why are you asking for tread?
Common misconception about bicycle tyres being like car tyres me thinks. Sheldon Brown has words of wisdom on the topic as does Josh Brandt.
Commercial aircraft, and especially motorcycles, demonstrate that a round cross section tire, like the bicycle tire, has an ideal shape to prevent hydroplaning. The contact patch, a pointed canoe shape, displaces water exceptionally well. In spite of this, hydroplaning seems to be a primary concern for riders who are afraid to use smooth tires. After assurances from motorcycle and aircraft examples, slipperiness on wet pavement appears as the next hurdle.
Benefits of smooth tread are not easily demonstrated because most bicycle riders seldom ride near the limit of traction in either curves or braking. There is no simple measure of elapsed time or lean angle that clearly demonstrates any advantage, partly because skill among riders varies greatly. However, machines that measure traction show that smooth tires corner better on both wet and dry pavement. In such tests, other things being equal, smooth tires achieve greater lean angles while having lower rolling resistance.
Tread patterns have no effect on surfaces in which they leave no impression. That is to say, if the road is harder than the tire, a tread pattern does not improve traction. That smooth tires have better dry traction is probably accepted by most bicyclists, but wet pavement still appears to raise doubts even though motorcycles have shown that tread patterns do not improve wet traction.
Andrew
As I think you probably appreciate, this isn't as full an explanation as it might be. Aquaplaning ('hydroplaning') is when water gets completely between the tyre and the road surface, reducing the friction to zero. As Sheldon says, the bicycle tyre's shape makes it less prone to aquaplaning, but perhaps more important is its width (or lack of it) and relatively high inflation pressure. As aquaplaning isn't a factor, there is no need for a tread that dissipates water (the main reason for a tread in road car tyres). Any tread pattern reduces the maximum amount of tyre in contact with the road surface, and it's the interaction of the tyre's material with the road's material (the coefficient of friction) that provides the grip.
More important in what makes a good cold- and wet-weather tyre is its ability to distort in low temperatures and thus extract the maximum friction from that road/tyre interface, and this is a function of the compound construction of the tyre. Summer tyres (car or bike) harden in cold weather, and give poorer performance. Despite the unlikelihood of aquaplaning for bicycles, the presence of water will still reduce the coefficient of friction between a tyre and the road, and thus the maximum available grip.
On bike tyres (but not car tyres), tread is for snow and/or mud.
