Why do motorbikes' tires have very little to no tread, while cars' tires must have deep treads? I have noticed that on the road, most motorbikes' tires have little to no tread. Even when I look at parking motorbikes that are used in everyday roads, some have even slick tires, or very little tread.

On the other hand, cars' tires must specifically have very deep treads. Naively thinking, this sounds strange, since cars are much more stable and safe, having four wheels, and more balance, and having more connection (friction) to the road. The only thing I could think about was that there are certain racecars, with specifically no treads on their tires, but most cars on the road are not like that, and must have deep treads. Now the motorbikes I see drive on these same roads, with tires that have very little to no tread, so it must be something different (other then in the case of racecars).
Question:

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*Why do motorbikes' tires have very little to no tread, while cars' tires must have deep treads?

 A: What you do not to happen is for the vehicle to aquaplane which will happen if there is a significant film of water between the tyre and the road surface.
Because motorbike tyres are much rounder than car tyres the contact width across the tyre for a motorbike is much smaller than than for a car so any water between motorbike tyre and road surface is comparatively easy to squeeze out whereas with a car tyre the water has to be channelled out along the grooves in the tyre as the contact distance from the centre of the tyre to the outer edge is comparatively large..
To go around a corner a motorbike has to lean over so an outer part of the tyre becomes in contact with the road.  On a lot of motorbike tyres the rubber compound is harder in the centre (used when the motorbike is vertical) than on the outer part where the grip has to be better and here there are some grooves just to make sure that water is channelled out.  The outer softer rubber compound wears faster but is in use for less time than the inner harder rubber compound.
For a car as much of the width of the tyre is used when the car is going in a straight line  and when a car goes around the corner the tyre does lean but very much less and so only the edge of the tyre is curved and I think that the rubber compound across the tyre is of uniform composition.
