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Race season is nearly upon us, and I'm ready to trade in my winter training tires for something that will land me on the podium (or at least get me to the starting line). I wanted to know exactly how the differences in tire diameter (i.e 23, 22, 20, 18 mm) affect the bike. Do wider tires corner better? I'm sure they soak up road shock, but do they also have substantually more rolling resistance? I've already checked out my mainstay, analytic cycling, but they don't have any information regarding tires. What is the prevailing wisdom? I need to get a set for road racing (clincher) and a set for the veledrome (tubular). I currently have 23mm on the road, and 18F 22R on the track.

Thanks in advance,
Doug
 

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From what I've read in studies, the differences in rolling resistance between 20mm to 24mm is very small ends up being less in the larger tires, all other things being equal (One of the reasons I've read the Conti Competition 22mm has less rolling resistance than the 19). I do feel more confident cornering on 25s then I do most 23s. At 90kgs I stay away from 18mm and 20mm. When you look at the math, yes the contact AREA is the same between the different sizes for same pressure and load, but the shape is different. The narrower tires are going to have a longer contact patch which can actually increase rolling resistance (but depending on condition and rim and mass and narrower cross section to the wind may offset somewhere else). If the wheel is rolling in the xy-plane (viewed sideways in plane on a sheet of paper on your desk) look at the angle from the front most portion of the contact patch to the rear of the contact patch from the center of the axle. The rolling resistance seems to be proportional to the amount of energy to deform the tire and the angle shown. By increasing tire width, you would be decreasing the angle.
Of course you seem to be able to run higher pressures in the smaller tires to shirk the contact area and therfore the angle theta of the contact patch. But then if you are on rough pavement, each bump is going to translate a small amount of your kenitic energy into gravatational potential energy. In other words slowing you down as you bounce over the road surface. A larger tire at a lower pressure will be able to deform to the bumps better and allow you to roll over them without changing your height relative to the ground. The capacity of slightly larger tires only works to a slightly larger degree of course.

The above is just what I've read. In personal experience I run 25s of my favorite tire when I can find them because I enjoy the smoother ride and feel better cornering. It's like the pavement just feels smoother and I don't bounce around, which is unnerving for me, in the corners.
 

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Your answer: it depends

General rules on tire size vs. performance:
- smaller tires can be pumped harder, which can deteriorate cornering due to lack of rubber on the road and lack of resiliency in the tire (bouncing off road irregularities)
- smaller tires have to be pumped harder to avoid pinch flats
- at the same pressure, a smaller tire will have higher rolling resistance due to more casing flex, but they are pumped harder so rolling resistance is about the same regardless of tire size
- smaller tires are lighter
- smaller tires, pumped harder, will wear faster (same amount of power transferred through a smaller contact patch)

All that said, there is a lot of difference between brands and models of tires, so it may well be that a supple tire will significantly out perform another tire of the same size. In the top of the line clinchers (Conti, Michelin, Vittoria, Vredestein, Veloflex, etc.) there are not many differences beyond the color scheme. Also, if you are very light, you can run a narrow tire at lower pressure and do just fine. 23 mm is very much the standard in road tires, with heavier riders gravitating to 25s or even 28s. For most people, a 23 gives the best compromise of comfort, traction, weight, wear, and pinch flat resistance.
 
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