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749/999 Fork Internals

8K views 45 replies 8 participants last post by  xracer 
#1 ·
I modified a set of Monster S4 Showa forks to use the fork cartridge and valving from some Showa SBK forks (I was told they were from a 999 - they are 53/53 at the triples and have TiN sliders and have GD161 cast into the lower stanchion). In the original S4 fork the spring sits at the bottom of the fork tube around the cartridge with a spacer tube on top. In the SBK forks the spring is much shorter and there is a washer that sits on the top of the cartridge that the spring sits on and then there is a short spacer tube above. When I look at the parts book for the 999 forks, the spring looks like it should be at the bottom around the cartridge. Can anyone tell me what the correct configuration is?
 
#3 ·
Does the spring sit down in the bottom around the cartridge or does it sit above the cartridge on a washer? The SBK fork I have looks different than this image. #7 is the entire compression valve with the adjuster and it is inserted through the lower fork stanchion and threads into the bottom of the damper tube. The damper tube does not have a ring at the bottom like is shown and the spring sits on a thick aluminum washer that is placed over the damper rod and sits on the top of the cartridge. Then there is a short preload spacer with a black plastic washer at the lower end that sits on the top of the spring and extends a little bit into the spring and into the spacer tube to hold it in position. At the top of the spacer tube is a white washer that extends a little bit into the preload tube and on top of that a small metal washer that the preload adjuster pushes on.
 
#9 ·
999 cartridge has the compression piston as part of the adjuster that goes in from the axle recess, and the cartridge tube screws onto that. in the 996 and most other (older) showa the compression piston slips into the bottom of the cartridge and is held in by a round circlip and a couple of dimple peens.

apart from that i think yes, the same. i have one here i can split if you really need a photo of it.
 
#7 ·
i'd say it really doesn't matter, as long as the cartridge tube can support the load and the spring is preloaded correctly.

but from memory - did this a couple of weeks ago - the 999 fork is like the other sbk forks in stacking, in that it has a steel tube maybe 100mm long under a washer under the spring, then the steel tube with plastic ends on top of the spring.

someone may have been in there before you too.
 
#10 ·
When I get time I'll take some pictures of what I have and post them up for comment. I don't think it really matters either way how the spring is installed as long as nothing is rubbing and the fork is setup properly. What I did was have the lower stanchions from the S4 forks machined out to fit the compression adjuster from the 999 fork and I installed the 999 cartridge into the S4 fork tubes. I reassembled everything using the S4 spring and preload spacer and everything seems to fit ok but there was less preload on the spring because the 999 cartridge has more extension. I reinstalled the forks on my bike (a 2007 GT1000) and everything seems ok except I am getting a metallic clank from the forks on extension (like when dropping off the backside of a bump) and I was wondering if the spring is hitting the cartridge.
 
#12 ·
the showa forks will often make a single sharp noise when they start to rebound. i walked around the workshop one day at my previous employer and pulled up on every sbk showa fork bike i could find to illustrate it to a customer. never found a cause of it, and once they're apart they don't seem to do it.
 
#13 ·
Hello,
older SBK forks have steel cartridges and the compression adjuster nut from below in the stanchion, as in the picture above. 999 onwards has the complete stack / adjuster nut, that also holds the cartridge in place.
If you intend to take the compression piston off (to clean / change shims etc), be extremely careful when loosening the nut, as the alu tower is extremely brittle and breaks very easily.

While you have the forks apart, I´d trow away the hydraulic stop on the cartridge rod; it will give you a few mm of working travel.

It does not matter where the spring sits, I usually make a new bottom spacer from alu tubing if I need to adjust the total length of the package, and use the original top spacer tube.

Kind regards,
 
#15 ·
Hello,
older SBK forks have steel cartridges and the compression adjuster nut from below in the stanchion, as in the picture above. 999 onwards has the complete stack / adjuster nut, that also holds the cartridge in place.
If you intend to take the compression piston off (to clean / change shims etc), be extremely careful when loosening the nut, as the alu tower is extremely brittle and breaks very easily.

While you have the forks apart, I´d trow away the hydraulic stop on the cartridge rod; it will give you a few mm of working travel.

It does not matter where the spring sits, I usually make a new bottom spacer from alu tubing if I need to adjust the total length of the package, and use the original top spacer tube.

Kind regards,
torbjorn, do you put the aluminium tube in the bottom and a washer on top of it then the spring? what thickness aluminium tube?
 
