KYB Fork Oil Level

Questions and comments about converting to beefier forks..
jafo
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KYB Fork Oil Level

Post by jafo »

I've changed oil on a few forks now and I'm getting the process down. this bring to mind a subject I'm not totaly up on. that'd be fork oil levels. Up til now, I've been going by the recommended levels in either the manual or on the team green site. Only now I'm wondering if I can go a step further and use the oil level as a tuning tool. So if you drop the oil level that should soften the froks some correct? Higher oil levels should stiffen the forks somewhat, correct? Basicaly, I'm getting ready to put new seals in this bike i have and it's really set up for MX'n but I'd like to soften the front end alittle. If I drop the fork oil level after the rebuild, that should soften the compression of the forks more than if the oil is at recommended factory levels right? These babies work pretty good on flat tight trails but the minute your into baseball size rocks, it like to bounce you onto another route. I don't want to take to much stiffness out of the front end, just alittle. I've already adjusted the compression screws out all the way. It helped but on a landing off a jump, I get a full compression of the forks, it's fine that way, but it's the first quick hit on the rocks that really spooks me. it actually bounces the front end up and to the side.

So what do you think about lowering the fork oil, and what do you think is a good amount to drop? The stock measurement for this bike is 134mm stock factory.
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Post by Indawoods »

The oil level will only affect the very bottom of the stroke. You would want to pull a few large shims from the valve to get what you want.
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Post by jafo »

So pulling a few larger shims out would increase the oil flow then right? What shims do you take out, there are shim on top of the valve and bottom also. I can't remember exactly the order they are in. I think I remember equal shims on the top and bottom. So you could take a large one out on the top and bottom right?
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Post by Indawoods »

Yes if you have a stock shim stack. What does you shim stack look like?

It should be pyramid shaped.....
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Post by IdahoCharley »

The larger shims on the base valve closest to the piston affect the low speed hits the most. Those are g-outs, faces on ramps, etc. The middle of the pyramid of shims to the smallest shims have the most affect on high speed hits - like baseball size rocks, tree roots, etc.

Removal of some of the large shims will soften the point where the the high speed shim come in but if you are deflecting off of baseball size rocks at a trail pace of say 12-17 mph then it is likely you need to soften your high speed portion of the shim stack and/or possibly go to a smaller diameter clamp shim.

Your BV clicker is basically a fluid bypass around the shims and has very little affect on high speed hits - it has most affect on low speed hit like g-out, landing off a jump, etc. and very little or no effect on high speed hits. (the speed factor is not vehicle speed but the velocity speed that the fork needs to compress to absorb the hit) If you are running a mid-valve set-up then fork tuning over the internet could get interesting. :mad:

Lower or raising your oil height primarily affects the last 1/3 of the fork travel. If your getting full travel but not bottoming hard I would say leave the level along for now and concentrate on valving. Suspension tuning is not easy to understand primarily becasue of the various variables.

Oil viscosity and height play roles so it is generally best to settle on a oil then find a height that works assuming that your springs are close to correct for your weight. Focus on valving to get the response you want - trying to stay somewhere in the middle of your clicker range for adjustablity. Then go back and fine tune your oil or oil height adjustments. At least that has been my approach.

Indawoods asked for your valve stack info - please be sure to supply the information on the mid-valve set-up or check-plate set-up you are using and also the float.

P.S. The mid-valve/or check valve is located on the opposite side of the rebound valve(piston). How far open your clickers are on rebound will affect your mid-valve/check plate operation since the majority of the oil displaced by the rebound rod and piston is driven through this valve while the remainer is handled by the Base Valve. It is important that your rebound control is close before spending alot of time on a mid-valve or checkplate float setting.

Alot of off road tuners like to use a check plate set-up instead of a mid-valve. It does make additional changes to the active compression (BV) easier and quicker. i.e. can be done in the field or garage without removing forks from bike. Also use of a checkplate set-up with adequate float - seems to result in less shim maintenance necessary on forks and allows for quick initial reaction of the forks to trail junk since the float determines when the checkplate or midvalve will come into play.

FWIW - My mid-valve shims do need to be replaced every 750 to 1000 miles of trail use. This means a complete tear down and probably 20 dollars worth of shims every season for me.
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Post by jafo »

OK for starters, I'm confused on the terminoligy. When you are refering to the BV or base valve, is this what I'm calling the "Compression" valve located at the bottom of the forks? I'm thinking it is. Then theres the mid-valve, that I'm thinking is located in the cartridge part of the forks right? Or is that the rebound valve? I'm still learning all these terms and locations of the parts and what affects what. I understand and realise that suspension is a touchy trial and error sometimes thing. Thats why I'd like to learn as much as I can about it, i'd like to set up my own forks and shocks. If it does'nt feel right, then I would have some idea of what might needs to be adjusted or replaced.

