Tyre pressures and speed.

I have always believed that higher tyre pressures offer less rolling resistance so make bikes quicker. The counter argument to this is that lower pressures give you more grip on corners so allow you to maintain more speed on a twisty trail. As usual I’m only thinking of tyre pressures of mountain bikes, here. I have no recent road bike experience so would not like to offer an opinion. On recent rides I’ve been looking at the effect of shifting your body weight fore and aft and the resultant pressure put on the handlebars. This has allowed me to find a good balance between front and rear grip and hence more speed without either wheel causing me to slide to the ground!

In the course of my rides I decided to lower my tyre pressures. I’d set them at over 30 psi in the belief that I’d be faster across country, even if I sacrificed a little of the grip on corners. The result surprised me because not only did I get more grip but when a tyre did start to slide it did so in a gentler and more controllable way. I suppose I’ve always known this but hadn’t really considered it for a while. Having ridden with my tyres at 25psi I’d liked the grip and found that the app. Strava was telling me that I had been climbing the hills as fast as ever. How could this be with the extra drag from lower tyre pressures? What I decided to do was to go for a ride today and find a controlled, scientific way to test the effect of tyre pressures on speed.

Way back, when I was in school, I showed little interest in arts and history. I think I saw science as the way to explain the world and thought that arts explained nothing and were only there for interest. Times have changed. I’m now very interested in history and arts and see that whilst science may explain the physical world it has nothing to say about the spiritual side of things and matters of the soul. Fortunately, with a formerly scientific background, I know how to set up a fair test to discover the effects of different tyre pressures. Amazingly, for a mountain bike blog, I’ll return to the effects of tyre pressure on the soul later!

What’s needed for a fair test is to find a way to ride the bike where the only variable between different runs is tyre pressure. This means that pedalling can’t be involved because you’ll never put exactly the same amount of energy in through the pedals on successive runs. Similarly there can’t be any braking for corners or other obstacles. The only way would be to roll from the start and for the whole length of the test, measuring the time taken between 2 points. I rode to Healey Nab, my local hill, with a segment in mind. It had a good mixture of typical mountain biking surfaces but it was just too fast! I had to brake for the only corner. Starting closer to the corner I got round OK but at the end had to brake hard before I could click my stopwatch, due to a lack of space before a gate. I moved on to the top of the hill and tried to use the top loop. Without pedalling I ground to a halt part way down so went back to the top and rode down the main downhill track. To get round the many corners I needed to brake several times but thought that the section from the last corner before the jumps section would be suitable. I had a trial run and didn’t need to brake. The surface was smoother than I would have liked. I thought that a bumpier or rocky surface would be more typical but couldn’t think of a better place for my experiment.

I’d decided to only change the pressure of the rear tyre, to make things easier and had set it at 20 psi. This is a bit too low for a tyre with an inner tube like mine. You risk pinching the tube over rocks unless you are using more modern tubeless tyres, where you can run much lower pressures. I did 2 runs at this pressure and both were a fraction over 28 seconds. I pumped the rear to 30 psi, a rather high pressure I’d chosen for long cross country rides. My first run at this pressure was the slowest so far by a fraction of a second and my next was the fastest at a fraction under 28 seconds.

Every scientific experiment needs a conclusion. From my test, within the range of tyre pressures I tested, speed down a trail is not impacted! You might wonder what would happen on a section where you input power through the pedals but, let’s face it, gravity did the work for me. My section drops by around 9 meters. The weight of bike and rider is around 100 kg and with acceleration due to gravity being 9.81 I gain almost 9,000 joules of energy in the drop due to potential energy being converted to kinetic energy! Over 28 seconds this is around 320 watts of power, equivalent to a decent amount of pedalling.

If the surface had been rougher I accept that there may have been some impact but can’t help thinking it would have had little effect and may have been more helpful to the lower pressure option. I won’t run higher tyre pressures again because I believe I’ve found that it won’t improve my speed over using lower pressures. When it comes to corners the lower pressure will definitely help me to be slightly faster. But what about the effect on the soul? Lower pressures, like 25 psi, mean less vicious slides. It feels like the bike is more on my side, rather than fighting me! This is good for my soul.