Reworking forks ...

Been reading a lot, and riding some , and it seems a general consensus is the GRiSO forks are under sprung and over damped.

Fortunately for me, I'm 155lbs (70kg) so the springs actually seem about right.

I've got the compression all the way out (full open) and this works well on everything except sharp impacts (so a high speed compression type event). Unfortunately the roads in my area are pretty beat up and so this happens a lot on rides.

Seems options are:

1 - revalve (different shim stack)
Pros: inexpensive, might do the trick
Cons: little info, probably end up doing a few revisions/iterations, will it be good enough

2 - piston/shim kit (like Ohlins, Race Tech, Penske)
Pros: still not that expensive, maybe some thought/engineering went into it, might do the trick
Cons: I'm not sure the stock pistons are so horrible, so why toss them away for a different ones, probably still need a few revisions, not super impressed with the kits (like Ohlins, they have one valving stack, so same for a 155lb guy vs a 225lb guy?, Race Tech/Penske have generic stack recommendations, not really well thought out GRiSO ones)

3 - full cartridge kit (like Matris)
Pros: supposedly well engineered/designed, work in one shot(maybe)
Cons: getting pricey at ~$1000 US (kit only price), would include springs that I don't really need, a lot of money to spend for better valving

4 - replace the fork with other (like Ohlins complete fork)
I'm not going this far

So ... Option 1 ... revalve.

From the awesome Uzidzit post, the stock compression stack is:

17 x .15
17 x .15
10 x .3
10 x .3

Wow. This is a short and odd stack.

Our forks are Showas that are similar to a 06/07 Suzuki GSXR 600/750. But there is no guarantee our pistons are the same or similar to the GSXR ones. For example, Honda also sold bikes with similar Showas on them ... but the Honda & Suzuki pistons are very different ... so you would expect the shim stacks to also be very different.

So not as easy as just finding and replicating a GSXR or Honda shim stack as those pistons might be different.

Anyone ever dug into this? Any suggestions for a different shim stack?

I would be looking for lighter compression (flow more fluid) and lighter high speed compression (better control on those sharp hits).

Here is an example GSXR shim stack, similar 20mm cartridge Showa fork:

3 x 17mm, 0.1mm
3 x 16mm, 0.1mm
2 x 15mm, 0.1mm
1 x 15mm, 0.15mm
1 x 14mm, 0.15mm
1 x 13mm, 0.15mm
2 x 9mm, 0.2mm

Lots of shims!

If you really want to go nuts, there is this: [You must be registered and logged in to see this link.]

But lots of debate that the Excel calculations don't match real world hitting bumps.

Also seems more dirt bike oriented.

Here's a good pic showing different pistons ...

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I too experienced the over dampening in the front forks, and also wound the compression dampening all the way out. Despite being about 24 kG heavier than you (I'm on a diet, honest!), I haven't managed to hit the end of fork travel in either direction, so not contemplating any spring revisions at the moment.

If you're finding the compression still a little harsh, why not experiment with a lighter oil in the forks? A lot cheaper experiment than the other options you've listed.

Well, shims are like 0.80 each.

I think I'm going option 1. All the Matris cartridge owners can laugh at me for the next couple of months. I am mentally prepared to pull my forks apart 3-4 times. 5th and 6th time will be like "Oh brother". 7th time? I might be giving up by then, not sure.

Unfortunately it seems most shim stack info on the internet is dirt bike related

If anyone has any shim stack suggestions I'd love to hear them. I'm thinking between the stock one and that GSXR one I posted.

As for fork oil change only ... that is way easier than re-shimming for sure.

Here is a decent video of the oil change process: [You must be registered and logged in to see this link.]
And yes that fork isn't the same as ours, but exact same concepts/steps.

But my thinking is ... fork oil weight change is not going to make up for a crappy shim stack. So that will be far far away from a piston/shim kit (Ohlins/RaceTech/Penske) and the full Matris Cartridge.

But a good shim stack in our stock forks (same pistons) I think will be very close to options 2-4. That's my theory.

Looking for the emoji that shows watching this with interest & eating popcorn

You mean this one?


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Yep !

Ok, this pic is from a mountain bike suspension company, but was a very easy to understand diagram.

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So things to play with ... number of shims, diameter of the shims, thickness of the shims.

