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#11
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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 |
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#13
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What makes soft tires creep on rims?
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 |
#14
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What makes soft tires creep on rims?
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 |
#15
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What makes soft tires creep on rims?
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 |
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