Twin chamber fork damping

Can someone Please describe in detail how a cartride in a twin chamber fork works? Preferably a fork cartridge that uses springs, not a bladder. I'm not sure how to word this so I'll do my best. What I mean is, in what order do things take place in damping on a compression stroke when the rod enters the cartrididge? Including mid-valve, base valve, every type of bleed, and ICS springs, what is active 1st, 2nd, 3rd etc.

Yes. There is a missconception that parts are active in different parts of the stroke. It is not so, all valves are active over the entire stroke and they act the same everywhere. To make sense of it I have to, whenever I read "first part of the stroke", to translate it to low speed damping etc. Also the spring is linear, so that everywhere an additional centimeter of travel requires the same amount of additional force e.g. 0.47 kg. The air obove the oil acts as an air-spring that is progressive, it gets stiffer deeper in the stroke. That is why oil height is important, it controls the amount of progressiveness, i.e. how much stiffer the fork is near bottoming.

The the damping is equal over the entire stroke (exceptions exist) but depend on the speed of fork compression. Springs absorbs energy but give it back later, sometimes at a bad time :smirk:. Damping absorbs energy through flow losses in the oil and converts it to heat. Flow past a shimstack gives a damping force approximately linear with speed. A free flow (orifice) gives a damping force that increases four times when the speed is doubled, until "oil lock" occurs. Oil lock does not stop the oil but the flow will no longer increase with increased speed.

The base valve flow is only the oil displaced by the rod and therefore acts in the region where the linear shimstack is dominant. The midvalve flows more oil, therefore it exhibits a combination of shim and orifice damping. Float is a flee flow in the mid valve that makes it less linear i.e. less low speed damping but more high speed damping in relative meassures. But in absolute meassures, increased float gives less damping all over.

Also the highspeed and lowspeed parts of the stack pretty much is the same. Both parts affect the damping over the entire speed range with only minor diffrences. To get a speed dependent shimstack a crossover is needed.

I think I give it a rest at that, for now :rant:. Hope I am clear and this helps you and many other.

Would it be accurate to say hydrolic lock occurs in the orifice of the mid-valve stem before the fluid can begin to bend and flow past the compression stack? And this hydrolic lock is timed by the compression adjuster wich allows more or less fluid to pass through the orifice in the stem causing the lock to happen earlier or later. The earlier this lock happens, the sooner the shim stack controls the damping? If this is correct then this hydrolic lock may not happen before the damper rod displaces fluid and causes it to flow through the base valve, but it does happen before the mid-valve shims open/bend. If there's no air in the system, then as soon as the damper rod enters the chamber it displaces oil. I'm thinking I am still confused. I'm just starting to learn and am trying to just understand one thing at a time so please bare with me:blush:

I am a physicist, I understand the physics of it. The detailed timing and interplay is design specific and I willingly admit that I am not qualified to give a perfect answer. After reading your text many times I conclude that you are on the right track, mostly. The compression adjuster is not in the mid valve rod though, it is in the base valve.

I guess this is good simply guidline to understand what happens in the cartridge.

-rod goes in and it dispaces oil and pressure spring will compress

-at low rod speeds oil goes through the bv stem and clicker (some models use low speed stack to control and stiffens this flow)

-mid valve float is critical at lower rod speeds also and when more rod speed mv stack will come in the play (mid speeds)

-higher rod speeds bv stack has a big role

This was short and simply (I hope so:smirk:) So there are low, mid and high speed adjustability in these forks. Pressure spring lenght and rate creates cartridge pressure and overall damping. Everything needs to be in harmony.

Remember overall damping is critical and is pretty good on later model MX bikes. This is like to adjust carburator (slide cut, air screw, pilot jet, needle position and profile and main jet)...

there is also damping generated by the spring perch (high speeds i would say)and its position dependant, and the bottoming cones the same (position dependant)

the fork has a huge mixture of speed and position dependant damping and spring effects, pretty amazing when you think about it.

Yes, bottoming cones definitely pos dependent. Also ICS, if ICS spring has float, would be pos dependent (I conclude from thinking about it). Pos dependence of spring perch I am not knowledgeble enough to say anything about. Can't see how though.

im not sure of the spring perch is constantly submerged in the fork oil?

I used last year special spring seats made by KYB and they worked pretty well. I used a bit lower oil volume, not as much mid stroke harshness but still same bottoming resistance and fork was riding a bit higher than normal.

Awesome! this is what I was hoping for, all you smart guys to respond. I've already learned a ton from you guys and many others that hang out in this forum since I joined TT.:smirk:

Create an account or sign in to comment

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

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now