Are wider road bike tyres faster?

In a white paper entitled Total System Efficiency: Making You Faster, engineers at Zipp put forward the idea that there are ‘five principal impending forces acting on a cyclist… wind resistance, gravity, inertia, rolling resistance and vibration’.

To move forward, you must overcome these forces by putting more energy into the rider-bike system than the sum forces acting against the path of travel. So where do tyres fit in? The simple answer is everywhere. However, to find a way into this extremely complex subject, let’s start with that most touted term in regard to tyres: rolling resistance.

‘As a tyre rolls along it deforms and relaxes, resulting in energy loss – that is rolling resistance,’ says Oliver Kiesel, tyres and tubes product manager at Specialized. ‘This is elastic hysteresis loss, which happens inside the compound of the tyre as it rolls through its contact patch.’

As a tyre rolls along the ground it deforms – flattening at the bottom, bulging at the sidewalls – before springing back. As this deforming-relaxing movement happens, molecules inside the tyre compound slide past each other, which creates internal friction, which is energy lost to heat. In theory, wider tyres can limit such losses.

‘If we had a 23mm, 25mm and 28mm tyre of the same type and at the same pressure, rolling on a perfectly smooth road, the wider tyre would offer lower rolling resistance,’ says Kiesel.

This is because the wider tyre has a shorter, wider contact patch that results in less material flexing compared to the thinner, longer contact patch of a narrower tyre. However, that’s just one component of rolling resistance.

‘Imagine every time a tyre rolls through a longer contact patch, there is more rubber in contact with the ground, and this leads to more frictional losses through traction – the friction that occurs between a drive wheel and road surface,’ says Schwalbe’s product manager, Felix Schäfermeier. As such, rolling resistance is primarily defined as losses through elastic hysteresis and traction, and a wider tyre can make useful savings.

‘We observe that at 60psi, our 29mm tyre is essentially twice as efficient as a 27mm tyre,’ says Jake Pantone, Enve’s vice-president of product. But this is just the beginning.

Vibrational losses

‘Taylor Phinney did this experiment in Paris-Roubaix,’ says David Morse, advanced development engineer at Zipp. ‘He tried to maintain 35kmh over different surfaces, and in the velodrome it took him 175 watts to achieve 35kmh, while on a dirt path it took him 210 watts. But on the Arenberg cobbles it took him 339 watts.’

The reason for such vast differences is that it takes more energy to travel over a rougher surface. Quite a lot more, thinks Morse.

‘Let’s assume you are a 100kg rider-bike system. You’re riding over the cobbles and you get bounced up in the air by 1mm for one second. It takes roughly a joule of energy to raise 100kg by 1mm, and a joule per second is a watt. So you can see it doesn’t take a lot of movement before those losses stack up. It might look smooth but your average asphalt road can produce a lot of bumps.’

This led Zipp to study vibrational losses in relation to tyre pressure, using an 85kg rider-bike system on a rolling road ‘that simulated poor-condition asphalt’, riding at 32kmh on 28mm Zipp Tangente Speed RT28 tyres. The observed result was a ‘48-watt decrease in power required between 90psi and 30psi’.

Admittedly these numbers did raise eyebrows among some of our experts (this is Zipp’s research after all, and each manufacturer will point to its own), but all agreed with the basic theory: a smoother path of travel is a faster one. So surely all you need to do to unlock some free speed is to drop your tyre pressure, no matter the width?

The effect of volume

‘A tyre needs to support the rider,’ says Enve’s Pantone. ‘Go too low with tyre pressure and you start sacrificing aspects of ride quality – standing up and sprinting you will feel the bike wallowing, or going into a corner you’ll feel the tyre start to roll sideways. Wider tyres allow you to run lower pressures before this happens, and that’s because there’s an inverse relationship between pressure and volume.’

In other words, the higher the tyre volume, the lower the pressure needed to provide adequate support. It’s the reason a car tyre might be 38psi while a track cyclist might be running 160psi, and to understand this we need to know two things.

