More on Fat tyres and the coeficient of friction.

Well, I got a response from Bridgestone re post Other Ideas on Tyres

Just like we tought
- THE MORE RUBBER YOU GOT - THE LESS RUBBER YOU LEAVE ON THE ROAD.

Hi Robert,

Quite an interesting thread on the web forum. Thankyou for your enquiry. There are certainly some good points of interest.

I believe that the thread has been started by someone who has been studying pure physics. Looking back on one of my text books from my university days (Serway, Physics for scientists and engineers) it does state that co efficients of friction are nearly independent of the area of contact between the surfaces. One would imagine that this is correct for relatively smooth and flat surfaces.

If you consider the tyre / road contact patch then there is definitely a degree of mechanical influence from tread design interacting with the typically coarse road surfaces which we have in Australia. A wider tyre will allow more flex free grooves and ribs to be incorporated into the design, providing additional mechanical grip with the coarse road surface. Sidewall strength will also play a large part just to make things a little more complicated.

Tread compound will generally be one of the biggest factors affecting the friction co efficient of a tyre. If we considered F1 slick type tyres pre groove days acting on a smooth road surface then the pure physics would come into play where contact area would have little effect on friction force. Additional width in such a case would be of benefit to increase the shear force required to overcome the shear strength of the tread rubber compound (Shear strength is equal to the Shear force divided by the contact area. If area is increased then more force ie engine power is required to shear the tread compound of equivalent shear strength.) This is why the skinny tyre on the XR would leave a lot of rubber on the ground due to rapid acceleration. The power of the engine easily overcomes the shear strength of the tread compound acting on a small surface area and subsequently leaves a large amount of rubber on the road surface. Also, in general a sticky race type compound will wear at a more rapid rate. By increasing the width, the tyre footprint size will increase, and road pressure will decrease for improved wear life.

Tyre design in real world conditions is definitely much more complicated than outlined above due to many factors including expectations on wear life, vehicle design etc

Hope that this helps.

Happy motoring.

Darryl Moliere
Technical Field Service Engineer (Qld)
Bridgestone Australia