers’ function is to dissipate the energy absorbed by springs. Their effect is important to a vehicle’s ride behaviour – Gillespie (1992) divides damping ratios as shown below:
Low damping (10%): Results in a very high response peak for low frequencies and good isolation for higher speeds. The low speed peak is unacceptable and this order of damping is not used.
Medium damping (40%): Common range for road cars, combining good low frequency and high frequency responses.
Critical damping (100%): Good low frequency behaviour and poor high frequency responses.
Overdamped (200%): This set-up actually makes the damper so stiff that it does not allow the vehicle to move. The wheel hubs vibrates by themselves, generating resonant behaviour in the range of 3-4Hz.
Besides its importance to ride behaviour, the damper is one of the main tuning possibilities for vehicle handling in a corner and can have an effect when entering or leaving a corner, when the vehicle is rolling. For example, if the vehicle has too much oversteer on the corner entrance, reducing the damper rear stiffness would reduce the amount of weight transferred to the rear and could help to reduce oversteer.
Figure 11 – Damping variation effect on suspension transmissibility (Gillespie)
This is a common tune for a vehicle still in the development phase. Stiffening or softening the vehicle`s engine mount can drastically change its ride behaviour. For more sophisticated results (and more complexity in design) hydrodynamic engine mounts can be used instead of the common rubber bush.
You must be careful when tuning based on tire size and compound. As much as a bigger tire will indeed provide more grip, it will highly influence the ride comfort. Also, bigger tires mean a bigger rotational inertia to be moved by the powertrain and additional study is required to estimate the outcome on performance. Tire pressure is also a main tuning parameter. It can influence the understeer gradient of a vehicle and should be carefully observed.
Suspension geometry and steering geometry
Here is where you will find the most parameters to play with, although they are harder to change when the vehicle is already built. The most important parameters to change are…
Camber is the amount of lateral rotation the tire has from the vehicle`s front view. Camber is normally around 1° for road cars to compensate for body roll in corners. This causes the tire to have maximum contact in the corner, leading to maximum lateral acceleration.
Figure 12 – Camber compensation (Smith, 1978)
Warning here! “More is better” is not valid here. Sometimes you see “self-tuned” cars on the street with visible negative camber. Road cars will achieve their maximum camber thrust on the designed value. Increasing it will actually decrease your grip capacity since the car will have a shorter tire thread to apply traction.
Steering ratio, caster and scrub radius
The main objective when changing the steering ratio is to make the car more responsive and agile. Be careful though, the steering ratio will affect the understeer gradient as well.
Caster is the front wheel inclination from the side view. This provides a self-aligning torque on the steering system which makes the wheels come back to central position when you let the steering wheel go.
Scrub radius is the ground level offset of the wheel centerline and its rotating axle. The bigger the scrub radius, the more tire is offset and less steering effort will be needed when parking the car. Since this also highly affects braking stability, a trade-off must be found.
Other geometry considerations
The vehicle suspension can be set-up to provide anti-dive/anti-squat geometries which will improve the vehicle’s performance during acceleration and braking.
This constant change in stiffness in search of the best vehicle set-up is the daily job of a vehicle dynamics/race engineer. A great book to learn more about suspension systems and suspension elements design and tuning is Milliken and Milliken (1995). Figure 9 shows the influence of several tuning parameters between each other.
Figure 13 – Suspension tuning influences (Milliken and Milliken, 1993)
- Balkwill, J. (2013) “Advanced Chassis Engineering Student Handbook”, Version 8, Oxford: Oxford Brookes University.
- Caroll Smith (1978) “Tune to Win”, California: Society of Automotive Engineers.
- Daimler Media Website (http://media.daimler.com)
- Gillespie, T.D. (1992) “Fundamentals of Vehicle Dynamics”, Warrendale: Society of Automotive Engineers.
- Milliken, W.F. and Milliken, D.L. (1995) “Race Car Vehicle Dynamics”, Warrendale: Society of Automotive Engineers