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I've read that treads don't affect traction very much. This is why NASCAR cars have smooth tires.
But, the road is not clean. They say you can't hydroplane on a bike, but what about a thin patch of mud on the road? Or spots with some gravel on them?
Everyone says treads don't matter for roads, but they still keep engineering road tires with treads, so I'm curious the degree of benefit that this really provides?
If you are riding on the road, you don't need treads. Even with mud. Even with sand, or gravel. Unless you are riding on the street in the middle of a volcanic mudslide or torrential rain washing out the road, it won't matter. The curved cross section of your tires coupled with your mass will instantly cut right through any patches of water or mud as you ride through. A bicycle under human power won't "rarely" hydroplane, it will never hydroplane.
You can slide if you ride over sand or fine gravel. The gravel can get between the rubber and road, and cause you to lose traction. However, having treaded tires will not save you. The layer of sand or gravel will be just as slippery to the treads. The treads can't cut through the sand or gravel, because the underlying road cannot deform. The treaded tires will just slide along the pavement, lubricated by the sand or gravel, just like a slick.
A behavior you didn't ask about, but you can encounter, is a squirming feeling, that you get if you ride an aggressively treaded tire on the road. This won't happen riding on dirt or sand, because the ground can deform. This squirming and deformation becomes especially troubling when cornering. The sudden transition from smooth to treaded in those silly combination tires, right during a hard corner, is especially nasty.
The reason companies make treads on pure road tires, like Blam suspects, is simply because the majority of bicycling customers don't think about these things in great detail, and think that their bike is just like a car with 2 wheels, and since their car tires have tread, so should their bike tires.
The takeaway principle is that conditions that will result in a loss of traction on the road, will do so regardless whether you have treads or not. Treads reduce the consistency of your tires surface, and in doing so, makes for a worse road tire.
But there is not one traction
Water has basically no traction. Hydroplane is a combination of speed, viscosity, liquid depth, weight, and tire contact surface area. A bike rarely generates enough speed to hydroplane. A bike is typically going with small tire diameter for rolling resistance. Even in a car if you slow down to 40 mph you don't hydroplane. In a loaded down truck you can 80 mph and not hydroplane. You still get less traction on wet asphalt as wet asphalt is not as sticky. You did not get lifted off the asphalt (hydroplane) but you still have some loose water molecules on the surface and in the cracks acting as independent ball bearings. You pushed through the water as a mass but water is still on and in the surface. Like after no rain for a while and even a little rain it combines for slime. Water on an oil patch stay away. No amount of tread is going to hold you on water on oil at even 5 mph - hold on.
As for why you see tread on road tires. You assume the tread is "engineered". For the most part I submit they offer tread because it sells.
Mud is a whole different beast.
I think a treaded tire would cut through mud on asphalt better than slicks. A total slick with no biting edges on so is less likely to grip on the surface because it is smooth having less friction.
Tread gives somewhere for mud to go, kind of like in safety shoes where the water is squeezed out between the grooves. Likewise the mud is pushed between the gaps and the edges of tread can bite through mud to reach the hard ground. I would prefer to reach the hard ground underneath me than "mud"plane on mud.
I don't think hydroplaning with water could happen on a bicycle, this has been proven though a lot of road pro tires now have tiny ridges or a fine texture which micro interlocks with concrete to assist traction in wet conditions. An article can be found on the website velonews by various tire companies talking about how a fine tread can affect wet traction to a degree but this is different than cars and motorcyles
As for Jobst Brandts spiel on slicks; yes, maybe in a lab on tire machines slicks grip better on wet ground and corner better. However in the real world we ride over slimy concrete, dirt over concrete and sand; through gravel and over rough asphalt and uneven ground. So a slightly treaded tire might help with these surfaces. So most tires have some tread not for water dispersion but for non-uniform surfaces.
As the surface get deeper, softer and looser so does the need for more aggressive tread features.
As an additional note I disagree with Brandt that motorcycles should use full slicks on wet roads and tread grooves are there for artistic purposes. This may be so with bicycles but street motorbike tires differ greatly than slick racing ones, they have different rubber and must cope with different conditions. I would like to see if test results from motorcycle tire companies that actually prove his hypothesis if so most street tires would be bald as well as motorcycle gp rain tires.
