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Tire Pressure problem?

Old 08-15-10, 02:59 PM
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Fullforce
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Tire Pressure problem?

I rode 1600 miles on my original Specialized Mondo Pro tires that came with my bike with only one flat (on the rear). I inflated those tires to 100 PSI every time. I was told when I got a second flat, that I needed new tires so I went with Vittoria Open Corsa tires. When I told the LBS that I usually inflated my tires to 100 PSI, they told me that 120 PSI was the proper pressure. So, I have been following their advice, and I have had four flats in the last 300 hundred miles (all on the rear). Every time I have brought my bike back to them.

Is PSI causing my frequent flats? I have not changed my riding routes or mileage, and I ride on wide multi-use trails exclusively. The last flat I had (this morning), the tire appeared to just lose pressure suddenly at the rim.
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Old 08-15-10, 04:18 PM
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More information needed.
What part of the tube fails? Where are the holes, rim side or tread side?
Underinflation often causes "snake bite holes", a pair of slits caused by the tube being pinched between the tire tread and rim, but not always in pairs. You probably do need more than 100 psi, depending on the total load on the tires. More pressure is needed in the rear tire because it carries well over half the load.
Holes on the rim side of the tube are often due to inadequate rim tape, or rough handling of the valve stem.
Blowouts are often due to a small portion of the tube being pinched between the tire bead and rim, any part of the rim.
In the southwestern U.S. many flats are caused by goathead or grass burrs. These cause tiny round holes on the tread side of the tube and are more common in the late summer and early fall.

Last edited by Al1943; 08-15-10 at 04:24 PM.
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Old 08-15-10, 04:57 PM
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Also what size tyres are you using and what is your weight? There's no outright proper pressure to use; it's a criss-cross balance of numerous factors.
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Old 08-15-10, 05:00 PM
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As Al said there are too many variables. Anyway, I've never been convinced that tire pressure in any way correlates to flats, except snake bites form extremely low pressure.

Tread side flats could be from bad luck, softer tread compound, seasonal (thorns) issues, or any number of other causes. Belly side flats are usually the result of a bad rim strip, poor mounting, or crappy tubes.

Questions, why did you tamper with success? You were doing fine before, were the new tires narrower?
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Old 08-15-10, 05:37 PM
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Maybe your LBS thinks that everyone should use 120 psi in their bike tires ( I see that number tossed around a lot on BF). There's a formula for proper tire psi that has been presented here (on BF) before (I did a quick search and couldn't find it right away) that takes your weight and tire size into account. I weigh around 210 lbs, and mostly ride 700 x 25 tires. Using the formula I came up with around 112 psi for the rear tire and 102 for the front. That has worked well for me. My take (from reading) is that you want enough pressure to prevent (the already mentioned) snake bite flats and after that amount is reached you can adjust for comfort/less rolling resistance. If you keep adding air, at some point, the benefits of less rolling resistance will become less and less apparent, until, some argue, you actually get more rolling resistance because of excessive "bounce" in the tires.

I would think your biggest issue is either just a string of "bad luck" or your new tires are just not as flat-resistant as your old tires. I don't know if I would have "given up" on my old tires so quickly (holes can be booted etc.) but I'm known for trying to get every bit of "use" out of my stuff. You could examine your old tires (if you still have them) and compare them to your new tires (you would have to take a tire off the bike to examine it inside and out). After having a similar experience as you when returning to more-regular riding a few years ago (getting a flat about every 50 miles, especially on the back tire) I started really looking at the tires I was using and I concluded that the relatively thin-skinned (especially thin in the sidewall) Michelin Dynamic "training" tires (that came with my bike) was just not going to work for me as a rear tire (on the front it's okay). I've had good experience with the Marathon Plus tire (though it is quite heavy and many avoid it for that reason) and the relatively light Panaracer Pasela with TourGuard that has a kevlar belt. If you haven't gotten a plan and equipment like a mini/frame pump and patch kit (and learned technique) to fix your flats (especially on the road) then this experience might motivate you in that direction. Hope some of this has helped and that you find a solution.
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Old 08-15-10, 05:44 PM
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Originally Posted by mawtangent View Post
There's a formula for proper tire psi that has been presented here (on BF) before (I did a quick search and couldn't find it right away) .
Here's a link to the chart you might have had in mind. I use it a bit differently; to find the cross section width that is correct for the weight at 95-100psi.
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Old 08-15-10, 07:34 PM
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Yeah, I find 95-105psi to be a sweet spot for pressure. Then I get tyre size to work with that pressure and my weight.

