Jump to content

Clamp shims

Recommended Posts

I know there is no 100% correct answer to this question. Does anyone have a rule of thumb between the amount of face shims vs the clamp when changing the clamp size. For example only 

32x0.11 x 10

30

28

26

24

22

20

18

16

14

vs the same stack with a 12 clamp. I know the clamp effects the whole stack but does anyone use a rule of thumb like a 2mm change in clamp needs plus or minus x amount of face shims to keep the same cracking pressure. The reason I'm asking is I have a YZ250f stack I use offroad clamped on a 14 and have a Wr250F clamped on a 10 and want to minimise the shim purchases. Don't say use ReStakor :) and yes I could copy my known stack :)

Cheers 

MM 

Share this post


Link to post
Share on other sites

Hey Marty, I don't think a simple rule would be reliable to apply.

As per the Race Tech Suspension bible, I consider the face shims to be just just another shim above a clamp/hinge shim. Regardless of the diameter. It's the thickness that matters. Taper is there for durability, to avoid deforming shims closer to the clamp. This simplifies thinking about overall stack stiffness.

The stiffness of a shim is roughly equivalent to its thickness cubed. So if a 0.10mm thick shim has stiffness = 1, then a 0.15 = 3.4, a 0.2 = 8, 0.25 = 15.6  and a 0.30 thick shim is 27 times stiffer than a 0.1.  Just add up the stiffness of all shims to get total stack stiffness.  For the same stiffness total, more thin shims are more durable than less thick shims. But more thin shims ends up being physically thicker in total.

The rim of the clamp shim is the hinge point. It's the fulcrum for the lever which is the amount that the face shims extend past the clamp shim rim. So for your example, the 14mm clamp has radius 7mm. The 32mm face shim has radius 16mm. Which gives an overhang of 9mm.  If I change a clamp I calculate the proportional difference to the resulting overhang.  If you install a 12mm clamp your overhang becomes 10mm. So roughly a 10% change in stiffness of the stack above it. Quite a lot.    In summary calculate the change to face shim overhang.

To bring this back to your original question, clamp vs face shims, you have 18 shims above the clamp. Let's assume each are the same thickness/stiffness. So to make the stack 10% softer you could install a 12mm clamp, or keep the 14 clamp and instead remove 2 face shims. Approximately, but I think close to correct and seems to match my ride testing.  Note that's not a good rule if the shim stiffness doesn't add up like your example. That's why for an unknown stack I think it's best to do the calcs. 

Edited by numroe
  • Like 1

Share this post


Link to post
Share on other sites

Cheers Numroe for the answer I do realise it was a curly question :) I  sort of guess its around 2 face shims based of feel but this is subjective I know. I thought there maybe some secret tuners rule of thumb however most tuners are physics guys and don't like rules of thumb. :)

I understand the cubed rule or should I say accept it, also get shim width where I fall down is accurate measurement of one stack to another. If a rider says the bike is to soft "here" I can usually make a change to help however the effective measurement of one stack to another with differing clamp shims on non uniform stacks is hard and lets not even enter port or rod sizes. I guess that's why Dyno's and shim programs have become so popular. I do have a good ohlins manual that helps with this but its stuck on a dead hard drive. Maybe I should by the suspension bible never read it. 

 

Marty 

Share this post


Link to post
Share on other sites

It alters as you get closer to the face shim size , it's not uniform eg. Going from a 10 to 11 clamp is no where near as stiff as going from a 18 to 19

Sent from my XT1650 using ThumperTalk mobile app

  • Like 1

Share this post


Link to post
Share on other sites
7 hours ago, MartyMOOSE said:

I understand the cubed rule or should I say accept it, also get shim width where I fall down is accurate measurement of one stack to another.

What you (or at least I) do is add up a stack stiffness for a common/typical clamp shim. Then for any other stack with a different clamp, you can add up its shim stiffness, and then normalise (scale) that value to account for the different clamp. 

  • Like 1

Share this post


Link to post
Share on other sites
49 minutes ago, numroe said:

What you (or at least I) do is add up a stack stiffness for a common/typical clamp shim. Then for any other stack with a different clamp, you can add up its shim stiffness, and then normalise (scale) that value to account for the different clamp. 

You are ahead of me. How do you work out total stiffness to do this I'm assuming total shim thickness ? 

 

Marty 

Share this post


Link to post
Share on other sites
12 minutes ago, MartyMOOSE said:

You are ahead of me. How do you work out total stiffness to do this I'm assuming total shim thickness ?

As I wrote earlier. add up everything above the clamp. Ignore tapering. So in your example (assume 0.11 is the same as 0.10) you have 18 shims above the 14mm clamp. So that 18 units of stiffness (relative to a 14 clamp).  If you change a clamp shim then calc the % change to the face shim overhang (beyond the clamp edge) and scale the stack stiffness (normalise it).

Simple for single stage stacks. Some people put different thickness shims at different positions in the stack or taper with various theories. I'm not sure but I don't think that really makes any difference. Just add up their stiffness units and normalise for some clamp size.

For multi stage stacks with x-overs you might choose to compare say just the first stages, or second vs second stage. Or for very high speed strokes add them all up.  If the x-overs are the same thickness in compared stacks that helps, but don't get too pedantic. This is an approximate guide and the real test is on the track. You wanted a rule of thumb.

  • Like 1

Share this post


Link to post
Share on other sites
41 minutes ago, numroe said:

As I wrote earlier. add up everything above the clamp. Ignore tapering. So in your example (assume 0.11 is the same as 0.10) you have 18 shims above the 14mm clamp. So that 18 units of stiffness (relative to a 14 clamp).  If you change a clamp shim then calc the % change to the face shim overhang (beyond the clamp edge) and scale the stack stiffness (normalise it).

Simple for single stage stacks. Some people put different thickness shims at different positions in the stack or taper with various theories. I'm not sure but I don't think that really makes any difference. Just add up their stiffness units and normalise for some clamp size.

For multi stage stacks with x-overs you might choose to compare say just the first stages, or second vs second stage. Or for very high speed strokes add them all up.  If the x-overs are the same thickness in compared stacks that helps, but don't get too pedantic. This is an approximate guide and the real test is on the track. You wanted a rule of thumb.

Got it cheers :) Makes sense. 

  • Like 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×