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Can wider tyres and lower pressures make you faster?

In-depth
2 Jun 2021
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Wider tyres run at lower pressures don’t only make your ride more comfortable – they can make you faster too.

Words Stu Bowers Photography Danny Bird

Road cycling purists are tearing out the last of their hair. First we had the advent of electronic gearing and disc brakes, and now it seems the traditional 23mm tyre is about to be consigned to history. The trend has inexorably shifted towards wider tyres run at lower pressure.

It seems to go against common sense. You’d imagine a harder, narrower tyre would roll faster and be more aerodynamic, but the evidence suggests otherwise. More than that, the experts that Cyclist talked to claim that the new thinking on tyres could herald the biggest improvements to road bike performance in a decade.

Systematic approach

Now, before you rush out to buy new rubber, there is a caveat. Wide tyres on their own won’t make you faster. This is because tyres don’t work in isolation but need to be viewed as part of the wheel system.

The shape of a tyre once mounted and inflated is the most critical factor in determining how it performs, both individually and as part of that system with the wheel rim. As such, the dimensions of the rim (most importantly its internal width) are paramount for optimising tyre performance. That means, oddly, that it has been wheel manufacturers, not the tyre brands, that have led the way in harnessing the potential gains from using wider rubber.

‘If a wide tyre is fitted on a narrow rim it will have mostly negative effects on performance,’ says Bastien Donzé, wheel product manager at American brand Zipp. ‘The tyre will be pinched at its base, creating a lightbulb shape, so the tyre isn’t well supported and can feel unstable as it can roll left and right.

This, especially if the pressure is also reduced, can lead to a squirmy feeling as you lean onto the shoulder of the tyre during cornering, which won’t inspire confidence. Pinching a wide tyre on a narrow rim also fails to maximise the potential for the increased air volume inside it.

‘A wide [internal] rim profile allows the tyre beads to sit further apart, ideally making the sidewalls more upright, like an inverted U. This immediately increases the air volume inside the tyre and gives the tyre more structural stability, reducing that squirmy feeling and the chance of pinch punctures [if an inner tube is used] even at lower pressures.

‘What’s more, we have also found, contrary to what logic dictates, that this broader tyre shape can improve wheel aerodynamics overall, especially when stability in crosswinds is considered.’

A more aerodynamic wheel is a faster wheel, but there are also other ways in which a wider tyre can improve speed. Peter Marchment, co-founder of Hunt Wheels, highlights three.

‘The first is friction – what most people think of as rolling resistance. Second is the road surface effectively causing the bike to bounce up and down, and move side to side. Third is the energy expended by the rider as a result of the physical work muscles have to do to absorb road shocks and vibration.

‘Tests for rolling resistance are unequivocal, and it is now widely understood that wider tyres actually have lower friction at an equal pressure,’ he adds. ‘This is because the contact patch of a wide tyre is shorter and wider, or rounder, compared to the longer, slimmer elliptical shape created on a narrow tyre. This equates to less deformation of the tyre carcass overall as it rolls and less deformation is less wasted energy. In other words, less rolling resistance.

‘It’s true that a higher pressure effectively reduces how much a tyre deforms and hence lowers energy loss, but that’s only really of benefit if we’re riding on perfectly smooth surfaces, like a velodrome,’ Marchment says.

‘As soon as the surface has even the slightest roughness a high tyre pressure merely translates to a lot of minute unwanted movements as the tyre is deflected up and down and sideways, which are all taking away forward momentum.

‘The rider and bike system is almost continually hitting thousands of tiny bumps on real roads. The greater air volume of a wider tyre combined with a lower pressure helps the tyre absorb these deflections so they’re not passed on to the wheels, bike and ultimately the rider. In effect this reduces the unsprung mass that has to be moved out of the way on each surface impact and subsequently reduces kinetic energy loss.’

That sounds good in theory, but is there hard evidence? Cyclist contacted Wheel Energy, an independent test lab in Finland, to find out.

‘Our tests clearly show a narrow tyre and high air pressure do not provide low rolling resistance,’ says Wheel Energy’s president, Petri Hankiola.

‘For a 75kg rider, the tyres are loaded approximately 40kg on the rear and 35kg on the front. We’ve found in this example a 28mm tyre to be optimal, with pressures in the range of 79-84psi at the rear and 75-79psi at the front.

‘Also a 28mm tubeless tyre is the fastest setup [in rolling resistance tests]. At 40kmh a high-quality clincher tyre with a latex inner tube is about 2 watts slower. And a 32mm tyre is only about 2 watts slower than an equivalent 28mm tyre. Comparing rolling resistance, a 25mm tubeless tyre at 95-100psi is the same as a 30mm tubeless tyre at 72-80psi.’

