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2005 YZ250 forks - help me straighten these out for trail/desert use

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So these forks are on my 2006 YZ250, but appear to be '05s (title says '06, the parts on the bike mixmatch). Not sure what the deal is with that, but I'm going to work with it for the time being. Doing a complete bike teardown, and these forks need some serious love (or replacement with a newer unit - looks like newer forks on this bike might be a bit of a PITA).

Observations, background and complaints:

- Sprung for someone about my weight, sag is around 70mm, and I think they have 0.46 springs in them, which should be OK for someone around 180# (my weight).

- Seals are most likely due for replacement just out of the hours on them, not by leaking. Bushings too I'm guessing.

- When changing the lubricating oil, I had the outer leg compressed all the way, and the damper extended all the way. In that case, the spring had about 1/4"-/8" of clearance before it contacted its upper mount. This doesn't seem right. I noticed some "clicking" coming from the fork recently, but it wasn't always there. I assume this is the reason. I can't imagine the springs have "set" in the time they've been ridden. Could something in the damper assembly be unthreading and letting that extend longer than normal?

- The valving on these forks seems somewhat "hyper". Meaning, it will come springing off of rocks from deep in the travel and jump off of them. Seems like the rebound circuit wants to freeflow if it gets past mid-travel. People notice how quick the return is just bouncing around on the bike too. On high speed hardpack, the front end seems to get sort of "floaty". Spends more time off the ground or bouncing off of things than getting traction.

- Tall rocks deflect it the front wheel, a lot. As do medium sized rocks. And small rocks. The front end does not inspire any kind of confidence on rocky/rooty singletrack. However, it does seem to do well in whoop sections. The previous owner had it valved for "desert" from a smaller local tuner, but that can mean a large variety of things.

- Running 8 out on rebound, 10 out on compression for the most part. Backing the compression off all the way just makes it nervous and divey, and doesn't really seem to help the high speed compliance much. Dialing in the rebound makes it a little slower, but that deep-stroke freeflow still seems to be there.

Objectives:

- Record all the valving, and determine ICS and main spring rates (will be posting these up as soon as I have them).

- FIX THE SPRING SPACING.

- Make the recommended updates (free piston drilling mod, etc).

- Spend some time checking straightness of the tubes, polishing all surfaces on a lathe, installing some SKF seals with Slickoleum grease, blueprint the bushings for perfect fit if necessary.

- Get the valving sorted out to be more stable and safer and beat up my hands less.

Before I tear into these tonight or tomorrow, anyone care to speculate about the spring issue? Common problem w/easy fix hopefully? Any good starting points for offroad valving for these? Not looking for mega plush, just controlled and stable and without the jarring effect to my hands. Want it to hold up to "as fast as possible" on singletrack. In many sections, the forks are really the limiting factor to my speed, and wishing to fix that.

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Hey!

Make sure that these forks are really the 05 ones. Different floating piston und Base then 06 are the indicators.

Note your valving base, reb, mid (with float) and we will see!

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lots of info on the 05 fork do some searching on here and you will learn tons

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Some notes I had saved:

KYB Twin-Chamber mods

The problem that exists in the KYB TCs is related to the volume of oil being expelled from the internal chamber. Obviously the shape of the piston itself creates a pocket that traps air causing it's own havoc. KYB are clearly aware of the issues of cavitation and have taken extreme measures to control it, but have failed to address the contained oil volume oversight. The excess oil discharged is due to the limited travel of the IC piston. By placing a spacer under the IC spring, the piston is repositioned allowing further travel or displacement before exhausting any excess oil to the outer chamber. Clearly there is finally a legitimate case that warrants replacing the original IC spring with an aftermarket option. They need to be longer. Drilling the hole in the upper potion of the piston will reduce the total spring force and will make the fork more responsive to movement. I have been replacing the upper o-ring with a PTFE band which eliminates the seal but also further reduces drag. By placing a taper on the compression bolt shaft you can have the internal piston seal do the discharging rather than the external o-ring. This will efectively eliminate any trapped air. You would then require the previously mentioned hole in the upper portion of the piston to allow the discharged oil an escape path to the outer chamber. Showa have had issues in the past with their piston seal wearing the compression bolt shaft when they were made from alloy. KYB have also made the shafts from alloy. Time will tell if this was a mistake. There is a gaping hole for an aftermarket kit for this fork containing...a longer IC spring, a stainless comp bolt rod and a PTFE wear band.

