Science Questions

A friend of mine was going to get new tires and the guy tried to talk him into getting them filled with nitrogen. I can imagine it might be advantageous to do that on a high-performance racing car but if would it make any difference in a passenger car and why? Cole

Dave - I think there’s a couple of reasons why it might make a subtle difference. One of them is that there’s a lot of oxygen in air that we breathe. Oxygen’s quite bad for rubber. It will cause it to break down and get brittle and crack. That’s going to reduce the elasticity of the tire and make it last less long and make it slightly less efficient. The other thing is that ideally you want a gas inside your tire which if it compresses then expands again it doesn’t absorb any energy. The ideal gas for that is what are called ideal gases. The best ones are mono-atomic gases like..

Chris - Argon, xenon…

Dave - Helium would work probably because it would be quite cheap. It would escape very easily because it…

Chris - It’s very small and would escape through the gaps in the rubber molecules.

Dave - If you’ve got things like diatomic gases, things like nitrogen and oxygen then that’s slightly worse because they take more heat to heat up. They can’t just move – they can actually spin as well which is another way of absorbing the energy. Triatomic gases, things like carbon dioxide and water take even more energy to heat up. They are less elastic.

Chris - Just on a more pedantic scale, Dave. Isn’t it that a nitrogen molecule weighs slightly more than an oxygen molecule also worthy of mention? If you pump just nitrogen into the tire the actual gas inside will weigh slightly less?

Dave - It might have a minute effect. I think fundamentally that unless you’re doing a huge number of miles and really stressed about the efficiency increasing the pressure of your tires a bit more is going to have much more effect than filling them with nitrogen.

November 2008