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Pinch flats -- worse with certain rim/tire combi's, or(8 posts)

Pinch flats -- worse with certain rim/tire combi's, orbill
Aug 12, 2003 8:14 AM
solely a function of weight, pressure?
I never used to get pinch flats. I guess I've had a couple, but now I've had a couple in the last week, and I'm starting to wonder what I'm doing wrong.
I always ran my tire pressure up to 115 at least, up to 125 if the tire says it'll take it. Read a post last week said that 110 works as well as higher pressures and more comfy to boot. So, I tried it. Not great results. Comfy, yes. Flats are, however, a pain in the butt.
I'm running Axial Pro's on AC 420's. Pro's are rated to 110 -- I always used to pump them to 115 anyway. Nary a problem. Reduced to 110 -- problem.
Should I try a different, higher psi tire on the same rim? Is it a problem with the rim? The tire? The combi?
I weigh 160 lbs.
How do you know?filtersweep
Aug 12, 2003 8:23 AM
How do you know they are pinch flats, vs anything else? A rash of them suggests your tires have had enough.

Are your Axial Pros a bit long in tooth? They don't exactly make them anymore, and if they are like Pro Races, they will be fine for a few thousand miles, then it will be flat city. Finally, is your rim tape in good shape?

Realistically, I don't think you should be getting any pinch flats unless you are hitting something hard enough to wreck your rim...
The tires are pretty new -- I'd say maybe 400 (?) miles on thembill
Aug 12, 2003 8:34 AM
I guess I don't know for sure if they are pinch flats (which does bear double checking), but I've found nothing in the tires and ridden them a few tens of miles between flats, making me think that it's not something stuck in there.
I guess I could check the rim tape, too, but it's pretty new, as well (unless it has shifted around somehow).
Good ideas to check, though. Thanks.
There's pinch flats and then there's pinch flats ...Humma Hah
Aug 12, 2003 8:38 AM
... a true pinch flat, aka "snakebite", happens when you hit a square-edged bump and mash the tire flat against the rim. It produces a matched pair of small tears, about 1/8" long, on the tube. If you're suddenly getting these with good tire pressure, could be you've just developed a nasty spot on your favorite route that's worse than you thought.

Thinner tubes and skinnier, thinner-sidewalled tires may both be more sensitive to snakebites. Undersized tubes can also be a problem. If the tube has to expand to fit the tire, it is more likely to tear when pinched, and any hole that happens opens up and leaks very fast. A tube sized to fill the tire case with just a puff of air in it will be more resistant to flats.

One thing I've found that seems to help prevent snakebites is a little Slime in the tube. It seems to lubricate the inside of the tube so that it doesn't tear when pinched.

The other way of pinching a tube is if the tube is getting caught under the tire bead during installation. That can be a function of tire/rim/tube combinations ... a technique you have used for years may not work cleanly when you change one component.

Other non-pinch causes that might act like repeated pinch flats ... an imbedded piece of junk in the tire, a burr or other sharp spot on the rim.
One more thing to check...retro
Aug 12, 2003 8:51 AM
I'm not sure from your description that you're really getting pinch flats, especially at your weight and with the pressures you're using. I've got 70 pounds on you, and I haven't had a pinch on a road bike in years.
The location of the holes is a good clue to what's going on. Somebody else already mentioned the rim tape, but you might want to triple-check that. I had a rash of flats last year that I couldn't figure out, in the same general area of the rim side of the tube but not in exactly the same place. The tape looked good, but eventually I realized I'd gotten it dirty when I installed it so a 6-inch strip hadn't stuck to the rim. Under pressure it was slipping just a hair, less than a millimeter, and that was enough that a little piece of the tube could protrude into the spoke hole and eventually wear through.
the mystery either has been solved or has deepened considerablybill
Aug 12, 2003 4:52 PM
I was taking off the tire, and out of nowhere a stupid little valve cap appears. Really from nowhere, alhough the only place it could have come from, it seems, is from inside the tire. Now, I have no idea how such a thing could have happened, but somehow it seems as if maybe, just maybe, that cap could have been rolling around inside the tire even as I rotated it to clean it out. Could have happened, I suppose.
Weird one.
LOL! Looks for a screwdriver too! OTOH, latex tubes...Spunout
Aug 13, 2003 4:04 AM
could stop pinches, being a softer compound. They will bend around the pinch, expand, but will not rupture.

And, everyone should get Velox cloth rim tapes.
No way will 5 psi less cause pinch flats!! (nm)Kerry Irons
Aug 12, 2003 5:42 PM