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Rolling tires have an adhesion budget which can be expended resisting forces such as acceleration, turning, and braking. Drivers allocate their adhesion as needed, and in the case of drifting, deliberately exceed the adhesion available to spin their tires. It is a function of variables like the weather, contact patch area, the weight on the tire, its condition, and so on.

It is possible to measure lack of adhesion, slippage, as the difference between tire perimeter and distance traveled. However, I've never seen a gauge which would indicate how much road adhesion is available for braking and turning. Is it possible to build one?

  • Because the road surface is constantly variable, I don't see how this could be done reliably. You can sense when traction is lost, but then its too late in certain circumstances, but works quite well in many cases. That's what Electronic Stability Control does in modern cars. – JPhi1618 Jan 18 '18 at 19:36
  • From a mathematical perspective, the hardest part is ascertaining what the tire loading will be for each scenario; it is very dynamic and very much a function of suspension design. If you know how the tire is loaded under each scenario then determining how much friction is required to lose traction becomes much easier. – user35019 Jan 18 '18 at 19:45
  • There is no adhesion ; there is friction- static and dynamic. – blacksmith37 Jan 19 '18 at 16:22
  • good point @blacksmith37. if i understand rightly, adhesion is a less formal name for that static friction which exceeds kinetic friction. – Aaron Brick Jan 19 '18 at 18:50
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I believe that a predictive model would be the best thing you could create. There are so many variables from tire wear and temperature to road conditions and slope that having a perfectly accurate "gauge" for traction wouldn't be possible. Computer learning techniques could read as many sensors as you can pack on the car and develop a safe "handling envelope" for the car, but it would need to be constantly measuring and adjusting to keep itself accurate.

You could tune the predictive model to be somewhat conservative so you would almost always be safe, but there's always a chance for an unknown wind gust or slippery road kill to send you sliding. And since the system is conservative, your overall performance would suffer.

One issue is that static friction is always higher than kinetic friction. That means that once the tire starts to slide, it will continue to slide unless a force is reduced. That means that in order to really find the limit, you have to loose traction, start slipping, and then change the forces acting on the tire until the slide stops. Modern Electronic Stability Programme/Control (ESP/EPC) systems do just that, and they do a pretty great job. They can allow the car to operate just beyond the safe limits and adjust the forces to bring the car back under control faster than a human.

What you want to do is actually very similar to testing the strength of a wooden board. The only way to do that is to break the board. Unless you break it, you don't know the limit. But, once that board is broken your measurement is just an estimation because that exact, unique board no longer exists. You can measure 100 boards and get a pretty good model of when a board will break, but there will always be very hard boards, and very weak ones that defy your model.

So, in summary, the only way to measure adhesion is to push it beyond the limit and determine when adhesion was lost. You can then use that as a good predictor of when adhesion would be lost in the future, but it will never be 100% accurate because of the number of variables involved.

  • It sounds like ESC systems are the only possible source of this data stream. – Aaron Brick Jan 21 '18 at 2:37
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It wouldn't be too hard too test a tyre for it's roadholding performance. Here's how i would do it. Possibly it is done exactly this way by manufacturers.

If you apply a precisely dosed force on a wheel, you can push the surface it's standing on and measure how much force is needed for the tyre to break static friction threshold in various directions. Once static friction is overcome, you can measure how much force is needed to keep the tyre slipping at different speeds and angles, and there's your dynamic friction.

Vary the parameters road wetness, road texture, tyre temperature/pressure, tyre wear, tyre material etc. to get a complete picture of the tyre's performance.

If there's not already a standard to define your results for comparability, create a new one and suggest it to ISO.

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