Cheap tricks for the YZ shock

I really like my '06 YZ450 but it took a lot of time and work to make it a really fun bike for the kind of off-road mixed desert/woods riding that I do.

One of the problems I've been wrestling with for a couple of years is the rear shock. I've revalved it a couple of times and it works pretty damn good if I am riding the bike aggresively and have my weight on the bike correctly, but it's not very forgiving if I have a surprise hit or if I'm too far forward on the bike in breaking bumps/not on the throttle hard. In other words, I have to ride it like I'm racing all of the time.... not for leisurely riding. I've known what the problem is for quite some time (too much high speed compression damping) but no ammount of swapping shims on the compression stack would help (even did some porting on the piston). I could get improvemnts in the HSC, but changing the stack too much would always lead to a compromise somewhere else....until I figured this out!

In '06, the YZ's went to an 18mm shock shaft. For eons before '06 they were 16mm and the WR remain 16mm, even today. I recently had an '02 WR shock (16mm shaft) apart and noticed the HS compression adjuster had 8-2mm hiles in the end plate. I also had a shock for an '05 YZ (16mm shaft) apart and looked at it, it had 6-2mm holes. I also had another stock '06 YZ shock (18mm shaft). When I pulled the compression adjuster out and apart of the '06 shock, I compared it to the '05 and they were identical in every dimension. The only difference was the color of the anodizing on the HS adjuster nut.

Hmmmm.......

So, I decided to do a little experimenting. I took my spare '06 compression adjuster apart and drilled the six holes from 2mm to 3.8mm (#25 drill bit). About the biggest I could go without encroaching on any sealing surface. I reassembled it and swapped it with the one on my bike. The cool thing about making mods to the compression adjuster is you can remove and replace it without pulling the shock from the bike. Just lay the bike over at about a 45 degree angle on a stand, empty the N2 bladder, and unscrew the adjuster. If you have the bike at the right angle, you wont introduce any air into the shock.

After I put the modified adjuster in and recharged the shock to 145 psi, I went for a spin. WOW! what a HUGE difference on sharp hits! I could blast down a gnarly jeep road littered with grapefruit sized rocks and rain ruts while I was pinned in 4th gear and sit on the seat like I was in my recliner!

It worked great but with a price. The high speed adjuster had no effective differece if it was fully out, or in 2 turns.

Back to the drawing board.

I pulled it apart again and did a little research. SDI sells a lighter HS compression adjuster spring with a special washer/seat (Factory Connection sells one too, but it's only the spring). I have used the Factory Connection spring and didn't notice much difference. The stock spring seat sits on the clamping shim. The special spring seat sold by SDI has a relief and alows the seat to sit on the outside edge of the HS adjuster shim stack. I made my own spring seat like the one SDI sells and assembled it with the light weight FC spring. I also did another small mod to the shaft that would allow 3 full turns on the HS compression adjuster.

Back together and recharge the shock again.

This time on the check ride I had all of the adjustability I could ever want AND I finally have fixed the problem of too much high speed compression damping. All for under $50!

The WR orifice plate vs. the YZ

CIMG1207.jpg

HS compression spring with the modified spring seat.

CIMG1210.jpg

Adjuster assembly. The spring seat, shaft, and orifice plate are on the right.

CIMG1211.jpg

Brilliant!

I have heard that the entire 16mm shaft assembly can be swapped into the later (18mm) bodies; just use the 16mm cap, of course.

The smaller surface area of the 16mm shaft & seals results in a noticable reduction of "stiction" -- so the rear end responds easier and quicker to small bumps.

Great troubleshooting!

What I din't understand is why KYB left the compression adjuster alone after the switch to the 18mm shock shaft. Was the added HS compression damping something they were looking for? It appears that the added volume of displaced oil from the 18mm shaft causes the shock to border on hydraylic lock with the 6-2mm holes.

Now, with the larger flow area and the spring exerting more force on the shim stack, the back of the bike flows over objects (vs. sending the wheel in the air) when my weight distribution is less than perfect. The beauty of this mod is that I haven't sacrificed any bottoming control or caused a problem somewhere else.

Edited by 2grimjim

One of the things that Dave Johnson at SMART Performance has been doing for a few years now is opening up the ports in the exchange (high speed compression) piston. He specifies drilling to 2.72mm, which is a #38 drill. You could use a 7/64", also. The piston is easily damaged, so don't clamp it in a vise to do this. I clamp a pin in the vise and set the piston down on that, then drop a nail into one of the 6 holes to keep it from rotating. The holes should be chamfered.