Incidentally...We tested these mods with no other changes on a YZ250. The rider said the fork was clearly more stable. Any headshake was gone and the forks no longer fell through the stroke. As I mentioned previously KYB have taken extreme measures to counter cavitation. A bleed in the rebound / midvalve piston. A bleed in the rebound piston band. A clearence shim on the midvalve itself and a valve stack on the compression adjuster circuit. All valid steps to avoid cavitation but when you have a reduced oil volume or negative pressure situation, you have cavitation built in and no counter measures are going to be effective.

Terry

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#6 12-25-2004, 07:19 AM

russ17

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Terry,

Very informative. Here's the question I have. Is it the design of the piston it's self or the the shape of the pvc type band holder behind the piston? Secondly, does the size of the piston itself limit it's travel compared to a smaller one?

Third ,You spark my interest in your comments on the negative pressure situation caused by oil being expelled from the internal cart. I went out and checked mine.( I have not checked this after spending considerable time figuring out a bleed procedure to counter the negative pressure associated with the so called normal type bleed procedure). I using a bleed procedure that works and I am happy to report no negative pressure.

Your comments about the spacer under the ICS spring, I found very interesting. The thing that comes to mind here, would be ICS spring preload .( I know depending on the thickness of the spacer.) My question is, if we are creating an ICS preload here, (which would increase the spring forces) would drilling the bleed holes above the piston offset this equally? ( Example:by my calculations the overall spring rate for the 05 YZ 250 is .524 with zero ICS preload ).

Adding 5mm of ICS pre load ,the overall rate increases to.539 which comes to a 20.23% difference.

Russ

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Last edited by russ17 : 12-25-2004 at 04:14 PM.

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#7 12-25-2004, 06:18 PM

terry hay

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Russ

The pvc is the piston.

By adding a spacer you are simply reducing the freeplay in the spring itself. Which standard is appr. 12mm. What I am trying to achieve is a lower starting point for the pvc piston. Obviously you would increase the potential spring force overall as it would now have to compress further before the bleed process occurs.

Try this...

Fill the cartridge to just above the diameter change. Pump the rod to displace any trapped air. With the oil just above the diam. change compress the rod completely and measure the height change in the oil. You will find it is appr. 35-36mm. Now if you measure the travel in the IC piston from starting position to exhaust, you will find that it is also 35-36mm. This would be fine except for one thing. The comp valve and post have to occupy some of this space. This means the oil being displaced by the rod will now raise the oil height above the original 35-36mm exceeding the travel of the IC piston. So in fact, the reason the rod fails to fully extend is due to the valve and post volume being discharged. I have seen the rods full extend with a trapped air situation. Obviously the compressed air adds to the spring force assisting the extension of the rod. When you invert the cartridge and compress the rod you can both see and hear the lack of damping at the end of the stroke.

Terry

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#8 12-26-2004, 06:39 AM

russ17

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I saw the free play that you talk of. It was just my observation that once the IC piston entered the the cart it moved and the free play was no longer there. When moving the IC piston by hand, it seems to lock into 2 positions 1 were there is no tension on the IC spring ( the 12mm you talk of) and position 2 were there is a slight tension on the IC spring.( were it takes up the 12mm)

Upon entering the base into the cart, the IC pistion always moved to position 2 were there was tension on the IC spring. Seeing this, my thought was this was the starting point of the IC pistion movement( with the tension on the spring) and then moving from this position. Upon removal of the base( even with suction ) the IC piston was always in position 2 with no free play on IC spring. From seeing this, is were my opinion was coming from.( if the IC piston was indeed in position 2 from the get go then adding the spacer would add preload.)

thanks for the insight.

Russ

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#9 12-26-2004, 01:09 PM

russ17

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Terry ,

I did invert the the cart and you are right on the money. there is indeed a small amout of air, enough to extend the rod.

Thanks

russ

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#10 12-26-2004, 05:32 PM

terry hay

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It's extremely difficult to get the air out completely unless you modify the compression post. You can of course lay the cartridge on an angle appr 45 degrees. have one bleed hole uppermost and cover the other with your finger. The top o-ring seals first which will allow you to have the lower portion of the piston enter the oil, displacing most of the air prior to it sealing. This wont work however if the top of the pvc piston has been modified (drilled)

Terry

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#11 12-26-2004, 05:56 PM

russ17

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I will get some spacers made

Russ

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Last edited by russ17 : 12-26-2004 at 06:32 PM.

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Silver. I believe I looked into this before when I first got the bike and determined that they were the '05 units from the cap color and the design of the lower lugs.

Edit: Looks like I can't disassemble the inner chamber without an inner cap socket tool. Is this correct? Any way around this?

Edited by GHILL28

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Edit: Looks like I can't disassemble the inner chamber without an inner cap socket tool. Is this correct? Any way around this?

