Reworking Sachs shock ...

So I am reworking my stock suspension.  Partly from being thrifty, partly because I don't think it is sooooo horrible that it needs total replacement, and partly because it seems interesting to try this.

I am far from being a "suspension person", but I did grow up racing dirt bikes, I've pulled forks apart and added emulators, pulled a Suzuki shock apart before, rebuilt a KTM PDS shock ... so some experience but not much really.  I have spent a decent amount of time setting up my dirt bike suspensions (clicker settings, spring swaps, etc).

So my theory is the Sachs shock isn't that bad a shock.  It doesn't deserve to be tossed on a dusty shelf in the garage to live out the rest of it's life.      And I don't really want to spent $1200-$1700 for a cool new one.

Due to my lack of experience I didn't want my fairly new GRiSO to be out of commission while I putzed around.  I was very fortunate that woodrow53 had just such a shock gathering dust and he shipped it out for me to hack away at, I mean, to disassemble and revalve.  

Here is the victim:

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The little red circle.  My first "hummm".  I am used to having a schrader valve (or similar) on the bottom most part of the reservoir.  This one only has this little bolt and it doesn't make sense to me (yet) how it works.  Even if I replace it with a schrader, wouldn't the nitrogen need to go into the bottom of the reservoir?  Anyway, something to worry about later.

The red line points to the dust cover cap that needs to come off after the spring.

So in the dirt bike world, you loosen the spring preload, slide out the retention collar(s), take the spring off, easy peasy.  Hummm.  That doesn't work here.  Even with the preload rings spun full loose there is still a bit of tension on the spring and no way to get the collars off.

Ok, google shock spring compressor.  Crap, $100 tool.  But wait, youtube video of a guy using a ratchet tie down strap.  I have that!  

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Ok, spring off.

Cleaned things up a bit.

Now at this point I would want to use the schrader valve to release the nitrogen from the shock.  But, don't have one.  Just that little allen screw.  Hum, a screw with nitrogen pressure and oil behind it.  What could possibly go wrong with that!    

Off to google I go.  No great info.  Saw some crazy youtube videos of guys drilling into a shock body!  So if they can drill into it, I guess I can remove that screw.

I get 2-3 shop rags, working in an oil drain tub, fit the allen wrench between the rags, slowly loosen the screw, a bit worried/scared ... it was a tiny pissssss and a tablespoon of oil.  Whew.  I pumped the shock and very nice looking blue oil comes out.  Nothing bad in the oil, looked really good actually.  Shock had ~5000 miles on it.

Next step is to remove the seal/dust cover.  Use a small punch & hammer and lightly tap on one side and then the other to remove it.  The metal is very soft so need to be a bit careful here.  

Now you push the shock innards down a bit, that exposes a clip in a grove on the shock body.  Oh boy.  On a dirt bike shock, you have plenty of room to get into that clip.  On this shock, wow ...

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So I need to get into that little opening and get the clip out of the grove.  It isn't a snap ring with the loopy bits on the end.  

It is more like this:
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Feels like I need a dentist to help!  

Ok.  It took me about 15 attempts and 2 trips to stores for tools to get this.  

The final tool winner was:

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The one on the far right.

And you do need to be careful.  An o-ring and your shock piston need to slide out of here ... so if you mangle the shock body up you could possibly damage those when removing it.

So clip removed, cleaned up inside that tiny space as best as possible, and pull the shock apart!

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The bolt on the Sachs shock is not peened.  That is kind of rare.  But very nice and easy to remove.

And then ... shims ... the valving part of our shocks.  

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Wow, nice progress...

That's great, I was afraid to tackle mine.

Kiwi_Roy wrote:

That's great, I was afraid to tackle mine.

Me too! That is where woodrow53 came in and provided a shock!

You're 5/8ths of the way there.
3 main issues with the Sachs

1. Spring is too light for the majority of riders.
2. Hi-Speed compression is too hard, hydraulic locks as it cannot flow enough oil.
3. The compression adjusting needle (predominantly Lo-Speed) has a very blunt end.

Here is the piston that the oil flows thru. I don't know much about fluid dynamics, so I'll say "Looks fine to me"

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Here is the compression stack ...

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Measurements:  (ID for all shims is 12mm)

3  38 x .15
   36 x .2
   34 x .2
   30 x .2
   26 x .2
   22 x .2
   18 x .2
   15 x .5

Here is the rebound stack:

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Measurements:

18 x .5
22 x .3
24 x .3
26 x .3
28 x .3
30 x .3
32 x .3
34 x .3
36 x .3
38 x .3

ghezzi wrote:

You're 5/8ths of the way there.
3 main issues with the Sachs

1. Spring is too light for the majority of riders.
2. Hi-Speed compression is too hard, hydraulic locks as it cannot flow enough oil.
3. The compression adjusting needle (predominantly Lo-Speed) has a very blunt end.

Any suggestions on shim changes for #2? That is my main reason for even messing with this.

So now that I know what is in it, I've got some reading and studying to do to figure out how I might want to change it.

From reading I've heard guys say the shock has too much rebound damping and seems most run that clicker pretty far out.  Some have said not enough low speed comp damping and too much high speed damping.  

Also I need to read up a bit on springs.  I know about the space limitations (a higher weight spring is thicker, so some are so big they can't fit in the space), but I weight 155 lbs (70kg).  I did find a local suspension guy that knew a bit about these ... he suggested I go up in spring weight even though I'm kind of light and my sag numbers were good. I've read the stock spring is a .82kn rating, but not sure, and I don't have a tool to check that. I'll see if there is anything written or stamped into the spring.

So things are covered up in the garage and it is research time.

Any suggestions, or pointers to info greatly appreciated!

Great work ! Thanks for documenting it so well !

