Thread: 25s vs. 23s
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Old 02-14-19, 09:49 PM
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Originally Posted by Maelochs View Post
When one wants to see where the latest thinking on absolute high-speed performance is leading, look to the pro peloton.

Not everything they do translates to regular road riding but some of it does.And the trend there is towards 25s and 28s---for all the reasons @HTupolev stated in his thorough post.

I can remember when 15-mm tires at 190 psi or some such was considered 'fastest" because obviously less drag and friction. Now we have gone to almost twice the width and half the pressure ... and the guys in the peloton aren't doing it for comfort.

Also, studies have shown that what really affects tire aero isn't so much frontal area but rim width and depth. An ogive rim profile with the rim and tire perfectly matched in width creates less drag--drag being a function of turbulence more than frontal area.

There is a ton of research out there, I am sure, for people who want to go looking. I have seen a few small bits here and there, including one great piece on rim width and shape and tire width .... but when the pros started racing on wider tires and deeper, ogive-section rims, I didn't need to see a lot more studies. Those guys really pore over the research because there are careers and sponsorships riding on it.
As long as you match the rim width and cross section to the tire, the narrower tire will always win on air resistance. Air resistance is Area X Speed squared X Air Density X Coefficient of Drag. Keep that coefficient of friction constant by simply scaling down your super slippery fat tire and rim combo but scale down the size and width by say 76% (going from a 25c tire to a 19c) and yo9u have a wheel that has 76% of the aero dynamic drag. (Well no, the spokes and hub don't scale.) Yes, the new fat tires and rims are faster than the old skinny tires and very poor rims. but those old skinny tires on equally sexy but appropriately narrower rims would be fastest yet.) So if you keep the coefficient of drag of the combined tire and rim constant, the skinnier tire always wins the aero race. Always. Aerodynamics 101. Freshman year.

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