Tech question: Fork brace design goals? Jinx?

Discussion in 'Old's Cool' started by kevbo, Apr 13, 2004.

  1. kevbo

    kevbo Rubbery-Lip Flappin' PHI

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    One downside of the MZ Baghira is the all but non-existance of aftermarket accessories, especially on this side of the Atlantic.

    So I'm thinking I'm going to be making my own fork brace, and have been looking at what is available for other bikes.

    This has caused some confusion as to what the brace is trying to accomplish.

    I started out thinking the idea was to make the two fork sliders act more like a single piece, free only to telescope in syncronism. To do that the brace needs to attach solidly to the sliders, and have significant stiffness in a least two shear axes, (against rotation of the tubes sliders, and against differential teliscoping motion) and good torsional stiffness as well. (raising the torsional spring rate of the wheel to handlebar connection)

    Then I look at this thing:

    http://www.dual-star.com/index2/Brand/Kawasaki/klr 650 fork brace.htm

    or this:

    http://www.happy-trail.com/productinfo.aspx?productid=45&categoryid=1&startpage=1

    These have shear webs only against axial rotation of the sliders. There is very limited stiffness against differential extension of the sliders, and almost zero torsional stiffness. It seems the main intent of these designs is to maintain the spacing between the sliders, and little else.

    So I talked to one of my KLR-riding friends who has the K-9 brace (or a clone) and he said that the stiffer braces are prone to causing binding of the front suspension.

    I can see how that would be the case if the mounting surfaces aren't true, or the spacing is off....but when you get it right, isn't stiffer better? I wonder if the popular products are compromises to avoid binding when applied to the inevitable variations in machines, bent fork tubes, etc.

    I have access to a machine shop, and enough mechanical design background to make the thing work, I just need to know what it is supposed to be doing, and what potential problems I need to be avoiding.
    #1
  2. bigrichard

    bigrichard Long timer

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    Sounds like you've got the right idea.

    Before starting work might be good to see if fork tubes are straight and parallel.

    If you'll be playing in the mud and the brace has to be close to the tire you could try making the trailing edge thin and the distance to the tire progressively greater going forward.

    Take pics.
    #2
  3. Jinx

    Jinx Call me Renzo

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    Kevbo - I think most of the slider-to-slider alignment, in the direction of fork travel, is controlled by the axle. I think what you want in a fork brace is better torsional resistance, which will also help the fore-and-aft alignment (90 degress to the direction of fork travel) of the slider tops that can lead to binding. When you turn the bars, you want the wheel to follow immediately, not after a small delay as the fork tubes "wind up". Those flat plates actually do accomplish this by essentially halving the effective length of the fork legs Think of them as a cross-braced box structure, though like you I am not real impressed with this design..

    I have actually always liked the EGLI design, with it's two tubular cross members. And while Fritz makes his mounts integral with the fork legs, I would think a couple of bolt on clamps would work nearly as well. I have attached a few picks of Egli forks to get you started. Good luck.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    OTOH, Fritz's line of riding gear leaves a bit to be desired! :evil

    [​IMG]
    #3
  4. Ricardo Kuhn

    Ricardo Kuhn a.k.a. Mr Rico Suave

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    I wonder about this topic many times
    is my own impression most companies that makes this devices,make them base on the "Perseption" of what a fork brace is soppose to do not on the tecnical realityes or fisical parameters.
    also you need to remenber in most cases this brazes are Aftermarket and most buyers will not be willing to modified(weld drill,,,not even damage the OEM finish)of their current bike sometimes not even take the forks apart,so is not much that the companies can do with this kind of limitations.
    so the torzional capavility only go as far as what a bolt clamping on a tube can do--->AKA Not much.

    [​IMG]

    the EGLI fork braze you show on the picture above makes much more sense,,two torzion bars separated from each other so they can "Coil up" and stop the legs from misaligment in a much better way,but still if the legs are flexible like on the old R80's the whole fork is going to flex backwards when you are braking and oviuslly much more if you are turning at high rates.

    another thing you need to remenber is that the chassys place a big part on the handling of the machine,well the V-Max is a pretty good example,(I use to ride one and you feel the delay and the flexion was really strange to turn the bars but see the wheel still going straight until the forks woundup and decide to move the wheel)install upside down forks or a really efficient fork braze and then the frame flexion is even more evident,the whole head stock will flex side to side,I also see many of this Upside down or really masive convencional forks install(adapt) to many frames from a era with diferent design criterias that make the bike even more dificult to handle and sometimes even damage the frame because of the torxional loads

    I will say if you want to do it,start with the chassis and reinforse it to decrease the headstock area deflexion and then figure out a way to make a "trellis" type braze(very torxionally rigid)and very well attach(no rotational movements) to the forks,not just a straight line conection that more or less brings nothing to the table..
    also will be nice to fix the deflexion issues on the fork legs,,but all this things increase the mass and defeats the porpuse of the whole thing,,,
    think big diameter thin wall structures.
    #4
  5. Stephen

    Stephen Long timer

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    Kevbo, I think you're on the right track--you point out all the things a fork brace as sold by most won't do. And you're right. I've seen a few, and heard of many many more that don't line up with the forks. Some because of imprecise mfg, some because the forks weren't lined up to start with. I've seen fork braces cause binding and sticking and funny wear--some from the factory.

    So what's a fork brace for? Only this: to make up for an inadequate fork. It's a very poor solution, which is what everybody above has been hinting at.
    #5
  6. Robert

    Robert KickAss Adventurer

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    Moto International makes a brace for the MZ Skorpion...maybe they have something for the Baghira...there seems to be more of them arond
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  7. kevbo

    kevbo Rubbery-Lip Flappin' PHI

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    Thanks guys, for the sanity check.

    It's exactly the "windup" that I think most aftermarket fork braces do not address.

    The MZ does have 44.5mm forks (v. 38 on a KLR) so it starts from a stiffer place. I think it is actually pretty good but I am probably just spoiled by riding telelever and leading link bikes so much.

    Thanks for the Egli pictures Jinx.

    The design I have in my head was for a single large diameter (and yes ricky, thin walled) torsion tube, but using two like that might solve some of the "issues".
    #7