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Commuting Bicycle commuting is easier than you think, before you know it, you'll be hooked. Learn the tips, hints, equipment, safety requirements for safely riding your bike to work.

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Old 11-30-07, 01:20 AM   #1
kmac27
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Pinch test gone horribly wrong

So I was installing my mountain bike tires on for winter, due to the fact that my road tire just took a huge bolt today. I filled up the tire and was installing it on my bike and I was getting it on the chain and as I went to grab for a tool, BOOM!!!! I guess I pumped it up too much. Without a pressure guage how do you determine how much air the tube can take?
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Old 11-30-07, 02:55 AM   #2
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Are you sure the tyre was seated correctly?

I've destroyed a few tubes when I've mistakenly left the tube in between the tyre bead and the rim. This either led to a blow out as the pressure forced the tyre out of the rim or the bead pinch flatted the tube during inflation.

http://www.sheldonbrown.com/tires.html

edit - also check your rim strips as, especially with the old yellow michelin ones, they eventualy split and can expose the sharp spoke ends.
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Old 11-30-07, 04:36 AM   #3
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Was the tube folded inside the tire?

buy a pressure gage.
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Old 11-30-07, 04:43 AM   #4
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Tires shouldn't blow even when CONSIDERABLY over their max rated pressure... you most likely didn't have everything seated correctly.
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Old 11-30-07, 06:39 AM   #5
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I did that recently and felt it from upstairs. I'm glad I wasn't still working on the bike at the time.
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Old 11-30-07, 07:37 AM   #6
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Quote:
Originally Posted by gear View Post
Was the tube folded inside the tire?

buy a pressure gage.
Best way to avoid that is to stretch the tube out before installing to get rid of the folds. I did that once, and it held for 10 miles. Fortunately, it got a slow leak at the fold instead of blowing on me.

And yeah, definitely get a pressure gauge. Or better yet, a pump with a gauge on it.
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Old 11-30-07, 09:24 AM   #7
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Best way to avoid that is to stretch the tube out before installing to get rid of the folds. I did that once, and it held for 10 miles. Fortunately, it got a slow leak at the fold instead of blowing on me.
I don't understand... folds?

Don't you pump the tube up some before putting it in the tire? If so, I can't imagine what type of fold could exist.
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Old 11-30-07, 09:44 AM   #8
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Jeff,

The folds come when the tube is pinched between the rim and the tire. Rarely, tubes can get twisted. You are right that pumping up the tire some minimizes folding, but if one is using larger tubes for a given rim/tire combination, it makes it more difficult to seat the tube if it is inflated such that it will definitely not get pinched.

Jeff
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Old 11-30-07, 10:39 AM   #9
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Ok thanks for your help. Yea I was working on it and BOOM!! I felt the air from it, good thing it didn't have things flying. Should I not pump the tire up as much when seating it inside the rim with the tire? Maybe my tire lever pinched it. Anywho what should the pressure be on mountain bike tires? Also what should the pressure be on street tires, street tires for a mountain bike and I'm only riding on cement.
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Old 11-30-07, 11:54 AM   #10
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I don't understand... folds?

Don't you pump the tube up some before putting it in the tire? If so, I can't imagine what type of fold could exist.
This was definitely a 'one time only' kind of mistake. I thought I put enough air in it, but clearly I was wrong. But yeah, now I stretch the tube out and put a decent amount of air in it before mounting.
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Old 11-30-07, 12:14 PM   #11
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Should I not pump the tire up as much when seating it inside the rim with the tire?
Normally you only pump the tube up a tiny bit to give it a little shape when installing it, but not so much that you have difficulty stuffing it into place. Then you should be able to mount the tire by hand without a tire lever, finishing by pushing the last bit of bead over the edge of the rim with your palm or palms, or with both thumbs. Then push the tire bead in all the way around the rim to make sure no piece of the tube is snagged between the bead and the rim. Pay special attention near the valve stem: push the valve stem partway into the rim to make sure the reinforced section of the tube around the base of the stem is well inside the tire at that point. Then pull the stem out once you're sure the tube is inside the tire beads. Then pump part way up, check the sidewalls of the tire to make sure they are even all the way around indicating the bead is seated properly, then finish pumping.

The pressure varies by tire but is usuallly around 40-50 psi for knobby mountain bike tires. It should be marked somewhere on the sidewall.

Last edited by cooker; 11-30-07 at 12:19 PM.
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Old 11-30-07, 12:26 PM   #12
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I had a slime tube do that to me. I had installed the tube about a week prior so I do no think it was a folded tube. My when I inspected the tire after washing the slime off me the bike and everything roughly 5 feet to my right I could see where the bead had seperated from the tire. I think the tire sidewall had just failed and everything let loose. The tires had been mounted and remounted and there where a couple of areas where the wire bead was starting to show through. I will say it sounded like a shotgun had gone off my ears where ringing as where the guy who was walking his dog about 10 feet away from me. I am not sure who was more scared me or him.
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Old 11-30-07, 04:22 PM   #13
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Back in the day, we had a tube go at about 60 psi inside our concrete-walled shop. I swear it took 5 years off my life [hopefully just the crappy ones at the end].
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