Car tyres have complex, deep tread patterns, presumably to channel water away in the wet. Yet the tyres on my friend鈥檚 large motorcycle have hardly any tread pattern, though they are as wide as a car鈥檚. Their surface, more curved than a car tyre鈥檚, seems lightly punctuated by shallow, elongated S-shapes, with large smooth areas between. I would have thought a bike would need more grip on wet roads than a car, so why are the treads different?
鈥� A motorcycle, having two wheels, needs more traction on a wet road than a four-wheeled car does, not because it slides more freely but because the consequences of even a minor slide can be disastrous. However, what your correspondent is describing is a tyre that is designed for a high-performance sport bike that would seldom be ridden in the wet.
Sport-bike tyres tend to be made from soft, sticky rubber compounds and have the minimum tread necessary to comply with legal requirements, sacrificing both tread life and wet-weather roadholding to maximise dry-road traction. Their ability to grip a dry road is staggering, but sport-bike tyres may only last 3000 kilometres. Riders may wear out a set in as little as a week or two of hard riding, or a single day at the track.
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鈥淪port-bike tyres have a staggering ability to grip a dry road, but riders may wear them out in a week鈥�
Tyres designed for touring motorcycles that will be ridden in all weathers have more car-tyre-like tread patterns and are made of harder, longer-wearing rubber. They don鈥檛 have the extreme dry grip of a sport-bike tyre designed to adhere even when leaning at 60 degrees, but they maintain safe levels of traction on a wet road and may last up to 25,000 km.
Phil Stracchino, Gilford, New Hampshire, US
鈥� Cars sit flat on the ground and their tyres are similar to a flattened cylinder. This means the tyre makes contact with the road along a line across its rim. Motorcycles, however, have to bank to go round corners. Their tyres are shaped more like a ring doughnut, and at any instant only one point on the rim is in contact with the road.
Any water that is trapped between a car tyre and the road acts as an unwanted lubricant, and it cannot escape without travelling a certain distance sideways. It therefore makes sense for the tyre to have grooves into which the water can quickly escape. Provided that these grooves run diagonally rather than straight across, it is easy to ensure that the tyre鈥檚 area of contact with the road stays constant as it rotates.
Water can escape easily from beneath a motorcycle tyre given the small area of contact, so grooves would not help. What鈥檚 more, as the tyre rotates, the point of contact sweeps out a circle around the tyre鈥檚 edge 鈥� a circle that moves from side to side as the motorcycle banks. Any treads would turn this circle into a cogwheel, increasing noise and vibration.
Motorcycle tyres are generally made to be much softer than car tyres so that they can spread out more and maximise what contact area there is. This also makes them less hard-wearing.
Alec Cawley, Newbury, Berkshire, UK