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  1. #1
    ajd
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    Shimming a stem clamp

    I have a 26mm handlebar and a (claimed) 26mm handlebar clamp on the stem, but they don't fit. No matter how tight the binder bolt, I can still rotate the handlebars in the clamp with my body weight when in riding posture.

    I know that there are commercial shims available, but I'm wondering if it's good enough to simply sacrifice a soda can, cut a strip to size out of the sides where the metal's thinnest, and use that instead.

    Is there any reason not to?

  2. #2
    Senior Member sherpa93's Avatar
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    Beer can shims?
    http://www.velonews.com/tech/report/...es/3288.0.html

    Should be .1mm (or .2mm if you go all the way round)
    I used them for seatpost shim 25.4 to 26.2. I had to use
    4 actually and it worked OK til my shim arrived. It did slip
    a little maybe because I had 4 shims...

    Not sure I would trust it on a handlebar but if you only need
    1 shim .2mm might be fine. Definately worth a try. Good excuse
    to have a beer anyway

    PS: If I do it again I would rough it up with emory cloth or #320
    so its not so smooth.
    Last edited by sherpa93; 05-20-06 at 12:51 PM.

  3. #3
    2_i
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    I have used cans, commercial stem shims and seat shims for the purpose. They all work, better or worse. The problem with cans is that they are out of alu and their metal is thin so it gets easily deformed and squished under pressure. Partly for the purpose, I eventually bought a quantity of stainless steel and brass sheets of different thickness and I cut out my own shims. Both metals can yield decent shims.

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    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by ajd
    I have a 26mm handlebar and a (claimed) 26mm handlebar clamp on the stem, but they don't fit. No matter how tight the binder bolt, I can still rotate the handlebars in the clamp with my body weight when in riding posture.

    I know that there are commercial shims available, but I'm wondering if it's good enough to simply sacrifice a soda can, cut a strip to size out of the sides where the metal's thinnest, and use that instead.

    Is there any reason not to?
    The only issue with shims is that the same clamping force gets spread out between two surfaces instead of one. Or three surfaces with two layers of shims. This reduces the friction at any given surface-area, even though total friction is the same. You end up rotating the bar with less force since the weakest interface will slip. So rather than two layers of beer-cans, get a thicker single layer. And the knurled surfaces of pre-made shims really help to keep the grip high.

    On your stem, when the binder-bolt is tight, is the slot in the clamp open, or has it been compressed so that both sides are touching?

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    ajd
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    Quote Originally Posted by DannoXYZ
    On your stem, when the binder-bolt is tight, is the slot in the clamp open, or has it been compressed so that both sides are touching?
    The slot is filled with a rubber gasket, which I haven't tried removing, but due to the shape of the clamp I don't think it's a good idea to use the stem without the gasket.

    I suppose I could try slicing the gasket but that would be very awkward to do evenly. After your description of the texture on commercial shims, I'm wondering if the problem might be the relatively smooth surface of the handlebar in its clamping area (a matte-rough surface, as if from a light sandblasting, rather than etched grooves).

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    Quote Originally Posted by DannoXYZ
    The only issue with shims is that the same clamping force gets spread out between two surfaces instead of one. Or three surfaces with two layers of shims. This reduces the friction at any given surface-area, even though total friction is the same. You end up rotating the bar with less force since the weakest interface will slip.
    BS There will be no reduction of friction since the friction force os the clamping force times coefficient of friction. The clamping force is transferred from one shim to the other and not shared between them.

    Why not applying some loctite to the bar-stem interface.

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    Senior Member Grand Bois's Avatar
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    The Nitto stainless shims I've used are smooth. They're ready made, but you still have to do some grinding on them to shape them so they don't show. Then you have to deburr them so they don't scratch your bars. A beer can shim might even be easier.

  8. #8
    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by AndrewP
    BS There will be no reduction of friction since the friction force os the clamping force times coefficient of friction. The clamping force is transferred from one shim to the other and not shared between them.
    True, the clamping force generates a fixed amount of friction as dictated by f=mu. The amount of total friction is based solely on the clamping force. However, this friction is then spread between the total surface area clamped. If you double the surface area, the friction per square inch is reduced by 1/2. The outer and inner surface on the shim has only 1/2 each of the total friction.

