What makes soft tires creep on rims?

On 7/12/2017 7:34 PM, Doug Landau wrote:
On Wednesday, July 12, 2017 at 6:12:00 AM UTC-7, AMuzi wrote:
On 7/11/2017 8:12 PM, bob prohaska wrote:
What is the process by which soft tires creep on rims when the
tire is underinflated, even with no applied torque?

I have a CycleTote trailer that uses standard 26" wheels. I tend to
keep the tires quite soft, just a few psi, partly out of neglect and
partly because the trailer is less inclined to bounce at low tire pressure.
The beads seem firmly seated, the tire can't be slipped on the rim by hand.

Over time, the valve stems become inclined in a direction that suggests
the rim is rotating backwards relative to the tube. There's no torque
applied to either tire or rim, so what is driving the motion?


It's not specific to torque[1]. The deformation at the
bottom where your tire squishes out is cyclical, changing
the pressure outward against the inner face of the rim,
briefly higher, then lower. This effect is seen more
dramatically at below-optimal tire pressures. One might
suspect torque because the rear is usually more pronounced
but I believe that's proportional to load.

[1] Wheelbarrows and other carts do this as well.

How about underwear? Is this the same mechanism by which, when you're pretending you're mario andretti, and driving 95 up such-and-such canyon-road, your underwear creeps up until you are in a wedgie, about to be strangled?

Another mystery is how a stone in your shoe migrates against the
pressure gradient to the spot where it hurts the most.

--
- Frank Krygowski

On Wed, 12 Jul 2017 11:28:17 -0700 (PDT), wrote:

Jeff, the exact same thing will happen if you use the
locking screw on the filler value. Test it. Install the
tire with the name centered on the filler. Ride it with
less than minimum pressure for any distance the tire tube
will remain vertically though because of sticking to the
rotating tire it CAN tear the filler out. But the name
can move a long way from where it started.

I yield, you're right. Tire creep is caused by insufficient tire bead
to rim friction induced by insufficient tire pressure. Some reference
mention that the inner tube and tire are locked together by the larger
surface area. The main danger of tire creep is damaging the inflation
valve stem.

I didn't want to ride around on under inflated tires so I didn't try
the rim and tire mark test.

This article recommends using tubular cement to glue the tire to the
rim:
http://forums.mtbr.com/fat-bikes/tires-creeping-rims-832205.html

Tire creep is also a problem on aircraft tires:
http://www.airmech.co.uk/forums/showthread.php/255-Tyre-Creep
https://www.youtube.com/watch?v=iSt3weFb_vU

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 15-07-17 18:03, Jeff Liebermann wrote:
On Wed, 12 Jul 2017 11:28:17 -0700 (PDT), wrote:

Jeff, the exact same thing will happen if you use the
locking screw on the filler value. Test it. Install the
tire with the name centered on the filler. Ride it with
less than minimum pressure for any distance the tire tube
will remain vertically though because of sticking to the
rotating tire it CAN tear the filler out. But the name
can move a long way from where it started.

I yield, you're right. Tire creep is caused by insufficient tire bead
to rim friction induced by insufficient tire pressure. Some reference
mention that the inner tube and tire are locked together by the larger
surface area. The main danger of tire creep is damaging the inflation
valve stem.


Could there be another reason for tire creep? I sometimes have this on
my mountain bike, even though the tires are always inflated to what the
manufacturer gives as the maximum pressure (about 3 bar).

My explanation/guess: On a bumpy downhill trail the tire can bounce into
the air while braking. At this moment the wheel locks, and then the tire
is pulled around when the wheel hits the ground again.

I had a valve stem break off when I first started mountain biking. Since
then I have tried to release the brakes while going over a bump, which
seems to help a lot. Occasionally it's still necessary to deflate the
tire and shift it a bit on the rim, but the tire creep is never enough
to damage the valve stem. I never use the locking nuts.

Ned

On Sun, 16 Jul 2017 11:21:35 +0200, Ned Mantei
wrote:

Could there be another reason for tire creep?

Yes.

- Grease, oil, gravel, or water between tire bead and rim causing tire
slippage.
- Bad mating between the rim and the tire bead.
- Ultra thin rim that bends.
- Locking up the brakes while skidding on pavement.
- Baby powder between the tire and the rim.
- Slime or anti-leak goo between rim and tire bead.
- Solid or foam (no air) tire. With no air pressure to push the tire
against the rim, the tire should slip quite easily.
- The tube moving instead of the tire might still be a possibility.

I sometimes have this on
my mountain bike, even though the tires are always inflated to what the
manufacturer gives as the maximum pressure (about 3 bar).

3 bar = 43.5 psi = 300 kPa

My explanation/guess: On a bumpy downhill trail the tire can bounce into
the air while braking. At this moment the wheel locks, and then the tire
is pulled around when the wheel hits the ground again.

Yes, that should be a problem. It's the same as locking the tire and
skidding while braking. The forward force of coming to a stop or
hitting the ground are transferred first to the tire, through the rim,
to the spokes, and to the hub. At the same time, the locked brake
disc also transfers the force to frame. If the disc brakes are not
locked, then some of the forward forces are converted into heat. The
weak link of this chain is initially the brakes. However, if your
brakes are in great condition, the weakest link is probably the tire
to rim interface, which is what's slipping. The locked brakes while
airborne is the key because most of the force NOT involved in bending
something in the disc brakes, it now transferred to the tire bead to
rim interface.

I had a valve stem break off when I first started mountain biking. Since
then I have tried to release the brakes while going over a bump, which
seems to help a lot. Occasionally it's still necessary to deflate the
tire and shift it a bit on the rim, but the tire creep is never enough
to damage the valve stem. I never use the locking nuts.

Installing the tire valve lock nuts probably won't help prevent creep.
What the nuts will do is keep dust and dirt out of the inside of your
tire. When you change tubes, do you see any dirt or dust INSIDE the
tube? If so, it might be entering via the gap between the valve and
the rim. If this dirt or dust gets in between the tire bead and rim,
it might slip. I've never built a wheel without the tire valve nut,
so I don't know if this really is a problem.

Ned
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sun, 16 Jul 2017 11:20:54 -0700, Jeff Liebermann
wrote:

When you change tubes, do you see any dirt or dust INSIDE the
tube?

That should be inside the tire, not inside the tube.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558