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Thread: Frame Cracks 1976 R75/6

  1. #1
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    Frame Cracks 1976 R75/6

    Here's another that I think is not uncommon. In other experience, is this just a simple weld job or is there some reinforcement suggested?
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    Administrator 20774's Avatar
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    Lew will jump in here momentarily...he has great pictures of cracking in this location if I can tell from your pictures. Here's a picture he included in a previous thread:

    http://www.bmwmoa.org/forum/showthre...l=1#post859169
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    Registered User lmo1131's Avatar
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    You rang?

    A reinforcing plate (mending plate) is absolutely necessary. If you just weld the crack it will again.

    Shane Balkowtisch posted a fix that Josh came up with. 1/8" thick cold roll is probably sufficient (it will certainly be stronger than the original). You need to get down to bare metal. As noted on the photo; do not weld "across" the bracket, it will just result in another crack (at some point). There was another discussion a year or so ago but I can't find it in the archives. The discussion was about whether to weld, or braze the joint. I think the final agreement was to TIG weld it.

    I also think one of our member was manufacturing mending plates... but can't remember who.

    You'll need to carefully check both sides, top and bottom of each bracket.

    Note in this image that "stud" welded onto the bracket is for the rear sets.

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    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  4. #4
    Bluenoser
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    This bracket holds the exhaust & the rear foot peg. I'm pretty sure this is way more common than one would like to admit. I know my project bike had both sides cracked and it really looked like it has always been that way. It likely won't hurt anything to leave it alone as all the stress on this bracket is downwards, weight of passenger on pegs & exhaust. I think it looks worse than it really is.

    This is pretty good metal in this area and you have to use fairly high heat to get a good weld. When you install a brace, you transfer the stress to another location on the affected metal.
    1971 R50/5 SWB with R75/6 drivetrain
    2013 DL650

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    Registered User lmo1131's Avatar
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    I found the image of the reinforcing plates that one of the members was offering... maybe it will jog someone's memory.

    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

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    Thank you for the advice. I can make the part and TIG weld. If the repair is post worthy, I'll put up some pictures. Thanks again.

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    Registered User krpntr's Avatar
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    comon cracks

    I have the same cracks on my frame. They were welded once but cracked again as mentioned in the previous post. That was about six years and bucu thousands of miles ago. Nothing has fallen off in that time.

  8. #8
    James.A
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    I would get in there with a die grinder and hog out some material from the crack, then weld it up, and then weld on the reinforcing bracket. I would think that MIG welding would be more than adequate for that repair. It would be a good idea to throw down a few beads on a practice piece to get the heat and wire feed settings just right. It shouldn't be much trouble to shape out a gusset if you have some flat stock and a bench grinder.

  9. #9
    John D'oh
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    Just forget about it... Cycleman2 is correct.

    Those tabs were used by the factory as an assembly jig to help align the muffler / foot peg brackets during the
    construction of the frame. They serve a dual purpose in that they are broken when the bike falls hard on one side
    or the other - which tips the mechanic or potential buyer off to the fact that the bike has been down... When an airhead
    goes down it typically hits on a handle bar, headlight, valve cover and/or the muffler. Weight of the bike and momentum
    during a spill momentarily flexes the muffler mount (to which the muffler is rigidly attached) and the small tabs crack...
    This is simple "lever and fulcrum" physics.

    I don't bother with them. More important is if the muffler hanger bracket is bent out of line from the spill. If you absolutely
    MUST weld in doublers, 'inert gas' is as good as it gets - quick and clean BUT, be sure to heat the surrounding area with a 'rose head'
    gas torch and let it cool naturally to relieve stress and then repaint right away. Do NOT quench a BMW airhead motorcycle frame...they are MILD steel
    from the start for a good reason. Put your money and time into gasoline and throttle management.

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    Thanks Salty Dog, I like that answer. The split wide open valve cover was another indication that the bike had been dropped but interestingly enough, the muffler is not damaged. Anyway the beg mount is not bent and the cracked parts are still aligned with one another so i guess I'll leave it alone and keep an eye on it. Thank again.

  11. #11
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    I'm waiting to hear more While I know I hear good info above on the welding/metallurgy part- the leave the crack alone, it's only a mfg jig part anyway ,hmm- I'm waiting to hear from other experts on that one
    "If I had my life to live over, I'd dare to make more mistakes next time...I'd relax,I'd limber up... I would take fewer things seriously...take more chances... take more trips...climb more mountains...swim more rivers...eat more ice cream." Jorge Luis Borges at age 85.

  12. #12
    Registered User lmo1131's Avatar
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    hmm- I'm waiting to hear from other experts on that one
    We may be in for a wait, I imagine the only real experts on this subject are retired by now and living comfortably in F??nfseenland.

    Personally
    , I'm not buyin' it. Like aircraft, motorcycles are designed to be driven, not crashed. Notably "German", to keep overall weight down, BMW's engineers used the lightest gauge material suited to the application. With nothing to restrict transverse flexure of the joint, which is what the welds on the "tabs" do, the rest of the joint is continually stressed.

    But, to each his own.
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  13. #13
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    Quote Originally Posted by Lmo1131 View Post
    We may be in for a wait, I imagine the only real experts on this subject are retired by now and living comfortably in F??nfseenland.

    Personally
    , I'm not buyin' it. Like aircraft, motorcycles are designed to be driven, not crashed. Notably "German", to keep overall weight down, BMW's engineers used the lightest gauge material suited to the application. With nothing to restrict transverse flexure of the joint, which is what the welds on the "tabs" do, the rest of the joint is continually stressed.

    But, to each his own.
    Do they fish, hike & ride bikes in "Funfseenland"? Maybe something to shoot for if my frame falls apart...
    "If I had my life to live over, I'd dare to make more mistakes next time...I'd relax,I'd limber up... I would take fewer things seriously...take more chances... take more trips...climb more mountains...swim more rivers...eat more ice cream." Jorge Luis Borges at age 85.

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