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Old 08-14-09, 11:04 AM   #1
intx13
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Blew up rear tube supposedly at 20 psi below max

Hi folks.

I was pumping my tires today with a pump that has an integrated digital pressure-o-meter (terminology?). These are 90 PSI tires. At 70 PSI the tires were feeling very stiff and did not want to be inflated any more. I hopped on and road to campus. While walking the bike from the road to the rack, the rear wheel suddenly "stuck". I leaned my face real close to it (maybe it was bulging near the brake? I only had about 5 seconds to look at it.) when the tube went off like a gunshot - I jumped 3 feet in the air!

I bought the bike (1969 Raleigh Super Course) with new tires (and presumably tubes) on it a few weeks ago and had noticed the "firm before full" issue, but had no problem riding it. I'm confident that I didn't puncture the tube; the road was clear the whole way and the 20 feet I walked it to the rack was clear pavement.

Do you think the pump is innacurate, or is it possible that I had "bad" tubes that didn't match the spec on the side of the tire?

I'll take the wheel to the shop to get the replacement tube and get a feel for what 90 PSI feels like on the fingers. I can pick up an analog pressure-o-meter-thingy while I'm there too if the pump seems to be the culprit. Is there a way I could quickly test the pump? It's just a cheap Bell gadget, nothing fancy.

Any advice is appreciated!
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Old 08-14-09, 11:09 AM   #2
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No, your tire was not seated properly is my guess. Your tube worked it's was between tire and rim in that area and kablamooooooo. No problem with pump, too much pressure, etc.

Next time, get a bit of pressure in the tube/tire and make sure tire is seated befire fully inflating.

Also on a 1969 bike, the rims may not be hooked and may not be able to handle high pressure tires. Did you change the tires?
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Old 08-14-09, 11:23 AM   #3
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Pressure gauge. Although I do like pressure-o-meter. I pronounce it presherahmetur, emphasis on "ah".

Your gauge may be wrong, may be OK. Get another gauge.

Tire seating may indeed be the problem. Whether it's a tire/rim incompatibility is another question.
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Old 08-14-09, 11:30 AM   #4
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Thanks for the help, jsharr.

Quote:
Next time, get a bit of pressure in the tube/tire and make sure tire is seated befire fully inflating.
So if the inflating problem has been going on for the few weeks I've owned the bike, that would imply that the tube has been off-kilter that whole time, right? Not just at the moment of explosion. I'm afraid I'm not quite sure what an improperly seated tire would look like; I'll have to look around.

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Also on a 1969 bike, the rims may not be hooked and may not be able to handle high pressure tires. Did you change the tires?
The tires and tubes were installed new by the previous owner. The bike was advertised in 1969 as coming with "Dunlop high-pressure racing tires" (but I can't seem to find the actual number listed). I'm not sure if the rims are hooked or not as I haven't had the tires off yet (and can't do so right now).
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Old 08-14-09, 11:40 AM   #5
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Unseated tire would look like this.



Could be an issue with a twist in the tube, rim tape being worn out, etc, leading to explosive failure of tube as well.

If the tire and tube have been holding pressure for a while, and the only thing that changed is that you aired the tire up, then I would look at the rim tape carefully.

Before you take the tire off the rim, make note of where the label on the tire is versus the valve stem in the rim, so that you can figure out where the damage to the tube is in relation to the tire.
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Old 08-14-09, 11:51 AM   #6
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I think I would have noticed the tire being unseated like that as I've been spending a lot of quality time with the bike recently. I'll note the label position and yank it tonight and see what I can see.

Thanks for the help; I'll probably post back with updated information / questions this evening.
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Old 08-14-09, 11:57 AM   #7
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What size are the tires? Sounds like you'd probably do better running them at 70 on that bike.
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Old 08-14-09, 12:50 PM   #8
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What size are the tires? Sounds like you'd probably do better running them at 70 on that bike.
To be honest, I'm not sure (I haven't measured). The bike sold with 700Cs on every later model but I'm not sure about the 1969. I'm assuming the same. Wait.. that's the wheel, and you're asking about the tire. I have no idea about that, I'd have to check.

I'm not sure why running them at 70 would be preferred if the tire is to be believed. And it blew up at 70 anyway, so that wouldn't help me in this case.

The other thing is that the front wheel is doing the exact same thing, but didn't blow up (yet). Is it likely that both tubes are twisted or whatever? And you know, I felt like I was really having to strain to pump up the tires on my old bike... it could be my imagination, but maybe the pump's pressure gauge is just bad.

I might be better off running down to the shop with both wheels and have them poke at them for me before I do more damage
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Old 08-14-09, 12:57 PM   #9
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I believe that bike has straight sided rims not nook beads rims like new bikes and if you used hook beaded tires 60 to 70 psi is the max that should be used. Same problem happens with Schwinn rims that are straight sided. Roger
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Old 08-14-09, 01:04 PM   #10
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Sounds like non hooked rims and too much pressure.Lifted the tire off the rim.
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Old 08-14-09, 01:06 PM   #11
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I'm not sure why running them at 70 would be preferred if the tire is to be believed.
Just because the tire says it CAN go to 90, does not mean it SHOULD go to 90. It's a common misconception.
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Old 08-14-09, 01:12 PM   #12
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i had a similar problem with a bike i had.
there were other underlying issues, but it seemed all was resolved with a new pump.
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Old 08-14-09, 01:13 PM   #13
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One good way to keep twists out of the tube is to put just enough air in it to make it round out, prior to installing the tube in the tire. Start with the tire on the rim, but only one bead of the tire in the rim. Install the tube into the tire. Once the tube is in the tire and you visually inspect it for twists, you can put the tire back on the rim. You will most likely need to deflate the tube in order to get the tire bead all the way back on. The proceed as described in my first post.
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Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.
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Old 08-14-09, 01:18 PM   #14
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Thanks l_bRAD, rhenning, Booger1, I'll take better care with the next tube. (And let some air out of the front tube before I end up with another surprise!)

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Just because the tire says it CAN go to 90, does not mean it SHOULD go to 90. It's a common misconception.
Yep - I read online somewhere that maximums are understated, and that good practice was to inflate to the stated max. I failed to think about the fact that these are modern tires on old rims (and also that I shouldn't believe everything I read online!). Ah well, live and learn!
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Old 08-14-09, 08:42 PM   #15
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Well, I got a new tube (actually, I had the shop do it, I didn't have levers; I picked some up, plus an extra tube, while I was there). While I was there I confirmed that I have non-hooked rims. The fact that the tire caught on the brakes just prior to explosion is a pretty clear sign of an unseated tire - so sayeth the bike shop (which makes sense to me).

The front wheel is no worse for wear, though I have both at 75 PSI now. I think my pump reads about 10 PSI low; the shop put 75 PSI in the rear tube but when I checked it at home it read 50, then 65 after a single pump. I'm sticking to the squeeze-test now that I know how 7x PSI feels. I probably had both tires at ~100 PSI this morning... and on non-hooked rims I now know that to be a no-no.

Thanks for the help folks
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