As you would expect a shim that is 0.1mm thick is "softer" than a shim that is 0.15mm thick.

But what if you had 2 17mm (diameter) x 0.15mm thick shims vs. 3 17mm x 0.1mm shims? Both of these add up to 0.3mm thick. Seems the 3 0.1mm thick shims would be "softer" than 2 0.15mm thick shims. Just reading and learning here.

One not so intuitive thing ... a 17mm shim is "softer" than a 14mm shim for example.

Most info I'm finding is from dirt bikes (motorcycles) and mountain bikes (bicycles).

Ok, had an "aha moment".

One thing that was bugging me, why are the forks so harsh and the shim stack is so whimpy?  

I've been reading and looking into this work, and a comment from a guy on another forum explained it.

We have 2 17mm x .15 shims ... they should bend and flow a decent amount of oil.  
Behind them are 2 10 x .3 shims.
Behind that is the reset of the compression valve.

So these 17mm shims can only bend back .6 mm (the thickness of the 2 10x.3 shims) and then they hit the rest of the comp valve.

So moderate bump, fluids flow, 17mm shims bend, all is ok.  Smaller dips in the road, small amount of fluid flows, all ok.

Then a sharp bump comes along, lots of fluid needs to flow, the 17mm shims bend, but can only go .6mm and hit the "stop" so not enough fluid can flow, so the forks stops moving and sends a jolt up to the rider.  

Aha!  

So with some of these other shim stacks the shims can bend lots further than .6mm , so more fluid can flow, fork can work, jolts not sent to rider.

Ok, semi boring update ... ordering tools/parts.

Biggest set back ... the Becker Technik stand is going to take ~3 weeks to get to me.  

I ordered the shims from these guys:  https://www.suspensiondirect.com/shop/

Got this from Motion Pro: ***EDIT***DO NOT BUY THIS TOOL. IT WILL NOT WORK*****
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This is used to hold the Damper Rod when loosening the bolt on the bottom of the forks.  Some guys leave the fork together and hit that bolt with an air wrench, but that seems too violent to me.  I'd rather hold the damper rod properly.

Got this from Motion Pro:
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This is helpful when putting the fork back together ... you can more easily pull the rod up ... and you can use it to raise the rod up/down to get oil into the cartridge.  Probably easy to rig something up to do this, but this just looks so nice!  

Got this off ebay:
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This is used to push the spring pre-load tube down, exposing the a nut that allows you to remove the fork cap.  

So when the stand shows up I'll be ready to try this out!  

looking good, i will chip in with some info as you go :-)

So before changing any of the shim stack, i would look at different oil so that you have an understanding of what works with your stack, sort of a base to work from.

The other condideration is air gap, based upon the oil level. I was going to work on this next, as i don't know if the shim stack flows enough oil to actually use the compression of the air as the final bit of compression damping.

If you are going get an oil as a base then some work on oil levels to see if or what difference it makes would again give you a basis to work from.

There is software round that will help you with shim oil flow, as opposed to a big sreadsheet.

try HERE

Whew, finally my Becker Technik stand showed up.  About a month.

Ok, where was I, fork revalve ...   (I also started a Sachs shock rework thread, working on some "what to do" parts of that, so what the heck, switch back over to the fork)

I pretty confident the problem is what I described a few posts back ... our shim stack is so short (thin), on hard rapid bumps the shims start to bend and then hit the backer plate/washer and can't flow enough oil.  

I gotta go back to my notes ... I have the sample GSXR shim stack and Uzidzit's post on here ... but all three of these have different pistons so that makes it a bit tricky.  

I am sticking with stock springs.

Hopefully will dig into this in the next few days.

Ok, got to work on the forks - finally!  

First ... get bike on stand, jack up front so front wheel is ~1" off the ground

Next ... prep the fork - I opened the compression and rebound full open (not sure you have to).  I also wound the preload all the way out (most number of lines showing).  Then loosen the top cap.  I do this while the forks are still on the bike, top triple clamp bolt loosened, bottom 3 clamp bolts still tight.

Next - remove forks from bike (brake calipers, wheel, fender).  Pretty easy to do, bolts are all out in the open and easy to get to.

Then - loosen for cap all the way and you have:

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Then connect the $15 eBay tool and a ratcheting tie down strap, ratchet it down until you see the bolts:

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The GRiSO Service Manual goes into really good detail on this too.

