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Small front tire larger back

Old 04-20-07, 08:44 AM
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Small front tire larger back

Has anyone ever put a small high-pressure tire on the front, say 700x20 145-psi, and a larger back tire 23 120-psi? It seems to me that the tradeoff of a large tire is air resistance but better rolling resistance because of the shorter contact point along the circumference.

The back tire is blocked from the air by a larger frame tube. The front tire carries less weight and would deflect less, with a smaller profile the sir resistance would be less.

Anyway just a thought, I was looking for the best clincher tires and tubes for racing only, any suggestions would be appreciated.

Rich
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Old 04-20-07, 09:19 AM
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People do this. In fact, Continental Attack/Force come spec'd as 22 front and 23 rear.

One additional consideration however is narrower tire = smaller contact patch. Given that washing out the front tire is generally a bad thing, the wider 23 front may be a worthwhile insurance policy, particularly in a crit.
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Old 04-20-07, 09:20 AM
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Ive seen it on a couple of track and TT bikes because smaller tires are faster.. but i dont know all the physics of it.. I guess i could if wanted being a physics major but yeah.
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Old 04-20-07, 09:30 AM
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Just be careful you don't go over the top with the front tire's pressure. The higher the pressure the less contact area it has with the road so you have less braking power, especially since you don't have much weight over the front. Many people keep their front tire at a slightly lower pressure than the back one for this reason - obviously you'll need it higher than the back if it's a 20, but better to inflate it to the lower end of the limits given by the tire imo.
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Old 04-20-07, 10:29 AM
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If you're averaging 30mph in a time trial on a smooth road, then there may be some aero benefits to a narrow front tire. On a rough road, a wider tire can actually be faster because it doesn't bounce around as much. If you're not averaging 30mph, then work on your engine before you start thinking too much about silly things like this.
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Old 04-20-07, 10:51 AM
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Originally Posted by johnny99
If you're averaging 30mph in a time trial on a smooth road, then there may be some aero benefits to a narrow front tire. On a rough road, a wider tire can actually be faster because it doesn't bounce around as much. If you're not averaging 30mph, then work on your engine before you start thinking too much about silly things like this.
Ouch
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Old 04-20-07, 11:13 AM
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For rolling resistance and traction, it would actually be better to go fat in the front and skinny in the back.
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Old 04-20-07, 11:30 AM
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How so??? I thought wider tires have less tolling resistance because of deflection and the back tire has more weight and would deflect more

Originally Posted by waterrockets
For rolling resistance and traction, it would actually be better to go fat in the front and skinny in the back.
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Old 04-20-07, 11:43 AM
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Originally Posted by johnny99
If you're averaging 30mph in a time trial on a smooth road, then there may be some aero benefits to a narrow front tire. On a rough road, a wider tire can actually be faster because it doesn't bounce around as much. If you're not averaging 30mph, then work on your engine before you start thinking too much about silly things like this.
So based on your theory, if we are not averaging 30mph on a 40km TT then we have no business on this forum or starting/replying to threads. (Because 99% of this forum is asking about "silly little things like this.")
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Old 04-20-07, 11:47 AM
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Well, wider tires have less rolling resistance at the same pressure, but smaller tires generally run at higher pressure, so they have the capacity to be lower rolling resistance. I guess the max psi for a given tire model is the same across all sizes, so at the listed max, the wider would be lower rolling...

If you look at the following chart, and check out the Avocet 30 28c vs the 25 c, you can see how they are very close in RR at a given pressure, but you can't run the 28c at as high of pressure as the 25, so the 25 will generally have lower RR if you keep it pumped up.

A lot of people leave their tires a little underinflated for comfort, and the skinny benefits aren't realized in this situation.
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Old 04-20-07, 12:00 PM
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Originally Posted by waterrockets
Well, wider tires have less rolling resistance at the same pressure, but smaller tires generally run at higher pressure, so they have the capacity to be lower rolling resistance. I guess the max psi for a given tire model is the same across all sizes, so at the listed max, the wider would be lower rolling...
If you're using the same pressure in wider and narrower tires, then one of them has the wrong pressure. The ideal pressure is always lower for a wider tire and the max is also usually lower. See sheldonbrown.com or roadbikerider.com for some good discussions are correct tire pressure.
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Old 04-20-07, 07:00 PM
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Gave it a try

Well I gave it a try Cont Force in back 23-120psi and Veloflex Record in front 20-140 psi (9-10 bar recommended pressure). It was smooth pavement 22 mph but I’ve got to say it felt a little faster, not very scientific. I tried some hard corning with no problems and the ride was fine. I’ve got a club ride this weekend and I’ll let you know how it feels in a pace line.

Rich
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Old 04-20-07, 07:04 PM
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Originally Posted by johnny99
If you're not averaging 30mph, then work on your engine before you start thinking too much about silly things like this.
Since reducing drag saves more time for slower riders, it seems one could argue if you're going 30 mph, there's not much to be gained by thinking about silly things like this.
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Old 04-20-07, 08:15 PM
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Originally Posted by SaddleBags
So based on your theory, if we are not averaging 30mph on a 40km TT then we have no business on this forum or starting/replying to threads. (Because 99% of this forum is asking about "silly little things like this.")
I agree that the tone is not the best. But he does have a point, the 23/25/28 makes no noticeable difference unless you're in a TT.
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