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  1. #1
    Doesn't ride enough Lamabb's Avatar
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    Fat tires on narrow rim = untrue wheel?

    I don't know if I believe this, but when I had my wheels trued on my touring bike my lbs mechanic told me that having a wide tire (1.75in) on a narrow rim will cause my tire to be more prone to losing its true. He explained that this was because when I stand (which I do often), the wide tire pulls left and right more than a narrow tire would.

    Is he just pulling my leg?

  2. #2
    Senior Member Kopsis's Avatar
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    If you ignore all the other reasons why this is wrong and simply do the math, you'll see that at a 20deg lean angle, the lateral force on the rim is only 4% higher for a 45c tire than a 32c tire (due to the increased wheel diameter). If 4% is enough to force your rims out of true, you need to find a better wheel builder.

  3. #3
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    Quote Originally Posted by Kopsis View Post
    If you ignore all the other reasons why this is wrong and simply do the math, you'll see that at a 20deg lean angle, the lateral force on the rim is only 4% higher for a 45c tire than a 32c tire (due to the increased wheel diameter). If 4% is enough to force your rims out of true, you need to find a better wheel builder.
    Yes, and I'd expect this minimal effect would be overshadowed by the larger impact forces felt by the rim when you hit a pothole with a high-pressure narrow tire compared to a lower-pressure wide tire. That can lead to the spokes near the contact patch becoming unloaded and allowing the nipples to unwind. So I'd expect a greater tendency for a wheel to go out of true when using narrow tires - but it shouldn't be a big issue if the wheel is built properly.

  4. #4
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    That argument seems pretty specious to me. Jobst Brandt's The Bicycle Tire shows that the number one force acting on a wheel is simply radial load, i.e. supporting the weight of the rider and bike.

    It is possible that a larger tire exerts more force on the rim bead, because the tire has more surface area. But that all depends on how high you pump the tire. And it shouldn't have anything to do with how well the wheel holds its true.

  5. #5
    coprolite fietsbob's Avatar
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    Off road .. Dirt .. for light weight racing , K Bontrager, now a brand name owned by TREK.
    took the thin road rims in more spoke drillings, and took out a section

    to reduce the 622 rim to 559 and put MTB tires on them ..

    bike mechanics entertain a lot of theories , it is a form of keeping your mind engaged .

    but at those wages you really cannot set up the engineering test facilities
    that you can at a University engineering department, to run the tests and write up the Thesis.

  6. #6
    Ghost Ryding 24/7 Ghost Ryder's Avatar
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    If this were true, my wheels would need truing on a daily basis, maybe hourly.
    I run 40-41mm tires on my gravel grinder, that I regularly pull bunny hops, & ride down stairs.
    My rims are OEM, not even hand built. I bunny hop on some of my road bikes too. Maybe Mavic's are magically bulletproof or something...
    Giant Defy Dura Ace : Rip/Hammer-Specialized Allez Ultegra/105 : Recovery/Spinner-Specialized Allez Red : Trainer-Kona Major(Rad) Jake : Down & Dirty

  7. #7
    Senior Member Blue Belly's Avatar
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    Only thing pulling a rim out of true is a change in spoke length. There are many causes for this. None have anything to do with rubber.

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