#14 ·
Althought not directly linked to the original post, I'll post my question here since the title matches. I was under imperssion that 749S and the 749 base model (2004) have identical front forks but then I read from some old Ducati review they don't have. I would need some spare parts for my base model forks and I was planning to buy 749S fork for a donor.
 
#16 ·
I don´t have any of it here, but if memory serves, it´s 38 mm outside Ø, 2 mm wall thickness. I´ll check next time I´m in the workshop and correct if necessary.

I crossdrill them at the bottom to make sue there´s no restriction to oil flow; don´t know if it really makes any difference, but id I do, it does not matter, so to speak.
I also crossdrill towards the top, so I can fish them out with a piece of welding rod or similiar, without draining the oil.

I put them in the bottom, use the stock support washer between the spring and the tube, and the stock pre-load tube on top.

I once tried smaller Ø tubing (approx 31 mm, with a 39 mm "foot" towards the spring. Not good, since it allowed the spring to wiggle around, rubbing against the cartridge.

2020-05-08 10.20.52 by torbjörn bergström, on Flickr
 
#17 ·
Here Is a picture of what the original fork configuration was

985529


and her is how I configured it in the S4 fork

985530


I’m assuming from the comments here that the original configuration isn’t the standard configuration for a 999. Any ideas of what bike it might be from? I assembled the fork with spring and spacer from the S4 fork and since the 999 cartridge extends farther there is almost no preload on the spring. Should I cut a longer spacer so there is 10mm of preload on the spring in the assembled fork?
 
#18 ·
Also, what is the correct torque value for the compression valve bolt? I found a service manual for the S4 forks that shows 30-40Nm but that is for the style where the adjuster bolt is bolted into the compression adjuster. It seems excessive for the 999 style where the compression valve is bolted into the cartridge tube.
 
#20 ·
Thanks for the info. I think I may have found the source of the noise. The SBK cartridge has about 25mm more extension than the S4 cartridge and when I assembled the fork using the spring and preload spacer from the S4 fork, there was still about 12mm of space above the preload washer and the fork cap when the fork was at full extension so when the fork would extend, the spring and preload tube could bounce inside the fork and I think that is what I was hearing. I bought some tubing today to make some longer preload spacers to match the SBK setup so hopefully that fixes the issue.
 
#21 ·
I put the longer preload spacers into the forks and the noise is gone so it must have been the spring bouncing in the fork.

The forks had felt a bit harsh also so while I had them off the bike I changed the fork oil to Motorex Racing 5W (22.60 cSt) from Motul Factory Line Medium 10W (36 cSt) and the forks just soak up bumps now. I would have never figured that the oil viscosity would make such a difference. They’re like a whole different set of forks.
 
#23 ·
fuck me, 10 weight in those forks is just nuts. they'd be crappola grande. i usually use maxima 5 which is 16cst.

i had a set of ohlins from a 998s apart once that had been played with and one spring had no preload on it - forget how it actually was now. but the forks were full of aluminium slivers, and the oil was silver.
 
#24 · (Edited)
I had initially used the Motul 10W as the viscosity was similar to the Showa SS8 (36.8 cSt) that is specified as the correct oil for the forks. I was thinking that lighter oil than the Motorex 5W might be even better so it’s interesting to see your comment about that. I initially thought that the lighter oil might be better because I’m using a 0.95kg/mm spring which is a little softer than the standard SBK spring but after doing some reading here in the forums I found that the harshness is a common complaint with the SBK forks and lighter oil is often used to combat this.
 
#29 ·
This is a very difficult topic, and you have to remember that any fork (or shock) will have a certain amount of static friction that will have to be overcome before any movement happens.
With that said, assuming you have the stoc Showa pistons, there is n need to invest in an other fancy parts:

P4250083 by torbjörn bergström, on Flickr

This is the stock Showa piston, and a Gold Valve. The GW looks fancier, but I don´t think anyone would be able to tell the difference ..... so if this is what you have, it´s perfectly OK.

For road use, I like to use a cross-over shim; if you list the shim set-up you have, I could propose an alternative.

Finally, if you want to learn, I think that Racetechs "Suspension bible ....." book is a good introduction, as long as you remember the suspension world does not end north of Recatech :)
 
#30 ·
This is a very difficult topic, and you have to remember that any fork (or shock) will have a certain amount of static friction that will have to be overcome before any movement happens.
With that said, assuming you have the stoc Showa pistons, there is n need to invest in an other fancy parts:

P4250083 by torbjörn bergström, on Flickr

This is the stock Showa piston, and a Gold Valve. The GW looks fancier, but I don´t think anyone would be able to tell the difference ..... so if this is what you have, it´s perfectly OK.

