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  1. #1
    mtbr member
    Join Date
    Oct 2018

    Repairing seat tube crush

    One of the first instructions/warnings I was given as a budding bike shop mechanic some 30 years ago, was when you clamp a bike into a Park stand, be sure the seat post is in place and be sure to cushion the clamp with a shop rag. The reason is that thin tubing walls can be deformed by the pressure, and the paint or decals can be damaged. On some bikes, itís adviseable to only clamp the seatpost to avoid crushing the seat tube.

    (Some people suggest clamping the top tube instead, but that has the same risk of tube damage, and if the brake cable runs along the tube, the pressure can interfere with the cable operation. This is aside the fact that it is much easier to true a wheel and adjust the chain line if you can invert the frame when itís clamped to the seat tube.)

    I recently bought a used steel frame for a project retro bike, where it looks like someone didnít get the word. After cleaning up the frame and checking it over i found the seat post I intended to use would only go halfway down. I suspected the tube was deformed, but it wasnít visible. After taking a measurement of the seat tube diameter with calipers I found that about halfway down (right in the area where the seat post would stick) the diameter measured 28.7mm if taken on the left and right sides of the post, and 28.6mm if taken on the front and back sides.

    The measurements proved the tube was a minor oval and too tight for the seat post to slide down as designed. This confirmed to my mind some uninitiated mechanic or owner had clamped the seat tube without the seat post in place.

    But how to fix it? Iím really kind of picky about things like that and it was bugging me. I donít have frame blocks (and didnít want to damage the paint, as frame blocks will do). Then I thought, well, if the damage was put in by a Park clamp, why not reverse the damage with the same type clamp? My son has a nice Park frame stand and so I greeted him on the front porch with my frame in hand and asked if I could borrow his frame stand for a few minutes. He looked at me quizzically, and then said ďOk, I guess soĒ.

    I put the bike in the clamp (soft rag in place) positioned right above the water bottle bosses, with the frame rotated 90 degrees from what you would normally mount it. This meant that the head tube was touching the standís vertical post. Then I worked the clamp gently up and down the crushed area until the seat post would slide nicely down to proper position. This meant that the seat tube was now round again, and I was very happy with how easy it was, and that the ďdamageĒ was only temporary. The seat post now glides down smoothly, stopping only when it bottoms out on the water bottle boss.

    Metallurgically, steel has good memory and will return back to itís original proportions and position with the right kind of coaxing as long as it wasnít creased. Creasing steel will work-harden it and makes removal of damage exponentially more difficult. Fortunately for me the tube deformation was minor and for this reason it came out easily.

    By the way, the shop owner I worked for is Eric Gooden of Cape Bicycle in Cape Girardeau, Missouri. Heís a great mentor and taught me a lot in the four years I worked for him.


  2. #2
    mtbr member
    Reputation: DethWshBkr's Avatar
    Join Date
    Nov 2010
    Oh wow.
    One of the first things I always heeded was "never" clamp a tube. Seat post only. If it was absolutely necessary, no pressure and a rag!

  3. #3
    Magically Delicious
    Reputation: Cleared2land's Avatar
    Join Date
    Aug 2012
    An impressively creative fix!
    A bad day of cycling is better than a good day at work

    Work Truck - Dassault Falcon 7X

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