Not without making a mess of the cap.

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If you are in a hurry and doin't want to order the inner cap wrench you can make one from a piece of alumnium or rock hard maple on a band saw and use a adjustable wrench to remove the cap.

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So I popped out the cartridges today. The outer walls of the damper look very worn from spring rub I assume. The springs have a hand marking of "42" in one end, which sounds like they're on the light side for me.

I was using a 13mm wrench as some instructions stated to hold the rod out of the lug while undoing the jam nut. That worked great except that it shifted, retracted with the wrench attached, and gouged the shit out of the lower end of the damper rod.

Also, a 3" long piece of white plastic tubing with a black end to it fell out of one of the stanchions. I assume that's part of the bottom out system, and is supposed to be fixed in place.

Between correct springs and a new damper rod, that's probably most of the cost of some '06+ forks with the correct springs already installed. What hardware is needed for that swap? I've read that the triple clamp spacing is slightly different. I could just machine some wheel spacers that could line things up again.

My '06 YZ450 seems to have the same axle spacing though..

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Get yourself a set of '06 - up SSS forks, you'll be way ahead with less work in the long run. Triple clamp spacing on '06 - up is 2MM wider, theoretically that throws the wheel offset to centerline by 1MM or .039". Put it together and check it out (measure wheel offset between tubes) to see if it's enough to worry about. I put a set of '07 250f forks in my kid's YZ 250 2t and had no problem with offset.

Note: The caliper changed on the '07 250f and '08 250 2t, need the smaller caliper from those or a '04 later CRF caliper if using those forks.

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Well, I have an '08 CRF brake system that I'm throwing on with this bike rebuild, so does that make the year of fork a non-issue?

Yeah I don't want to be dumping time and money into something that could be potentially way better with some new parts to begin with, and still not have it perform as well as the new stuff.

The wider face shims on the new forks would tell me the pistons probably flow oil a heck of a lot better on the compression stroke.

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Get yourself a set of '06 - up SSS forks, you'll be way ahead with less work in the long run.

I have an 05 yz250 and have considered going with 06 forks but have not found a way to know what is inside a 5 year old fork. The fork could have been revalved multiple times and for the wrong type of riding. Usually the ebay ads just tell you what year bike they came from. I would love to find a low usage stock 06 fork, but its not likely.

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That's the issue with most of the EBay forks. Plus I don't know what the hell springrate is in there. Not terribly useful descriptions on those ads.

I'm not 100% confident of my sag numbers for this thing, plus they're probably greatly skewed by the spring gap issue. 180# rider, YZ250 with guards and desert tank, wear a Camelback while I'm riding. I'm guessing 0.46 springs or so? RaceTech says 0.49 and a 5.1 rear, but I'm getting correct rear sag with a 5.3 rear spring. Mx-Tech says 0.44 and a 4.9 rear.

Anyone with real experience on this?

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The '08 CRF caliper will mount to '07 - later YZ250f or '08 - later YZ2502t/YZ450 forks. Won't mount to your '05 forks.

The .46 sounds in the ballpark for your described conditions (assume desert riding with 180lbs, camelback, desert tank).

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Weird, seemed alright with a mock-up fit on the '05 forks. Guess we'll see.

I remember I got correct sag out of my old KTM 380 2T with .44 springs. Put on a little bit of weight across my shoulders since then, about 10#.

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I suppose at this point it's prudent to ask: Is there any way a set of Twin Chamber Showas off an '06 CRF would fit? I have a set of these that I ran on my old KTM for a while.

I don't suppose the CRF stuff is the same diameter and same axle as the KYB's, is it? I don't have them at my place unfortunately. Loaned them out but can get them back whenever probably.

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I suppose at this point it's prudent to ask: Is there any way a set of Twin Chamber Showas off an '06 CRF would fit?

I don't have a database of all showa/KYB forks, but if all showas have one size, the upper triple clamp diameter would be 57mm, while KYB cc's have either 56mm (at least first generation KX) or 54mm (newer).

so the showas seem too big in diameter...

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Some notes I had saved:

It's extremely difficult to get the air out completely unless you modify the compression post. You can of course lay the cartridge on an angle appr 45 degrees. have one bleed hole uppermost and cover the other with your finger. The top o-ring seals first which will allow you to have the lower portion of the piston enter the oil, displacing most of the air prior to it sealing. This wont work however if the top of the pvc piston has been modified (drilled)

Terry

can you elaborate on the modification that needs to be done?

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Forgot to mention, if you need spares or anything I have a set of 05 yz125 forks laying around that can be pieced out.

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