By UK law, car tyres must have a tread at least 1.6 mm deep. It is about 9 mm on my new tyres. At what point would deeper treads worsen road holding and why?
鈥� Tyres do not need grooves to improve grip in dry weather. This is demonstrated in motor racing, in which smooth 鈥渟lick鈥� tyres are the rule. These are used unless it is wet, when the addition of grooves allows a tyre to squeeze away water that would otherwise lubricate it and remove grip. The heavier the rain, the deeper and more numerous the grooves.
鈥淭he deeper the grooves, the less tyres will grip because less rubber touches the road鈥�
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New road tyres have sufficient grooves for all but very heavy rain. But as they wear, their capacity to shift water drops because the grooves become shallower, until they are insufficient even for light rain 鈥� which is the limit for legal tyres as noted in the question (with a safety margin added).
The deeper the grooves, the less grip the tyres have because less rubber touches the road and the parts between the grooves move about. So on a dry road, any groove inhibits grip. But unless you can do a pit stop as soon as it rains, you are still better off with grooved tyres for road use, purely to ensure you can drive in all conditions.
John Davies, Haverbreaks, Lancashire, UK
鈥� The maximum tyre tread before the ride becomes too rough depends on the kind of road on which you鈥檙e driving, and what suspension you have. Bicycle tyres are a good example 鈥� a thick knobbly tyre grips mud and loose terrain well, but is heavy and noisy on tarmac. The heavy tread means that it won鈥檛 rebound so quickly to the relatively small stones on the road, so its grip is poor. Modern tyres on racing bicycles are thin and light. The tread layer has low mass so it can follow small road undulations, providing good grip. Because the internal pressure of the tyre is high, it pushes through any water, so even slick tyres grip well in the wet. On a good road you can do over 40 miles per hour in comfort. But the tyres have zero grip on slimy mud when cycling.
Simon Dales, Oxford, UK
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This article appeared in print under the headline 鈥淭reading lightly鈥