Thread: Slipping fork tubes

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  1. #1

    Slipping fork tubes

    I've got a problem which throws up several questions.
    I've fitted some aluminium yokes to my Guzzi with the standard 34mm fork tubes.But the Marzochi yokes are designed to grip 36mm tubes.I made some sleeves from steel tube to adapt the yokes to hold smaller diameter tubes. The sleeves are slit vertically to match the slit in the yoke and to allow for the pinch during tightening.Lock-tightened them to the yokes.3000 miles later the left hand tube slips about 7mm.The pinch bolts in the yokes are m10 stainless cap heads,is there a torque setting for steel into ally ? I am concerned about stripping the thread.Would replacing the steel sleeves for ally help them grip better ? The previous owner did just that and I 'heli coiled' it. I've considered using lock tight to in effect glue the tubes in,but this would cause problems if and when I need to drop the front end out.It has also been suggested to roughen up the internal surface of the sleeves to provide a key for increased grip.But anything along those lines would scratch the tubes during removal and replacement.

  2. #2
    Senior Member mhbsmark's Avatar
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    The obvious solutions would be to either swap the forks for the correct size or change the yokes, but I suspect you're looking for a cheaper fix?
    If there is enough material around the pinch bolt threads you could drill them out and use use a bigger bolt but with a finer pitch, say 11 x 1 or 12 x 0.75.
    With a finer thread you would get more friction on the threads and they should stay tight. Not sure about torque settings but this company should be able to help
    http://www.tracytools.com
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  3. #3
    Thanks for replying.They are the correct forks.The yokes came with a load of other parts when I purchased the doner bike.I decided to use them as the standard originals were ugly.
    As a solution to my problem I've decided to drop the forks out,a light rub with emery paper where they make contact with the yoke face,thorough clean and an application of bearing retainer.
    I have a few other ideas,but I'll see how plan 'A' works out.Watch this space.

  4. #4
    If you want to sleeve yokes down to fit forks the sleeves really need to be turned up on a lathe so you get a good fit on both the fork and the yoke,rolling up a piece of handy steel sheet will very rarely do the job.
    Roughing up the fork tube ? Would you score your brake disc surface to get more friction ? No, you are just cutting down the surface contact area.
    Bearing retainers and other such "glues" are exactly what it says on the tube.
    Bigger bolts so you can do them up tighter ? Nope, barking up the wrong tree.
    Take the forks and Yoke(s) to a friendly machine shop, explain what you want and you should be fine.
    Steel or Ali will be fine but I prefer ali for the anti corrosion aspect.
    The sleeve need to be a "top hat" shape with the wider diameter going under the yoke, this may seem odd but it stops them pushing through the yoke IF there is any movement under braking (that is when the biggest load is applied on them).
    Hope that helps you out.

  5. #5
    Senior Member mhbsmark's Avatar
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    Turning up a set of sleeves is always going to be the best option, and the "top hat" makes sense.
    However a rough surface is best for any glue or adhesive application (in any situation) simply because there is more surface area for the glue to stick to. I agree that roughing up a brake disc would give less braking surface but by the same principle it gives a greater area for glues to stick to (all those little peaks and troughs)
    And you can't argue with the maths as far as bigger bolts are concerned. The bigger the bolt, the tighter it'll do up!
    If you don't learn from your mistakes then they are just mistakes

  6. #6
    A rough surface for gluing/bonding is good but I wasn't talking about either in this instance. I wasn't arguing with any maths, just that it was the wrong path. The slipping is down to an ill fitting shim between mis-matched items, having bigger bolts done up tighter or using a "glue" is just trying to mask the problem rather than fix it.

  7. #7
    Senior Member mhbsmark's Avatar
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    bluedog, You hit the nail on the head when you say mis-matched parts.
    renegade, if this company can't machine what you need then I don't know who can. https://www.fastec-racing.co.uk
    I saw their stand at the bike shed show and the quality of some their machined parts on display was second to none.
    If you don't learn from your mistakes then they are just mistakes