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Old 10-06-09, 06:22 AM   #1
A.Gronow
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Beware of pressure

Just a word of caution for everyone.
I've had my track pump for a couple of years now(Branded Oxford) and have used it without thought on all types of bikes/tyres.
Yesterday, I checked my road bike and topped it up to 100 psi and of I went for a training ride.
A mile out and the front wheel started to wobble and within 5 seconds the inner tube exploded forcing the tyre off the rim. I was only coasting at the time so I was ok.

This really spooked me but after replacing the tube I rode to my LBS to check the pressure.
I used thier track pump and inflated it to 90 psi to get me home.
At home I check the pressure with my pump and it only showed 40 psi.
So, when I'd put 100 psi in earlier I'd actually inflated to 150 psi. No wonder it went pop.
Moral of the story? If you have a old pump, be a little wary of the accuracy of the gauge.
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Old 10-06-09, 07:55 AM   #2
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I don't have a pump with a gauge.However I do calibrate my gauges.I have one that dead nutz,1 at -2 and one at +2.It takes a bit of special equipment but I already had that.
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Old 10-06-09, 08:33 AM   #3
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I'm sure there are considerable inaccuracies in pump gauges, new and old. But this sounds like a case of the inner tube being caught between the tire and the rim. Even if it is a very small pinch of tube between the tire bead and the bottom of the rim the result will be exactly what is described in this post. At least that has been my experience.

Al
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Old 10-06-09, 03:32 PM   #4
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what does it mean when my rear tire has to be constantly pumped up every morning before my commute? i do plenty of skidding, so would that make a difference? im running a 48/16, so therefore id get one skid patch.. is that necessarily bad? sorry many questions at once.. but could someone give me info on any theories. thnx
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Old 10-07-09, 01:45 AM   #5
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Sounds like you had a pinched tube. Over-pressure failures typically result in the rim's sides being blasted apart.
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Old 10-07-09, 04:59 AM   #6
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If you don't have a pressure-gauge on your pump, or suspect the one you have has seen better years, I've found this one to be highly accurate and reliable:

http://www.rei.com/product/756362?cm...:referralID=NA
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Old 10-07-09, 06:16 AM   #7
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Originally Posted by gdelacruz View Post
what does it mean when my rear tire has to be constantly pumped up every morning before my commute? i do plenty of skidding, so would that make a difference? im running a 48/16, so therefore id get one skid patch.. is that necessarily bad? sorry many questions at once.. but could someone give me info on any theories. thnx
You have to pump your tire because it has a leak. My tires will typically only need a top up once or twice a week.
If you have worn your tire through by skidding it will burst, not leak slowly.
If you insist on skidding lots, change your gear ratio to get more skid patches, or regularly dismount your tire and turn it to skid another bit bare. A better idea would be to use brakes to stop.
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Old 10-07-09, 06:40 AM   #8
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Quote:
Originally Posted by Al1943 View Post
I'm sure there are considerable inaccuracies in pump gauges, new and old. But this sounds like a case of the inner tube being caught between the tire and the rim. Even if it is a very small pinch of tube between the tire bead and the bottom of the rim the result will be exactly what is described in this post. At least that has been my experience.

Al
Makes sense to me but I've been using that tube/tyre combo for a while now with no problem.
When I removed the blown tube there was a slit that must have been 5-6 inches long.
There must have been some force to pop the bead from the rim?
Regards,
Andrew.
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Old 10-07-09, 07:06 AM   #9
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Originally Posted by A.Gronow View Post
Makes sense to me but I've been using that tube/tyre combo for a while now with no problem.
When I removed the blown tube there was a slit that must have been 5-6 inches long.
There must have been some force to pop the bead from the rim?
Regards,
Andrew.
It sounds like a typical case of the tube being caught between the tire bead and the rim but if the tire had not been remounted or flat in some time then I don't know what happened. Mystery flat.

Al
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Old 10-07-09, 02:08 PM   #10
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Originally Posted by gdelacruz View Post
what does it mean when my rear tire has to be constantly pumped up every morning before my commute? i do plenty of skidding, so would that make a difference? im running a 48/16, so therefore id get one skid patch.. is that necessarily bad? sorry many questions at once.. but could someone give me info on any theories. thnx
I don't see how skidding would cause a tube to get a slow leak.

Having one skid patch will wear the tire out relatively fast (well just one spot, but once that spot gets too low, the whole tire is shot). Consider changing the cog to a 17 (for 17 skid patches). Or, for a smaller change in ratio, get a 47 or 49 chainring (for 16 skid patches) if you can find one for your crank.
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Old 10-07-09, 11:36 PM   #11
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Originally Posted by A.Gronow View Post
Makes sense to me but I've been using that tube/tyre combo for a while now with no problem.
When I removed the blown tube there was a slit that must have been 5-6 inches long.
There must have been some force to pop the bead from the rim?
That force comes from the pressure-differential between the outside air versus inside pressure in the exposed part of the tube that's caught between the tyre and rim. Usually there's no pressure differential because a completely enclosed tube is pushed back on the outside by the tyre casing and rim with the same force as air-pressure on the inside. There's no movement because all the forces are balanced.

However, the section that's pinched in between the tyre and the rim has no outside covering to push back, only outside air at 14.5psi absolute. Compared to 150psi on the inside and the expose section of tube is pushed outwards by the air-pressure. The seeping tube gets in under the bead and lifts the tyre off. The long slit in the tube is a trademarked and patented sign of a pinched tube.

The only way to be sure you didn't pinch the tube is to let out air after installing the tyre and check. Pull the tyre sideways so that you can see down into the gap between the tyre and rim-edge. No tube should be showing. Then move over about 6" and repeat. Work your way all around the tyre making sure no tube is visible. Then repeat for the other side. You didn't do this did you?

Last edited by DannoXYZ; 10-07-09 at 11:49 PM.
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Old 10-08-09, 12:37 AM   #12
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I find the best way to prevent this is to partly inflate the tube, go around it with your thumbs, deflates it - then bring it up to full-pressure. Things to look for: The valve-stem not being straight up and down and side-to side. That's grounds to pull the tire off and start over. Just pulling serves to make a pinch-flat more likely.
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Old 10-08-09, 05:07 AM   #13
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I seen a car show on tv. they got an old ford with cast alloy wheels. pumped its tyre upto 300psi. blew the tyre and rim to bits.

I guess since bikes are so much lighter construction. it would take less pressure to destroy them
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