# 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 . 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 . 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.

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• Hey guys, I'm wondering what suspension settings you all are running front and rear?  I have a 2017 crf 250r I'm a 210lb newer rider, my settings are currently set at,
rear suspension low speed  at 12 clicks
rear suspension high speed 2 turns
rebound is 12 clicks.
Left front fork inner chamber 150psi, outer  chamber 10psi, balance pressure 150psi
Right front fork compression position 7 clicks, rebound 29 clicks
Sag is 4"
Thanks guys

• One of the most common calls we get here at the shop is “how do you recommend I go about tuning my suspension?” Good question! The second most common call is “I started turning clickers on the front/back/bottom/top etc. and now I don’t know what I did, help!” The amount of improvement you can gain from proper suspension setup is significant, if you go about it in the right way.
Look at it this way: if you are 6ft. tall and get in your truck after your 5 ft. 3in. wife just drove it, the first thing you are going to do is adjust the seat/steering wheel, etc. for your size. The same thing goes for your bike’s suspension-last week we had (3) 2010 bikes in the shop for revalves at the same time. Identical models, one guy was a 150lbs. pro racing MX, the next guy was a “B” level GNCC racer and weighed 220, and the third guy was a 180lbs. trail rider. Same bike, three totally different setups!
So with that in mind, here is where to start:
• Grab your owners manual, a computer, clipboard and a scale. You cannot effectively start tuning until you determine if the correct springs (fork and shock) are on the bike for your weight. Put down the double cheeseburger, hop on the scale, and get your weight in street clothes. Add in for your gear, which typically runs between 20-30lbs. You can easily check recommended spring rates by visiting www.racetech.com under their spring rate calculator. Look in your manual (or ask your tuner) and see what rates are on the bike.
• If you need to change spring rates-do it first. Trust me on this, trying to tune suspension with the wrong rates is not only frustrating, but you will be short changing yourself on the results. On most bikes the shock spring is easily changed, fork springs can be a bit more difficult-get qualified help if you need it.
• Even if you are familiar with what “clickers” are, take a moment and read your manual. Determine what style of forks you have (closed cartridge or open cartridge), where the compression and rebound clickers are, and check to see if your shock has both a high speed and low speed compression adjustment.
• Grab the right tools to adjust, load up and go find a typical piece of terrain to test on. By typical I mean your MX track, hare scrambles course or favorite singletrack. You don’t need to ride a 30 mile loop in order to adjust your bike, rather focus on finding a section of track/trail that has all the different types of jumps/bumps/whoops you encounter.
• OK here is where I will preach a bit-everybody has a buddy or two that claims to “know suspension” and setup. This is YOUR bike, and unless you plan on dragging him around on the back of the seat the end result of your tuning should be focused on what feels good to YOU. Trust the feedback the bike gives you…..
• If you do not have an idea of where to set your clickers, put them in the middle of their adjustment range. This is your baseline setting.
• Gear up and get warmed up. It is important to be loosened up on the bike BEFORE you start tuning, or you run the chance of mis-diagnosing how the bike is feeling (I never start testing until I have at least 15-20 minutes of warmup time on the bike-I always ride stiff initially and sometimes do not get into a groove until then). Some guys can just jump on and pin the damn thing right from the truck. You know who you are, Wattsy……
• Remember, this is a tuning session not the MXoN. Use you head and ride at a pace UNDER you max speed-there will be plenty of time to “fang it” once you have zeroed in on some good settings.
• OK-ride and get a good feel for the bike with the clickers in the middle of their range. Now it’s time to really find out what “too soft” and “too hard” means.
• Take your clickers and turn them all the way out, full soft. Go ride the bike, but take it easy-it will feel ALOT different. Then come back in and turn everything all the way stiff-go ride again, being careful as this will feel totally different again. For guys that have tuned a bit, these two steps might seem pretty basic, but you will be amazed at the difference in how the bike feels. This is especially helpful for guys who are just starting out.
• Set everything back to baseline. FROM THIS POINT ON YOU WILL ONLY MAKE ONE ADJUSTMENT AT A TIME!!!!!!
• So now you will want to determine your tuning range. The tuning range is what settings you will use to adjust for different conditions. For example, If you are an MX racer as well as an occasional singletrack rider you will want to use different settings for those conditions.
• Fork compression is a good place to start. Ride your test section at baseline, then go about 3 clicks softer. The question to ask yourself after each adjustment is: Does it feel BETTER, WORSE, or THE SAME????
• There are no right and wrong answers, only what you feel. So let's assume that the 3 clicks softer felt better-go 3 more clicks softer each time until it does not feel as good. You have just found the soft end of your fork compression tuning range. Now return to baseline and do the same thing, only this time go stiffer. After you have found the best compression setting, work on rebound. Remember, one adjustment at a time ONLY or you can become confused!!! Do the same testing with your shock. Once you have both comp and rebound individually adjusted, you can fine tune them to work together-just make one adjustment at a time!
• As a final test, when you have what you would consider your best setup, write it down, then go back and compare that to your initial baseline, riding both setups back to back. Might surprise you…….
I could go into some advanced tuning topics about the interrelation of compression/rebound, high and low speed comp, tuning for extremes, etc. but we will save that for another newsletter. Take your time, tune by how your bike feels to you and have fun. You will be surprised by how much better you will ride with well adjusted suspension.