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If you have a question that only an expert can answer, then why not talk to us. Even better, if it’s published you could win Amazon vouchers. John Pendrey from the Isle of Barra in Scotland asked this month’s winning question.

“Why do wider tyres have more grip?

My school physics model says that friction depends only on the type of surface and load.”

Expert, Dr. James Balkwill, a Motorsports Engineering Lecturer from Oxford Brookes University in the UK, provided an answer:

“Yes indeed, we are told that frictional forces depend only on the coefficient of friction and the vertical load - it isn't true. It’s not a bad approximation for smooth stiff surfaces but tyres are neither of these things.

The way a tyre generates grip depends primarily on two mechanisms. The first is the mechanical purchase gained as the tyre adopts the shape of the road. On a fine scale this features lots of undulations from the aggregate in the road. The tyre moulds itself over these and engages with them much like a cog in rack and pinion. This effect increases with increasing contact patch area but trails of as the contact patch gets very big. The second mechanism is that the rubber actually chemically bonds to the road surface even when the tyre is rolling along quickly. The chemical bond is very strong and the larger the area, the greater the amount of bonding that can happen and so the more grip.

In this way rubber tyres and road surfaces as the highest friction pair of materials you’re ever likely to meet. Most materials have a coefficient of friction between nearly zero and around 0.7. Rubber-road values can be as high as 4 for a Formula One car!”

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