K&C Testing Tips
What is Compliance?
Through the years, working with K&C testing and explaining it to various customers, I’ve become aware that there is a great deal of confusion surrounding the very words “kinematics” and “compliance”. In this article, I will attempt to explain what “compliance” means in the context of K&C testing, and how that relates to vehicle performance either on-road or on-track. Here are a few contexts in which you might have heard about compliance:
In some contexts, “compliance” might refer to compliance with rules or regulations. But that’s not the meaning we are after. For engineers, “compliance” is the inverse of stiffness. So a higher stiffness value corresponds to a lower compliance value. That’s getting closer to what we are talking about, but it is still too general. You also hear racers, especially road racers, talk about “compliance” when describing the compliance of the suspension. In this context, they are really talking about how it behaves over bumps or when going up over curbs. A more compliant suspension in this context is one that has softer springs and/or softer anti-roll bars and/or softer shock settings. This can be measured on the K&C rig, but it’s really just vertical stiffness. Getting the best compromise for vertical stiffness is important, but there’s much more to compliance than that.
The “much more” I’m referring to is the compliance that we can accurately measure on the K&C rig. It is how much camber you lose under cornering loads, as well as how much steer in the wheels under braking loads, to name just two compliance parameters of interest. Vehicle manufacturers understand and utilize this type of compliance in the design of their vehicle suspensions. Rubber bushings are tuned, not just for isolation, but also for the desired steer characteristics under load. This allows them to design-in understeer to help make the vehicle forgiving and predictable for the average driver. But these same rubber bushings, when pushed into racing applications, can reduce performance. In production-based racing, when the rubber bushings can be replaced with something stiffer, this is often the first thing to do. But don’t let yourself believe that there is no more camber compliance or steer compliance, simply because you took out the rubber bushings. Sure, there will be less. But even on cars with metal-to-metal joints all around, you still see significant camber and steer compliance. This is the case because no material is infinitely stiff. Therefore, when loads are applied at the tire contact and reacted up through the suspension, the suspension members and the chassis to which it is connected will flex, leading to changes in camber and steer, as well as motion of the wheel center. This can be accurately measured on the K&C rig.
What is the impact of this compliance? Camber compliance and steer compliance lead to different camber and steer under load than you would predict based on the kinematics of the suspension. Camber compliance results in a loss of camber under cornering loads. So you have less camber in a loaded condition than you would predict. Steer compliance can go either direction, and so can contribute to either understeer or oversteer, depending on the suspension and whether we are talking about the front or rear of the car. In any case, this kind of compliance is very important to the dynamic behavior of the car, and it can be easily and accurately measured in a proper K&C test.
With the availability of independent, affordable K&C testing at Morse Measurements, there’s no reason suspension behavior should be a mystery. Find out what’s going on in your suspension and optimize your setup for improved performance.
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