[ChrisRobin]

I think it's also called a "check spring." I know how the different shim stacks work, but not sure what the spring does. I'm thinking that when the shims bend to the point where they can't bend anymore, the spring compresses slightly to open up for more oil flow?

 

[Flo33]

It works as a preload to the shim stack actually. Watch Steve explaining it in episode 2 of his Tuesday Tune Series:

Preloaded shim stack starts at 5:59

 

[Jm_]

Gives you "adjustable" high speed damping by preloading the spring on some kind of threaded insert, but this doesn't give anywhere near the tuning adjustment of different thickness and arrangements of the shims (and valve orifices), so in practice the usefulness isn't very much IME.

 

[ChrisRobin]

Ok, so technically if you have a highspeed spring on shims that cover a piston, then you're making the the highspeed damping more adjustable and more independent from the low speed, right?

 

[Udi]

Ok, so technically if you have a highspeed spring on shims that cover a piston, then you're making the the highspeed damping more adjustable and more independent from the low speed, right?

Yes, and contrary to post #3, a spring loaded compression stack with HS adjuster is very useful.

With a regular shimstack (no preload), the damping curve is relatively linear. That may seem counter-intuitive at first, but if you had a ported-only damper, the damping curve would actually be progressive in relation to shaft speed - which would cause spiking. So the shimstack linearises it, great, that's a basic problem solved.

However the problem with having a fairly linear compression damping curve (again, this is all w.r.t. shaft speed, not position) is that to get enough damping support in the mid-speed range where it's most beneficial (helps hold the rider up under g-outs, hard cornering, etc) you end up being forced to have more damping in the lowest shaft speed ranges - which, contrary to popular opinion, doesn't provide much useful support but does decrease sensitivity substantially.

Practically, what that means is that you end up with a harsher ride than you should ideally need to get a given level of support OR you have to sacrifice more support (than you should need to) to get the compliance you want.

Being able to preload the stack (on a correctly designed damper - which not everyone does unfortunately) allows you to de-linearise that curve in exactly the way that benefits the above situation - i.e. allows running less damping at lower shaft speeds, while simultaneously allowing more support in that mid-speed range where you need it. Keep in mind that while the adjusters are called LS and HS, what they really do is influence parts of the total curve, with a lot of overlap.

You *can* achieve similar things on a non-spring-preloaded stack by using ring shims, but having to do it internally makes practical tuning very difficult / time-consuming and removes the ability to fine tune the setting for different courses and conditions too. One setting will not work for all riders nor all courses, so an external adjustment is far more beneficial.

FYI a check valve is something completely different - this just allows oil to pass unrestricted in one direction - a practical example is ensuring a compression piston does not generate any rebound damping.

Hopefully that's of some use. Also I doubt anyone would complain if these questions were posted in the DH forum, would get faster responses. It's full of enduro crap now anyway, and this was a very good question.

 

[ChrisRobin]

Thanks for the great info. I initially asked that question because Push Industries advertises they upgrade that during their tuning service, although don't go into detailing what it does. I've had that service done a little while and wanted to know a little more... plus I felt the fork wasn't working as well as I had hoped. Turns out though it was the Fox air spring that was causing the issue but after a tweak to the negative air spring, it all feels good now.