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Old 11-05-09, 11:43 AM   #1
ValVal
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Tube too small ?

Strange thing happened to me today.
Yesterday i changed my tube because the former one was no longer patchable (damn glueless patches).
My tire is a 700x25c, so i put a bontrager 700/18-25 tube and start to pump. The tube doesn't seem to fill the tire very well but i haven't reach the recommanded pressure yet, so i keep pumping to 90psi... Looks good.
This morning i take my bike, and i feel a bump every wheel turn. Now i have spend half an hour trying to put the tube and tire back to place but the only thing i can do is move the tire bump. It seem that the tube isn't pushing the tire radially enough to place the tire beads in the bead sits, causing an unround tire.

Tonight i will swap tube, but have i done something wrong with this one ?
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Old 11-05-09, 11:48 AM   #2
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The rubber at the valve stem may be caught under the bead.
Are you using the valve stem nut?
Undo it, deflate the tire, push the stem into the rim then reinflate.
Most users of that type of valve don't use that nut anyways.
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Old 11-05-09, 11:57 AM   #3
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Yeah, i have thought of that, so i deflated, pushed the valve in, checked both sides of the beads to make sure the tube wasn't pinched and reinflated, but i still have a bump somewhere on my tire.
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Old 11-05-09, 12:00 PM   #4
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A too-narrow tube wouldn't cause that.
Maybe the tire casing has been damaged.
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Old 11-05-09, 12:58 PM   #5
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i'm not sure i've been clear enough, so here are a few more details :
1) The bump is caused by the beads being too much inward, resulting in a lower tire
2) By replacing the tire i can move the bump around but can't seem to remove it
3) The tire is a pretty new Continental ultra sport, which has never done that before
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Old 11-05-09, 01:30 PM   #6
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I'm pretty sure it's not the tubes fault, if only because I'm running the same width tube in my 35mm tires to no ill effect.

Just an idea, is this on a fixed gear bike that gets a lot of skidding?
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Old 11-05-09, 01:35 PM   #7
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that's a fixed gear bike that takes no skidding
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Old 11-05-09, 01:37 PM   #8
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90 PSI may not be enough to seat the tire. I've got one set of rims that take over 100 PSI before the beads seat properly.
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Old 11-05-09, 01:39 PM   #9
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tyre bump

I ended up swapping tyres over between 3 bikes (don't ask) and found that two sets of wheels accepted all 3 sets of tyres with the bead even all round. However, the F.W.Evans bike (with, I presume, Evans own wheels) had the problem you describe. the bead was even for 90% of the circumference, but the bead disapeared into the rim for the remainder. My mate, who has more patience than me spent about 20 minutes on each wheel until he had an even bead all round. I had never had this problem before. One final thought, I'm pretty sure the wheels were original equipment on the bike (Evans have a good reputation for their wheels) but they were specced in the catalogue as having 28 section tyres and we were fitting 23 and 25 section. Another thought, could the Evans wheels have been 27"? We were fitting 700C......
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Old 11-05-09, 01:46 PM   #10
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One final thought, I'm pretty sure the wheels were original equipment on the bike (Evans have a good reputation for their wheels) but they were specced in the catalogue as having 28 section tyres and we were fitting 23 and 25 section.
That may be part of my problem as well. My rims (ZAC 19) are rated 28-38, but since the bike came with 25 tires, i've always put 25.
It is the first time i've had this problem although many tube change, and that's why i blame the tube
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Old 11-05-09, 02:19 PM   #11
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It is not the tube's fault.

Make sure the tire bead is seated as well as possible, and that the tube is not pinched between the rim and tire, then pump it up to 125 psi... it should seat itself.

If it does not seat itself at higher pressure, deflate to ~30 psi and try to pull out all the low spots. Eventually you will get it round.
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Old 11-05-09, 03:38 PM   #12
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you just haven't seated the bead correctly. you can pull the tire with your hands and massage the bead into place - you might have to lower the PSI to do it.

I run 26" tubes in my 29er wheels and have no trouble its not the tube, its the operator.
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Old 11-06-09, 07:39 AM   #13
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90 PSI may not be enough to seat the tire. I've got one set of rims that take over 100 PSI before the beads seat properly.
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Originally Posted by LarDasse74 View Post
It is not the tube's fault.
Make sure the tire bead is seated as well as possible, and that the tube is not pinched between the rim and tire, then pump it up to 125 psi... it should seat itself.
This worked, thank you. I thought 90psi would be enough pressure to seat the beads but it wasn't.
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Old 11-06-09, 08:40 AM   #14
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90 Psi should be about the lowest pressure you ever run in 700X25 tires... Those tires probably have a pressure rating of 125, and can take much more.

PS I just looked at the Continental website and the reccomended pressure is 95 PSI and max pressure is 120.
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