Why do you ride faster when it’s warmer?

This weekend, it will be unseasonably warm for many of Britain’s cyclists meaning the summer kit might just get one more run out – plus a waterproof layer or two. One thing you might notice though, is that you tend to go a little faster in the heat. Why is this? 

The first reason is quite an obvious one, you’re wearing less clothes. A jersey and shorts is much tighter fitting than a winter jacket. That flapping in the wind costs energy – as the jacket presents a larger surface area and increases the amount of turbulent air. But, cold air itself is actually slower too!

Why is cold air slower?

If we have a think about the aerodynamic equation which governs how the force on something changes as it moves through the air under various conditions we notice that the power re quired to move our bicycle is proportional to the density of the air. Air density is not static, some air is more dense than other air. Remembering our GCSE science, we know that density is directly proportional to pressure – and pressure is proportional to temperature! This means, on a hot day, the air is less dense.

Why is less dense air faster?

Density is a quantity which describes how much of something takes up a given space. The density of air describes how many air particles are in any given square metre of space. If the air is less dense, there is less air to ride through. This is the reason many record attempts take place at altitude. With less air to ride through, it’s much easier to go faster!

If you’re out on your bike this weekend, you might see a notable uptick on your average speed. If you’re interested in how the weather can impact your riding or want detailed, up to date forecasts – sign up to Mywindsock for free here.