BIG IT UP! The 25mm Tyre Takeover!

BMC Racing, Team Blanco, FDJ, Omega Pharma, Orica-Greenedge, Argos Shimano, Sky: just some of the teams that were running 25mm tyres this year. 700c x 23mm is no longer the norm, pro teams are switching up to bigger profile rubber all over the peloton, with research showing that it can provide significant gains both in terms of aero-dynamics, grip and comfort.

I’ve long been running 25mm tyres on my winter/touring bike as it helps to cushion the load better, provides a bit more grip, and is less susceptible to pinch flats when the bike is heavily burdened. However, for racing I have always used Conti GP4000S tyres with 23mm profiles, or at the most extreme I tried a 21mm Vittoria for a time trial once. That might no longer be the case...

The thinking was that the smaller profile tyre has a lower surface area, so lower rolling resistance and also less weight (rotating weight as well, so even more significant). 25mm tyres were only to be used on particularly rough terrain and for heavily laden bikes. New research has proven this wrong.

From Schwalbe
Firstly there’s the rolling resistance argument. BikeRadar has a great article explaining why a 25mm tyre may in fact have lower rolling resistance. The argument runs along these lines: If a 23mm tyre and a 25mm tyre are run on the same pressure, with the same rider, then the contact area will in fact be the same; it is just that the 21mm style will have a longer narrower contact area and the 25mm will have a shorter fatter contact area. (See diagram from Schwalbe)

The interesting thing is that the longer flatter profile of the 21 or 23mm tyre is less rounded than the 25mm profile, which means it doesn't roll as well. Therefore at the same pressure (say the recommended pressure of most clincher tyres), there will in fact be less rolling resistance from the 25mm profile than the 23mm profile.

The second factor that comes into play is aerodynamics. In the pro peloton there is also an increasing tendency to use wide profile rims; these are considered stronger, but more importantly more aerodynamic. A wider profile is seen to “roll” through the air better than a thinner profile (think Zipp Firecrest). To match with these wider rims it is better to use wider tyres, as it lessens the chance of pinch flats and creates a better join between rim and tyre (less of a curled shape).

Cycling is a sport where every tiny gain counts for something, and wider tyres and rims are one of those little gains that seem to have been taken on board. The days of 21mm or even 18mm tyres on racing bikes are apparently long gone. Certainly for 2014 I am going to be making the switch, more out of curiosity than anything else. I’ll report back on the blog to let you know if I have noticed the “marginal gains”.


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