I have'nt taken the forks apart yet. When I do i'll be back with the needed info on the shim stacks.

I have'nt yet torn into the cartridge or base valve yet. All I've done so far is replace seals and oil and some general cleaning of the parts though not thorough, but good enough I figure for an oil change.

I'm eager to learn this stuff and may need some pics or something so I know what I'm looking at. I've refered to the link for the KYB fork overhaul but it does'nt show or does'nt label some of the parts your refering to, but I'll get it clear in my head before long.

Thanks for the good info guys, and I'll be back probably this week sometime with that shim stack info, I should be getting my seals for the forks this week. :supz:
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Post by IdahoCharley »

Your understanding is correct - Base valve is at the bottom of the fork leg on the KYB forks and it affects compression dampening.

The midvalve and rebound dampening shims are located within the cartridge. Basically there is a valve which looks similar to the BV but is shimmed for rebound control on the side closest to the BV: Shims controlling the midvalve dampening are located on the upper side of the same valve. (A check plate is just a particular mid-valve set-up)

Once you have the forks apart it should be clearer. Indawoods has a fork service procedure pinned which should help you if you have problems. I don't remember if the procedure or pictures specifically identify the individual components that we are discussing or not.

UPDATE - I just reread your original post and in reality Indawoods gave you the best advice! I tend to go nuts and look at the whole valving scheme but maybe pulling or changing a few shims on the BV is all you need.

You can do this pretty quickly without removing the forks from the bike provided you have a electric or air powered impact wrench. Just turn off the gas, pinch off the gas vent line (or remove the gas tank from bike) and then lay the bike on its side - lift up the axle end of forks and place on a five gallon bucket or something similar so the fork legs are angled upward. Clean the end of the fork of dirt - insert appropiate size allen wrench into BV assmbly and use powered impact wrench to remove the BV assembly.

NOTE - once the BV assembly is broken loose - turn the BV assmbly out slowly with the air wrench to prevent loosening the nut which retains the shims on the BV. The base valve assembly should slide right out. (Provided your fork legs are slanted upward at a 30 degree angle or so you should not lose any oil from the fork legs.)

Now you can examine the BV shims and make changes as appropriate. I think Morie posted a shim stack in a note to you or Ryan that did not look too bad to me. (Check related threads) Just based upon what you wrote concerning your suspension it would appear to me that you could go softer (fewer) on your low speed shims because you compression adjuster is fully open - that would be the shims that Indawoods was talking about and should get your compression clicker affective again.

Removing the larger shims will soften your low speed hits but do little for higher speed hits. Higher speed hits displace fluid quickly and the forks will likely still be harsh due to overly restrictive fluid flow capability for your riding conditions - without some additional changes.

The high speed changes necessary are likely to be a steeper slope of the shim pyramid or removal of an additional shim to allow for a higher volume of oil to be displaced quickly when you hit those rocks (which allows the fork legs to conform to the trail junk) and not deflect.

Anyway - once you have removed the shims and performed what ever other changes you want to the shim stack you just assembly the base valve assembly torque the retaining nut to 48 inch lbs (but check that torque figure to be sure) then screw your BV assembly by hand to get it started into the fork tube - once you have a few threads on it - I use the air wrench on slow speed to get it halfway tight.

Repeat procedure on the second fork leg and then turn bike right side up - run a tie down strap between the handle bars and the axle - compress the forks and tighen the tie down strap. Now torque your BV assembly to spec. Release the tie down and try out the new fork valving.

The first time you change the BV shims using this procedure may take you an hour or more to document and perform the BV shim shuffle - but after the first time it should take you less than 15-20 minutes to perform this whole operation.

Good Luck and hope this was helpfull - Charley
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Post by canyncarvr »

From:
http://www.strappe.com/suspension.html#level

This write-up basically uses oil level to set the maximum travel of the fork. If a test (described in the link) gets you less than 1/4" from 'bottom', add oil. If more than 1/4", remove some.

Obviously an incorrect spring rate will end-run that whole idea.

His rec is no more than 10mm change at a time.

For those too busy to do some research..here's an important point considering the air shock: As the forks compress the air shock '...acts like a spring but with a rising rate..' (from the posted link). The metal spring is generally considered to be much more linear in rate.


A BTW concerning oil level: I've read some..understand most of it...but still it seems to me that, the oil in a fork not being moved through ANY control (low or hi speed) until the air shock is compressed enough force oil through the restriction would make the air shock a working factor in the upper part of the stroke.

Or...the air shock does not compress much until the oil has moved though the system? Is that it? It's oil first, air shock next?

