23 mm combined with 25 mm tyre in racing

Question

I met a guy who mixed 23 and 25 mm for his front and rear tyre respectively.

Now thinking about it, I believe it's a pretty good idea since the rear wheel bear a higher proportion of the rider's weight. Wider rear tyres would support more weight with the same pressure as the front. It also helps reduce the wear and tear on the tyre itself.

So why don't bicycle (especially racing) run a slightly wider tyre in the rear, as the rear wheel mostly bear the weight of rider? It makes the ride more comfortable, and not aerodynamically terrible as the front is still 23 mm.

Answer 1

I would be genuinely surprised if it made any real-world difference. Some racers commit all sorts of voodoo... err... questionable practices on the off chance it might give them an advantage. In terms of 23 vs 25 mm tires, both sizes are frequently used by pro tour teams, if one had a distinct advantage over the other you wouldn't see both sizes being used at the same events. It terms of aerodynamics, even for sizes up to 32 mm there doesn't seem to be much difference in aerodynamics drag.

In terms of comfort getting the right tire pressure for your body weight will likely have a bigger effect than 2mm gain in the tire width.

Other disadvantages include complications introduced into the assembly and supply chain for the manufacturer, which would eat into profits.

Answer 2

The bigger question is, what would be the benefits of this setup? If you are willing to sacrifice the few grams of weight and square centimeters of cross section that come with wider tires, why wouldn't you use a wider tire in the front too? The benefits at comfort, grip and possibly rolling resistance apply to front, too.

In my opinion, the bike also looks nicer if both tires are similar. And if you keep spares, the same one goes for both.

Answer 3

I've seen time trail racers do the opposite and run a 22mm or 23mm tire in back and a 25mm up front.

One reason not to have a larger tire in back is because it's comparatively heavier and all the extra weight is around the circumference of the wheel. When you are putting power through the crank and trying to spin up the rear wheel, that extra weight is felt and you need more power compared to the effort required when riding a lighter, possibly smaller tire.

One other reason I can think of is that it might actually be more aero to have a larger tire up front when paired with a wide rim, which is the current industry trend. Every wheel manufacturer seems to find evidence that whatever they are doing is the best.

Answer 4

Racing is all about speed. 25mm tyres have a larger contact area so there is more rolling resistance than a 23mm. It doesn't make a huge difference but it is significant. You wouldn't be competitive on 25s with everyone else runnings 23s.

But for some very tough terrain, such as going over cobblestones, most teams will often run 25s front and back. They also deflate slightly from the maximum pressure to avoid some of the ride harshness.

I started on 28mm tyres on a commuter and soon went down to 23mm. Didn't notice any difference in tyre wear or comfort. Bike was less effort to ride. Only downside is is that it is more likely to catch a rut in the road, but you ride with a bit more caution.