How Low Can You Go? Tyre Pressures and Widths Posted on 25 Aug 15:48 , 1 comment
There is a lot of debate at the moment about tyre pressures and the trend for fatter tyres. We all know that with the old 23mm tyres, the advice was to keep them "board hard". In fact when I was time trialling as a youngster, we used to put an extra 10 strokes of the pump in for the event.
But now we are using 25mm tyres and even 28mm for heavier riders, can we lower the pressure and how much? And for those of us who don't ride at all competitively and spend our time riding for pleasure and cycle touring, the advice is to use even wider tyres 35-42mm typically for comfort without sacrificing speed.
It seems that all of the traditional advice has gone out of the window and we are all confused! This is compounded by the research recently published and publicized by Jan Heine et al.
Apparently fatter tyres do not slow you down and neither do under inflated tyres. The rolling resistance of a wider tyre is not any higher than a narrower one, nor is the wind resistance. To compound the confusion, they say neither do mudguards give any appreciable increase in wind resistance.
But what tyres and what pressure? Obviously the lighter the tyre the better, but what really counts is the flexibility of the side wall. A wider heavier tyre with stiff side walls is definitely going to slow you down! The theory (and practice) shows the rolling resistance of the tyre depends on how stiff or flexible the tyre sidewall is. The contact patch of the tyre and road depends on the combined weight of the bike and rider and the tyre pressure. A 200lb load at 100psi is 2 square inches in contact with the road at any time and as each part of the tyre contacts the road the the tyre wall squishes and unsquishes. That is what cause rolling resistance. Now consider this: a wider tyre will have a wider but shorter contact patch, so less of the tyre wall has to compress and de-compress. This means a wider tyre is better. So we are looking for a light flexible wider tyres. Not many manufacturers produce these. Grand Bois and Vittoria are the 2 best known.
But why do thin rock hard tyres just "feel" faster? This is because the buzz and vibration from the road surface feed back to the rider and give greater impression of speed.
So as to pressures- how low can you go? It seems as low as you can without getting pinch punctures, so maybe 60-65psi for most riders on 25mm tyres and a bit lower on gravel surfaces. If you are running 35-42mm tyres on your touring bike, then maybe as low as 35psi, If the road surface is horrible and pot holed then you will have to pump them up a bit to avoid rim damage.
So that's the theory, but in practice? Well I was persuaded to swap the 37mm Schwalbe HS-159's on my Velo Orange Polyvalent for some Grand Bois Hetres at 42mm. I turned up for a club run on the bike and the other riders were joking and commenting on my "balloon tyres". During the ride we went along an unmade road with soft sand and deep puddles. The other guys were swerving and bogging down while I cruised through. Then on the tarmac the low rolling resistance meant that I was hardly pedalling on the flat stretches and rolling away from them down the hills. At the end of the ride all of the questions were about the tyres - how much what sizes. No teasing about balloons. Now I know I was comparing a £50+ tyre with a £7 one, but most of the other guys were on respected mid-priced rubber. I also fitted 32mm Grand Bois Cypres to the new tourer I built for for my son, who jumped off his carbon fibre road bike to try the new bike out. When he came back I asked "how was it?". "Fast" he said.
So I am fully persuaded that wide, light and flexible tyres are the way to go.