Thread: 25s vs. 23s
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Old 02-14-19, 08:52 PM
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Originally Posted by El Davisimo View Post
Can someone please tell me why the trend right now is 700x25, instead of 700x23? I always thought that 23s are thinner tires, which means less contact with the road, which means less friction, which means more speed.
Contact with the road doesn't cause rolling resistance: the contact patch isn't sliding against the road at all, unless you're skidding!

There are two major sources of rolling resistance that have been identified:

1-When a section of tire reaches the contact patch, it deforms as it presses against the road. When it leaves the back side of the contact patch, it springs back into shape. But some amount of energy is wasted in this process; the amount of energy that sprung back into forward motion is never as much as much as was used to compress the tire; the lost energy was turned into a small amount of heat within the tire's casing when the tire deformed and sprung back. Higher tire pressures reduce this loss because the tire doesn't deform as much. The construction of the tire also has a big effect on this loss: tough stiff tires tend to waste a lot more energy than supple high-performance racing tires.
For a given tire pressure, wider tires don't deform as much, and so experience less of this loss. However, wider tires are generally run at lower pressures, which largely negates this difference.

2-The reason that bicycles use pneumatic tires is to serve as a suspension system. Roads are not perfectly smooth: they're made up of little irregularities, tiny bumps and valleys. If a tire is pumped really stiff, it will chatter off those irregularities and transmit them to the bike and rider, rather than smoothly deform around them. Even when this buzzing is subtle, it can still waste a meaningful amount of energy.
The rougher the surface, the softer the tire needs to be pumped in order to roll smoothly across the surface irregularities. But it's relevant even on very smooth roads: Silca did a test with a 190lb bike+rider on 25mm GP4000 tires, and even on a very plush fresh rolled asphalt road, the bike started to lose performance when the tires were pumped stiffer than 110PSI.

So, putting these two effects together:
You want to pump your tires soft enough that they're able to behave well as suspension. But narrower tires need to be pumped firmer in order to be well-behaved and minimize the losses from deformation. So, a wider tire can be pumped softer and achieve better performance as suspension, without suffering from greater losses to deformation.
Furthermore, a wider tire pumped softer can provide a more comfortable ride, and feel stabler on rough ground.

The compromises that wider tires face on road bikes are that they tend to be heavier and suffer from increased aerodynamic drag.

So, what's the optimal tire width? I dunno. In theory, it depends on the riding surface(s) and how heavy the bike+rider is (and a whole bunch of other things like the construction of the particular tire, and the wind conditions).
One reason behind the drive toward wider tires is that, in the last decade or so, a lot of people have been finding that the penalties for going wider are a lot less significant than they expected. In a sense, the reason behind the increase from 23mm to 25mm on racing bikes is that there wasn't much reason not to do it.

It's important to emphasize the importance of tire construction. My gravel bike is usually wearing tires which - aside from being 53mm wide - are built like high-performance road slicks. I occasionally take that bike to spirited road rides, and can largely ride with the same people as when I use a skinny-tired road bike. But I've also got a pair of cheap beefy ~46mm tires with very tough sidewalls: they slow the bike down by an absolutely massive ~2mph, despite being quite a bit narrower!
The point being: if you want a road tire to be fast, getting a tire that's fast is a lot more important than 23mm vs 25mm vs 28mm.

Another oddity worth mentioning is that most people "feel faster" on tires that are pumped stiff. When you go faster on a bicycle, the vibrations you feel from the road increase in frequency. So, when you go really fast, you're feeling lots of high-frequency vibrations. If you then damp these vibrations with a tire that's pumped appropriately-squishy, that "feels" similar to going slow.
I think that this is something that you can acclimate to, though. The first time I rode my gravel bike with those 53mm tires, I felt as if I was moving slower than my speedometer said I was going, but that sensation went away quickly.
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