Vehicle dynamics and chassis design
Vehicle chassis dynamics is a vast and all encompassing subject that incorporates absolutely every aspect related to making a vehicle chassis operate desirably.
Having an implicit understanding of exactly how and why a car chassis is designed the way that it is and being fully abreast of the very many important chassis design considerations puts us in the best possible position when we are asked to diagnose a customer’s problems in the aftermarket. Of course once the problem can be positively identified it is then possible to take good positive action and adjust a customer’s vehicle in order to achieve the desired results.
The main aspects that require consideration when calibrating a chassis are:
Being able to manipulate the exact angle of each wheel in the vehicles “at rest” state, this is a very powerful and very direct method of influencing the way that the tyres are presented to the road as the vehicle moves along, we can use this to create very specific effects.
This is the study and manipulation of the way that the geometry (above) changes as the suspension system is articulated throughout its range of motion and is subjected to loads, cornering, accelerating or braking.
Spring rate selection
Spring rate selection is an area of conflicting interest. A stiff spring system gives a fast and hard link between the chassis and the tyre’s contact patch, so gives fast responses. But on the other hand the stiffer the link between the contact patch and the chassis (i.e. a stiff spring) the greater the body acceleration will be if the wheel is disturbed (hits a bump) this can lead to un-comfort and in extreme cases user fatigue or even sometimes a net reduction in grip.
Damper rate manipulation
The original and main job of the damper is quite literally to damp any unwanted harmonic oscillation of the suspension spring. However over the years the dampers roll in the suspension system has been expanded to include partial control of the load that is transmitted to the tyre contact patch from the car body. It is this secondary but highly important function which makes suspension damper calibration a vast and complicated subject.
Roll force centre analysis and manipulation
When a car body has lateral forces acting upon it (i.e. the tyres forcing it to change direction or speed) the body’s reaction is to roll in a way closely related to its centre of gravity. These roll events change the way that the chassis presses down on to its tyres and is therefore of great importance when calibrating a vehicle chassis.
Body roll as is mentioned above has the effect of changing the load distribution and the suspension articulation of the chassis in a dynamic fashion. Being able to control these roll events gives us the ability to harness body roll and use it as a handling modifier.
Having a vehicle’s entire mass distributed evenly around a car is always ideal in terms of consistent handling. But many vehicles are designed and produced with unfortunate mass distributions, which are often near impossible to correct. In these cases simply being aware of the exact mass distribution pattern of the chassis and then in turn its short comings can become powerful knowledge when tuning the chassis.
Inertia match theory is the study of the chassis movement whilst turning. By looking at chassis wheelbase length, yaw inertia and centre of gravity it is possible to understand the relationship between the vehicles front and rear tyre slip angles as the chassis is yawed. The output of Inertia Match Theory is the chassis dynamic index number.
Ride height and suspension travel manipulation
Adjusting vehicle ride height is often the easiest way of making slight adjustments to the chassis roll force centres and weight distribution. Whilst having adequate suspension travel in order to accommodate the largest likely to be encountered obstacle is crucial in order to prevent both poor handling and suspension component damage.
The dynamics of the pneumatic tyre are massive and a very complex subject. Understanding the many different ways in which the tyres generate grip and having an intimate understanding of exactly how a pneumatic tyre works is absolutely crucial as it is this knowledge that under pins every aspect of chassis dynamics.
Braking and driving torque variations
Chassis dynamic behaviour can be greatly influenced by traction effects, whether it be braking or driving torque it is all transmitted through the tyre contact patch and is therefore very important to both understand. Being able to modify these traction effects by ways limited slip differentials, brake pressure control or bias mechanisms, traction control systems or active differentials is as much a part of the chassis setup as the calibration of the dampers.
The ultimate aim is to tune a vehicles entire chassis in order for it to perform it’s required function better whatever it might be, for example:
- Load carrying
- Long distance road travel
- Sports road driving
- Regular track day use
- Full motor sport