Chassis Dynamics – Understanding The Basics.

Chassis Dynamics – Understanding The Basics. (Copyright Torque Developments International)

Torque Developments International’s Technical Director – Sam Borgman explains the concepts surrounding this complex subject.

Vehicle chassis dynamics is a vast and all encompassing subject that incorporates absolutely every aspect related to making a vehicle chassis operate desirably.
Having a detailed understanding of the “how and why” of vehicle chassis design helps put us abreast of the many often conflicting chassis design considerations and gives us the best possible chance when we are called upon to diagnose a problem. Only once a problem can be positively identified it is possible to take good positive corrective action and actually adjust the vehicles dynamics to achieve the desired results.
The main aspects that require consideration when calibrating a chassis are:
• Static Geometry:
Being able to manipulate the exact geometry of each wheel on the vehicle whilst it’s at rest, this is the primary method of influencing the way that the tyres are presented to the on-coming road as the vehicle moves along, we can use this technique to create accurately targeted effects relating specifically to the pre-disposition of the tyre’s loadings.

• Kinematic Geometry:
This term refers to the way that the static geometry (see above) changes as the suspension system is articulated throughout the range of its motion and also when it deflects as it is subjected to various loads, e.g. 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 the greater the average vehicle body acceleration will be when the wheel is disturbed, this can lead to an uncomfortable time for the people inside the chassis and in extreme cases user fatigue.  In terms of pure chassis performance a balance must be struck between, ride comfort, system frequency, load path response time, and the control of vertical load force to the tyre’s contact patch.

• Damper rate manipulation
The original job of a damper was quite literally to damp any unwanted harmonic oscillation of the suspension spring. However over the years the dampers function in the suspension system has been expanded to include partial control of the vertical load as it is transmitted between the tyre contact patch and the vehicle body. It is this secondary but highly important function which makes suspension damper calibration a vast and complicated subject with very much to be gained if calibrated correctly.

• 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. In some cases it is possible to move the positions of the roll force centres and therefore use them as a tuning tool, in other instances they are rather fixed, but in all circumstances it’s important to understand where they are and how they will affect your chassis in operation.

• Anti-roll rates
Body roll as is mentioned above has the effect of changing the load distribution dynamically and also the suspension articulation of the chassis in operation. Being able to control these roll events gives us the ability to harness body roll and use it as a handling modifier. This can be a very powerful tool in order to make large broad strokes changes the cars balance when out in the field.

• Mass distribution
Having a vehicle’s mass distributed evenly around chassis is ideal in terms of consistent handling. However because of the pressures of component packaging many vehicles are designed and produced with unfortunate mass distributions, these imbalances can be impractical to completely correct. And in these cases simply being aware of the chassis’s mass distribution pattern and it’s inherent effects can become some powerful underpinning knowledge when tuning the chassis for performance.

• Inertia matching
Inertia match theory is the study of the overall chassis movement whilst in motion. By looking at chassis’s wheelbase length, it’s yaw inertia and it’s centre of gravity it can be possible to understand the relationship between the slip angles generated by the front axle as the chassis is yawed and slip angle at the rear axle.

• Ride height and suspension travel
Adjusting vehicle ride height is often the easiest and most direct way of making slight adjustments to the chassis roll force centres and can effect mild changes in the static weight distribution.  Having adequate suspension travel in order to accommodate the largest obstacle that is likely to be encountered is crucial in order to prevent very poor handling and damage to suspension components. Ride height also becomes a serious variable in the generation of underfloor aerodynamic effects and for vehicles employing performance enhancing aerodynamics there is often a very careful balancing act between needing suspension movement for chassis dynamics, but then perhaps not wanting any suspension movement in order to achieve better aerodynamic performance, in these cases simulation and track testing are used to settle the argument.

• Tyre dynamics
The tyre is the most important part of any vehicle and the dynamics of the pneumatic tyre are a massive and a very complex subject. Understanding the many different ways in which rubber tyres generate grip and having an understanding of exactly how a pneumatic tyre works is crucial, this knowledge under pins almost every aspect of chassis dynamics and drives the design of a cars suspension from the very beginning. Due to size of this subject I’ll cover the basics of tyre dynamics at a later date in it’s own post.

• 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 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 set-up as is the calibration of the dampers.

Overview
The ultimate aim is to view and tune a vehicles entire chassis in a holistic way to enable it to carry out it’s intended function better, what ever that might be, for example:
• Competition vehicles (maximizing the technical regulations whilst keeping the car usable by a human being) • Fun vehicles, Track-day or Fast road cars (purely optimizing the chassis/driver relationship) • Working vehicles (Optimising for the specific job, off-road traction, load handling, etc) • Comfort vehicles (Finding a safe compromise between vehicle dynamic behaviour and passenger comfort)
If we look after the aspect of chassis optimisation properly then safety will follow as a natural consequence.

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