# Experimentally determining tire friction coefficient

1. Nov 21, 2013

### marellasunny

In a recent lecture,I learnt that the friction coefficient "mu" between the tire and road surface is determined experimentally by using a graph. i.e a graph between the longitudinal tire force and the longitudinal slip. Well,how did engineers emperically arrive at such a graph in the first place?

I know that longitudinal slip is a function of the vehicle velocity and wheel longitudinal velocity.I guess it is easy to estimate. But,how could the longitudinal tire force be estimated? The lecture slides say that this could be made possible "... by measuring the tire profile deformation,from which the information of the potential transferred forces are determined." The connection on the relations is not quite clear.

Any ideas and experiences would be appreciated.

2. Nov 21, 2013

### Simon Bridge

How does the tire deform?
Is there a material property that could relate the deformation to a force?

3. Nov 22, 2013

### jack action

4. Nov 22, 2013

### marellasunny

I do know that automotive engineers model a tyre as a combination of a spring+damper system, except that the spring+damper(s) are spread out along the circumference of the tyre.

Since the stiffness of the tyre rubber material and the deformation(from experiment) are known,I guess one could calculate a force acting on the tyre. This force being longitudinal like a traction force is hard to imagine to me. I still don't quite know what equations they use to find the friction coefficient in the end.

5. Nov 22, 2013

### Simon Bridge

It's not going to be a simple relationship... in practice, the calculations are done using a machine.

In principle it is much the same process as the exercise where you have a block of rubber sitting on a surface and you drag something across the top surface.

6. Nov 28, 2013

### HowlerMonkey

You also have to factor in "load range" since testing is usually done within the range on a "per tire" basis meaning that you could have two nearly identically tires show drastically different friction because one has a different load range and was tested with "more weight" against the friction surface.