Shim stack taper, what's the point?

[Jasons]

I have been thinking about this for a little while and can't really come up with an answer. I am interested in hearing what you all think.
Is there any real reason to use a taper on a shim stack, other than to provide room for the shims to move?
It seems that other than the last shim in the stack the remaining shims in the taper really don't act much differntly than the largest shims in the stack. That said you could use all large shims with a thicker small shim (I think RT calls this the clamping shim). I believe the taper could add some benefit if you don't want to run a little more or less damping than a straight stack could provide. I will try to give an example of what I am saying.

Take this stack(totaly made up)

10 .1 x 24
1 .1 x 12
2 .1 x 24
1 .1 x 22
1 .1 x 20
1 .1 x 17
1 .1 x 14
1 .1 x 13
1 .1 x 12

Now it seems that since a model of the stack could give you a displacement(amount of bend) for a given force on the stack.

Could you get the same displacement / force curve from a straight stack by varying the number and thickness of one diameter of shims. Something like this:

10 .1 x 24
1 .1 x 12
4 .1 x 24
1 .5 x 12

The only reason I increased the thickness of the last shim was to give more room for the stack to move.
I would think it would be easier to model this system mathematically but would you lose some level of ride quality.
What do you think?
Jason

 

[marcusgunby]

This is a good thread and i hope we all learn from it-my input;
I have heard of someone using a shim stack on a shock(i think it was on DRN)and it was all the same size shims
eg 40.2*12
I know all shim stacks will have high speed and low speed damping no mater what the shim stack
ie they dont need to have a taper to produce the high speed effects.
Im guessing its just easier to manupulate shim stack strengths with a tapered stack.
:confused:
I would love to be able to input all shims into a pc program and it give me the total shim stack strength but i remember a tread on this and it seemed very hard even to maths grads.

 

[Jasons]

That was one benefit that I anticipate with going to a straight stack, you should be able to compare thicknesses and the relative stiffness and at the very least get a ratio of current stack vs a change. If you have a 10 shim stack and add 2 shims of the same thickness I would guess it would be a 20% increase. This may give a way of quantifiing a change which with testing you could get a feel for how much it changes and use that for future tuning. Any mechanical engineers out there that could clear up weather I am off base with this theory.

 

[Road/Dirt Rash]

I believe it gives the shim stiffness and response a more progressive action/ I like to think of it as the same principle as a torsion bar which is tapered to the end versious one which is the same diameter to the end. The old shocks (around 83) had only a few shims and were backed up by a convex shaped washer which probably did the same thing as it forced the shims to bend from the center out progressively.

 

[mxneagle]

With the stack that you are proposing, I would think that the shim stresses would be very high at the 12mm dia. The taper to the stack help provide better support and spread out the bending stresses in the shims above. Also you would loose a lot of hsc damping. That's not a bad thing in certain situations. On certain years of honda 250's we had good luck with a HS stack that had very few shims with alot of taper (like going in 3 - 4mm steps in the HS stack)

 

[SoCal Steve]

The reason to use a taper in the shim stack is to give a more progressive damping curve. In other words, for the fork to be able to react differently for differing impacts; high speed vs low speed etc. A completely straight stack I would predict would be overly harsh on small stuff, because it will take a certain amount of force to get any deflection, and then once it is open it won't give much increase in resistance. More shims of the same diameter at the valve face serve like an additional preload.... it takes more force to get them to move at all.

When the stack is tapered, each shim starts to pivot off of the next one; until that next one is deflected and then they pivot of of the following one, etc. So for small, rather low speed, impacts the deflection may not move all the way thru the stack. The clamping shim, or the last shim, creates the final pivot point for the stack. Normally there would then be a hardened washer to follow so the clamping shim never deflects. So if the pivot is a 12mm od., and the stack begins with 24mm ods., then the outer 6mm of each side of the stack will be off of the valve face with the highest speed hit. But if there were 16mm ods in the middle, the 24s would pivot off of the 16s for lesser impacts so only the outer 4mms would be deflecting.

So bottom line, the higher speed the hit, the further thru the stack deflects and the more fluid flows. This is what we want to make the damping progressive. For low speed impacts we want low deflections for a controlled feel and not blowing thru the stroke or bottoming. For high speed hits, we want the valve to flow much more so the fork can compress and absorb the impact and the feeling is not rigid or harsh.

Make sense?

Steve

 

[mxneagle]

Sounds about right to me. Basically we are building a leaf spring. Look at the configuration on a car vs a heavy truck (and then think about the resulting ride).

I suppose that one could accomplish the same effect by using tapered shims (thick towards the center and thin at the edge) but manufacturing those would be unrealistic.

 

[Jeremy Wilkey]

MXneagle,
Intresting idea, but when we look at the goal of effciency, it may not be the best route..

Regards,
Jer

 

[cp380sx]

Actually a leaf spring is is very similar to a shim stack in concept. Most leaf springs have bars of the same thickness but varying lengthd stacked on top of one another. They are close cousins.

 

[Jeremy Wilkey]

CP,
I agree completly.. Mace found a patent on system that used a wing type shim arangement. That would put us even closer..
Jer

 

[Road/Dirt Rash]

In theory you could have a flat spring, like half a car spring pinned on one end, sitting over a straight slot in the piston to do the same job. One set could have a small slot with a weak spring system for the slow speed, and a larger slot with a stiffer spring for the high speed bumps,, with a single set for the rebound on the other side. ???