Under pressure:
how much is too much for your tyres?
It wasn’t too long ago that the only pressure you were supposed to inflate your tyres to was the maximum recommended: normally 120psi and more. You were given this advice by any experienced cyclist and this was often accompanied by dire warnings of catastrophic pinch flats if you ran anything lower, and of course that old ‘fact’ that the higher the tyre pressure, the faster the bike.
This belief also coincided with a time when 23mm was as wide as most ‘proper’ cyclists would go and a 25mm tyre was seen as a boat anchor more suited to commuting or touring use.
Now, 25mm is seen as ‘narrow’ and the reserve of race bikes – with 28, 30 and even 32mm tyres being specced as standard on endurance road machines. So with this in mind, do we need to update our thinking in terms of tyre pressure? Is maximum still the way to go?
Why is finding the correct tyre pressure important?
It’s probably more appropriate that we talk in terms of ‘ideal’ tyre pressures rather than ‘correct’, as tyre pressure is a moveable feast – a single tyre pressure is not a fix-all for every rider.
Tommaso Cappella, Vittoria’s service and quality manager, and self-professed tyre pressure enthusiast, explains why pressure is so important: “The ideal pressure allows the tyre to sag properly, optimising the ground contact area, therefore impacting manoeuvrability and performance.”
Finding the ideal pressure is also about striking a balance between reducing rolling resistance and increasing comfort.
Laboratory research using a smooth surface has proven that higher pressures result in a lower coefficient of rolling resistance (Crr). However, real-world road conditions are so far removed from the laboratory as to actually result in a cut-off point where higher pressures actually impede efficient power transfer, due in part to an increase in energy loss as bike and rider are bounced around and break contact with the road surface. Lower tyre pressure allows the tyre to deform and roll more smoothly over broken road.
What are the factors that affect ideal tyre pressure?
According to Cappella, “The fundamental factor is the actual width of the tyre – let’s see it as an air chamber – its size depends on the rim width and the tyre itself. As a simple rule of thumb, the wider the tyre, the lower the ideal pressure.”
Your choice of tyre is also incredibly important, namely its specific construction.
A cotton casing deforms and behaves differently compared to nylon; also the TPI (threads per inch) make a difference. A higher TPI implies the yarn is finer, making the tyre feel smoother and roll faster.
Rider weight, normal riding position, riding style and road/atmospheric conditions all need to be considered too when calculating tyre pressure. A 100kg cyclist riding their bike on the smooth roads of Mallorca will want a higher tyre pressure than a 60kg rider covering the rutted, potholed lanes of the UK.
“At lower pressures, tyres roll more smoothly”
So where do I start?
While the various calculations involved can appear daunting, with too many variables and pitfalls, it can be quite easy to achieve a starting point from which to determine your ideal pressure.
Many tyre and wheel manufacturers are beginning to introduce pressure guides or even easy-to-use online calculators, to help you get started.
Zipp (SRAM) has a very detailed calculator that enables you to plug in a range of data in order to get your starting points, Vittoria likewise – although it is working on a new app with even more detail.
Cappella has some simple advice to get started on your ideal tyre journey: “If you used to ride 116-130psi (8-9bar) on a 23mm tyre you might want to drop the pressure in a 25mm tyre by 14psi (1bar), dropping another bar for 28mm and going down to around 72psi (5bar) for a
30mm tyre.”
Tubeless?
One of the oft-touted advantages of going tubeless is the ability to run lower pressures.
Indeed, this can be the case – just look at the low 72psi limit Zipp has imposed on its latest 303 tubeless wheels for all tyre sizes. Add to this the fact that a tubeless system offers a lower Crr when run at the same pressure as a tubed tyre, thanks to elimination of the friction between tyre and tube.
However, the pressure differences are not as marked as you might think. Zipp recommends just 1.5psi lower for a tubeless set-up over inner tubes with a hooked tubeless rim. This does change when using a hookless tubeless rim, though, where pressures can be reduced by nearly 6psi.
Tyre pressure calculators, such as those provided by Zipp and Vittoria, are a good place to start. After that, it’s all about experimentation and tweaking: “I would suggest every rider experiments with tyre pressures to find out the ideal,” said Cappella.
Tyre pressure has such an impact on performance that it really should be something every rider takes time to consider; it’s not just a question of banging in the maximum air pressure anymore in order for a tyre to feel fast. With so much research data pointing to the positive impact of lower pressures on both rolling resistance and comfort, we think it’s time all of us experimented with pressure.