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am I underinflated?

Old 02-13-13, 12:56 PM
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am I underinflated?

I have Vittoria Randonneur 700x28 tires. The max pressure is listed as 85 PSI, which I have been using. The ride and handling seem fine, and no flats since installing them 10 months ago. But I see recommendations for that size to be more along the lines of 100 PSI, especially carrying a 230 lb rider.

Should I be pumping the PSI higher?
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Old 02-13-13, 01:19 PM
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I used to have a set of 28 mm Ruffy Tuffy (Rivendell by Panaracer) tires, I ran mine at 105, and the stated max was 95 I think. But most tires can be run at higher than recommended inflations without incident. But I am not a safety engineer, nor was I involved with testing of any tire, so as they say, ya pays your money and ya takes your chances.
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Old 02-13-13, 02:41 PM
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I use the chart at https://www.bikequarterly.com/images/TireDrop.pdf as a starting place. Assuming about 2/3 of your weight is on the back wheel, that chart would suggest 100 psi, maybe 10 psi less if you've got long chainstays and the distribution is closer to 50/50.
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Old 02-13-13, 03:02 PM
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I always find myself pumping my tires a good 5-10 PSI over the suggested amount. Anything lower then that and it just seems like there is too much deflection from bumps and I don't want to risk getting a pinch flat.
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Old 02-13-13, 03:06 PM
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Originally Posted by ThimbleSmash
I always find myself pumping my tires a good 5-10 PSI over the suggested amount. Anything lower then that and it just seems like there is too much deflection from bumps and I don't want to risk getting a pinch flat.
+1
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Old 02-13-13, 04:27 PM
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Originally Posted by pdlamb
I use the chart at https://www.bikequarterly.com/images/TireDrop.pdf as a starting place. Assuming about 2/3 of your weight is on the back wheel, that chart would suggest 100 psi, maybe 10 psi less if you've got long chainstays and the distribution is closer to 50/50.
There's an app for that. Found one for Android, so I assume there must be an Iphone version as well.
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Old 02-13-13, 04:29 PM
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The problem with larger tires at high pressure is you're putting a lot of stress on the rims. The rims have to hold against that pressure, and for any given pressure a larger tire will require more force to hold it in place, as pressure is force divided by area and larger tires have more area.

100 PSI for a 28 seems a bit high. If you're worried about pinch flats, I have to wonder what you're riding over.
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Old 02-13-13, 04:38 PM
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I cant get over the fact you have been riding it for ten months without issue, but now feel you need to do something different because of a chart? Not to mention that the manufacturer also seems to think the rated pressure is fine (since they wrote it on the tire)
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Old 02-13-13, 05:28 PM
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Originally Posted by achoo
The problem with larger tires at high pressure is you're putting a lot of stress on the rims. The rims have to hold against that pressure, and for any given pressure a larger tire will require more force to hold it in place, as pressure is force divided by area and larger tires have more area.

100 PSI for a 28 seems a bit high. If you're worried about pinch flats, I have to wonder what you're riding over.
This is not correct.There is not change in either the pressure or the the area of the rim subject to pressure from the tire, no matter the size of the tire. A larger tire does not exert any more force on the rim. If you subscribe to the tire drop concept from Frank Berto, then 100 is not too high for a clyde rider. Many do not seem to believe in the tire drop idea, I think they just stick with the recommended pressure. I just looked at my chart and anything over 132 pound wheel load will lead to an excess of 100 PSI for a 15% tire drop.

But as always, you just find what works for you.
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Old 02-16-13, 09:44 PM
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So I used to always make sure to run my tires at the highest PSI. I was firmly in that camp and kept in it.

Then I started to not care. I found out that running at lower PSI I was more comfortable and somehow faster. Running at 55 or 60 PSI on 700x42's I was faster than running at 90 on 700x35's or 90 on the same 700x42's. I didn't feel faster at first, but sure enough.