That would cover it.

Thanks.

Thanks IC for an excellent write-up on the whole shebang!

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Post by IdahoCharley »

CC - I'am not an expert in suspensions or anything but the way I view the action of the forks associated with air spring and fluid movement is a little different (maybe just with an additional twist).

When the fork leg is being compressed that rebound piston/valve/mid-valve assembly is being driven through the column of oil that is occupying the inner fork tube so fluid and the velocity of the valving within the cartridge is absorbing part of the 'hit'. Has to be so (says I - :lol: :lol: ) since the rebound rod is stationary and is holding the piston/valve/mid-valve assembly: When the fork leg is compressing it is forcing the trapped oil up through the rebound/piston/valve/shims assembly that is within the cartridge portion of the fork. A portion of the fluid is also forced through the stationary BV compression valving via the air spring.

It has been my understanding that the cartridge area of the fork (at the valve assembly) sees somewhere from 2-3 times the oil displacement as the BV assembly.

I agree the air shock plays a factor though out the complete stroke of the fork. But the air spring portion (psi) is a function of trapped air verses shrinking space. If you had an trapped air column and doubled atmosheric pressure then the air column would be 1/2 as long. Double it again and the air column would be 1/4 the length as the original column. Double it agin and the air column is now 1/8 the original air column. Near sea level you would be applying a force of around 120 psiA at this point. :? I certainly agree that air spring forces are very important component to consider an use to tune forks to your liking.


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Post by jafo »

Thanks a bunch guys. This has clarified awhole lot. I know what exactly I'll be doing now.

Jon.
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Post by jafo »

Well here is a good reason and lesson on why you should go completley through your forks. I went ahead and tore down one fork. On KYB forks on Yamaha's, they put a bottom out bushing in the top ot the fork in the cap area above the spring. I took the cap off the fork and found that the bushing is completley destoyed. It's in a million pieces. Then I took the rest of the fork apart and took the BV out and apart only to find that one of the main bled holes completley blocked off with old gunk and probably minute parts of the bushing that had been ground up while in the fork.

Anyways here is the shim stack broken down. It's from right to left and continued above the lower line of shims ( ran out of room on the towel).

Image

These are all below the base valve. there was only one large shim on top of the valve along with a collar, spring and some other metal doohicky and then the nut. Some one has had the base valve apart already, because I did'nt have to file the top of the stud to get the nut off.

Well......ideas?
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Post by canyncarvr »

The largest ones...about 24x15? (24mm dia. x .15mm/.006" thick or so).

I'd take one or two of the next size (22's) out.

Gee...looks to be not quite a single stage stack.

You will notice a big difference just cleaning the things up!!

For comparison...my stack config is in my red !clicky! profile..but it's:

24x15 (4)
22x15
20x15
17.5x15
16x15
11x13 (2) (measured .005")
17.5x50 (2)

No. That's not some magical potion or anything...just what the last tuner thought worked. Just listing it to show how varied opines can get. It's one heckuva lot simpler than yours..it IS a single stage stack.

If you haven't seen this done or figgered it out...it's a very good idea to loop a piece of wire (or tie wrap) thru the whole stack when you first take it off. Measure them all while they are tied up...to make sure nothing slips out of line.

It is VERY easy to miss a couple of the thin ones stuck together. Then, there is an extra one all of a sudden and who knows WHERE it came from!!??

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Post by jafo »

Thanks CC.

I used a couple of zip tyes to keep them all in order. I'm not up on the sizes of the shims, really have'nt got that far yet. I've been running down some new bumpers for the forks and finally found them on Bikebandit. The other bumper was ok but seemed kind of brittle, so it's got to go.

I'm going to go out now and attempt to measure all these suckers.

My next question in all this is how does removing some shims, make the oil flow easier making the fork softer?
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Post by jafo »

Since I don't have anything to measure the thickness, I did measure the diameters. Here they are in order.

4-24mm

3-22mm

1-17mm

1-24mm

1-22mm

1-20mm

1-19mm

1-18mm

1-16mm

1-14mm

1-11mm

Then two thick washers.

I'm going to just take two of the 22mm shims out of each stack. I'll start there. I don't want to take all the stiffness out of the forks just alittle. So I'll take your advice CC and go with a couple of the 22mm out of each stack.
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Post by IdahoCharley »

Jafo - glad you got your forks apart and found the messed up stuff - floaters within the forks really are hard to compensate for. LOL

If I refer to "above" or "below" a shim in the following parable it will be referenced from the face on the valve and associated with the oil flow path. Generally larger diameter to smaller diameter shims.

