Re: 25/28 Inner Tubes

Pete Biggs said:

I don't see how an inner tube can deform a tyre or unseat it. A problem
with the tube may affect its durability or rolling resistance but how can
it put the tyre out of round? Does it not do whatever it has to do to
conform to the tyre?

Not if it's doubled up somewhere. That'll give you a really bad
out-of-round spot (out of round as in whole-wheel round, not casing shape)
plus a good opportunity to push the tyre off the rim if you inflate it to
the right pressure.

Jasper

Jasper Janssen said:

Quoted message said:

I don't see how an inner tube can deform a tyre or unseat it. A
problem with the tube may affect its durability or rolling
resistance but how can it put the tyre out of round? Does it not
do whatever it has to do to conform to the tyre?

Quoted message said:

Not if it's doubled up somewhere. That'll give you a really bad
out-of-round spot (out of round as in whole-wheel round, not casing
shape) plus a good opportunity to push the tyre off the rim if you
inflate it to the right pressure.

I've used too large a tube (700c) in a 22" child's tire by folding it
and had no trouble. The same goes for a MTB fat tire in which a road
tube is too long but small in cross section. Neither of these
mismatches caused problems in use. Of course the folds aren't good
for the tube and the uneven distribution makes wheel balance worse
than usual, but even that caused no problems.

Unless the tube gets caught under the bead of the tire (the usual
cause for static blow-offs) so that it doesn't seat correctly, I don't
see how an oversized tube would push the tire off the rim.

Have you tried this?

Jobst Brandt

Quoted message said:

Jasper Janssen said:

Not if it's doubled up somewhere. That'll give you a really bad
out-of-round spot (out of round as in whole-wheel round, not casing
shape) plus a good opportunity to push the tyre off the rim if you
inflate it to the right pressure.

I've used too large a tube (700c) in a 22" child's tire by folding it
and had no trouble. The same goes for a MTB fat tire in which a road
tube is too long but small in cross section. Neither of these
mismatches caused problems in use. Of course the folds aren't good
for the tube and the uneven distribution makes wheel balance worse
than usual, but even that caused no problems.

Unless the tube gets caught under the bead of the tire (the usual
cause for static blow-offs) so that it doesn't seat correctly, I don't
see how an oversized tube would push the tire off the rim.

Have you tried this?

Not so much tried as 'been bitten by it'. I can't explain it either, but
several times a tube that was slightly too long ended up with a short
folded section (3-7 cm I'd say), and it resulted in a bad 'bump' at that
spot. One possible explanation is that I may have been less than diligent
about getting the bead properly seated, and that part of it was still in
the rim well thus allowing the tyre to bulge at the opposite end. Usually
when you inflate a tyre it'll pop into alignment naturally, but maybe
somehow the doubled up section interferes with that. These were all 622/37
tyres, incidentally, with tubes either of the 622/35-45 region or the
28-35 region, if I got the wrong one.

I just don't know how it works.

Jasper

Jasper Janssen said:

On Mon, 07 Nov 2005 17:21:37 GMT, [email hidden]

Quoted message said:

Jasper Janssen said:

Not if it's doubled up somewhere. That'll give you a really bad
out-of-round spot (out of round as in whole-wheel round, not casing
shape) plus a good opportunity to push the tyre off the rim if you
inflate it to the right pressure.

I've used too large a tube (700c) in a 22" child's tire by folding it
and had no trouble. The same goes for a MTB fat tire in which a road
tube is too long but small in cross section. Neither of these
mismatches caused problems in use. Of course the folds aren't good
for the tube and the uneven distribution makes wheel balance worse
than usual, but even that caused no problems.

Unless the tube gets caught under the bead of the tire (the usual
cause for static blow-offs) so that it doesn't seat correctly, I
don't see how an oversized tube would push the tire off the rim.

Have you tried this?

Not so much tried as 'been bitten by it'. I can't explain it either,
but several times a tube that was slightly too long ended up with a
short folded section (3-7 cm I'd say), and it resulted in a bad
'bump' at that spot. One possible explanation is that I may have been
less than diligent about getting the bead properly seated, and that
part of it was still in the rim well thus allowing the tyre to bulge
at the opposite end. Usually when you inflate a tyre it'll pop into
alignment naturally, but maybe somehow the doubled up section
interferes with that. These were all 622/37 tyres, incidentally, with
tubes either of the 622/35-45 region or the 28-35 region, if I got
the wrong one.

