Wheelsuckers

The 'science' of tyre pressures

When I got my first ‘real’ road bike with 700x23c tyres, the first thing I did was to blow those skinny threads of rubber up to the max. My thinking was that if it said 120 psi on the sidewall, then inflating to that pressure would surely give me the lowest possible rolling resistance.

The guys that I rode with fell into two camps on the question of correct pressure: some used the thumb-on-tyre technique as their preferred pressure gauge; others, like me, attempted to push the limits, thinking that harder tyres meant a faster ride for the same effort.

It was only after I started reading-up on the geekier side of cycling (engineering background) that I came across a lot of sources on the web that argued that the ‘correct’ pressure for a tyre is based on its profile, the rider’s weight and his/her weight distribution on the front wheel and rear wheel. Makes sense if you think about it.

One respectable source (Michelin) even had the handy graph below to work out the correct tyre pressure based on the tyre’s profile and the rider’s weight.

The chart appears to be a good generalisation and plugging in my weight (63.5Kg), it shows that my tyre pressure on 700x23c’s should be about 96psi front and back, considerably less than the 120 psi max. However, even when on the hoods, more weight will be through the back wheel so in practice the front tyre pressure should be less than the rear; I reckon front-rear distribution is about 45/55 front to rear. So reducing the front by 10% and increasing the rear by the same gave me 86 psi front and 105 psi in the back.

My first outing on the new set-up showed that the ride was certainly more comfortable and, could it be, just a little faster/easier…I couldn’t help but bore the group to death about it. But, my argument was ill-prepared and failed to convince: one question in particularly fired back at me was “if that’s true, then why do track cyclists run on 140+ psi”? I had no answer.

More research on the subject (did I mention geek?) gave me an answer and, thankfully, in layman’s terms: the more a tyre/wheel moves vertically, the more energy is wasted in that direction rather than used productively in the horizontal direction i.e. forwards. So, a tyre that is over-inflated for a specific rider is more likely to ‘bounce’ on a poor road surface (which, on British roads, is more often than not). Track cyclists have a perfectly smooth surface so this is not a consideration – high tyre pressures then make sense to minimise rolling resistance.

To get a sense of what it means in the real world to get your tyre pressures spot-on, consider that feeling when you cycle from a rough road-surface onto a lovely smooth piece of tarmac. Suddenly the amount of vertical bounce has been reduced drastically – zipping along feels so much easier. It’s the same principle. 

OK, optimising your tyre pressures won’t necessarily make you any faster (I’m a case in point!) but it will mean you use less energy over a long ride.