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This is a discussion on cb750 racing frame within the Project Builds forums, part of the Caferacer.net Forums category; Moto Martin frame: ( Or a very good copy. ) Danger, is my business."...

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Thread: cb750 racing frame

  1. #11
    Banned Witworth's Avatar
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    Moto Martin frame:

    Name:  motorstek_69064_original.jpg
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    ( Or a very good copy. )

    Danger, is my business."
    Wydglyd likes this.

  2. #12
    Banned Witworth's Avatar
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    Quote Originally Posted by roccitycafe View Post
    I would imagine they didn't spec one because they used off the shelf engines and the frames were designed around that fact, using the head to help triangulate the neck is used quite a bit, but if you've got a ton of reasons I shouldn't, then go for it
    The valve cover and it's screws are too weak to have chassis stresses applied to it.

    The valve cover will flex and cause it to leak oil.

    The motor will expand when hot and that may cause the head gasket to fail.

    A bolted on valve cover is a maintenance nightmare.

    It would be far better to design the bike so you can easily pivot the motor forwards and downwards on the bottom back engine mount like a CBX1000. Then you can get the head off in minutes for maintenance.

    A valve cover motor mount on a sohc is about zero benefit for a big box of HeadF^%$s.

    Danger, is my business."

  3. #13
    Banned Witworth's Avatar
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    Quote Originally Posted by roccitycafe View Post
    either .08 or .06 on the 1.25, and .06 on the 1"

    I'll box in the neck area with sheet gussets
    Gussets? Why on Earth?

    If you have good mitred joints and good welding they are completely unnecessary. In fact they will concentrate stresses and may cause cracks. Also all that welding will weaken the tubing and cause everything to flex all around the place when welding it up.

    Bike frames are not bridges or hotrod chassis.

    Danger, is my business."

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  5. #14
    Senior Member Cyorg's Avatar
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    Unfortunately I have to agree that a SOHC valve cover is not a good place to attach mounts. The thin die cast alloy won't take it and you would have to allow for the expansion and contraction of the cylinder and head, which would defeat the original purpose of the mount.
    When you do get around to fitting and welding the tubing, I would recommend using the swing arm pivot mounts as datum.
    Last edited by Cyorg; 02-27-2015 at 04:29 PM.
    "Non urinat in ventum"

  6. #15
    Senior Member bmartin's Avatar
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    I like the idea - carbon fiber for the rear - very trick. I'd make sure that valve cover & Head cover can be removed with the engine in the frame. I do like the Martin Frame..
    Bob - Palmyra NY
    2 - 69 CB750, 1 Turbo
    1 - 71 CB750

  7. #16
    Senior Member brad black's Avatar
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    can you lower the steering head? it seems awfully high above the engine.

  8. #17
    Senior Member Geeto67's Avatar
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    Rock, here is my concern:

    where is the swingarm pivot?

    If you follow the prevailing notions of what makes a good handling frame the overwhelming consensus is that you need to tie the swingarm pivot and the headstock in the most direct way possible. Look at that Moto Martin frame whit posted, or any japanese wish bone frame and you will see: either a direct connection to the pivot or the most heavily reinforced area is where the pivot meets the backbone spars. Its in tonys book too.

    now, the other reason I am curious about the pivot is, there is this other prevailing. Opinion that the pivot should be as close to the front sprocket as possible. Why? To counteract driveline forces acting on the chassis.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

  9. #18
    Senior Member Geeto67's Avatar
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    Oh and I agree, the valve cover, however strong they are is not a good place for a mount on a cb750. Now the head....you could drill vertically through all the cooling fins, insert spacers in between them and have a pretty good mount.....maybe.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

  10. #19
    Senior Member roccitycafe's Avatar
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    Name:  cb750 frame-Layout1.jpg
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    surprisingly the neck needs to be that high with 30" forks and a 17" wheel, it's deceptive looking but when you go and measure things out, that's where it's gotta be

    i don't particularly like the morini frame because if you really look at them they're wide as hell, the tubes wrap around the engine, so instead of triangulating the neck with short stiff tubes, you've got long gangly things hanging around, as far as the swingarm, it's going to be built off the rear 4 engine mounts, which tie into the top tubes which go to the neck and valve cover mounts, so all the tube section going from the rear mount to the valve cover is doing is adding additional stiffness to the engine structure
    Last edited by roccitycafe; 02-28-2015 at 09:07 AM.

  11. #20
    Senior Member jaguar's Avatar
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    There are a bunch of interesting frames out there.

    Check this place out.
    Old site, but good information to start more research.
    Satanic Mechanic - Bakker

    Also I would seek out Tony Foale's book on frame design.
    I know that you are not looking to make a replica of a frame, but looking at what others have done and reading about how it worked will give you a leg up.

    Way done the line, but have you thought about quantifying the results of the new frame?
    I have always been impressed with your body work and thrilled to see you trying out frames. I actually dont even have a good idea as to how to show if the new frame is better, worse or indifferent of the stock frame though.
    Just thinking via my fingers.
    I am Derby\'s Bitch


    Some times things come around that are so singularly inept it gives you a whole new appreciation for the ept

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