Pulling together some data:
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The surface dictates what tire pressure you should run; larger tires support lower pressures.
https://blog.silca.cc/part-4b-rolling-resistance-and-impedance
Running too much pressure for a given surface will kill your aero gains easily. For “Naples, FL” perfect but realistic pavement with a equipped 160lb rider, that pressure is ~90-110psi; for a common brushed concrete sidewalk ~60psi, and a weathered country road is probably ~80-95psi. -
A larger tire costs about .15w per size increment 25,28,32,35… for a given tire at the right pressure.
Ok… you’re rolling… what about aero
3) If you’re tire is too wide for the aero rim, you’re not aero. The 28mm wide bontrager was made for a 25c tire. Larger than 25c, you won’t get any benefit. This really only matter for the front tire. The right size tire on a regular rim is faster than the wrong one.
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Tire size affect on aero on a non-aero rim. About .3w per mm @ 22mph over a 25c (32c cost about 2.1w)
Rennradreifen: Ist "breit" gleich "besser"? | roadbike.de -
you can make big tires aero with the right rim. remember that the tire size counts in crosswinds though. A 35c tire automatically add 10mm of crosssection over a 25c in gusty winds. I think Hunt, 3T, and DT Swiss make a wheel that can make a 32-35c tire aero.
Comfort:
6) Tire pressure and spring rate - It’s pretty linear. As pressure decreases, the ride gets more compliant, but there is a diminishing return. Moving from 110psi to 90 is huge, but 90 to 70 not so much… until the surface calls for it. Moving from 60 to 40psi on a gravel trail is HUGE.
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Underpressure is not good - you’ll bounce and your body will bounce on the saddle or you’ll strike the rim , in which case spring rate goes put to whatever the wheel/bike spring rate is.
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Same pressure, different size - 90psi rides like 90psi no matter what size tire you’re using.
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Tires themselves - road tires are not built for durability and will ride better than gravel tires. A fast rolling gravel tire will cost you about 2w over a comparable road tire (the prior Schwalbe g-One Speed vs Pro v-guard)
So, figure out the appropriate pressure for the surface you’re riding and work from there.
On that rim going from 25c to 32c on a GP5000 tire would cost you (22mph, perfect road):
- +.5w rolling watts
- +2.1 tire watts + 7w wheel aero
So, 9.6w - that’s 1mph loss on perfect road. Judging the road wrong (10psi too high, new asphalt vs country road) would cost you 6w. On smooth pavement 85psi is just as comfortable as 50psi, so you’re better off on the 25c/28c( f/r) setup. As soon as you hit some surface that’s not asphalt, you should be 30c+. As soon as that surface is dirt and not perfectly smooth, you want to be under 45psi meaning 35c+.
The cost of running a smooth gravel tire on the road is about (BWR California scenario) - 25c @85psi 12.1rolling vs 38c @ 45psi 18w & 4.2w tire aero & 7w wheel benefit = 17.2w road penalty and about 20w on-road advantage (minimum, that’s on bumpy trail but doesn’t account for irregular bumps, sand pits, climbing traction/braking downhill).