    In a bar/stem combo with no shims, you have to overcome ALL of the friction at the interface. However, in a bar+shim+stem combo, you only have to overcome 1/2 the total friction, just one surface of the shim has to slip, either the outer surface or the inner surface of the shim has to slip which has only 1/2 of the total friction.

    It's like springs in series stacked on top of each other. It reduces the spring-rate by 1/2.

    Devise an experiment and find out for youself, it's true. Try shimming a 25.4mm bar into a 26.6 or 27.2mm stem with multiple layers of beer-cans. Use a torque-wrench to clamp down the pinch-bolt to the same value each time. You'll find that the force required to spin the bars goes down with each additional layer of shim material.

    Most people don't experience any slippage with shims becasue 1/2 the friction is still enough. With no shims, it may require 300lbs of force on the bar-ends to spin the bars. With shims, it's 150lbs, still more than sufficient to hold the bars in place. However, if you've got a sub-standard interface, like the stem bore is not perfectly parallel, or if teh bar's a little distorted at the clamping surface, then the mating surfaces may not match evenly enough. Commercially-made shims have knurled or stripes machined into them to actually dig into the bar & stem, thus uses a mechanical interlock that holds tighter than just friction.

    Quote Originally Posted by ajd
    The slot is filled with a rubber gasket, which I haven't tried removing, but due to the shape of the clamp I don't think it's a good idea to use the stem without the gasket.
    You can take off the gasket and measure the width of the slot at the binder-bolt before and after tightening. It's very possible that the clamp is being squeezed far enough that some parts of it is touching the other side, thus preventing any additional clamping-force from developing; you're just torquing the bolt down tighter, but teh clamp ceases to close further. At least with the gasket off, you can measure the gap. Then get a thinner gasket if necessary.

    Could it also be that the threads on the bolt of stem is stripped? Using a torque-wrench on that bolt?
    Last edited by DannoXYZ; 05-20-06 at 10:44 PM.

  9. #9
    2_i
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    Quote Originally Posted by DannoXYZ
    CLamping force generates a fixed amount of friction as dictated by f=mu. The amount of total friction is based solely on the clamping force. This friction is then spread between the total surface area. If you double the surface area, the friction per square inch is reduced by 1/2. The outer and inner surface on the shim has only 1/2 the friction of the total amount.

    In a bar/stem combo with no shims, you have to overcome ALL of the friction at the interface. However, in a bar+shim+stem combo, you only have to overcome 1/2 the total friction, just one layer of the shim has to slip, either the outer layer or the inner layer of the shim has to slip which has only 1/2 of the total friction.

    Devise an experiment and find out for youself, it's true. Try shimming a 25.4mm bar into a 26.6 or 27.2mm stem with multiple layers of beer-cans. Use a torque-wrench to clamp down the pinch-bolt to the same value each time. You'll find that the force required to spin the bars goes down with each additional layer of shim material.
    DannoXYZ you have it wrong and AndrewP is right. Da Vinci was one of the first people who studied friction systematically as the friction was hampering his devices. To beat the friction, da Vinci eventually invented the ball bearings. One of the da Vinci's laws states that "the areas of contact have no effect on friction".

    There are two points to remark, though. Thus, the laws of friction are generally coarse. Second, the experiment you propose is likely to produce the effect you describe, but not in proportion to the increase surface area. Simply, for one reason or another, e.g. dirt, the coefficients of friction will be different for different contact areas. In a stack, as you have earlier mentioned, the weakest link, i.e. contact area with the lowest friction coefficient, will give first.

  10. #10
    The duda man Knudsen's Avatar
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    Regardless of the laws, why not try it? Werx on Naval jets
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  11. #11
    ajd
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    Quote Originally Posted by DannoXYZ
    Could it also be that the threads on the bolt of stem is stripped? Using a torque-wrench on that bolt?
    No t-wrench but the stem is NOS so it's unlikely to have arrived stripped. With the handlebar, the bolt goes in and comes out securely; no slippage. Without the handlebar in, the bolt can be tightened down to the limit the gasket allows it.

    I'll try a shim this afternoon. Thanks.

  12. #12
    Packfodding 3 caloso's Avatar
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    ajd:

    How did it work? I ask because I am considering sacrificing a Pepsi can so I can use a beautiful Ambrosio 26.4 mm stem with a favorite Ritchey 26.0mm handle bar.
    Cyclists of the world, unite! You have nothing to lube but your chains!

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