For my desert racer ('06 YZ450), I modified the exchange (HSC) stack this way:

Stock:

21.4 0.11 18.4

21.4 0.80 17.5

18 0.30 6

18 0.30 6

16 0.30 6

16 0.30 6

11 0.30 6

11 0.30 6

9 0.30 6

Modified:

21.4 0.11 18.4

21.4 0.80 17.5

9 0.20 6

18 0.20 6

18 0.20 6

16 0.20 6

16 0.20 6

11 0.30 6

11 0.30 6

9 0.30 6

The main compression stack is tweaked just a little:

KYB Piston

40 0.20 12

40 0.20 12

40 0.20 12

40 0.20 12

40 0.20 12

40 0.20 12

36 0.15 12

30 0.15 12

40 0.25 12

38 0.25 12

36 0.25 12

34 0.25 12

32 0.25 12

30 0.25 12

28 0.25 12

26 0.25 12

24 0.25 12

23 0.25 12

41 4.50 12

...And the rebound heavily stiffened. Weak initial rebound is a huge fault on the YZ shock, and changing that makes an enormous difference in the stability of the rear end, to the extent that it makes the front more stable, too:

Lock Nut

26 3.00 12

24 0.30 12

26 0.30 12

28 0.25 12

30 0.25 12

32 0.25 12

34 0.25 12

36 0.25 12

28 0.11 12

36 0.15 12

36 0.15 12

36 0.15 12

36 0.15 12

36 0.15 12

36 0.15 12

36 0.15 12

36 0.30 12

36 0.30 12

36 0.30 12

36 0.30 12

36 0.30 12

36 0.30 12

36 0.30 12

KYB Piston

One of the things that Dave Johnson at SMART Performance has been doing for a few years now is opening up the ports in the exchange (high speed compression) piston. He specifies drilling to 2.72mm, which is a #38 drill. You could use a 7/64", also. The piston is easily damaged, so don't clamp it in a vise to do this. I clamp a pin in the vise and set the piston down on that, then drop a nail into one of the 6 holes to keep it from rotating. The holes should be chamfered.

Nice! You're a gentleman and a scholar sir.

At one time I was really good about writing down changes I made and having paper notes scattered everywhere. It's only been in the last 3 years that I have started to save stuff on my computer. Now I save notes on my phone!

I went digging through my pile of paper notes and found the shim stacks from my 1995 CR250 that I owned 15 years ago! I can't read half of them though, paper got saoked with coffee and fork oil. Not so sure which one was in the coffe cup.

The modified spring seat I have in the pic is available from SDI (with the lightweight spring).

http://www.suspensiondirect.com/

It can be found in their downloadable PDF catalog on page 8.

The part number is SDEHSAL

That part number appears to be just for the spring (according to the description), or does it actually include the seat once it arrives?

That part number appears to be just for the spring (according to the description), or does it actually include the seat once it arrives?

I believe that it includes the seat too. At least that what it says in the description. They call it a retainer though.

Edited by 2grimjim

2grimjim, & Grayracer513,

Thank you guys for your info you put out here! :bonk::lol:

Now, 2grimjim, while I think I understand what you did, I'm afraid this thick skull of mine still doesn't quite understand how you did it.......

You wanted the HSC on your shock to be slightly "softer", or flow more. I think I got that. The lighter spring & seat seems to me as if it would do that. Did you do that in conjunction with another drilled out orfice plate you had, or the WR plate, or a stocker with the lighter spring seat?

You gave a very good verbal of your R&D process, but I can't figure out what combo of parts you settled with? :lol:

Gray, are the ports in the piston that get drilled out just slightly visible in the pic that 2grimjim gave? Can I assume that drilling out the holes in the piston are done instead of using the lighter spring, and the different or altered orfice plate? Or is there a combination of these things that would work well?

Sorry for my not quite understanding here..... I actually have worked on KTM's early PDS/double piston shocks a few years ago, thought I had shocks understood to a small degree, and kinda thought KYBs would be a snap, comparatvely speaking....

Jimmie

You wanted the HSC on your shock to be slightly "softer", or flow more.

The initial purpose was to reduce the ammount of high-speed compression damping, i.e. resitance to sharp impacts.

Did you do that in conjunction with another drilled out orfice plate you had, or the WR plate, or a stocker with the lighter spring seat?

I ended up doing three things; opened up the 6 holes from 2mm to 3.8mm; removing .04" from the nd of the shaft with the o-ring (to get another turn of adjustment); and modified the spring seat with a lip that contacts the outer most periphery of the shim stack instead of the clamp shim.

You gave a very good verbal of your R&D process, but I can't figure out what combo of parts you settled with?

You could duplicate this mod by purchasing the SDI spring/washer and drilling your stock valve base. trimming the end of the shaft was only done so I could get more adjustment but it isn't really necessary.

As far as drilling the holes in the valve base (orifice plate?) you might want to start out with something closer to 2.5mm and work your way up from there. The 3.8mm that I used may have been to much but the added adjustment seems to have made up for it.