I can only repeat what my suspension guru said. After modifying a Sachs I gave it away as a bad joke and went the full house "bells n whistles" of a Matris R.

I never discussed individual shim stacks and sizes with Scott. However, the new owner of the Sachs considered it a vast improvement. For me, having previously owned a well sorted Ducati, my expectations may have been on the dark side of the moon.

I've even had the Matris rebuilt twice, to get the exact ride quality I was seeking, then I added plastic wheels, just for good measure.

Yes, I've had a Ducati with Ohlins suspension, spoiled for life now I guess. It controlled chassis movements so well, yet if you hit a section of ripply pavement it just floated over it.

On my KTM 990 SMT, I can set it up comfy, floaty ... but then if you pick up the pace the bike is bobbing all over the place. If you work out the bobbing and dramatic chassis movement (at fast-ish pace), then the suspension is harsh on ripples and stuff. But that's another project.

On my GRiSO, if I had a high speed compression clicker for the front and rear, and could dial that out, I think I'd be happy. But no, I gotta go swap some shims.

Can someone explain how the shim stack works, why do they get progressively smaller?

Here is a simple explanation (these are mountain bike guys, but same principles):
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Here is a TON of info on shim stack concepts ...
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Here is a write up from Ohlins:
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For the people that don't like to click links, key points from the first link:

"The common answer to this question is that the tapered shim stack adds a more progressive feel as they flex. This is not true. Both tapered and straight stacks are linear, meaning they gradually increase in stiffness as they are loaded"

"Tapered shim stacks also prevent permanent distortion of the shims. When the shims on a tapered stack flex, they all bend at multiple points spreading out the stress. Tapered stacks allow more clearance before they come in contact with the base plate. Shims on a straight stack all flex the same amount and bend on the clamping shim. The stress on the shims are concentrated at the bending point and the chance of permanent distortion is increased."

With the other links, it starts to sound like launching a space probe might be easier than getting my valving right.  

For the math folks out there ...

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Yes, yes, I see.  

So should I replace some .2 shims for .15 shims?

So certainly it comes down to some amount of guess work, trial and error.  

One thing that really complicates things is the piston and the holes for oil passage, both size and shape.  So if you find a shim stack example for a Ohlin shock, well that exact same shim stack with a stock Sachs piston will work differently since the holes & ports in the piston are different.  

And when people talk about the "low speed stack" and the "high speed stack", it is more of a concept vs. something like "after shim #4 you are into the high speed stack".

So you hit a bump, shock piston moves thru the oil, oil flows thru the piston, shim stack bends back to let the oil thru.  

Theory is at slow shock shaft movement speed (low speed comp) only those first shims react to that.  So in this case, the 38mm dia shims bend back.  I guess the 36mm and maybe the 34mm bend a bit.  Maybe the 30mm bends some.  Not sure.  But if you wanted a firmer reaction to this type of situation, you could stack on more of the 38mm shims, or use .2 thickness shims.  

If you have a large, hard, sudden hit, so the shock shaft moves rapidly, so high speed comp time, then theoretically the 38mm shims bend quickly (don't play a big part in it) and the smaller diameter shims play more of a role.  But ... if you stacked on 10 38x.3 shims (a dumb idea) then that would not open quickly and you'd get a worse jolt than our stock bikes give us.  So that dumb stack would mess up the theory ... high speed comp would be handled (prevented?) by that first set of large diameter shims.  

So another dumb example ... let's say I decided to pull out all 3 of the 38 x .15 shims and put in 1 38 x .1 shim ... more oil flow baby!  Then the low speed comp would be almost non existent, any bump would flow a ton of oil,  but you basically bottom on everything.  

Another fun thing to toss into this, the shock spring.  So you put on a stiffer shock spring.  If you did not change the valving, then that would make the rebound faster (stronger spring pushing the shock back) and the compression firmer (more force to compress the stiffer spring).

And people are complaining that the stock rebound is too slow (stiffer spring helps this) and the slow speed comp is too light (stiffer spring helps this).  

So is it possible to add a stiffer spring, and then lighten up the High Speed Comp and call it done?  That is my current thinking and theory.  

I'm thinking to swap out some of the .2 comp shims with .15 shims, somewhere in the 18mm to 34mm range.  

But another thing to be careful about, the shims need room to bend back ... if you make the total stack too tight, then the shims will hit the big washer behind all this and not have room to bend.

Like the picture on the right most side here:

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Have you considered buying a Race Tech gold valve to replace the piston in the Sachs, then it may also be possible to use a recommended shim stack by Race Tech.

ghezzi wrote:

Have you considered buying a Race Tech gold valve to replace the piston in the Sachs, then it may also be possible to use a recommended shim stack by Race Tech.

Race Tech does not list anything for the shock. Dan Kyle (super experienced Ohlins tuner) is about 1.5 hours away from me. I am a bit tempted to go see him ... see if there is an Ohlins kit for this. These Sachs shocks are similar to some that came on Aprilias and Ducatis. Note: there is no official Ohlins kit for our shocks ... but just thinking ... show him the piston and stacks and he might say "try this!".

But ... first approach of this plan was to be thrifty. So oil, shims, nitrogen recharge, like $50 usd + a spring.

I figure the GRiSO guys with money and willingness to spend it are well covered ... there are ~8 shock options for our bikes! That's pretty cool. But they range from $800 - $1800.

Approach 1 - self revalve, $50 ish + spring
Approach 2 - suspension tuner revalve, $250-$300 + spring
Approach 3 - self install a kit, $300 - $400 + spring
Approach 4 - suspension tuner installs a kit, $500-$550 + spring

I'm still good with approach 1. Even if this turns out to be a failure, I can then go to Approach 2, and I only blew ~$50.

But ... I am tempted to call Dan up ...