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  1. #1
    Senior Member acupuncture Doc's Avatar
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    Double Butted Frames Question

    I recently purchased a mountain bike for touring conversion.

    It was pointed out to me recently that this bike did not have a double butted (or even a butted) frame. I am a larger guy and weigh around 250lb. (think former college lineman here). My question is this: Is the lack of double butting on this frame going to be a serious drawback to mostly on road touring for a guy of my size? I suppose I could swap the frame out for a Surly LHT in a pinch without too much trouble.

    The bike is a Giant Iguana 640 which was a cheaper knockoff of the Iguana. It has a 4130 chromoly steel frame/hi-tensile steel rear stays. I know not all Yukons are double butted and these seem to do OK for tourer conversions. It is a mountain bike after all, and I suppose I can count on some structural integrity due to the off road intention its designers had for it.

    So whaddya think? Is double butting essential for a touring frame? I want to be reasonably sure I can use this bike before I put any more money into it.

    Any info is appreciated as always.

  2. #2
    Last one to the top... Little Darwin's Avatar
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    This is based on general understanding, and is open to correction.

    Butting is done to reduce weight. Unbutted tubes tend to be the same thickness as the thickest part of the butted tubes.

    Therefore, if I understand properly, your bike will be fine, unless you think it is too heavy. If you need something lighter, then look for a frame with butted tubes.

  3. #3
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    The weight difference is trivial most likely something in the order of about 30 inches of steel 3.5 inches by .01" thick. Every little bit helps, but then if you are lineman like, you shouldn't have too much difficulty carrying that around.

    The butting is mostly in the top and down tubes. The seat tube normally is externally butted and that may have been done since a machined tube helps with fitting the seat post and the outer clamp. The downtube is under tension and the top tube is under compression. For the same external dimensions the butted tube would be weaker, but it is butted because experience shows that the extra material is not normally missed. Under heat, the thicker ends are an advantage during welding or brazing temps. When bridging as if you sit on the top tube thicker ends are required just as a fishing rod is thicker where you hold it. An under compresion the butted tube has a slightly higher margin than an unbutted tube for remaining in collumn (but the walls of the straight gage tube are stiffer.

    Overall, if you like the way the bike rides don't worry about it.

  4. #4
    weirdo
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    I can see how butted tubes would be slightly lighter than straight walled, but I thought the main idea behind them was to give a cushier ride without having to weld or braze on thin tubes. That isn`t the case?

  5. #5
    Senior Member brianmcg123's Avatar
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    Quote Originally Posted by rodar y rodar View Post
    I can see how butted tubes would be slightly lighter than straight walled, but I thought the main idea behind them was to give a cushier ride without having to weld or braze on thin tubes. That isn`t the case?
    No.
    Everyone's a roadie, they just might not know it yet.

  6. #6
    Senior Member acupuncture Doc's Avatar
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    Thanks !! I you I could count on an answer here. I appreciate the info !!

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