Monday, 1 September 2008

Tyres - Facts and Friction by Mike F.

In a recent issue of a high quality motorcycle magazine two “facts” were quoted.
First: The tread pattern plays no part in road grip but helps to channel water away from the tyre/road interface to prevent or reduce aquaplaning.
Second: Grip is due to a chemical reaction between the tyre and the road surface.
I understand the fact that the tread does not affect grip as such but channels water out from under the tyre to prevent or reduce aquaplaning. This is illustrated when you compare the grip achieved by “slicks” which have the largest rubber to road contact area and “wet” tyres with a pronounced tread pattern.
As a chemist I am baffled by the hypothesis that the tyre grip is due to a chemical reaction between the tyre and the road surface. I presume the alleged chemical reaction is between the rubber of the tyre and the tarmac surface of the road. As every tyre manufacturer will use a huge range of raw materials in different concentrations in addition to the common basic materials such as rubber hydrocarbon and carbon black. There will be a huge difference in the composition of “hard” high mileage tyres for trucks and vans and “soft” grippy tyres for bikes.
Likewise road surfaces vary according to the composition used. Bitumen is the classic binder used but modern polymers have replaced it in some areas and some roads like the A50 have a concrete surface. SMA (stone mastic asphalt) has been used to give better grip and macro and micro chips are used again to improve grip and hence safety.
These factors make the concept of tyre/road chemical reaction difficult to understand.
My understanding has always been that it is a physical reaction at the tyre / road surface interface where grip “happens”. Friction prevents the tyre surface from moving in relation to the road surface. (Don’t confuse this movement with the rolling action of the wheel.) The weight of the vehicle and gravitational force working vertically force the rubber against the road. Lateral force (centrifugal in case of cornering) acts horizontally. When the latter overtakes the former the tyre will be forced laterally i.e. it will slide. It is the hardness or softness of the tyre that will resist this action to a lesser or greater degree.
Wheel spin (acceleration) and skidding (braking) are other lateral acting forces which will induce tyre/road movement.
Friction is reduced when the surface is smooth, wet or covered with paint, ice or diesel hence the greater the risk of a skid or slide when these factors are present.

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