The Changing Shape of Tyres
Sure, they are still black and round. But have you noticed that the tyre and wheel assembly seems much larger than it used to be? Particularly the wheel part! It fills the wheel well in the mudguard. ‘Why is it so?’ asked the professor.
Partly it’s fashion. Vehicle designers don’t like gaping holes under the bodywork, it looks more functional to fill the wheel well, but there really is a good reason.
The ‘footprint’ of a tyre dictates the contact surface it has with the road surface.
Fat, wide wheels have a short contact patch, where the tyre meets the road. In the wet, this short footprint doesn’t allow much time for any water trapped under the tread to be displaced, either to the side, or straight through to the back of the tyre. If to the side it has further to go, also. So the risk of the tyre riding up on a wedge of water, and losing grip, is increased. In the dry, it really doesn’t matter- grip is good anyway. But if the tyre has to be run at a higher pressure, to stiffen the walls and allow high cornering speeds, the footprint is even shorter still. Then if it rains, you can be in big trouble grip-wise.
A long footprint allows more time for the pattern to do its job, displace the water, and allow the tread rubber to grip the road surface, not a film of water. Exactly the same principle applies to the farmer’s tractor. A long skinny tractor tyre footprint will out-pull a big widey, though you must agree that it doesn’t look as macho. There’s just more grip available. The ultimate extension of this idea is the tracks on an Army tank, and that’s what you would call a very long footprint, with so much surface area that the weight of the tank is distributed over a large area, enabling it to travel even where there is no road at all. This is called high flotation, but the idea of a car tyre is to have enough contact pressure to squeeze the water out from under the tyre so it can get down to the ‘nitty gritty’, the road surface.
But what if the road itself is smooth. Unfortunately, this is commonplace, on heavily trafficked roundabouts, at bus stops, and roads that are just plain worn out. Yes, roads wear out. The basalt, crushed rock, or screenings of a local rock, set in a bed of asphalt become worn and polished, and like water smoothed river gravel, (which is the last thing that a roads engineer would use), offer only reduced amounts of grip compared to when the road was re-surfaced. Incidentally, the rate of tread wear slows also. You see the signs ‘Road slippery when wet’, or a yellow and black signed graphic of a skidding vehicle, and you should realise that there is less grip available from the road surface at this particular spot, so take care.
This highly desirable “long footprint” is obtained by increasing the rim diameter. This allows the designer to use a wider, low profile tyre, which displaces more water, but without impairing the ability to shift water from under the tyre footprint, because of the longer time available for water displacement.
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