That shop needs to determine the actual cause of Fullforce's recent 4 flats. Without figuring out the real cause of the flats and fixing it, it seems irresponsible to charge him for the following ones. It could be something like a thorn or metal fragment through the tyre that was never removed. Or a sharp edge on a rim hole or a bad rim-strip or spoke sticking through. Pressure is the least of your concerns.
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Old 08-15-10, 08:55 PM
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My tires are 700 x 23, and I weigh 165, The tires have a 320 TPI rating.

I was thinking that a very bright light and a magnifying glass might be necessary,

The two flats on my old tire were in the center of the tread; the shop found a sliver of glass. The flats I've had on the new tire seem to look like pinch flats, although I ride about 100 miles before they happen.
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Old 08-16-10, 12:19 AM
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For most of my life I rode bikes with pretty fat tires and had never heard of a "pinch flat". I had a bike with 27 x 1 1/4 tires and one with 26 x 1 3/4. I didn't use gauges to measure the psi, I just pumped them up (every month or so) until they "felt" pretty solid. I'll admit during much of that time I was significantly lighter in weight than at present. When I got my first flat-bar road-bike it had 700 x 25 tires and I got my first pinch flat (before I ever heard of the concept or the phrase). I've probably experienced 4-5 pinch flats to date. Most of the time I've felt a conspicuous "hard contact" feel (a "bottoming out") that indicated that the rim had made contact where it shouldn't and pinch-flatted the tube. Sometimes it wasn't obvious at that exact moment, but within a minute of the contact (when the air loss was obvious), and after examining the tube and retracing the events right before the flat I was able to deduce that I had hit "something" too hard, I think it was usually just a rock (maybe just an inch in diameter).

I guess my point is: if you are pinch-flatting you probably will feel it happen (a hard contact feel). On the other hand if you have been pumping your tires to 120 psi then that should keep pinch flats to a minimum (because your rim has good support and it will be less likely to "bottom out" on something). Someone probably has already added that pinch flats leave two holes (like a snake bite) in the tube, so that is something to look for when making a diagnosis. If you eliminate a run of "bad luck", something stuck in the tire and/or something getting to the tube from the rim-side, then you are back (in my view) to just needing more flat-resistant tire(s). I run a flat-resistant tire on the back wheel (where, historically I get most of my flats) and find that most any tire will work on my front wheel (I often have a lighter "training" tire on my front wheel).
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Old 08-16-10, 07:36 AM
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Originally Posted by Fullforce View Post
My tires are 700 x 23, and I weigh 165, The tires have a 320 TPI rating.


The flats I've had on the new tire seem to look like pinch flats, although I ride about 100 miles before they happen.
I don't feel like typing so much today, so here's a link to a prior post (#3 in the thread) that may help you if the leaks are on the belly side of the tube.

If they're about 1-2" from the valve, the last note about properly pulling the valve back down against the rim may be the key. Too often the base of the valve hangs up between the beads in the tire and the tube is over-strectched as it bubbles out filling the gap below.
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Old 08-16-10, 08:44 AM
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I weigh 165 and run Michelin Pro 3 Race tires at 115 to 120 in the rear and about 110 in the front. I say about because I doubt that my pump gauge is very accurate. I fully inflate to those pressures before each ride.
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