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Road Cycling It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle. -- Ernest Hemingway

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Old 10-15-07, 11:55 AM   #1
HDK
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23 mm vs 25 mm tires

Hello everyone, I recently bought a Cervelo R3, and it came with Vittoria Diamante Pro Light 25mm tires. I have 2 questions:

1) I was surprised after riding the bike for 2 months with the tires inflated to 110 psi, that the reccomended pressure is 130 psi min. to 160 psi max. I have since inflated them to 140 psi (I am 185 lbs.) The ride is harder, but not to bad. Is this high pressure due to the fact that the tire is 25mm vs 23?

2) What difference if any would I notice if I went to a 23 mm tire. One LBS guy says the 25 mm has less rolling resistance because it is crowned more than the 23mm and thus actually has less of a contact patch. What say you???

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Old 10-15-07, 12:03 PM   #2
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You were right the first time. You don't need all that pressure....

Let's pretend your tire is rock and has zero deflection. If you hit a bump, some of your velocity will become vertical. The harder your tire, the more energy you waste bouncing around.

If you want to try a nice fast 23c tire, get a Michelin Pro Race. My favorite skinny tire is the Vredstein Fortezza, at 100psi it rides as sweet as a girlfriend's first kiss. But it's even better in a 25c. Btw, get the plain Fortezza, there is a zillion flavors, none of them ride as well.

I think the extra weight of the 25c tire would be more important than any potential change in the size of the contact patch. Having said that, I wouldn't expect much at all in the way of a difference either way.

When I was riding in Europe a racer/tour guide borrowed my Road Morph to pump up this guy's tire. He wouldn't put in as much air as the guy wanted. "Why do Americans always want so much pressure?" I didn't have an answer.
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Old 10-15-07, 12:05 PM   #3
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I didn't realize those came in 25's. Anyway I'd be surprised if your clincher rims are rated for 140psi. I have heard that 23's have less RR than 21's but I'd still stick with 23's over 25.
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Old 10-15-07, 12:17 PM   #4
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Tires rarely measure true to the stated size. I went from 25mm Gatorskins to 23mm Gatorskins and there is almost no difference in actual size. As for the high air pressure recommendation, it has little to do with tire size - in fact one of the reasons for going to larger sizes is to run at lower pressures without getting pinch flats. Note that even if tire manufacturers rate their tires for very high pressures, wheel manufacturers often do not. 140-160 psi is way over the recommended limit for many lightweight clincher wheels.
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Old 10-15-07, 12:18 PM   #5
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At 185, I'd go with the recommended pressure. It'll definitely help against pinch flats. As for the 23mm v. 25mm issue, I'd doubt the difference is significant.
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Old 10-15-07, 12:23 PM   #6
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At 185, I'd go with the recommended pressure. It'll definitely help against pinch flats. As for the 23mm v. 25mm issue, I'd doubt the difference is significant.
With a 25c tire and his weight, 100psi ought to be enough to prevent pinch flats.
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Old 10-15-07, 02:01 PM   #7
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I'm 220lbs and I run my Conti GP 4000 X 25's at 100psi and the ride is great.
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Old 10-15-07, 02:22 PM   #8
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"Why do Americans always want so much pressure?" I didn't have an answer.
Bigger / more is better. The American Way. Duh?

Seriously... wider tire <==> lower tire pressure is necessary to avoid pinch flats. Sheldon Brown's site has a nice table of suggested pressures given weight born by a tire and the tire's width. Obviously the rear tire has more weight on it so you can get away with a lower pressure in front but will need a higher pressure in back. There's really no reason (on most bikes) to have the same pressure in front and back. It's pretty much guaranteeing that one will be suboptimal.
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Old 10-15-07, 02:42 PM   #9
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Originally Posted by HDK View Post
2) What difference if any would I notice if I went to a 23 mm tire. One LBS guy says the 25 mm has less rolling resistance because it is crowned more than the 23mm and thus actually has less of a contact patch. What say you???

