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Advice: Fixing Cracked Frame

Discussion in 'Technical and Troubleshooting Torque' started by mattb, Oct 6, 2006.

  1. I discovered today that the Kwaka GT550 has two hairline cracks in the frame (showing her 130,000km age!). I don't know the technical terms, but basically you've got the part where the whole steering mechanism hooks on, and immediatley below that the frame divides in two, into two bars running parallel. A couple of cms into the divide, there is a hairline crack on each bar, going halfway round the bar, from left to right, on the front or forward-facing side of the bars.

    My question is this: I'll check out repair prices, but I'm tempted to wait a month till I'm back at the folks and get Mum's very able partner to weld the cracks. Is that an adequate fix? Should we weld an extra piece of metal to connect the two (as yet unbroken) sections instead. Anybody got any advice from experience, eg something we need to look out for, take into account etc?

    Anybody able to direct me to a good webpage in this?

    Cheers, Matt.

    (New tag: only ever having owned old early 80's bikes with lots of problems is fun and educational!!! :) )
  2. Matt, my bro is a 'specialist' welder and can weld shit to clay but even he would like to know a few things.,
    oh and i presume it's the head stem your talking about???
    What material is the frame?
    Any bearings, electrics, etc near the cracks that need removing?
    Do you want a 'touch up' repair? or dou you want it done properly?
  3. Hey Woodsy.

    Thanks for the response.

    Depending on how one defines it, I'd say the cracks are 5 to 10 cms below the head stem. So there are electrics, and the tank, right there. I could do some research to find out the metal of the frame, but the bike's an '83 model, basically a Z or KZ, and I'm guessing it's a pretty standard steel? (I'm guessing there are differences with fancier later sports bikes?)

    As regards what sort of job I need done, if I get someone else to do it I need to do something cheap, as I'm the stereotypical poor student, but I want to be able to do two-up touring, as well as be able to sell the bike in a year or two, with a good conscience, so I guess 'properly' is the right call, unless a touch up will fulfill those needs?

    If I was to wait and get a family member to do it, I guess I'd do a full job, but I think it might be wise to jump on the problem before it gets worse.

    Cheers, Matt.
  4. Get it welded up and it will be stronger than before, no worries.
  5. I'm reminded of Romulus' words. Rai Gaita (one of Australia's best - a Victorian lad - and indeed one of the world's best, philosophers) quoted his Dad, a welder, in the eulogy-cum-biography, Romulus My Father. Romulus gave this guarantee to his work: "If it breaks, it won't break where I welded it!"
  6. as a dood with a bg in structural engineering, get it fixed proper. saving money could cost you......

    dont short-cut, the thing with welding a frame is that the mechanical properties of the steel changes after it has been heated by the welding process. a good tradesman will look after all that.

    good luck with it :)
  7. When you get it welded, just be sure to protect anything nearby from heat
    (be sure to remove the petrol tank and drain the fuel out of the carburetors),

    Yes, cool the steel gradually by spreading the flame slowly, don't cool
    it fast with water or it will go brittle and crack again near the weld.
  8. The Kwaka GT750 was notorious for cracking at exactly the spot you describe, the 550 was much less prone to the problem. I assume that is because the 550 makes less power and is a bit lighter but has basically the same front down tube thickness.

    The later GT750's had a slightly modified down tube arrangement with reinforcement in that area, it might be worth seeing if you can find out exactly where (and how( they were altered from the earlier ones in order to make your welding reinforcement as effective as possible.
  9. i reckon he's been spending too much time getting up on the back wheel ;) :LOL:

    good idea ^^
    reinforcing would certainly help, give google a hit!
  10. yep, this is spot on advice. don't do a backyard weld job on this, get it done by an experienced welder who knows how to do the job without degrading the material. done wrong you could end up having a nasty failure down the track if the steel embrittles at the weld.

    also be aware that you have what sounds like two big fatigue cracks. With every bump, turn, brake or acceleration that stresses the frame they are growing and eventually the cracked tube sections will have insufficient strength and fail. my point simply being don't keep riding it like this indefinitely. good luck with the repair mate :)
  11. Top advice above. The crack will have been caused by fatigue. merely welding the cracks MAY work, but it is likely that the frame wil crack again near the weld, due to localised heat distortion and previously fatigued material.
    The correct fix is to have sleeves made up and welded in, with a long taper at the ends to distribute the load evenly over the existing tube and not create a weak spot. The sleeve can be a piece of split tube which can be opened up and closed up again around the tube. Then the split is welded up.
    Welding the crack would be a very temporary fix, and I'd probably do it as a TEMPORARY measure only, and check the area next to teh weld constantly for any further cracking. I don't know how thick the tubes are, I'd probably go MIG rather than arc welding, in case it's too thin.
    Are you sure the crack is not just the paint flaking due to frame flex?

    Regards, Andrew.