Rainy race weekend here in the Midwest. Why is it standard practice to reduce tire pressure in the rain? Wouldn't this increase rolling resistance? What about traction?
Decreasing tire pressure from the maximum suggested pressure may actually reduce rolling resistance, allowing you to go faster with the same effort. Until recently, rolling-resistance studies were done in a lab with a tire rolling on a large, smooth metal drum spinning under the tire. That's where most pressure recommendations came from. But recent studies that have better simulated actual road surfaces indicate that lower pressures often result in lower rolling resistance. To determine what's best for you, do roll-down tests (coasting downhill parallel to another rider to see relative speed) and experiment with different tire pressures.
As for traction in wet conditions, decreasing tire pressure does indeed help. By decreasing pressure, you increase the size of the tire's contact patch on the road. The lower the pressure, the more tire is touching the road, and the more grip you will have.
Additionally, proper air pressure is directly related to rider and bike weight; without knowing yours, I'd suggest you take out 10psi as a rule of thumb when racing in the wet.
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