Looking at the data, it becomes obvious that the frictional differences between tyres are very small, but that would be to miss the bigger point.

It’s Hankiola’s final statement that really opens up the wider tyre discussion – the opportunity to use a larger volume tyre at lower pressure for the same rolling resistance. This is potentially more valuable to overall performance than we currently understand, and where things start to get really interesting. Which brings us nicely to Marchment’s third point…

Going forwards

‘We know the benefits we’ve discussed so far to be true and quantifiable,’ Marchment says, ‘but there is a third level that is far less understood, and it could prove to be the biggest benefit of all.

We often hear reducing tyre pressure referred to in discussions around improving comfort, which stands to reason, and that’s a benefit not to be undervalued. But what is less considered is how much actual energy that saves, and the subsequent effect that can unquestionably have on performance.

‘All the time we’re experiencing vibration transmitted through the bike to our bodies, our muscles are having to do physical work to absorb this,’ Marchment adds. ‘And it’s not a passive process. Muscles have to burn energy reacting constantly to the vibrations, and that is ultimately energy that could be put into going forwards.

‘The difficulty is that it’s not easy to measure these losses. A power meter alone doesn’t tell us enough. That only tells us energy expenditure through the cranks and doesn’t measure the work done by your arms, neck or back muscles.

‘It’s our aim to start doing more to analyse the frequency and amplitude of the vibrations a rider’s body experiences, and try to quantify how much energy is used – or should I say squandered – as a result.’

Zipp is equally keen to dive more deeply into this hypothesis for improved performance. ‘From a wheels standpoint, Zipp has for some time focussed on aerodynamics to make people faster,’ says Donzé. ‘But now we’ve stepped away from looking at just that alone. We’ve identified greater benefits to speed by taking a more holistic approach to how rims and tyres interact.

‘Aero is still a big deal, obviously, as is weight, but these are gains that have already been mostly exploited. But how the tyre performs is a huge component of speed and is an area with the potential to unlock significant power savings.

Rough with the smooth

‘The first thing to accept is that we don’t ride on smooth roads,’ says Donzé. ‘For me, at this moment, anything smaller than a 28mm tyre doesn’t make any sense. It’s really interesting what we’ve found in researching the evolution of our rim shapes – actually, we have been blown away by what we’ve found. Being able to ride with much, much lower tyre pressures can potentially help far more to make bikes faster than we could have imagined.

‘We’ve built a rolling road – like a treadmill for bikes – to carry out very specific test protocols, but we’ve also done a lot of real-world testing and we couldn’t believe the numbers we were seeing.

‘Using our latest rims and comparing a 28mm tyre at much lower pressures – even down to the 55-65psi range [with a 75kg rider] – versus a 25mm tyre inflated up to 90-100psi, there is a profound effect on power.

‘The rim is so important in making this possible. With the right support from the rim shape, tyres become like a suspension system and really filter out the vibrations. The reduced sensation of fatigue allows the rider to focus more exclusively on smooth power input through the pedals. The benefits were astonishingly high; as much as 40-50 watts on rough road surfaces.’

Those are big numbers to bandy around – improvements that would take a considerable amount of training to achieve in terms of physiological performance, but Donzé is unequivocal on the matter.

‘We’re at a point where the rider can now save on every point. Our latest rim profiles are faster with 28mm tyres than anything that’s gone before, plus we know there’s less rolling resistance combined with a substantial rise in comfort and vibration dampening, which leads to significant power increase.

‘The latest disc brake wheels are lighter too, with most of the weight saving coming from the rim, which is the most important area to shed grams for wheel performance.’

Want a guide to the best robust tyres for winter? Check out the Cyclist guide here.

Tempted to convert to tubeless? Read our guide to how to convert and the best options here.

So given everything we’ve just discovered about the benefits, and with disc brakes eliminating a lot of the restrictions around rim size, why not opt for even wider rims and even more voluminous tyres? For the answer we’ll pass the baton back to Marchment.

‘There are of course some drawbacks to using even wider rims and tyres,’ he says. ‘First, there’s an increase in weight, plus the fact that going wider still would present more frontal area and hence we’d have to revisit new solutions to combat the increase in aero drag.

‘For the time being it looks like 28-30mm is the sweet spot in terms of getting the best of everything, but that’s not to say we would expect that to stay that way forever. Look at it this way: just two years ago we were debating whether a 25mm tyre was beneficial over a 23mm and already it seems like we’ve moved a long way on from that.’

It’s seems that the time has come to exile skinny rubber and those outmoded high pressures and accept that the research all points to one thing: go wide or go home.