I just don't know how it works.

Air pressure is normal to the surface of whatever is containing it. What
this has to do with this thread, I have no idea.

--
Phil, Squid-in-Training

Jasper said:

I can't explain it either, but
several times a tube that was slightly too long ended up with a short
folded section (3-7 cm I'd say), and it resulted in a bad 'bump' at that
spot. One possible explanation is that I may have been less than diligent
about getting the bead properly seated, and that part of it was still in
the rim well thus allowing the tyre to bulge at the opposite end. Usually
when you inflate a tyre it'll pop into alignment naturally, but maybe
somehow the doubled up section interferes with that.

I suspect that a bit of the excess tube may have gotten between the
bead of the tire and the rim, preventing it from seating properly in
that area. Having a wider and/or longer tube than appropriate
certainly makes it harder to mount the tire on the rim while making
sure that the tube is all properly stowed inside the tire.

But I've had the same experience as Jobst that once installed the tire
and tube worked fine. There was no detectable problem when riding even
with rather severe mismatches - such as a 700c tube inside one of the
20" tires on my folding bike or in the 26" mountain bike wheels of
other riders.

Pete Biggs said:

I don't see how an inner tube can deform a tyre or unseat it. A problem
with the tube may affect its durability or rolling resistance but how can
it put the tyre out of round? Does it not do whatever it has to do to
conform to the tyre?

Recently I had a very good example of how an inner tube can unseat a
tyre. I'm fairly sure that this is what happened, due to the history of
the tube and tyre. Here's what happened. I'm mad enough to run a fixed
gear "shopping bike" with 700c wheels. I have Vredestein Stila 32mm
tyres on it, which are rated to a maximum pressure of 75psi. These tyres
are a little prone to needing careful seating on the rims IME, so I was
extra careful to seat them, inflate to 20 psi and check the seating
before fully inflating them on first installation. After several months
of regular use with re-inflation to 75psi every 4 to 7 days and no
problems whatsoever, I accidentally inflated them to 95psi (my pump has
number markers every 20psi, and I had a bit of brain fade that day - I
later discovered that the little red marker I had set at 75 had slipped
around the dial by about a 20psi marker, which was how I worked out I
had over-inflated the tyre by that much). I then didn't get more than
half way along my regular two mile trip to the local supermarket before
the tyre in question managed to unseat itself from the rim. Amazingly,
the tube did not burst, and I let some air out, re-seated it and
re-inflated it to 75psi with my mini-pump and carried on without any
further problem. That was some months ago.

The tyre unseated itself over a length of about 8 inches on one side
only, which I would have to say meant that at the time it was unseated
it was out of round. The other (rear) tyre was on a different rim, but
still overinflated by the same amount. I did not have a problem with
that tyre, but it should be noted that the rear rim was somewhat
narrower than the front (hey, its a shopping bike, built for next to
nowt!). I built both wheels myself, and checked them for roundness after
the event. Both were within the 0.5mm spec I built them to, although I
have to admit I did not check rim roundness with the tyre blown off.

Anyway, my conclusion is you can unseat a tyre with an inner tube if you
overinflate it. I'm not going to stop using the tyres in question
though, because other than that incident they have been excellent.

JimP

Jim Price said:

Anyway, my conclusion is you can unseat a tyre with an inner tube if
you overinflate it.

I reckon that's caused by the air pressure being too high for the tyre &
rim (or the way the tyre is fitted) rather than anything to do with the
tube itself. In other words, the same thing will happen with any tube if
you pump it up as hard.

What I don't understand is how a fold in a tube causes any unseating,
assuming it's not trapped under a bead.

~PB

Jim Price said:

I accidentally inflated them to 95psi [20 psi over rating]...
I then didn't get more than
half way along my regular two mile trip to the local supermarket before
the tyre in question managed to unseat itself from the rim. Amazingly,
the tube did not burst, and I let some air out, re-seated it and
re-inflated it to 75psi with my mini-pump and carried on without any
further problem. That was some months ago.

The tyre unseated itself over a length of about 8 inches on one side
only, which I would have to say meant that at the time it was unseated
it was out of round.

Hmmm... more support for the theory espoused in the "time for a cookout
-- we're having crow" thread. I'm beginning to think that Peter Cole had
it right.

--
Dave
dvt at psu dot edu