Tuesday, 12 August 2014

Tyres — why thinner isn’t always better

To non-cyclists road bikes have two ingredients that almost always raise comments: saddles ‘like razor blades’ and super-thin tyres. Those razor-blade saddles aren’t that uncomfortable at all, at least not after novice bottoms have gone through a little bedding-in process. And thin tyres are pretty self-explanatory — the thinner they are, the quicker they must go, surely?

Er… well…

Look at almost any current range of mass-produced road bikes and you will find an increasing trend among manufacturers to fit tyres with a width of 25c rather than the established favourite of 23c. But why is this?

First of all, let’s understand why 23c became the standard choice on new road bikes. One notion you will often find us mentioning in this blog is that in the road bike market, public purchasing trends are often dictated by what pro riders use. That is to say, most people who hop aboard a road bike dream of being in the Tour de France and want to ride essentially the same kit as the Froomes and Wiggos of this world.

To top racers the advantages of thin tyres are relevant. Less rubber means lighter weight, which is particularly important because this weight is found at the wheel rim, which affects acceleration. The small width also reduces air drag, helping improve aerodynamics, particularly at speeds of 25mph or more. Of course the pros use top-end, often prototype versions of their tyre supplier’s products, but the 23c standard was a pretty good facsimile for amateur road riders.

However, now, with the prevalence of sportive bikes, speedy commuters, and more multi-purpose drop-bar machines, those catchall terms, and catchall spec choices look a little bit basic. We should mention at this point that even bikes now built for pro riders make a great play of their ‘comfort’ factor: recognising on one hand the idea that even an elite racing cyclist will expend less unnecessary energy just coping with road conditions if he or she is comfortable; and on the other, that the vast majority of amateur road cyclists now aren’t the experienced, one might say ‘grizzled’, club riders of yesteryear.

So 25c tyres are staking their claim as first choice on new road bikes. It’s easy to accept that with a larger capacity they might be a little more comfortable, but surely they’re equally a small bit slower?

Not necessarily.

Fatter tyres can actually offer better speed, at least below 20mph where the vast majority of sportive and leisure riders pedal. Writing in Cycling Weekly earlier this year, Richard Hallett pointed out that:
“A two millimetre increase in tyre size, equates to a nine per cent increase in tyre radius and, therefore, an 18 per cent increase in tyre air volume. For comfort, the fatter tyre can be inflated to roughly 5psi less for the same sensation of road surface imperfections.” This wider tyre with reduced pressure will also offer better grip than a 2mm-thinner tyre.
However, if you inflate both 23c and 25c tyres to the same pressure, they will actually share the same size contact patch with the road. But because the bigger tyre is wider, its contact patch will be correspondingly slightly shorter. This, Hallett says: “creates less tyre carcass deflection than the long, narrow patch of a thinner tyre, and reduces rolling resistance loss.” Which means wider tyres are just a little bit quicker.
It’s true, wider tyres are heavier, but there’s also another advantage with bigger rubber: you’re less likely to suffer from pinch flats. OK, it’s a slightly longer-term look at speed, but having to stop to change a puncture puts a significant dent on anyone’s sportive standard or Strava segment.
However, one word of caution. If you’re a road bike rider already, the tyre world isn’t entirely your oyster — road cyclists are still constrained by the tyre clearances already built into their bike frames. But with more and more mainstream road bike brands purposely building their frames with enough tyre clearance to fit up to 32c rubber, the option is there for many people to experiment with wider tyres.
So wider tyres = faster riding? Potentially yes — plus more comfortable and more secure riding, too.