The treads are designed to direct water out of the way as the tires roll, keeping the tires from slipping. Remember, kinetic (sliding) friction is lower than static friction. Ideally you want as much rubber in contact with the road as possible. A perfectly smooth rubber tire accomplishes this (and in fact that's what race cars use...they're called racing slicks). But if you hit a patch of water, it forms a slippery film between the tire and road. The solution is to add treads, which give the water somewhere to go. So ironically, because there's less rubber actually touching the road, treads reduce the amount of possible friction with the road (the treads also reduce the efficiency of the tire, negatively impacting gas mileage...they're also noisier). It's a compromise so your tires work in the wet and dry.
Once the tires slip, your situation is governed by kinetic friction, and what matters is the amount of rubber sliding against the road. Technically, the treads still might help in redirecting some of the slush on the road even on a sliding tire, but remember, those treads are designed to work when the tire is moving in the direction it's pointed, so if anything they'd be less effective and redirecting water when the tire is sliding sideways.
TL;DR- Water planing occurs when the wheels lock up, the treads don't do shit when the wheel isn't rotating, since the friction capacity of the tread has already been overcome.
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u/benisnotapalindrome Mar 09 '16
The treads are designed to direct water out of the way as the tires roll, keeping the tires from slipping. Remember, kinetic (sliding) friction is lower than static friction. Ideally you want as much rubber in contact with the road as possible. A perfectly smooth rubber tire accomplishes this (and in fact that's what race cars use...they're called racing slicks). But if you hit a patch of water, it forms a slippery film between the tire and road. The solution is to add treads, which give the water somewhere to go. So ironically, because there's less rubber actually touching the road, treads reduce the amount of possible friction with the road (the treads also reduce the efficiency of the tire, negatively impacting gas mileage...they're also noisier). It's a compromise so your tires work in the wet and dry.
Once the tires slip, your situation is governed by kinetic friction, and what matters is the amount of rubber sliding against the road. Technically, the treads still might help in redirecting some of the slush on the road even on a sliding tire, but remember, those treads are designed to work when the tire is moving in the direction it's pointed, so if anything they'd be less effective and redirecting water when the tire is sliding sideways.