| MadSci Network: Engineering |
Nasser, I believe that the phenomenon can be described with one word - traction. I was unable to find any quick reference as to why, so I have attempted to reason this out based on the indisputable facts we know and accept. (No one of any repute seems to want to give this info away freely!!!) In the wet, the (wet weather) tire works as a water pump. As long as the tire pumps the water out from between the tire and the track surface we will have (relatively) good traction. There are two ways to lose traction by stopping the pumping action - 1. By rotating the tire faster than it can pump water (driving too fast for conditions), and 2. removing the tire from the medium we wish to pump - reintroducing it to the medium does not afford instantaneous contact between the rubber and the road. From Bridgestone Tire's F1 website: "First," describes Mr. Hamashima, "water must be drained through tread pattern so that water does not sneak under tyres and form a film. This prevents tyres from rising and allows them to stay in contact with the road. Tread pattern must be designed taking account of how quickly water underneath tyres can be drained..." (Hirohide Hamashima - Bridgestone Motorsport Technical Director) [see entire text at link below] http://ms.bridgestone.co.jp/en/index_press.html If we allow that tire to leave the road surface it will quit "pumping" the water away and we will hydroplane (tire slides over water surface and is no longer in contact with the road). The only way to regain traction is to slow down and allow the tires to again begin pumping the water away. Stiffer suspension settings which work great on dry tracks would allow the wheels to leave wet tracks over bumps and would break traction with the wet road many times over during a single lap, forcing the driver to either slow down or simply wait out the loss of traction and hope the tire begins pumping again. The use of stiffer springs could be considered analogous to the old 'straignt axle' rear suspensions while the more pliant softer settings could be considered analogous to the more modern independant rear suspensions (IRS); we get better traction from the IRS over rough roads - in envisioning this loss of traction consider the entire car - not just a single tire/spring combination. I think the key to running in the wet is to be 'smooth' and minimize erratic input - softer spring rates could help with this. I saw several references to biasing brakes more towards the rear and disconnecting swaybars for wet setups as well - these steps would lead to less severe input to the tires during braking (well, less severe to the fronts anyway, but definitely more balanced over the four contact patches) and cornering. By using the softer setup the car is less susceptible to traction loss over bumps and lap times go down. Remember too that all of these references to "soft" and "hard" are relative. While in general, a 'wet setup' may indeed use softer spring rates than a dry setup, the rates in front and rear may also change with respect to one another. Softening the front spring rate while maintaining a heavy front brake bias would lead to a 'diving' front end and a loss of control/traction as well - you must look at the setup holistically. Testing is crucial in determining what works best for each specific vehicle. Sincerely, Steven Miller Undergrad - Mechanical Engineering San Diego State University
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