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  1. #1
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    Brompton frame hinge clamp and flex

    It crossed my mind the other day that the flex I'm feeling when I pull on the handlebars of my mark 3 M-type might be a lot to do with the frame hinge clamp design. I was inspecting it closely and there's no doubt that a lot of flex occurs in that region. I don't think too much has changed in the post-2004 hinge design, basically manufacturing tolerances are extremely loose, the plates have a very small area on the left outer edge where they meet and there's a gap from the top to the bottom due to a slight curvature of the inner surface. When I pull on the bars I can see the gap at the top shrinking, i.e. there's visible flex.

    In all other folding bikes I've looked at there's a large contact area that forms a complete circle, i.e. it's buttressed in 360 degrees. This, I'd imagine, is one reason why Bromptons tend not to suffer from creaking in the hinge clamps, not because they're better engineered but because there's a much smaller contact area and that contact area is directly under the clamping plate.

    Looking at the post 2004 hinge redesign I'm thinking it ought to flex a little less. . . a fractional improvement would be significant amplified through the length of the stem/handlebar.

    Has anyone here who's owned Bromptons with both the old and new hinge types noticed much difference in terms of overall flex?

  2. #2
    coprolite fietsbob's Avatar
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    Has anyone here who's owned Bromptons with both the old and new hinge types noticed much difference in terms of overall flex?
    I have a Mk 2, forged plate hinge, hand brazed fillet ,
    and a recent Mk4, cast hinge, machined, inserted tube, brazing penetration between the 2.

    I have not noticed significant flex in either, Riding.

    nor do I have a test fixture with instrumentation to create the Data
    that gives the flex a numerical value.

    Which I think is required to have any meaning..
    Kg of force , in a given Axis, for a mm of motion.
    have you access to a university Mechanical engineering department?

    or drop by the Factory in London and see what test facilities they have.
    Last edited by fietsbob; 08-16-11 at 10:51 AM.

  3. #3
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    Are they both M types, fietsbob?

  4. #4
    Senior Member kamtsa's Avatar
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    You can make a small experiment. Stick a laser pointer to one side, a paper target to the other, apply force and watch the laser points moves on the rarget. The farther the pointer and the target from each other the more sensitive your measurement is (or bounce through a mirror or two).

    I have a S model and is pretty stiff so I doubt this is the frame.

  5. #5
    Senior Member
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    Nice idea.

  6. #6
    coprolite fietsbob's Avatar
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    Are they both M types, fietsbob?
    Yes, ... though the actual bars have changed, over the years.
    Source and Spec.

  7. #7
    coprolite fietsbob's Avatar
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    I suppose the laser pointer .. if the bike is rigidly mounted on a fixture..

    but we don't ride bikes on rigid mounts, to get to where we need to go, eh?

  8. #8
    Senior Member kamtsa's Avatar
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    Quote Originally Posted by fietsbob View Post
    I suppose the laser pointer .. if the bike is rigidly mounted on a fixture.. but we don't ride bikes on rigid mounts, to get to where we need to go, eh?
    I was thinking loading with body weight, pulling the handlebar, whatever can reproduce the flex, nothing fancy. If you are a science teacher, assign it to the kids as a science project and let us know the results. ;-)

  9. #9
    coprolite fietsbob's Avatar
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    Ah, maybe there's part of the difference, I don't pull up on the bars, much.

    Having adequate range, I drop it into the lower gears to spin up the hills..

    mostly just rest my hands on there.. support the upper body a bit.

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