In the search for the fastest tyre, should you go narrow or wide? High pressure or low pressure? Tubs of clinchers? The answers are not as simple as you might think
One of the biggest ever leaps in bicycling technology came from an unlikely source: a Scottish veterinary surgeon by the name of John Boyd Dunlop. In 1888, in a significant departure from his day job, Dunlop created the first pneumatic tyre in a bid to rid his son of the headaches and discomfort that had troubled the lad as he rode his solid-tyred tricycle around the bumpy cobbles of Belfast.
Fast-forward to today and the basic concept hasn’t changed – a sealed chamber of air provides a layer of cushioning between the rider and the road – but that does not mean that all tyres are equal. Some tyres are faster than others, but it takes a bit of understanding of tyre technology before you can hunt down the best one for you.
Resisting a rest
‘While riding, a cyclist has to face different types of resistance: air resistance, weight (if accelerating or braking) and the rolling resistance of the tyre, which is the energy loss due to the tyre rolling forward,’ says Michelin’s road tyre developer, Nicolas Cret. ‘We measure rolling resistance with fixed parameters such as regulated pressure, constant speed, load and temperature. The measurement machine is usually composed of a drum, which has to be as big as possible to simulate flat ground. The tyre is rotated at a given speed/load/pressure during a warm-up session, and then we will stop
the drum power and measure the distance until the tyre stops rolling. The longer the distance, the lower the rolling resistance is.’
In basic terms, then, rolling resistance is the force that acts against the forward motion of a tyre rolling on a surface. In practical terms, along with factors such as air resistance, this resistive force means that when you’re freewheeling on a flat surface you will eventually come to a stop. But since energy can neither be created nor destroyed, only changed, where has the energy that was propelling us forward gone?
‘Rolling resistance in tyres is energy consumed to overcome tyre deformation,’
says Wolf VormWalde, tyre product manager at Specialized. ‘When a tyre is under load it deforms, and to deform a material requires force. When the tyre rolls the deformation is ongoing as the tyre tread and sidewall go through the contact patch [where the tyre meets the road surface] as the wheel rotates. The tyre is therefore stressed and deformed going into the contact patch and relaxes going out of the contact patch. But unlike a perfect spring, the tyre does not give back the energy put into it when deforming.’
Observe what happens to a stationary bike’s tyres under a rider’s weight and you will understand what VormWalde means. A tyre under the load of a rider will bulge out at the sidewalls and the tread will flatten to conform to the shape of the surface beneath it. When the bike is in motion and the tyre is rotating, this process happens over and over at the point at which the tyre meets the road surface. In an ideal world the tyre would ‘give as good as it got’, rebounding off the road surface with as much force as went in to squashing it onto the road surface in the first place, and hence the energy put into forward motion would be conserved. Unfortunately, rubber compounds in tyres are ‘viscoelastic’, meaning that as they deform under load the molecules in the compound’s polymer chains rearrange themselves, and in so doing, rub
up against each. This internal friction creates heat, which, sadly, is a useless by-product in the quest to propel your bike forwards. Just feel your rear tyre after an hour on the turbo trainer and you’ll soon get the picture.
It’s this deformation of the tyre that is key to its rolling resistance and hence its ‘speed’. There are various ways that you can affect the way a tyre deforms, one of which is to vary the pressure of the air that you pump into it.
Deformation of character
If the more a tyre deforms, the more rolling resistance it has, surely all you have to do is inflate a tyre to the highest pressure possible, making it all but impossible to deform, and energy loss through rolling resistance will be minimised? The truth – as always – is a little more complicated.
Christian Wurmbäck, product manager at Continental, says, ‘Increasing the pressure in a tyre will decrease rolling resistance, but only up to a point. As an example, if you take a 23mm tyre and increase the pressure from 85psi to 115psi you’ll have less rolling resistance. But if you take the same tyre and increase the pressure from 115psi to 140psi there is virtually no difference.’
