tire shifting on rim during mtb downhill

Ned Mantei said:

Recently I had a valve stem tear off while descending a fairly steep and rocky downhill section.
The tube, and presumably the tire, had shifted on the rim. Two weeks later I noticed that the
(new) valve stem was no longer pointing straight out of the rim hole, and had to unmount and
remount the tire. I assume that the wheel bounces into the air, the brakes lock up, and the sudden
impulse when the tire makes contact again can pull the tire around on the rim. Is avoiding this a
matter of technique, such as momentarily releasing the brakes, is there some way to better fix the
tire on the rim, or is this just the way it is? As you see, I am new to mountain bikes.

--
Ned Mantei Department of Cell Biology, Swiss Federal Institute of Technology CH-8093 Zurich,
Switzerland

use higher inflation pressure or glue one bead of the tyre to the rim

--
Marten

Ned Mantei may have said:

Recently I had a valve stem tear off while descending a fairly steep and rocky downhill section.
The tube, and presumably the tire, had shifted on the rim. Two weeks later I noticed that the (new)
valve stem was no longer pointing straight out of the rim hole, and had to unmount and remount the
tire. I assume that the wheel bounces into the air, the brakes lock up, and the sudden impulse when
the tire makes contact again can pull the tire around on the rim. Is avoiding this a matter of
technique, such as momentarily releasing the brakes, is there some way to better fix the tire on
the rim, or is this just the way it is? As you see, I am new to mountain bikes.

In my experience, for this problem, nine times out of ten, the problem is underinflation. I've also
seen it happen when someone sprayed down the rim, tube and tire with silicone lube to make it easier
to mount the tire.

--
My email address is antispammed; pull WEEDS if replying via e-mail. Yes, I have a killfile. If I
don't respond to something, it's also possible that I'm busy.

Ned Mantei said:

Recently I had a valve stem tear off while descending a fairly steep and rocky downhill section.
The tube, and presumably the tire, had shifted on

Mark the tire and rim, ride, and look to see if the tire is moving. Mark the tube and the inside of
the rim, too, and see if the tire and tube are moving the same amount.

Quoted message said:

can pull the tire around on the rim. Is avoiding this a matter of technique, such as momentarily
releasing the brakes, is there some way to better fix the tire on the rim, or is this just the way
it is? As you see, I am new to mountain bikes.

I've never had this happen, nor heard of it happening, but I'm sure it's not the first time
it's happened.

Are you inflating the tires enough? I imagine that the loose fit caused by underinflation could
allow that to happen.

Maybe some sort of glue or other surface treatment applied to the rim where it meets the tire bead
could help. Or, maybe roughing the rim where it meets the tire bead with coarse sandpaper (I suspect
there could be some downside to this, though I can't imagine what).

--
Rick Onanian

Rick Onanian said:

Ned Mantei said:

Recently I had a valve stem tear off while descending a fairly steep and rocky downhill section.
The tube, and presumably the tire, had shifted on

Mark the tire and rim, ride, and look to see if the tire is moving. Mark the tube and the inside
of the rim, too, and see if the tire and tube are moving the same amount.

Quoted message said:

can pull the tire around on the rim. Is avoiding this a matter of technique, such as momentarily
releasing the brakes, is there some way to better fix the tire on the rim, or is this just the
way it is? As you see, I am new to mountain bikes.

I've never had this happen, nor heard of it happening, but I'm sure it's not the first time it's
happened.

Are you inflating the tires enough? I imagine that the loose fit caused by underinflation could
allow that to happen.

Maybe some sort of glue or other surface treatment applied to the rim where it meets the tire bead
could help. Or, maybe roughing the rim where it meets the tire bead with coarse sandpaper (I
suspect there could be some downside to this, though I can't imagine what).

We used to run rim locks on our dirt motorcycles to keep this from happening at the low tire
pressures we used to float on deep sand. I'm guessing that hard breaking is causing the tire to turn
in the rim. You could make your own rim locks (a wedge that tightens against both beads when you
tighten a nut through the rim), or you could run enough pressure for this not to happen -- or --
maybe try tubeless 🙂. Of course, with tubeless, I wonder if the tire would simply deflate quickly
and immediately when this this happens!

David

In article <[email hidden]>, Werehatrack

Quoted message said:

Ned Mantei may have said:

Recently I had a valve stem tear off while descending a fairly steep and rocky downhill section.

Quoted message said:

In my experience, for this problem, nine times out of ten, the problem is underinflation. I've also
seen it happen when someone sprayed down the rim, tube and tire with silicone lube to make it
easier to mount the tire.

The tires are at 60 psi /4 bar, the maximum suggested pressure for these tires. Even the rim tape
seems to have shifted, at least the portion near the valve hole. I had to cut away a bit of rim tape
to have an opening over the valve hole again.The tires are seated properly in the rims. Since the
bike is new, it does seem possible that some sort of lubrication was applied by the rim manufacturer
(Mavic), or in the factory during assembly. (Except that these 1.9 inch tires fit on the rim with no
effort at all--it's not clear to me that lubrication would bring anything during assembly.) I will
check this.

--
Ned Mantei Department of Cell Biology, Swiss Federal Institute of Technology CH-8093 Zurich,
Switzerland

Ned, occasionally rims are made slightly undersized and even maximum inflation pressure won't
sustain large shocks such as you've described. perhaps you could buy, beg or borrow another wheel to
use to see if it's your wheel or your technique.

"Ned Mantei" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

In article <[email hidden]>, Werehatrack

Quoted message said:

Ned Mantei may have said:

Recently I had a valve stem tear off while descending a fairly

steep and

Quoted message said:

Quoted message said:

Quoted message said:

rocky downhill section.

Quoted message said:

In my experience, for this problem, nine times out of ten, the

problem

Quoted message said:

Quoted message said:

is underinflation. I've also seen it happen when someone sprayed

down

Quoted message said:

Quoted message said:

the rim, tube and tire with silicone lube to make it easier to

mount

Quoted message said:

Quoted message said:

the tire.

The tires are at 60 psi /4 bar, the maximum suggested pressure for

these

Quoted message said:

tires. Even the rim tape seems to have shifted, at least the portion near the valve hole. I had to
cut away a bit of rim tape to have an opening over the valve hole again.The tires are seated
properly in

the

Quoted message said:

rims. Since the bike is new, it does seem possible that some sort of lubrication was applied by
the rim manufacturer (Mavic), or in the factory during assembly. (Except that these 1.9 inch
tires fit on

the

Quoted message said:

rim with no effort at all--it's not clear to me that lubrication

would

Quoted message said:

bring anything during assembly.) I will check this.

--
Ned Mantei Department of Cell Biology, Swiss Federal Institute of Technology CH-8093 Zurich,
Switzerland

Rick Onanian said:

Maybe some sort of glue or other surface treatment applied to the rim where it meets the tire bead
could help. Or, maybe roughing the rim where it meets the tire bead with coarse sandpaper (I
suspect there could be some downside to this, though I can't imagine what).

The downside could be the roughened rim wearing through the casing prematurely and causing the tire
to separate from the bead.

I would suspect underinflation or a tire that fits too loosely on the rim. Some tires fit more
tightly than others