For road use, I like to use a cross-over shim; if you list the shim set-up you have, I could propose an alternative.

Finally, if you want to learn, I think that Racetechs "Suspension bible ....." book is a good introduction, as long as you remember the suspension world does not end north of Recatech :)
The valves looked stock when I had the forks apart before but I guess I have to open them up again and determine what the current shim setup is. Any tips on removing the bolt that holds the shim stack? Earlier you had noted to be careful with it as it breaks easily. Will I need to use some heat (from a heat gun) or is that a bad idea?
 
#32 ·
I had no idea you were Brad the Bike Boy. I have read all of your fork posts on your blog and they have been very helpful. Thank you for putting your work out there.

I made a very similar tool for holding the cartridge but mine is not nearly as pretty as yours is. I used a hand held grinder and a piece of old galvanized gas pipe I had in my toolbox that I used to use as a snipe. I should add a centering ring similar to what you did to make the tool easier to use as currently I have to be very careful that it doesn’t slip off the end of the cartridge.

Any tips on removing the nut that holds the valve and shim stack in place? Do I need to use some heat? I don’t want to break the valve body or do any damage
the valve or shims.
 
#33 ·
Well, me neither :)

I have not had too many of these, but when I do, I heat it enough to soften the (potential) Loctite, then feel gently if the nut will loosen. If it does not, heat a little bit more, and feel again ... and when you´re all done, putting it back together, don´t give it that little extra twist on the wrench "just to make sure" ....

I have not broken one yet, so I have not had to try, but if you break it, I think it is possible to take away the whole alu post, and replace it with a drilled-out screw. A bit of work, but I think it would be possible.

I also think it is fun to make tools, and I have some that I don´t know what they are for anymore. So I´ve I got myself a Dremel engraving tool, making sure I mark any tool I make before I forget what it´s for. More urgent the older I get .... but I prefer that to the alternatives :)
 
#35 ·
Good morning,

The 0.4 x 11 are just washers, partly to adjust the hieght of the stack (?), partly to provide a hard base tor the stack against the alu valve holder (?). I´m not sure, really.
The 0.2c10´s are the clamp shim, which dtermines the solid "post" of the stack and has a huge influene o the stiffness of the stack. Most Showas with a 10 mm cartridge rod uses a 9 mm clamp shim, either 2x0.2 or 1x0.3 mm.
The 11 pcs. of 0.15x17 mm shims are pretty normal fora Showa, maybe on the stiff side.

I am in no way an expert on this, and most of my experimenting has been on my Öhlins forks since they are much easier to work on (cartridge go out the top with the forks on the bike, and (most) of the oil remain in the forks).

For what it´s worth, I would try the following (if you want it softer):
1) 9 mm clamp. This will make the whole set-up softer, but will not really changfe the nature of the shim stack;
2) Try a cross-over shim below the first 0.15x17 (aka face shim), 0.10 mm thick and maybe 11 or 12 mm diameter. This will allow the face shim to bend slightly before the rest of the stack comes into play and will provide some softer initial action. It is possible to play with the position and diameter of the cross-over shim. If you should want to try a thicker cross-over shim, use several (I would say max 2, actually), since 2x0.10 is softer than 1x 0.15, and much softer than a 0.20 which would add to the general stiffness of the stack.
3) Remove a 0.15x17 or two. I´m not sure you would notice much difference unless you remove several, and that might then be too much ..... don´t know, really.
4) As a guide for option 3), use the Raceteh system (as discussed earlier, I think), to determine how many 0.15x17 shims you should use on top of the smaller shims in their tapered stack. The Showa big-port piston is similiar enough to the Gold Valve for this to be a reasonably good guide.

Be very careful with any Loctite if you use that on the valve holder nut, and clean the threads thoroughly of any old Loctite .... :)

Kind regards,,
Torbjörn.
 
#36 ·
good that you're playing with it, will be interesting to see how you find it. torbjorn is quite experienced at this.

158mm is a lot of travel. are these meant to have a plastic travel limiter or the like that has been removed. i would have expected 125mm or so.

i find the centring ring on a tool like the one i made usually makes a massive difference to the overall strength and success of the tool. ie, nut fucking shit.
 
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