Time trial in the rain: how much wet tarmac costs you and how to distribute power

In the dry, at 300 W, the example time trial takes 55.4 minutes with a Crr of 0.004. On wet tarmac, with a typical Crr of 0.0055, the time rises by 58 seconds. If a soaked skinsuit also adds 2 % to CdA, the total is 80 seconds. Holding the dry time would take 309 W.

Why does wet tarmac slow you so much?

The tyre has to displace the film of water and the rubber deforms differently when cold and wet. In the field, with a power meter and the coast-down method, it has been measured how rolling resistance changes with the state of the road²: in the wet it rises by 20 to 50 % depending on tyre and pressure. At 43 km/h, going from 0.004 to 0.0055 is about 14 W more at the same speed, almost 5 % of the power.

Humid air, on the other hand, changes almost nothing: at the same temperature and pressure it is slightly less dense than dry air, not more, because water vapour weighs less than the nitrogen and oxygen it displaces. What does slow you is soaked fabric, which loses the texture that made it fast and changes how the air separates from the body^3,4.

What changes in pacing?

• Target power does not change: the physiology is the same. The expected time changes, and you have to accept that so you do not set off chasing an impossible mark.
• Corners matter more: taking one at 25 km/h instead of 35 costs 3 extra seconds per corner. On a time trial with twelve corners that is 36 seconds, almost half of what the wet tarmac takes. Brake in a straight line and early, not in the corner.
• Pressure: half a bar lower gives more contact patch and more grip; rolling resistance rises a little, but safety in corners is worth more.
• Cold: with rain the temperature drops and the air gets denser, which does raise drag: at 10 °C instead of 20 °C, 3.6 % more density (see air density).

Rule of thumb: in the rain, expect about 2 extra seconds per kilometre on the flat and plan the corners as if they were the most important part of the course.

What is known and what is not

The rolling resistance effect is measured and large². The effect of soaked fabric on CdA is less well quantified: it depends on the suit and how hard it rains, and the 2 % in the example is a conservative estimate. What nobody can calculate from the sofa is how much you will brake in each corner on a wet road.

Frequently asked questions

How much slower is a time trial in the rain?

About 2 seconds per kilometre on the flat from rolling resistance and the soaked suit, plus whatever you lose in the corners.

Does humid air slow you down more?

No. At the same temperature, humid air is slightly less dense. What slows you in the rain is the cold, which raises density, and the wet tarmac.