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Whiteline Chassis Talk 1 – The basic suspension modes.

Preamble:

This is the first part of an ongoing series of posts discussing chassis and suspension tuning as seen by Whiteline. By that we mean that it’s our perspective based on the facts, as we know them.

This is not meant to form the basis of the definitive perspective or encyclopaedia on the subject as we have a lot more yet to learn. For example, our professional library has over 30 texts on the subject and that’s just scratching the surface. However, we hope that some may use this information to better understand the basics, the Whiteline world and why we do what we do.

Physics is physics but just as there are theory’s that are taken as fact till they are disproved, we too are open to new ideas, techniques and theory’s that may change our view and position. This is not meant to act as a disclaimer but rather as a reality check to hopefully highlight the fact that this does involve science and that the industry as a whole is constantly learning and changing as a result.

For example, front wheel drive race setup as a science did not exist 15 years ago. It’s only really in the last 5 years that the techniques and principles developed at the cutting edge of the sport have entered the mainstream suspension industry. This is not to suggest that someone’s discovered the equivalent of front wheel drive “gravity” but rather, enough thought and science has now gone into this area to properly understand the physics behind the very complex interactions involved.

This has lead to some dramatic changes in setup theory overall. Equally, its safe to assume that this is not the last time it will happen and we need to be ready to accept new ideas, question our existing ones and be ready to accept change. However, there are some things that are as fundamental as gravity and a good understanding of them will help illustrate and provide clarity in understanding suspension and chassis dynamics overall.

The 4 Basic Suspension Modes:

The attached table was actually taken from Racecar Engineering magazine (1997) and is very useful in showing the various and distinctly different modes of suspension travel. This is important because it highlights that we need to break down all movement into its correct mode and try to deal with it using the correct tool (or component).

Needless to say it will become increasingly obvious how much of a compromise any conventional suspension system and why “active” systems are so much more effective (and why they are banned in most motorsport). However, in the real world we have to recognise that the 4 main modes, in alphabetical order are Heave, Pitch, Roll and Warp. Some of these are obvious and some people will already know others but it’s worth going through each one separately in detail.

Heave is defined as a synchronous motion of all wheels in one direction. In simple terms, the vehicle body moves up and/or down as a whole.

Pitch is defined as front and rear wheel pairs move in a synchronous motion but in opposite directions. That is, the body of the vehicle rocks either forward or backward resulting in the front or rear wheels pairs compressing or extending as a pair but in opposite directions to each pair.

Roll is defined as the synchronous motion of each left and right pair of wheels albeit in opposite directions. Body roll in simple terms with one side of the vehicle extending while the other side compresses on its suspension.

Warp is a little trickier but is one of the most important to understand properly. It’s correctly defined as oppositional motion in opposite directions for either left and right wheel pairs or front or rear wheel pairs. Or, synchronous motion in opposite direction in the case of diagonal pairs. In simple terms, this describes what happens when one single wheel moves up or down independent of the others in response to a change in the surface. This may be a bump in the road or a pothole for that matter. However, it is distinctly separate from all the others and needs to be for better understanding.

All these modes are occurring constantly at any given time while the vehicle is moving. No surface is ever perfect and every change will result in one or many of these 4 elements coming into play. Separating them however helps us look at what components are involved in each, what they are doing and what is the optimal or hypothetical ideal to best deal with each. But before we start, its useful to jump ahead and quickly summarise the critical role of the wheel and tyre

Every performance outcome on a car is dependent on the tyres. Cheese cutters will not cope with delivering big power to the ground while the brakes will struggle to stop the vehicle. Big wide tyres set to toe-in or set to big negative camber settings will sit like boxes on an angle to the road effectively running on the edges.

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It’s obvious that the tyre needs to be appropriate for the job and so does the angle or alignment setting but even more importantly, it needs to stay physically connected to the road. This is one of the most misunderstood and overlooked elements of suspension and chassis setup but is absolutely critical to a successful outcome. You can’t have grip with out contact and the main role of a good suspension system is to keep the wheel in contact with the road surface. Not just touching it but maximising the size and quality of the contact patch. More on this in future issues so lets go back to suspension modes.

Springs are primarily designed to hold the car up leaving some compliance to allow the wheels to follow the changes in the road surface. In which case, we can ask the question what is the ideal spring rate needed for “warp” modes? Many, including Whiteline will argue that the theoretical ideal rate is effectively zero, none, zilch! Why, because any rate that reacts against unimpeded individual wheel travel will result in a reduction in the quality and quantity of the contact patch.

Note: Interestingly there is a model of a suspension system in existence that delivers just such an outcome using hydraulics. It is patented but is unlikely to reach mainstream production due to the relatively complex nature of the components. However, modern commercial examples surround us already. Racecars including F1 use separate springs for the various modes (done so for decades) while modern active dampers or shocks are designed to respond in different ways to different loads and modes.

Roll is best controlled by swaybars. Why not springs, you ask? Consider what happens when the car is loaded into a corner in roll and one or both of the loaded wheels encounter a change in road surface through a warp mode? Remembering the law of equal and opposite reactions, a loaded outside front wheel encountering a bump will encounter a significant rate through the pre-compressed spring (car is loaded in corner) resulting in the spring passing on the load to the body that will move away from the road. Result? Lost contact, even if only for a moment but potentially disastrous from a handling point of view when you consider its doing a great deal of the cornering work at the time.

Heave and Pitch modes need sufficient rate to stop the car bottoming out over uniform bumps or the front or rear axles running out of travel as this would have the same effects as excessive resistance on a loaded wheel as detailed above. Specifically, we run the risk of a tyre(s) momentarily losing contact with the road leading to loss of grip. In simple terms, this loss of grip is part of what you feel when the car understeers or oversteers. Understeer being a relative loss of grip on the front while oversteer is the relative loss of grip in the rear.

Whiteline believes in using as little spring rate as absolutely possible for these reasons. We start from a small amount and increase, as we need it, not the other way round. As a full range suspension manufacturer, we believe in using the right component designed for each job to deal with its chief responsibility. That’s why we use swaybars and not springs to control body roll. We would no more design a swaybar to hold a car up in pitch or heave than we would try to get the spring to hold the body up in roll.

If one accepts that the above modes are fair and logical, it follows that any increase in rate for a desired outcome must lead to an increase in other areas to maintain a balanced outcome. So how do we explain a 100 or 300% increase in the factory spring rate while leaving the roll resistance rate the same? For example, how could one justify using 400lb + springs on a road/race car while still using stock swaybars when the stock springs were only 200lb.

We don’t, because it doesn’t make sense. Assuming we ignore any “bandaid” or short term tuning remedies, a balanced and inclusive solution that recognises the unique role of each component will always be far more effective.

More in future issues but here's a quote from the head of one of the fastest touring car teams in Australia.

"The Shocking Truth" - an article in Motorsport magazine, No 192.

In this article, Larry Perkins is talking about their development program since the end of the '99 season, where he felt they were 3/4 sec off front running pace, to end of 2000, where the Castrol Team (his) were winning front row starting positions, races and setting fastest laps again.

... One major aim of all the changes was to be able to run very much softer springs, front and rear. They accept more roll in doing this, but the important thing to consider is the overall dynamic situation - if you've got the stiff chassis and suspension components, the suspension that is free to move without friction, carefull optimisation of the suspension geometry, and shocks that can control the chassis platform, it does work better.

Jim Gurieff

Whiteline Automotive

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