On Wednesday, July 12, 2017 at 10:06:17 AM UTC-7, Jeff Liebermann wrote:
On Wed, 12 Jul 2017 07:26:17 -0700 (PDT), wrote:

On Tuesday, July 11, 2017 at 7:46:46 PM UTC-7, Jeff Liebermann wrote:
On Wed, 12 Jul 2017 01:12:30 -0000 (UTC), 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?

When you under-inflate the tire, the inner tube loose contact at a few
points with the inside of the tire. That creates a situation where
the circumference of the inner tube and the tire are slightly
different. Visualize what happens when two circles of unequal
diameter and a common point of contact on the ground. As they roll
forward, the inner circle rotates at a different rate than the outer
circle causing the two circles to change relative position. The
difference in circumferences are very small, but large enough to cause
creep. When the inner tube and tire are properly inflated, the inner
tube and tire are locked together by the tire pressure, so no relative
rotation (creep) is possible.

Errr, no. The creep is a rather simple thing. It's when the coefficient
of drag (traction) of the tire on the ground exceeds that of the tire
on the inside edge of the rim. You don't need power in nor braking power
for this to occur. The tire is generally locked onto the inside edge of
the rim by the inflation pressure.

Yep, that's another explanation.

Please note that the OP mentioned that:
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.

As far as I can recall, the only tube type tires that I've seen creep
were on under-inflated tires. There may also have been a problem
where someone used far too much baby powder, or smooth surface rim
tape. I don't recall those. It is possible that the tire is slipping
on the rim with under-pressure. However, it's my contention that the
tube is what is moving relative to the tire, and that the rim and tire
remain in place. It would be easy enough to test by putting simply
marks on both the tire and rim to see they move relative to each
other.


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.