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possible results of modifications?

This is a discussion on possible results of modifications? within the Technical forums, part of the Caferacer.net Forums category; I posted my cb500t build a few years ago and I've been rideing it since as a part time commuter and on short trips around ...

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  1. #1
    Senior Member oldhondacafe's Avatar
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    possible results of modifications?

    I posted my cb500t build a few years ago and I've been rideing it since as a part time commuter and on short trips around the area. I broke the trans last night and was pulling the motor to see why I had no 3rd gear when /i fouund my frame was cracked. I'm sure the trans issue was from my exuberant rideing style but it seems the frame crack might be due to my modifying the frame to mount the single shock swingarm. These bikes were flexible by design and I guess that my mods. moved the stress point or maby didn't allow for vibration? Anyways it broke in less than 6000 miles and the bike only has 15000 total miles. I'm not positive it's from the modifications but it's worth thinking about. Other than a blown trans and broken frame the bikes been a blast and I'll have it going again soon. Name:  100_0702.jpg
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    \'78CB750f,\'76 cb500t,\'5-CB/CL450 basketcases,\'68 CL175,\'66 CA77/in a CL72 frame,\'1971 DT1f,1988cbr1000f,1987cbr1000f

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  3. #2
    Senior Member crazypj's Avatar
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    Is there a crack just before bend onto seat tube as well?
    I would weld up the cracks then plate over it, possibly all the way to steering head.
    All the engine mounts were tight? if front or rear were loose it puts stresses into different places and could happen even without shock mods
    I can't remember for sure but I think there is a specific tightening sequence?
    I dabble in rocket science, when I\'m not picking my nose
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  4. #3
    Senior Member KeninIowa's Avatar
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    Multiple angle pics of upper shock mount

  5. #4
    Senior Member nic579's Avatar
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    Quote Originally Posted by KeninIowa View Post
    Multiple angle pics of upper shock mount
    ditto..... ....

  6. #5
    Senior Member crazypj's Avatar
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    Upper shock mount should be putting that area in compression not tension so it's probably a vibration issue with slightly more rigid frame.
    I would check tube near lower engine mount as that's where tension would be
    It's also the area that flexes when cornering as rear frame 'bends' the front half
    I'm expecting something similar with my XS650's, looking to make some 'perimeter frame' tubes to go from swing arm pivot area to steering head plus a cross brace under the carbs
    Last edited by crazypj; 04-21-2013 at 01:03 PM.
    I dabble in rocket science, when I\'m not picking my nose
    The other night I gave my girlfriend an orgasm, but, she spat it back at me

    'you can take my word for it or argue until you find out I'm right'

  7. #6
    Senior Member oldhondacafe's Avatar
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    Thanks, I've already got it back together. I vee'd out the crack and welded and plated over with another piece of similar tubeing. I'm planning to do a complete do over of this soon. I used a XL500 swingarm and lincage and have picked up a complete xl500 frame with all the rear suspention. I may just graft the two together or try to copy it as close as I can. When I first did it I just looked at a lot of pictures and guessed. I also had a SX650 and copyed the frame brace tubes they have from the swingarm area to the center frame tube, I suspect this is whats causing the problem.
    \'78CB750f,\'76 cb500t,\'5-CB/CL450 basketcases,\'68 CL175,\'66 CA77/in a CL72 frame,\'1971 DT1f,1988cbr1000f,1987cbr1000f

  8. #7
    Senior Member KeninIowa's Avatar
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    I disagree with PJ's failure analysis, but without pics I'd just be talking.

  9. #8
    Senior Member crazypj's Avatar
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    Ken, why?
    I'm a mechanic not an engineer, ejumucate me
    Lets hear your theory
    I dabble in rocket science, when I\'m not picking my nose
    The other night I gave my girlfriend an orgasm, but, she spat it back at me

    'you can take my word for it or argue until you find out I'm right'

  10. #9
    Senior Member KeninIowa's Avatar
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    The backbone's not in compression. It's almost perpendicular to the compression forces the shock's putting on the chassis.

    The upper shock mount is tied to the two smaller diameter tubes(I'm guessing). Thus creatng a hinge point.

    It broke where it did because the way it's welded to the backbone makes a stress riser there.

    The frame works in it's stock configuration because the lower tubes like where the rearsets mount triangulates the tail. They're too far out of plane to do any good with the monoshock. And the motion ratio is all different twin shocked. Mono shock is like putting a fat girl on the end of a teeter totter. That's why the mounting points have to be exponentially stronger.

    I'm not an engineer either.

  11. #10
    Senior Member Tonnystark's Avatar
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    ..also not an engineer thou have built a few tube frame sand rails (rewelded a few also)..my thinking's with PJ that this is a side force flex crack at this point. The engine is contributing to the frames rigidity along with the down tube 'tree' in front of the mono shock and all look pretty stout 'cept at the point where its cracked..IMT gussets and welds do converge at this point ,yet do not by this view,have overlap..I would also give that front down tube just above the front mount ,a close look to be safe.. You have prob fixed this particular crack with welds and over plates ..

    Is it entirely possible to have made this frame too stiff with that additional vertical tree?

    Have experianced in Sandrailing that if having been built too solid, frames do not absorb punishment as well as one that has some 'give' or ability to flex ...... (absorb and release stored energys)
    Last edited by Tonnystark; 04-23-2013 at 11:08 PM. Reason: additional thoughts
    78\' SR500
    64\' YG1S

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