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13499Re: [rootsradicals] rim failure (was: Random stuff)

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  • David Chase
    Jul 9, 2011
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      On 2011-07-09, at 11:57 AM, jparks781 wrote:

      > David,
      >
      > what rim was that?

      DT Swiss XR4.1.

      > what tire pressure range do you try to maintain?
      60 psi

      > how many miles approx.?
      2500/year (but not all on those tires, snow tires in winter)

      > was it on your Dummy? Did you move it from your previous X to the Dummy?

      No, I had bought it used to replace the dead hub from last year. Guy who owned it before me had it on a recumbent trike for about a year, with skinny tires. Not a light guy either, but probably not so many miles on it (he has spinal stenosis, which makes many things difficult, and why he was selling the wheel).

      > Why do you fault the wide tire? I'm trying to figure out the mechanics / geometry of how the wider tire might have exacerbated the problem.

      My thinking is that the wide tire leaves the rim perpendicular to the rim edge, so the full tension in the tire is applied to the rim -- that is, pulling the rim apart, instead of pulling up off the rim. Because otherwise, there is "no difference" between a 60mm tire at 60psi, and a 30mm tire at 120psi (but note that 120psi 30mm tires are not usual case). The tension in the casing, is proportional to the diameter of the tire, times the pressure -- image slicing an inflated tire like a bagel, and the area of the bagel corresponding to an inch of bagel circumference.

      > I ask because I'm running Big Apples on Velocity Cliff Hangers, which are fairly narrow rims, 1.125" outside width. The Big Apples measure a little over 2.4" wide on these rims at 60psi.

      The Cliff Hangers are plenty wide, I think, 28mm outside width. The rim I broke was 17 mm inside width, 22 mm outside width.

      > Although I weigh ~145#, so that may keep me out of trouble. Also you are a much more faithful cycle-commuter than I, I'm sure your putting a lot more miles on the bike.

      I am not sure that the rider weight matters that much, though to the extent that the tire deforms to be even more perpendicular, that should spike the tearing force.

      David
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