Like CC stated you don't have a single stage stack. Easy to tell since your 8th shim is small than your 9th shim indicating a "crossover" shim. Cross over shims are placed in the stack to allow the shims above it to bend over further (flow more) prior to engaging additions shims further down the stack. Taking out the crossover shim will 'stiffen' low speed compression and would normally turn the "shim stack" into a single stage stack - However it appears you have a non-standard shim stack. Just from the picture it appears that the shim under the cross-over shim is larger than the shim above the cross over. That is very unusual because it can cause spikes or harshness. Normally you would either match the diameter of shim above the cross over or drop the shim diameter size.

Your clamping shim is the 3rd from the left in the upper line of shims. The shims below it are spacer or back-up shims (non-functional as far as bending but functional in the sense of limiting the bend of the shims located above them).

Referencing CC posted stack - his clamping shim(s) is the 11mm by .13 mm with his spacer or back-up shims being the 17.5mm by .50 mm shim(s)

Clamping shim diameter and thickness play a major role in overall BV performace and I would advice you to be careful when making any changes to it. (Ride the bike slowly and evaluate the fork action and slowly increase your aggressiveness so we don't read about you in the hospital reports section of the newspaper.) Other than that removal of a couple of shims or buying some shims to try different tapers, etc. can be a very rewarding experience if you have the time to experiment. It is surprizing what changes can be experience by adding or removing just a shim or two. Generally you normally see steps of 2 or 3 mm differences in diameter as you build your shim stack other than cross-over shims. Larger differences than 4-5mm in adjacent active shims normally shortens the shim's useful life.

The collar, shim, spring, and metal dohecky is the base valve check plate which is functional during rebound of the forks. Do not confuse this shim with the others since it thicker and has a specific purpose. This check valve should not need to be changed.


Good Luck and keep up posted!!
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Post by jafo »

Thanks for all the info guys. I'm totally inlightened. :blink: This should be interesting.

I'll try to take it easy the first ride, but what fun would that be and how bad could it get???? :partyman: Just kid'n. I've used up most of my sick leave from work so I have to be careful........ thats why I'm running a hare scramble on the 30th. :supz:

Thanks again and I'll be sure to let you know how the change feels.
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Post by IdahoCharley »

jafo wrote - "My next question in all this is how does removing some shims, make the oil flow easier making the fork softer?"

Answer - it is pure Black Magic!! Well maybe not. As you remove shims from the stack it takes less fluid pressure to bend the remaining shims thus allowing more oil to flow through the piston - which allows the fork to become more compliant in the direction which it is being forced.

A steeper taper to the stack will allow more oil to flow for high speed hits i.e. 24, 21, 18, 15, 12, 11 progression will blow through the stroke on a given hit faster than a 24, 22, 20, 18, 16, 14, 12, 11 shim progression. Of course there are trade offs - If you go too soft on the initial part of the stack or too few 24s in this case then the forks will tend to settle (knife-in or oversteer) in corners most notably downhill corners in off-trail riding situations. The forks may actually feel stiff because they are riding too low in the stroke - air spring/ fork spring and maybe the KYB fork gods punishing you for messing with the valving.

If you have too many 24s then the forks will feel stiff when landing a jump or in g-out type situations along with not settling enough in the first half of cornering situations. Understeering and lack of tire bite (rebound settings figures in here also) - of course you can compensate for this by getting up on the tank more.

I believe your headed in the right direction with removing the 22s so I give you some suggestions worthy of the price you paid for this advice - if your almost completely happy with the forks after removing the (2) 22 diameter shims from the forks but still have an issue with baseball size rocks: try removing the 24 and 19 shims which are under the crossover shim. This may make the forks too soft now on high speed hits. If this is the case then you could increase the oil level or better yet IMO replace the 17 crossover shim with the 19 which you removed assuming they (17 and 19mm shims) are the same thickness.

Have Fun

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Post by canyncarvr »

I'm feeling somewhat ill. A little flushed and a tad nauseous.

I've gotta be getting ill....'cuz everything IC said makes sense to me!!

I gotta go lie down!

I've never seen such a succinct, well written, understandable post (or any other writing) on stacks.

Dang! Well done!!

Have fun indeed!!

THANKS!! :grin:

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Post by jafo »

Oh, I agree CC. I've pretty much understood everything he's put out in front of me and thats not normal for me. I usualy have to read through it about 4 times before it starts to make sense, but the way IC explains things, it very easy to understand.

IC, thanks for the help and I'll keep the other adjustments in mind. Oh, and I'm not to sure about you not being a suspension expert, it sure seems to me your pretty sharp on the subject.

I appritiate all the help you guys have given also, even though I'm on a Blue bike now.
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Post by IdahoCharley »

Thanks CC and Jafo for the kind words. Sorry CC about making you sick - :rolleyes:
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