In theory wider tires should have less rolling resistance at the same tire pressure (to a limit) but typically increased sidewall deformation cancels that advantage. Going to wider rims restores it. The theoretical contact patch of least resistance is a circle.
Further to what djconnel says: you'll find the best narrow tyres to have the highest tpi count and a thin(-ish) thread. Up to 350 tpi using the best quality cotton or silk fiber tissue.
Studies have shown that on average a 23mm wide tyre is a better choice for road use since it exhibits the smallest set of compromises all factors considered.
To further optimise this you'd also need a rim that's a bit wider than the typical 19 to 21mm wide rim bed or else it would be an aerodynamic misfit.
The only tyres that can approach a circular shape contact area are latex track tyres. A decent road tyre will have an oval shape contact patch where ideally the edges are orientated lateraly across the tyre.
When the tyre is overinflated and/or has too low tpi count the edges of the oval shape will be found along the radius of the tyre.
This situation is unwanted as the thread (+ anti-puncture belt) of the tyre is much less flexible than the sidewalls hence increasing its rolling resistance.
The general jist of things is explained by this graphic from Conti:
http://velonews.competitor.com/files/20 ... 46x421.jpg
Hed has claimed this as a benefit from about 2009 but as I said, the only data that I've seen is very neutral. Al Morrison showed that the same tire on a traditional 20mm and a 23mm rim, back to back on rollers, had the same resistance within margin of error.
And from an article by Greg Kopecky over at ST:
"Does that automatically mean that the Crr is lower? Some manufacturers suggest that this is the case, but nobody is producing hard numbers to prove it – or at least showing them to any of us. Zipp Technical Director, Josh Poertner, challenges the idea. He suggests that there may be a very, very small improvement – but neither their testing, nor any independent testing they’ve seen can prove it outside of the margin of error of the test (on the order of .0000125 Crr improvement). My suggestion is that saying that the wide rim itself improves Crr, is like claiming that the chicken came first, when it’s really the egg. Yes, a wide rim allows you to run a wider tire and lower pressure – which lead to lower Crr. But it isn’t the rim itself that lends to this improvement; or at least not enough that we can actually measure it. I’ll go ahead and insert the voice of reason for you: Go worry about something else! Sleep more! Eat less Twinkies!"
http://flocycling.blogspot.com/2011/08/ ... 4-flo.html
Lower pressure - lower crr? To a certain point ofcourse...
DMF wrote:Lower pressure - lower crr? To a certain point of course...
Where is that point? Tom Anhalt did some measurements and found that point at ~120 psi... but it wasn't a really accurate test and would be specific to his tires and road.
On a smooth surface higher pressure always *reduces* Crr. But there is an optimal point on roads where the combination of resistance due to roughness and the resistance due to tire flex is minimized.
Plus, wide rims make the sidewalls more vertical, which inherently makes the ride more harsh. I believe that is why manufacturers tell you to reduce pressure. If your narrow rims feel right with 100 psi, and your wide ones feel right with 90 psi, I'm pretty sure you'll have higher resistance with the wide ones.
120psi may rule on track, but on asphalt it's just insane unless you're a real lardo... Rolling resistance has very little to do with road friction, it's all in the movement of the casing. The more supple the lower the crr. No tire is supple at 120psi.
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