The complete compression adjuster assembly is available from SDI too if you don't want to whittle on your stock iece.

Thank You, 2grimjim!!! :bonk:

That helped a lot! Mucho appreciated-o!

One other hopefully quick thing. You say that you can buy the HSC adjuster assy. from SDI. I'd imagine that'd be at some sorta OEM spec, and not modified? :lol:

Jimmie

Gray, are the ports in the piston that get drilled out just slightly visible in the pic that 2grimjim gave? Can I assume that drilling out the holes in the piston are done instead of using the lighter spring, and the different or altered orfice plate? Or is there a combination of these things that would work well?

The exchange piston is what Jim is calling an orifice plate, shown in the first picture. The drilling to 6x2.72mm holes takes older YZ pistons to the same spec as used in the '08-'09 YZ shock.

Don't miss the point that the exchange stack (also called the high speed compression stack) in mine is much lighter than stock. The use of .20 thick shims in place of .30's makes the stack less than a third as stiff as the stock set up. Also, the 9x.20 under the face shim creates a bleed. That creates a path for a certain amount of oil to flow without deflecting the shim stack, the same as opening the main compression adjuster does, but it's more specifically focused on the action of the exchange stack. This oil flow can accurately be thought of as a "lead-in", reducing the amount of pressure that needs to be created before the stack bends.

In a sense, both mods do a similar thing, but in a somewhat different way. As oil flow through the exchange circuit increases, the oil first flows only through the bleed created by the adjuster, and in this case, the bleed shim. Once it starts moving too fast to pass through these pathways without raising the pressure further, the valve stack first bends (or deflects, more correctly) to allow oil past the ports, then, as the flow rate continues to rise, the spring begins to compress and allow the piston to back away from the incoming oil, creating space in the bore to act as a buffer until the oil can flow through the piston. By lightening the stack, one can control the pressure curve with a degree of accuracy not completely possible by simply lightening the blow-off spring. The curve can be controlled to the same level, but the shape of the curve at its base (when pressure builds from low to high) can only be affected by working with the stack.

It would be interesting to see what the effect of the lighter spring in conjunction with the reduced stack with the bleed shim added would be, but it might be too much, too. Depends on how you use the bike. This mod costs about $6 for the shims.

Edited by grayracer513
details, details...

Thank You, 2grimjim!!! :bonk:

That helped a lot! Mucho appreciated-o!

One other hopefully quick thing. You say that you can buy the HSC adjuster assy. from SDI. I'd imagine that'd be at some sorta OEM spec, and not modified? :lol:

Jimmie

Correct, They only list one part numper for all YZ's from '06-'09. Couldn't tell you if it has 2mm or 2.7mm holes. I suspect that they are probably manufactured to the later spec and have the larger holes. This same part should work with '05 and earlier YZ's (maybe back to '96?). The only issue that I'm aware of is the earlier compression valve is 2mm longer and requires making a small spacer for the newer valve to fit the '04 and earlier shock body.


The cool thing about making mods to the compression adjuster is you can remove and replace it without pulling the shock from the bike. Just lay the bike over at about a 45 degree angle on a stand, empty the N2 bladder, and unscrew the adjuster. If you have the bike at the right angle, you wont introduce any air into the shock.


A word of caution for anyone not familiar with working on shocks: Even with the gas bladder dumped out, there can still be significant pressure in the shock, usually the result of gases released from the fluid, or intruding from other sources. The nitrogen pressure normally keeps this gas dissolved in the fluid, but when you let the gas pressure off, the gas expands and releases itself. If the shock has been recently serviced, there may not be much of this, but if it's been together any length of time, it's virtually a certainty there will be gas in the fluid, and residual pressure in the shock, so wear eye protection, cover the adjuster with a rag as you unscrew it, and keep your mouth closed. :smirk:

Also, if you chose to leave the shock on the bike, be sure that there is not enough weight on the rear wheel to cause the shock to compress at all, or fluid will be lost when it's opened. This will require the removal of the shock for proper bleeding.

Thanks to both of you guys for the good explains here! I'm truly grateful for that.

This is definitely something I need to try. And I like the fact that both of y'all essentially accomplished the same thing, two different ways.

Thanks again, Guys!

Jimmie

Please excuse me for continuing to add details to this in a piecemeal manner, but there are a couple of other things to keep in mind regardless of where the shock is while working on the compression adjuster:

The O-ring seal on the piston is very easily damaged during reassembly, and it's prudent to have a spare on hand. Keep the seal in a freezer for a while before assembling it and things will go better. I grab the valve stem by the flats with a small pair of "channel locks" and give it a gentle, circular rocking motion while bearing in on it.