First, a pressurised tyre is subject to ‘cylinder stress’, where the contained air is trying to push its way out with equal force in all directions across the inside of the tyre and rim bed. Second, pressure = force ÷ area, hence psi: ‘pounds per square inch’. A 90psi tyre functions as if 90lb of weight is pushing on every 1in × 1in square of tyre.

Now imagine a 23mm and a 28mm tyre, both cut in half across their width and laid flat like belts. The 28mm tyre is visibly larger, and it has a greater surface area (it’s around 1.3 times larger, but we’ll spare you the maths). Now imagine those tyres intact, on wheels and inflated to 90psi. Every square inch of each tyre has 90lb of force pushing on it, only here’s the thing: the 28mm tyre is made up of more square inches, hence more force is acting upon the 28mm tyre’s walls – the cylinder stress is greater, the tyre is stiffer.

The upshot is that a wider tyre can offer adequate rider-bike support at much lower pressure than a narrow tyre, which leads to lower vibrational losses. Flipping this on its head, it also means that to achieve the same level of comfort as a 23mm tyre, the 28mm tyre needs to be pumped up to a lower pressure – run both at 90psi and the 28mm will actually be the less comfortable of the two, which might feel counterintuitive.

There is one more clever thing that wide tyres do.

Spring rate

‘Imagine a 100mm suspension fork,’ says Vittoria’s Avery. ‘The spring rate curves up quickly [it goes from being easy to compress to much harder to compress] because you only have 100mm of travel before you bottom out. But if you have 170mm travel, the spring rate curve can be much shallower – it can be super-soft and responsive for the first 50mm as you have 120mm left to play with.

So a wider tyre is like a bigger fork – it is taller, so it has more travel, if you like – and it can be run at a lower pressure in a way that means its spring rate can be shallower or more progressive.’

It’s a bit like a tennis ball verses a child’s plastic football. The tennis ball is low volume and high pressure; squeeze it hard and it gives a little but soon resists further compression. The football is high volume and low pressure; squeeze it more gently and it gives a lot before becoming progressively harder to compress. You can stand on both without total collapse.

What we want from our tyres is the perfect blend of football and tennis ball – easier to deform (limiting vibrational losses), but not deforming so much as to lose stability or incur undue losses through rolling resistance. This latter point is very much one of conjecture – does rolling resistance matter in terms of pressure? It depends on who you ask and how you measure.

Pressure

‘At 48kmh our 27mm tyre becomes 17.4 watts more efficient by going from 40psi to 80psi,’ says Enve’s Pantone, although he points out this is on a smooth steel drum ‘most unlike the real world’. Conversely, Specialized’s Kiesel points to a difference of two watts between his 30mm tyre at 60psi and 80psi.

And then there’s Morse at Zipp, who says, ‘Rolling resistance is largely controlled by tyre compound and construction techniques – how thick the tread is, how many plies, the carcass material,’ – a point on which Schwalbe’s Schäfermeier also agrees. So what might be a sweet spot for pressure and width?

Zipp and Enve are two companies with online pressure calculators that claim to be geared up for real-world conditions, and throwing a few numbers into them elicits the following: an 82kg rider with a 27-31mm tyre should ride at 58psi, says Enve, while a 50kg rider should go down to 39psi. Zipp is perhaps strangely more conservative given its earlier research, recommending around 62psi for an 82kg rider on 30mm tyres, 51psi for a 50kg rider.

Still, putting this all in perspective, it’s clear that wider tyres at lower pressures are faster. So how wide should we go?

• Read our guide to bike tyre pressure

What about weight and aero?

The objection goes – wide tyres are all very well, but what about weight and drag? Again, there are various answers, but here is some compelling food for thought for starters. First, weight.

‘In terms of going up a hill, 100g is negligible compared to the 100kg rider-bike system,’ says Zipp’s Morse. ‘But when we’re accelerating we can feel the increased mass, right? But if you do the calculations, a WorldTour sprinter riding at 1,500W loses just 1.3 watts if tyre weight increases by 100g. And most of us aren’t riding at 1,500W, so I would argue weight doesn’t mean anything.’

This point is also echoed by Ken Avery from Vittoria, who adds, ‘The difference between 25mm and 28mm is around 10g. It’s hardly worth worrying about.’

