Why tire friction coefficient changes with load

As mentioned in video, friction coefficient is a function of weight (normal force) applied to tire ? As weight on tire increases, friction coefficient decreases. Why is it so ? What is the physics behind that ?

Temperature related effects on the tread materials.

Here is a book about that: The Racing & High Performance Tire by Paul Haney. I see it's out of print, and while interesting, I wouldn't say it was worth any huge investment (which out-of-print books seem to command).

Hi Andis, David's suggestion on Paul Hanley's book is a good one, I recommend reading it too.

Tyres are an extremely complicated part of the car, I really can't do them justice in a forum post. But I'll try give you some useful comments on it. As David said, the temperature has a big effect on the grip. The two main grip mechanisms for tyres are molecular bonding (between the tyre and road surface) and hysteresis in the tread which affects the surface pressure distribution. Both of these mechanisms are very sensitive to temperature.

Another element is how the tyres contact patch evolves as we increase the vertical load. When you increase the vertical load on a tyre, the contact patch can not increase very much laterally (simply due to the shape and construction of the carcass). The contact patch can start to increase longitudinally, though not enough to make up for the lack of increase laterally.

One part of the problem with this is that because the surface grip of a tyre does depend on surface area (different from most simple physics examples) we are not increasing area in proportion to the vertical load.

Another point related to the contact patch shape and area, is that when we extend the contact patch longitudinally, we are making it more difficult for the tyre to flex and comply at higher slip angles as we are distorting it more by forcing more of the contact patch to stay stuck to (and therefore aligned with) the road surface.

End result is that for practical combinations of vehicle mass and tyres, when we increase the vertical load on the tyre, we end up with a lower coefficient of fiction. More grip, but at a lower efficiency. The whole concept of coefficient of friction when it comes to tyres is a massive oversimplification. But, in the context of this driver course, it's still very useful.

Hope that helps

As has been said, there's the co-efficient of friction which is the interaction between the tyre rubber and the surface of the road/track - this is basically proportional to vertical load.

There is the physical interlocking of the tyre tread with the irregularities in the road/track surface. Think of it as being like a flight of stairs laid flat - the CoF may be the same as a flat surface, but the tyre can partially sink into the 'stair' and partially lock in place. If you're looking at "knobbly" off-road and rally tyres you may get the idea, as these are largely designed to 'dig into' the surface, making their own 'stairway'.

"Grip" is a combination of those two factors and is basically how much resistance the tyre has to shear forces at the road/track surface, and is what we're untimately interested in.

The third part is the actual rubber, or more accurately how resistant it is to micro-tearing, where the surface layer is physically ripped from the tyre when the "grip" shear force exceeds the bond strength of the rubber. You can see this as the lines where rubber has been left behind by skipping/sliding tyres.

As with most things, the trick is to balance 'grip' with wear. One needs to balance the CoF, the flexibility of the tyre to conform to the surface, the mechanical strength of the rubber, etc. Sounds easy, right? Except different rubber compounds will have different characteristics and these will also alter with temperature.

I'm not really a 'track' person, but a common error I see is people selecting the softest "knobbly" tyres in the belief this will give best grip, while in practice they will frequently overheat, because the loading and distortion of the tread builds heat, and rip apart - a 'harder' compount would usually give more overall grip, and for longer.