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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 07-12-10, 09:27 PM   #1
Trill_01
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A lilttle help with a tire issue please?

Hello everyone! I am new here and kind of a beginner at mountain biking. I have this problem with the tires on my mountain bike. About every month or so of regular riding the innertube will break in the same spot consistently. It always breaks right at the base of the valve. Whenever I get a flat I take off the tire and look at the inner tube and the base of the valve is always split like the whole valve is almost severed off, not completely but at least like one third to one half of the circumference of the rubber at the base of the valve is split open.

I wonder if anyone one here knows why this always happens and what could be the cause of it. Any help or insight would be greatly appreciated.
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Old 07-12-10, 09:50 PM   #2
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I'm guessing you have Schrader type valve (like a car's valve stem). There is probably a sharp edge on the rim's valve stem hole, which is acting like a knife against the valve stem when you ride. I'm not sure how to alleviate the issue, perhaps file away the sharp edge?
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Old 07-12-10, 10:03 PM   #3
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As Mudpie suggests it could be a bad valve hole in the rim, and filing the edge of the hole in the rim might help, but I suspect there's more at play.

I'm guessing that you're using tubes where the valve is molded into a conical section of rubber which comes out from the tube. Some of these are pretty poorly made and the valve ends short of where it meets the tube leaving only a heavy rubber section right where it passes through the rim. Try to buy a tube where you can feel that the metal valve goes all the way to the tube and ends in a little metal flange that you can feel through the tube.

If you can't find one like that, try punching a hole through a patch and gluing it on as added reinforcement at the base of the valve.

Also next time you mount the tire try to be careful that the valve is straight and points directly to the axle. Often I see people with crooked valves which means that the tube is shifted around a bit and straining the base of the valve.
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Old 07-13-10, 12:16 AM   #4
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Another possibility: low tire pressure causing tire to rotate slightly, thus causing the damage to your tube. The sidewall of your tire should show the recommended pressure, AND maximum pressure rating. If you are running the tire at LESS than the recommended pressure, it could cause this.
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Old 07-13-10, 06:49 PM   #5
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Any chance you're using a hand pump instead of a floor pump? Its easy to rip the stem off being a little too rough with a hand held pump. It can be done with a floor pump too if you're wiggling the pump head a lot to get it on or off the valve.
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Old 07-13-10, 08:35 PM   #6
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All four of the above excellent points should be checked. The only other that has partly been mentioned is change brand of tube.
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Old 07-13-10, 08:46 PM   #7
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Watch your stems for signs of leaning. If they start straight up and down and after your first ride they are leaning then you are not putting enough air into them and the tire is slipping on the rim. When this happen and it puts enough pressure on the rubber then it'll tear. But just in case it's cutting through you could use a file or small knife blade to chamfer the stem hole's edge on the inside.

Something else you can do to check for tire creeping on the rim is to use a felt pen or some easy to remove paint and mark a line on the rim to the tire. If the part on the tire isn't lined up with the part on the rim after a few rides then again you have proof that the tire is moving on the rim and that is your trouble.

Also do you use anything to aid you in mounting the tires? Like a tire lube similar to what motorcycles and cars use to aid in seating the bead?
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