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Thread: DIY Cranks

  1. #1
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    DIY Cranks

    I'm thinking about a custom crank configuration. Main reasons are orthopedic, but there are a few trick things I might try while I am at it.

    I seem to recall seeing some cranks that were welded more or less like a dog bone stem (end tubes parallel). Possibly German custom, though all I could see in that vein at the moment are cast. Anyone see something like that?

    My main concern is avoiding the need to machine the square tapers, or whatever interface. Seems one could hack off a steel socket from a donor, then I could mount it on a spindle in a lathe, and machine it round, and then fit it to a tube, and weld that to a 4130 crank arm.

    I only need a single ring up front, I will have an IGH in the rear.

    Anyone seen anything like this? What thickness of 4130 for the arms. It will be for touring. I'd be willing to take a hit on the weight to get a solid set-up.

  2. #2
    Senior Member ftwelder's Avatar
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    Bullseye tubular cranks are .035" wall 4130 sheet. They are welded with 4130 wire and heat treated. To reach a certain weight goal you would need to heat treat the structure which would require heat-treating and 100% 4130 material.

    Material engineered for forging may not be ideal for welding. It may also be fine. Constructing from tube would require a larger arm than solid but not too much. The square-taper interface is a bit narrow for directly attaching a tubular arm. People use fork legs for arms sometimes. I have pedal bosses if you need some. I have made many cranks from steel.
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    Randomhead
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    Grove Innovations (that used to be 10 miles up the road from me) used to make cranks. You hear rumors about Bill Grove or one of his ex-employees making cranks, but it's hit or miss. Cranks are a little scary, there are a lot of problems with fatigue.

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    Depending on the issue you are trying to address by making custom cranks, Profile Racing's BMX Cranks ( http://profileracing.com/profileraci...9&categoryId=1 ) may fit the bill especially since you only require a single front ring. They come in many lengths and are made in the USA from 4130, but require a spindle setup different from square-taper. Good Luck!

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    Thanks guys, some good suggestions there.

    FT welder, do you have the stubs that go on the BB, or are they the threaded stubs for the pedals? I need the BB part. I can make the pedal stubs, though it would be silly to do so normally, except I want to make some specialized pedals, due to the same orthopedic needs. That might normally sound complicated, but I have in mind a variation of the fly paper theme. Threaded stubs would still be useful, just in case my ambition flags. I could use 2 sets of the BB stubs if you have them.

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