YMMV but I'm now firmly of the camp who really doesn't obsess over tire PSI. In fact I'm petty sure my tires at are like 40 or 45 right now but who cares.
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Old 02-16-13, 10:01 PM
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Originally Posted by howsteepisit
This is not correct.There is not change in either the pressure or the the area of the rim subject to pressure from the tire, no matter the size of the tire. A larger tire does not exert any more force on the rim.
Incorrect.
See "Width and Pressure": https://sheldonbrown.com/tires.html
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Old 02-16-13, 10:10 PM
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I run 28's at about 85psi. If you go up to 100 you are losing the benefit of the higher volume tire. Most days I run my 23's about that too just to smooth out the chipseal, just ride light over curbs and potholes.
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Old 02-16-13, 10:14 PM
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Sheldon does not say anything about equal pressures causing higher rim stress than it causes in smaller tires. What he is saying is that a larger tire, because it has a larger surface area, is under more total pressure than a smaller tire at the same pressure. The rim however, stays the same size and is subject to the same pressure in pounds per square inch.
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Old 02-16-13, 10:30 PM
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Originally Posted by howsteepisit
Sheldon does not say anything about equal pressures causing higher rim stress than it causes in smaller tires. What he is saying is that a larger tire, because it has a larger surface area, is under more total pressure than a smaller tire at the same pressure. The rim however, stays the same size and is subject to the same pressure in pounds per square inch.
Still incorrect.

"Each sidewall of the tire bears half that load, and so each inch of length of tire sidewall will be under a tension of 50 pounds. Now let's consider a tire twice as wide, two inches across, at the same 100 PSI. Each inch of sidewall will be under a tension of 100 pounds. So, a wider a tire would ride harder, and need stronger fabric, if inflated to the same pressure."

The tension in the sidewalls has doubled. Now what is it that is holding the sidewalls at the bead of the tire?

Mavic maximum pressure chart for their rims, varying by tire size: https://www.mavic.com/sites/default/f...echart_eng.pdf

Last edited by Shimagnolo; 02-16-13 at 10:34 PM.
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Old 02-16-13, 11:00 PM
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Mavic's chart is for tire size, not rim width, and I think its based on issues of tire material strength, not the stress placed on the rim, which is what you said.

Edit: After drawing a bunch of diagrams I understand what you are saying, I am thinking now that your are correct, that you do increase the overall pressure on the rim with a bigger diameter tire. That said, I have run all kinds of tires at all kinds of pressure and never have I blown out a rim, nor have I seen it happen. I have sen rim seams fail, if that was caused but the pressures due to too large of diameter I don't know.

Last edited by howsteepisit; 02-16-13 at 11:54 PM. Reason: I was WRONG!
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Old 02-17-13, 08:36 AM
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Despite all the technical back and forth, I live by a very simple rule of thumb, which has served me well for my 59 years on this planet: "if it ain't broke, don't fix it".
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Old 02-17-13, 02:17 PM
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Originally Posted by mprelaw
Despite all the technical back and forth, I live by a very simple rule of thumb, which has served me well for my 59 years on this planet: "if it ain't broke, don't fix it".
Thanks for all the responses. I'm sticking with prelaw's program.
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Old 02-17-13, 02:18 PM
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Originally Posted by vesteroid
I cant get over the fact you have been riding it for ten months without issue, but now feel you need to do something different because of a chart? Not to mention that the manufacturer also seems to think the rated pressure is fine (since they wrote it on the tire)
I guess I don't know everything and feel like I can always learn and improve. Must be nice for you, though.
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Old 02-17-13, 02:42 PM
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Originally Posted by howsteepisit
Mavic's chart is for tire size, not rim width, and I think its based on issues of tire material strength, not the stress placed on the rim, which is what you said.

Edit: After drawing a bunch of diagrams I understand what you are saying, I am thinking now that your are correct, that you do increase the overall pressure on the rim with a bigger diameter tire. That said, I have run all kinds of tires at all kinds of pressure and never have I blown out a rim, nor have I seen it happen. I have sen rim seams fail, if that was caused but the pressures due to too large of diameter I don't know.
I've broken a Mavic mtn rim...*twice*...by running a 2" tire at 70psi, (the max listed on the sidewall of a semi-slick). This was a bike ridden only on the street. The first time it happened, (on a rim just a few months old), I assumed it was a bad rim, and moved the hub & spokes to a new identical rim. Then it happened again on the new rim just 6 weeks later. Some research turned up the tire-width/pressure/rim-stress issue. The cracks always formed in the center of the brake track exactly where the inner wall of the rim meets the back of the brake track. The tire would push the bead of the rim out so it was dragging on the brake pad.
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