Then, aero. Without delving too deep into this very complex subject, both Zipp and Enve – whose stock in trade is aero – agree changing tyre sizes by millimetres results in losses of just a few watts, which could be considered huge in the aero gains game, but for most riders might seem a fair enough trade-off. So given all this, what is a sweet-spot tyre size? Again, it depends on who you ask, and it also depends on one final factor: internal rim width.

Matching tyres to rims

‘For us, the Enve 29mm tyre on our 25mm internal rim hits the best balance,’ says Pantone. ‘It actually measures up close to 30mm.’ And this is the thing – the internal width of a rim directly impacts the width of a tyre, which is one of the reasons it’s hard to say ‘Xmm is the fastest tyre choice’. It also impacts its tyre profile, which has connotations for aerodynamics and vibrational losses.

‘From an aero standpoint, the rim-tyre interface is critical, so if you put a 28mm tyre on a 19mm rim you’ll end up with a bulbous tyre shape that just won’t be efficient,’ says Pantone in reference to aerodynamics. However, go the other way and there are problems too.

‘If you have a too narrow tyre on a too wide rim the sidewalls become pretty straight,’ says Kiesel. ‘This leads to a kind of bad behaviour in compression. A little bit of a balloon shape allows the tyre to flex better. So we see from testing, in terms of aerodynamics and rolling resistance, the sweet spot is a 21mm internal rim with 26mm or 28mm tyre. A 25mm internal rim is too much.’

Again, it’s all a trade-off, before one last curve ball thrown in by Kiesel.

‘There are those pushing hard for wider tyres because they make wider rims. They claim there are performance reasons, but a lot of it is down to manufacturing. They are making hookless rims because they are easier to manufacture, but the maximum recommended hookless tyre pressure is 72.5psi [according to the European Tyre and Rim Technical Organisation, ETRTO].

So if you are limited to that pressure, your rims and tyres will have to be wider, because only wide tyres will perform well at this pressure – a 28mm tyre at 72.5psi will be slower than a 30mm tyre – and wide tyres need wide rims to support them properly.’

On that note, we’ll return to those who just make tyres for the last word.

‘If you are using 17-19mm internal width rims, then 25mm is still a really good option for a good tyre shape. For 20mm, there’s a really good benefit with 28mm tyres, and when you get closer to 25mm, then 30mm or 32mm tyres are a good option,’ says Schäfermeier from Schwalbe. But if you really just want one answer to this question, just ask Avery from Vittoria.

‘In real life, from rider feedback, 28mm is the new normal because of the entire package. It’s the size that’s going to reduce the time it takes from point A to point B.’

• Read our guide to internal rim width and why it matters

How fast is my tyre?

Above is a table of a selection of some tyres that are pretty fast and some that are very popular but much less fast. This data comes from independent tester Bicycle Rolling Resistance and it suggests a few interesting things.

First, tubeless is speed king; second, in pure rolling resistance terms, higher pressure = lower rolling resistance; and third, construction and compound play a crucial role, as all other factors here are equal, even most of the tyre widths.

A note on tread

‘For grip on a road tyre, the tread pattern doesn’t matter nearly as much as the compound,’ says Schwalbe’s Felix Schäfermeier. ‘If you have a good compound, it will perform better than a really rough tread pattern. From our perspective we use the tread pattern to reduce aerodynamic drag.’

‘The tread is super-critical [for aerodynamics],’ agrees Enve’s Jake Pantone. ‘The tread not being smooth creates a little bit of turbulence as the air hits it, which creates a boundary layer of air that basically prevents there being suction between the air and the tyre. The turbulent layer lets the air slide really quickly over the tyre without getting stuck to it.’

But does tread offer more than just aero gains? The jury is out, but Vittoria’s Ken Avery certainly thinks so. ‘The grooves down the shoulders of our Corsa tyres play a big role. They go circumferentially so they allow the casing to flex more easily as they allow the tyre to collapse sort of inwardly as it rolls. It also helps with grip, it provides channels for moisture to be released from the contact patch, and it presents an effective edge that resists drifting out of a corner.’

This is an old article but we've republished it because it's very useful – especially as we approach the Classics.