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How does rim damage happen?

Discussion in 'The Perfect Line and Other Riding Myths' started by SteadyStare, Oct 30, 2016.

  1. SteadyStare

    SteadyStare Adventurer wannabe

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    Ok, so I've bent my fair share of rims but I'm curious as to what is actually happening. Usually when you run lower pressure, especially on a heavy ADV bike, bent rims seem to come out of nowhere. So what's the actual cause?

    1. Is it the actual tire sidewall deforming and being forced outward, bending the rim and compromising its strength?

    Or

    2. Is it that there is so much force that the tire essentially goes flat and the rim contacts the ground through the tire?

    I've heard arguments for both, but I'm curious if any has any actual proof like a still picture or slow motion video of a rim bending. The outcome is the same either way, just hoping to settle a late night bar argument.
    #1
  2. dddd

    dddd Long timer

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    can't be #1. how would we put on new tires over the rim...
    #2
  3. SteadyStare

    SteadyStare Adventurer wannabe

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    So you are saying with enough tire pressure you could prevent all rim damage (not including side hits our other odd angles)?

    Not saying you are wrong, I'm generally interested. Think about squishing a water balloon on the top and bottom, which causes the sides to bulge. Could that not be happening with a tire? Could the force of the impact be keeping the tire bead in place while sides are forced out wide enough to bend the rim?
    #3
  4. windblown101

    windblown101 Long timer Supporter

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    No, the force exerted by the air inside of the tire is distributed equally all the way around the tires circumstance so if it was just air pressure causing the bend it would not be localized. More air pressure does protect rims.

    In my area the rocks are largely to blame for tire and rim damage, landing jumps into rocks exacerbates the problem. Here's a pic of tire and rim damage from rocks on my 1190R, notice the rim isn't bent, that's because I tend to run pretty high pressure on big heavy bikes. It's also why I run a front tube, tire damage around here is rarely pluggable. Heres my front tire at the moment. If it didnt have a tube in it there is no way I'd continue to run it.

    20161030_181054.jpg
    #4
  5. shovelstrokeed

    shovelstrokeed Long timer

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    It is actual contact between the rim and the obstruction. Yes, the tire deforms to that extent. It can even happen on a square edge where the tire deforms on both sides. The distortion of the tire is absorbed by a momentary increase in air pressure throughout the tire which helps to keep the bead seated. Run enough air pressure in the tire and rim damage will be minimized but not fully eliminated. All it takes is the right angle of attack and a sharp enough edge and the obstacle will penetrate to the rim.
    The relative profile of a dirt tire does little to help in this situation as they typically run from 80 to 100% section to width and thus have a tall profile which is more conducive to collapse under impact.
    #5
  6. cyclopathic

    cyclopathic Long timer

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    Wrong analogy gas (air) compressible and liquid (water) is not. Even with liquid the pressure applied to one point would be equally applied to all.

    The main difference that in liquid you would have a high pressure high speed shockwave and gas lower pressure lower speed wave, and tire filled with air can reduce volume and one filled with liquid stays constant.

    Regardless the damage occurs from tire collapsing and rock hitting rim. On tubed tires you also getting pinch flat.
    #6
  7. aldend123

    aldend123 Long timer

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    It's number 2. Take a bicycle, lower the air pressure, and slowly press the tire on to the square edge of a curb. That type of action is the easiest way to do it, and rocks will do similar. Or pressing the wheel hard in to a rut/pot hole. On a dirtbike, easiest way to lose a tube is a pinch flat (also common on bicycles). The tire flattens enough to contact the rim, then the rim tears the tube.

    I know this is a bicycle, but same concept. What's happening could also bend the rim on a rim with a larger lip, like those on motorcycles.
    [​IMG]
    #7
    Idle likes this.
  8. High Country Herb

    High Country Herb Adventure Connoiseur

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    I agree. It is number 2. In the engineering world, they call that a "point load". Pounds per square inch has to get pretty high to damage a rim, and reducing the area where a load is applied is an easy way to do it.
    #8
  9. Maggot12

    Maggot12 U'mmmm yeaah!!

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    2. Bent a car rim on a pothole..
    #9
  10. Andyvh1959

    Andyvh1959 Cheesehead Klompen Supporter

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    My 94 BMW R1100RS is known to have "soft" rims. But in 188,000 miles of riding typical non smooth Wisconsin roads I have never experienced a bent rim on the bike. However, I consistently run at least 38psi in the front tire and most often I have it at 40 psi. So I'd say low tire pressure is a contributor.
    #10
  11. tlub

    tlub Long timer Supporter

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    If you bend a rim, be aware that you just used the tire sidewall to deform fairly strong metal, in a concentrated area. There probably is internal damage to the sidewall, and you should replace that tire, especially if you are going to run at any kind of speed with it, which will stress the sidewall further.
    #11
    GETTHUMPER2 likes this.
  12. KeithC

    KeithC Been here awhile

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    :y0!....Not enough lubrication?:hide
    #12
  13. Telemarktumalo

    Telemarktumalo Go Red Sox!

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    All things being equal, a higher pressure will result in less rim damage than a lower pressure when striking a sharp object. So, if you want to run low pressures for a better ride, then a larger volume tire will reduce the chances of a rim strike. As mentioned earlier, bicycle wheel makers are exploring different tire volumes and pressures that can be run. Some of the big volume fatties are running 5-6 psi with a good ride, traction and big hit forgiveness.
    #13
  14. eatpasta

    eatpasta Lawnmower Target Supporter

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    I think people under estimate how many G's that tires/wheels/forks experience during normal riding. I remember reading about it somewhere - can't recall the exact figure but I remember it being significant
    #14