VormWalde from Specialized agrees: ‘On a perfectly smooth surface the higher pressure is always faster. But this effect tapers off on real roads, such that we say at 130psi you pump the tyre to dead [ie, it can’t get any more usefully rigid]. The important thing to remember is that the relationship between the tyre and the road is symbiotic, and that roads are never perfectly smooth.
‘You don’t want the tyre so hard that when you roll over the road it can’t absorb the surface frequencies. It’s more efficient for the tyre to absorb roughness and bumps than it is to pass these amplitudes on to the bike and the rider. Lifting the bike and rider up will always consume more energy than squishing a tyre down. That’s one of the reasons why you see cyclocross and mountain bike riders running such low pressures,’ he adds.
He has a point. For instead of allowing a particularly bumpy section to launch
him into a the air, the seasoned mountain bike racer will try to keep his or her body on a flat plane, using his arms and legs to absorb all the bumps the terrain serves up. In layman’s terms, if you want to go horizontally forwards, you don’t waste your energy going vertically up and down.
The trick is to work out the best tyre pressure for the road you’re riding on – something that may require a bit of trial and error. And then you have to ask
yourself whether you’re on the right width tyres in the first place.
The small matter of size
In the good old days, racers thought thinner tyres were better, with most pro wheels being shod with anything from a 21mm wide tyre to a minuscule 18mm. Over time, riders have perhaps placed more stock in comfort and less in bum-numbing speed, such that 23mm tyres have become a road bike standard.
However, Schwalbe product manager Marcus Hachmeyer says studies into tyre behaviour have uncovered some rather surprising things: ‘If you compare tyres with different widths but identical specs – same compound, same rounded profile, same inflation pressure – one can say in terms of rolling resistance: the wider the faster!’
This sounds counterintuitive – after all, road bikes are way faster than touring bikes or mountain bikes – but analysis of a tyre’s contact patch has helped designers such as Hachmeyer see past the popular belief that ‘narrower equals faster’.
‘Wider tyres are faster,’ echoes Wurmbäck at Continental. ‘A 24mm rolls faster than a 23mm, but a 25mm tyre rolls even faster than that. In fact, our GP4000s tyre is around 7% faster in a 25mm than a 23mm version.’
The reason goes back to this issue of deformation. Although at the same pressure both the wide and narrow tyres have the same contact patch area, the precise shape of each contact patch will differ. In a narrower tyre this patch will be thinner but longer, forming a slim oval shape along the length of the bottom of the tyre, whereas for a wider tyre the contact patch shape will be more circular, as the tyre is flattened more across its width. The result is that the thinner tyre’s slimmer, longer contact patch encourages more deformation of the tyre – specifically the sidewall – than its wider counterpart. And as we’ve heard already, the more a tyre deforms the more energy is consumed by deforming it. But if this is the case, shouldn’t we all be riding around on 28mm?
The case against
‘Although a 28mm tyre will be quicker than its 23mm version in terms of rolling resistance, the weight of the 28mm will be higher than the 23mm as a bigger size means more material. This is likely to create a noticeable difference in terms of inertia, and it will have an effect during acceleration or deceleration phases,’ explains Nicolas Cret from Michelin. ‘Aerodynamic properties will also change from a 23mm tyre to a 28mm.’
If pushed, what would the experts choose? ‘We’ve found 24mm is the ideal compromise in rolling resistance, aerodynamics and weight,’ says Specialized’s VormWalde. However Ken Avery from Italian old guard Vittoria disagrees: ‘More [width] is not always better. Moderation is the key. Once you go over 26mm the subtle gains in rolling resistance start to dissipate. The formula
is thrown off, so to speak. Also, this assumes that all tyres have a consistent profile, which they do not. Often tread thickness [in cross section] makes the tyre more pointy than round, such that a 24mm tyre from one manufacturer may be faster or slower in a given scenario than a 23 or 25mm.’
To complicate matters further, on top of choices about tyre pressure and width come considerations about the suppleness of a tyre.