The other thing is, to exclude as much air as possible on assembly, I run the adjuster nut out of the housing, set it in place on the valve stem, then put the housing down over it, adding the last bit of oil as this happens. The method is described in detail in This Document by Dave Johnson. Read from step #20 on page 12.

Please excuse me for continuing to add details to this in a piecemeal manner, but there are a couple of other things to keep in mind regardless of where the shock is while working on the compression adjuster:

The O-ring seal on the piston is very easily damaged during reassembly, and it's prudent to have a spare on hand. Keep the seal in a freezer for a while before assembling it and things will go better. I grab the valve stem by the flats with a small pair of "channel locks" and give it a gentle, circular rocking motion while bearing in on it.

The other thing is, to exclude as much air as possible on assembly, I run the adjuster nut out of the housing, set it in place on the valve stem, then put the housing down over it, adding the last bit of oil as this happens. The method is described in detail in This Document by Dave Johnson. Read from step #20 on page 12.

Grayracer513, I don't think you should apologize for that info, Mah Man! Stuff like that helps tremendously. See, again, my only shock experience has been on KTM PDS stuff. The o-rings on those HS compression adjusters aren't fragile at all. I wouldn't have thought to come up with a spare o-ring for that until after I'd torn the old one..... :lol::bonk::lol::lol:

Again, I appreciate your help. If there's anymore little nuggets of info like that ya think of, it's appreciated, from all here.

Thanks,

Jimmie

Edited by Mr. Neutron

Sorry to dredge up an old thread...

Re the spring seat for the cadj spring, and altering if it applies the spring pressure to the clamp shim vs to the face shim. What is the theory behind this?

I can understand that aplying spring pressure to the back of the face shim would be a good way of having an adjustable amount of spring pressure that influences the entire valve stack. I've read the argument that the factory method of pressure against the clamp (the smallest shim) is intended to alter the effective size of the clamp. When there's no spring pressure, the effective clamp is the small diameter post machined into the stem. When there is spring pressure, the effective clamp is the larger diameter of the smallest shim, making the stack stiffer.

The Restacker page argues that all shims in a stack (at least a conventional non-crossover stack) flex as one unit, not progressively from the face shim down. If that's accurate, wouldn't spring pressure on the clamp be transmitted through the whole stack, and so effectively be the same thing as pressure on the face shim?

The restackor page is fundamentally correct, if not absolutely.  The first initial lift will flex only the face shim(s) at their outermost edges, and the rest of the stack will begin to follow this pattern as they are impinged upon.  That sounds contradictory to the Restackor statement, but the reality is that such independent flexing of the face shim group only takes place within the first 2-3% of total shim stack flex at most, depending on stack configuration, even with a single face shim, so it can be disregarded as significant.  An exaggerated example would be a three shim stack with a 30mm face, followed by a 15, then clamped with a 10.  The edge of the 30 will actually lift a little without disturbing the 15, but it won't be very much, even with that setup, and you won't see very much of that sort of thing being used.

 

The pictures here show the layout of the KYB HS adjuster and HS comp valve.  Notice that the piston does not move at any time; it's clamped in place by the sleeve and the adjuster housing. (any time you see a piston or other component with an O-ring as a seal, assume it doesn't move in operation)  Not shown in this illustration is the fact that at rest, a gap exists between the face shims and the piston, which creates a bleed that allows very low speed fluid flow without stack deflection.  As stroke speed increases, the stack begins flexing.  As it increases further, the force on the face shims starts to significantly deflect the two 11 mm shims, which are the effective clamp shims, since the 9 under them never applies force to the spring seat.  When the 11's begin to bend, they bear against the spring seat, which is free to move along the HSC valve stem.  The adjuster can be tightened or loosened to alter the pressure that resists the deflection of the 11 mm shims, which is how the HSC is adjusted.

 

Note also that the concept of applying spring pressure to the back of the face shim does not apply to this setup, as the spring pressure is borne by the two 11's, the effective "clamp", and not the face or other larger shims. 

 

highspeedUnitLS-HS-1_zpsd225380f.jpg

This is a good thread, I need to get my shock figured out ASAP before it throws me off the bike!

- Gray, I was planning on just copying your setup listed at the front of this thread for desert/woods singletrack, but I weigh 150 pounds. As for the rebound, should it still work? And I'm a little confused by your specs, what do the shims in bold mean? Added shims?

I have an '07 with stock settings, so maybe my shims differ from your '06. I think I can manage to figure out the HSC, but my concern was with the rebound and if I just stiffen it via more face shims. Need it to behave similar to the others on here, i.e. absorb square hits and rocks and fix rebound. Mine seems to kick up unless I go to about 8 clicks out, then it starts packing on large stuff. No happy medium.

Thank you.

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