High thread count is the main factor in rolling resistance is tires.

http://en.wikipedia.org/wiki/Rolling_resistance
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Old 10-15-07, 02:43 PM   #10
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So I am a bit google challenged. Is there some links to some research or studies on this? Tiresize/pressure, etc?
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Old 10-15-07, 05:10 PM   #11
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http://www.sheldonbrown.com/tyres.html
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Old 10-15-07, 07:40 PM   #12
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The marked size of a tire is not an indicator of its actual size. Many tires marked as 28mm are actually 27mm, and some are 26mm.

Likewise, most tires marked 23mm are not 23mm wide, they are 24mm wide or 25mm width.

The width of a tire is not a guarantee of to its rolling resistance or shock absorption abilities. Back when Schwinn was a "real" bike company, their tests showed 200% differences in rolling resistance between two brands of tires that were of identical width and appearance. When "Bicycling" was a real bike magazine, their tests of rolling resistance got the same results Schwinn did.

Generally, among the "top five" brands of tires, the best clue to rolling resistance is weight. The lightest tires of a given size have the least rolling resistance. The heaviest tires of a given width tend to have the best flat resistance, and the best resistance to damage from road debris and UV exposure.

The one guarantee with tires is that tires with a "real" width of 28mm will absorb road shock much better than tires with a real width of 23mm. At Paris-Roubaix, every year there are pro cyclists who refuse to ride the bike their sponsor supplies them unless it is modified to use 28mm tires...

The correct PSI for a tire is the PSI level that provides 15% deflection. With a tire with a "real" width of 25mm, that 15% deflection might be obtained at 140 PSI by a 220 pound rider, and at 80 PSI by a 120 rider. With 15% deflection, both riders would obtain an identical contact patch, identical shock absorption, rolling resistance, handling, and braking characteristics.
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Old 10-15-07, 07:46 PM   #13
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Bicycle Quarterly has been doing a lot of studies on this topic. They are looking at it more from a long-distance riding perspective. Since, for at least 90% of us, the only "racing" we will ever do is the weeknigth hammerfest, this is our usual riding, too. Their point seems to be that optimal inflation pressures are not as high as we use and rider comfort counts for a lot on long riders, and tire construction (thread count, for starters) has more to do with rolling resistance than width and inflation pressure.

If your riding is more like a crit race with lots of fast cornering, hard acceleration, and manuevering, you may get a different answer. As always, YMMV.

To the OP's questions - I doubt you'll feel any difference between 23mm and 25mm. Especially if the tires are different makes/types. I'd try pressures at the lower range, closer to where you started. If you aren't getting pinch flats and the ride is more comfortable, then it works for you.
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Old 10-15-07, 07:58 PM   #14
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You got 2 months use with the Vittoria Diamante Pro Lites on 110 PSI?! I had a pinch flat within a week on 120 PSI. I thought they were kidding when they said "130MIN".
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Old 10-16-07, 06:52 AM   #15
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Yeah, I got 2 months of riding at 110 Psi with no flats. However, keep in mind these are 25mm, and yes the do look a lot larger than the 23's I see around. I liked the ride at 110 psi and had no flats so I guess I will back it down to that pressure. The wheels BTW are Capagnolo Vento's.

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Old 10-16-07, 09:13 AM   #16
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Experiment with the pressures. When I tried 23mm lights I used 125 in the rear compared to the 115 I use in the regular version. Any less pressure made the tire feel to squishy for my liking. 110 does feel better on chip seal.
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Old 10-16-07, 10:57 AM   #17
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I think your right, a lot of the roads around here are chip seal or crack and repair seal.

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Old 10-16-07, 11:43 AM   #18
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I've been riding a 25c with 70 psi these days. I like it better since the ride is smoother and I get a better workout. No pinch flats at this PSI and it does better on gravel.
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