What lies beneath
If deformation causes loss of energy from heat, then a tyre that is more supple will take less energy to deform in a given way than a tyre whose carcass is more rigid. Under the rubber compound of a tyre tread lie thousands of tightly-woven fibres. Depending on the tyre, this ply carcass could contain as many as 320 threads per inch (tpi), all of them a very fine cotton, or perhaps as few as 60, made from a decidedly thicker nylon. The upshot, say manufacturers such
as Vittoria and Challenge, is that the higher the thread count the more supple the tyre, and hence the easier it deforms, and thus
the lower rolling resistance it will have.
‘The greater the tpi count, the more flexible the tyre,’ says Simona Brauns-
Nicol from Challenge. ‘Over time, suppliers have delivered higher and higher quality threads that have made it possible for tyre manufacturers to go from a max weave of 280/300tpi to 320tpi. The more supple and flexible the casing, the more comfort and, most of all, the more adherence to the road, achieving therefore the most speed.’ However, in the world of tyres nothing is simple, and so more threads do not automatically mean a faster tyre.
VormWalde at Specialized says, ‘A 60tpi tyre with a good casing compound can be as fast as a 100tpi tyre. The material is important too – some polycotton casings are fast but that’s not because of the thread count, it’s because of the latex impregnation which makes it very elastic. A high thread count does not necessarily mean a faster tyre.’
The likes of Specialized and Challenge could likely go on arguing about thread count and casings for days (it’s no surprise that Challenge prides itself on producing tyres with a thread count as high as 320tpi, while Specialized seems content with producing a maximum 220tpi), but their opposing points of view highlight the very crux of this ‘fast tyre’ issue: there are no definitive answers. Sure, there are basic parameters – size, pressure, suppleness – but such things are so inextricably linked to both each other and questions of rolling resistance, aerodynamics and inertia that it’s pointless to focus on just one aspect at the expense of the others.
As Cret at Michelin puts it, ‘Designing a tyre should be seen as trying to improve many conflicting performance areas at the same time. A tyre is always a compromise of performance. What is a fast tyre? Well, that depends on what you mean by fast.’
Travel by tube
Will swapping out your inner tubes gain you speed?
If more supple tyres means better rolling resistance, then the same should be said for inner tubes.
‘An even more supple and puncture-resistant ride can be achieved by using a latex tube instead of a butyl inner tube,’ says Simona Brauns-Nicol at Challenge. ‘Ours can be inflated to around 300 times their original volume. Latex is strong and elastic at the same time, and doesn’t puncture as easily, as
the elasticity means a latex tube tends to go around foreign objects.’
As well as being an inherently more supple material, latex is lighter too – so it will outperform butyl tubes in terms of rolling resistance. However, this suppleness comes at a cost: latex is more porous than butyl, meaning air will leak out perceptibly over the days.
All sewn up
Tubs or clinchers? The argument rolls on…
For years tubulars have been touted as the best tyre a serious rider can get, with supporters claiming the only reason not to ride them on a daily basis is down to the inconvenience and cost of puncturing. However, there are a couple of companies out there willing to upset this particular applecart.
‘Clinchers are faster than tubulars,’ declares Specialized’s Wolf VormWalde. ‘This is because half of the effective air chamber is the rim. The rim sidewalls do not deform when rolling and thus consume no energy. You thought we pushed Tony Martin to use clinchers for commercial reasons, didn’t you? No! They’re simply faster.’
This flying in the face of conventional wisdom isn’t just from one man (albeit one at centre of a rather large bicycle corporation), but rather it’s a sentiment shared by the likes of tyre giants Schwalbe and Continental too. But if that’s
the case, why aren’t the pros riding clinchers? Well, says Continental’s Christian Wurmbäck, that’s a no-brainer.
‘A tubular wheelset is light but, importantly for pro riders, it affords run-flat-ability. In the event of a high speed flat, a tubular stays on the rim because of the glue, unlike a clincher, which has a tendency to fall off, making for a very nasty accident.’