There is a lock nut tightened up against the fork cap ... loosen than, then unscrew the red fork cap ... the long rebound rod comes out too.  Then a washer and a plastic spacer.  Then the long metal spacer (tube).

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This pic shows the spring out and a black zip tie I used measure fork oil level.

So now a few odd things.

First, the oil level was 85mm with the spring in (fork compressed all the way, measure from top of fork tube to top of oil). Most places I read and looked said the oil level should be between 100-110mm with the spring in. The GRiSO shop manual says 110mm with the spring out. So this seemed like a lot of oil was in my forks.

Second, the oil was really really thick as I poured it out. Like 30 weight thick. It was red and really coated everything well.

This was the left fork. When I did the right fork the oil color was like engine oil, clear to light brown and the weight seemed much lighter like maybe 10 weight or so.

I bought my GRiSO new and this is the first time the forks have been opened up. So ... why a different brand/type/weight oil for the left and right sides?

I wonder is Showa ships them with oil in them ... or if Moto Guzzi preps the forks with oil? Who knows.

One really cool thing about this work ... you do not have to pull the fork upper and lower tubes apart.  You could do that if you need to replace seals or check bushings ... but my bike is embarrassingly new.

So next step is to remove the bolt at the bottom of the fork that holds the damper rod in place.  I had bought the Motion Pro holder tool ... it is useless!!  Not long enough and not really designed right, waste of money.

So out with the impact wrench.

Oh ... you need a really long 8mm hex allen socket, like this: (this way longer than you need)

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Impact wrench made quick work of it.  Then pulled the damper rod out.  The bottom has a washer cap, remove that, push the compression valve up in the tube a bit, undo the clip ring ... and you have this:

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Put the bolt back into the compression valve and use that to help pull that unit out of the damper rod.

So now you have:

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To remove the allen bolt I used this soft jaw insert in my vice:
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All pulled apart:

This stuff is small!  

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Shims are: (all 6mm ID)
17mm dia x .15mm thick
17 x .15
13 x .15
8   x  .2
washer (11.3 x .4 - not a valving shim)

So my theory that our 17mm shims can't bend back far enough ... nope, I was wrong.  All kinds of room for them to bend back.

But this pic might show a source of our issue.  The right side is the top side of the compression valve/piston.  So when you hit a bump, oil comes rushing at this allen bolt (right to left in this pic), goes over that shim/washer, and thru the compression piston, then as the oil comes out of the compression piston it bends the shims.  

So you can modify the shim side of things (normally what is done during a "revalve") ... but I wonder if this "front side" is too restrictive.  Not letting enough oil thru the piston.  

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So I came up with this comp shim stack:
17 x .15
17 x .15
16 x .15
14 x .15
13 x .15
12 x .15
9   x .3

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I was thinking of re-doing the rebound stack too ... but then I thought:
- rebound has seemed ok for me
- I am not changing springs
- I am going to a lighter oil (5 wt) (and way lighter than the red stuff)
- I am going with less oil in the fork
- I am going with a different comp stack

So lots of changes in Round 1.  

So I decided to leave the rebound stack alone.

So that's it for pics.

I put those shims back on, put valve back in vice clamp, tightened bolt, valve back in bottom of damping rod, wire clip back in place, washer/end-cap back on damping rod, slide back in fork, tighten bolt at bottom of fork.

Then add oil to fork, pull the rod up and down 5-6 times, kind of slowly, gets the air out of things.

I went with a oil height of 105 mm with the springs in.  Between the 100 & 110 I had read about.

I should have took a pic of the motion pro damping rod tool, it worked great.  You screw that on the top of the rod, then put on that long spacer tube, then the motion pro tool lets you pull that rod up.

Then ratchet strap to get the spacer tube pushed down, put the fork cap back on and tighten the lock bolt.  Then slide the fork tube up and tight the fork cap on.

When I finished the left one, I did the "push it up/down on the floor" test ... and compared it to the right one (that wasn't done at that time).  Hum.  Seems rebound is faster and compression a wee bit easier, but not dramatic.  But this is the human fork pushing dyno vs. a real dyno.  

So after doing this, I'd say it really is easy.  It would probably take me ~20 minutes to do 1 fork tube from start to finish, and not really rushing.