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Discussion Starter · #1 · (Edited)
Hi

I have ridden a few gsx-r 750's and I've always liked the feel of the showa gsx-r forks. I think they are some of the best oem forks in any street bike (the öhlins equipped bikes not included). They never feel harsh and have enough firmness for sportier, even track riding.

I have installed some k-tech internals in my 900-ss forks but had a cocked thread in the stock damper rod and I haven't been able to get the k-tech valving to work exactly as i would like, so I thought I would like to try and transplant some gsx-r fork internals to my 900 superlight forks. The k-tech valving seems to have too much high-speed rebound and that would be possible to correct by removing a shim or two from the rebound stack, but because the cocked threads I don't feel like putting any more work to the 900-ss cartridges.

I gathered some information from the net and found the complete internals including fork caps for k4-5 gsx-r 600/750 on ebay quite cheap. They are made by Showa and should be quite high quality compared to the 900ss forks. They have a proper rebound circuit too.

The gsx-r internals are almost the same length as the 900-ss internals. The bottom bolt is exactly the same and so is the fork cap. The gsx-r has blue/gold anodized fork caps and I wanted to keep the stock SS look so I took the caps apart and found that the only difference in the fork cap parts is the rebound adjuster screw/rod and the middle part the damper rod screws in. The damper rod in the gsx-r cartridge is 12mm and 900ss is 10mm, so the middle part has to be taken from the gsx-r. The fork cap body and the preload adjuster nut of the 900-ss forks can be used in with the gsx-r cartridges. You can keep the aluminum-colored cap parts from the 900-ss and swap them to the gsx-r cartridges.

The gsx-r cartridge outer diameter is 24mm and the 900-ss 22.6mm. The 900-ss forks have a 1,3mm thick cup-shaped spacer below the cartridge bottom end that needs to be removed to fit the gsx-r cartridge.

The lengths of the cartridges from bottom to fork cap lip are

900-ss bottomed out 55,9cm, topped out 67,2cm
gsx-r k4 55,3 - 66,9cm

The bottomed out depth of the 900 fork from the bottom of the fork leg to the top of the top tube is 55,2mm. So the stock cartridge will bottom out before the fork leg does. Only if you remove the plastic bottom-out piece (it is removed in the pic below), will the fork leg bottom out before the cartridge. The gsx-r cartridge will bottom out shorter than the 900-ss cartridge (and 1mm before the fork leg) so it should not be an issue.

I assembled the fork legs today with gsx-r cartridges and they seem to fit ok. However, I forgot to take the gsx-r springs and spring spacers with me so I had to leave them out. So no test ride yet.

The whole swap can be done without removing the forks from the bike, only the front wheel needs to come off, so it's quite an easy job.

I'll report the riding impressions as soon as I can get the springs (I forgot them to another town so it will take a few days).

The fork length will be about 4mm shorter after this mod

a pic from the cartridges side by side (the fork caps are already swapped)


I think this could be a cheaper and easier alternative to swapping to sbk forks or re-valving the stock cartridges, if you can just find the gsx-r internals cheap. I think some other year gixxer fork internals would probably work too (k1-3 and srad 750) but I can't confirm it because I don't have the parts. If someone has them, please post the measurements.
 

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Evolution suspension did a similar conversion on my Multi with great results. Totally changed the character of the bike.
 

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Howdo!

Be interested to see how you get on :D

Totally agree with you, the K5 forks are a good piece of kit on the road, I've got loads of confidence in mine. If this feel can be transferred to the SS forks I would be impressed.

Best of luck Pablo
 

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Discussion Starter · #4 ·
I have the forks together. The gsx-r spring diameter is too big for the ss fork tubes (38mm and the fork tube internal dia is 37mm). The length of the 900-ss springs (I have race-tech 0.85kg/mm, the same rate as the gsx-r stock springs) is 340mm. They fit to the gsx-r cartridges but require only 10mm spacers to get 15mm preload. The hydraulic stop rings on the gsx-r cartridges will rub on the inside of the 900-ss springs, so I removed those (race-tech recommends removing them anyway when re-valving forks, so I dont believe it is an issue as long as you keep enough comp damping to prevent bottoming).

I had a test ride and I think the forks now feel a lot like the gsx-r. They seem to just swallow most of the bumps and still feel settled enough.

The modification really requires aftermarket aftermarket springs, the stock gsx-r springs are linear 0.85kg/mm but wont fit to the 900-ss forks. The stock 900-ss springs are about 0.6kg/mm (I measured it with a scale and progressive, the rate changes to about 0.9kg/mm at roughly 70mm of travel). I wouldn't try this conversion with the stock 900-ss springs.
 

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I was thinking about sticking the whole forks from a late GSXR in to my SS. I think they use the same diameter yoke so should slot in. Not sure about front wheel though.

Would be great if you could do a full picture report on the fork build, this sounds like a great cheap-ish upgrade
 

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Discussion Starter · #6 ·
I have now disassembled a pair of cartridges from K0-3 gsx-r 750 and they seem to be better built than the k4-5 cartridges.

pics here:

https://picasaweb.google.com/102766334876883285574/Gsxr750K2Forks#

The reason I did this was that I found out that there still is a design fault in the rebound circuit of the k4-5 forks, might be easily fixed, but if someone is planning this mod, the K0-3 are the cartridges to get.

reasons:
- the bleed circuit on them are flawless
- the bottom cup washer diameter is the same as on 900ss, so no need to leave it out

More on that later, when I have tried them on my forks.
 

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Discussion Starter · #7 · (Edited)
Well, the K2 internals went into my forks today and they fit without any modifications, as long as you have the fork caps from the gsx-r.

No spacers at all, just the metal washer over the 900ss spring gives 10mm preload.

If you want to keep stock looks, use the 900 ss fork caps and preload adjusters, but the center part the damper rod screws into must come from the gsx-r because the damper rod diameter is 12 versus 10 mm. You can use the 900 ss rebound adjuster screws, but the part below the threads must be cut off to work with the gsx-r rebound adjuster rods.

With the k0-3 internals, the cartridge bottom to fork cap distance is exactly the same as on the 900 ss internals, so the fork length doesn't change.
 

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I kept the GSXR fork caps when I did mine. I thought they were a little nicer looking. What oil level are you using ?
 

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pajazo, thanks again for the help with my SportClassic Marzocchis!

You mentioned you did Shim Restackor calculations on the stock stacks... and that the stock GSXR piston port-size different from Gold Valve made the RaceTech shim setups too stiff. (Not enough "flow".) You also said the K5 GSXR shim stack was too stiff as well, and the K3 was a lot softer.

Could you share those calculations here?

Also, how should I modify my (K5 shim stack/GSXR-showa piston) to be the equivalent of a (RaceTech C34 stack/ "gold valve" piston) in terms of your stiffness calculations? You said how to fix a K3, not a K5.

Or can I just work off the picture you took of a disassembled stack? (I think K3/K5 share same base shims, only top is different.)

In a K5, I also found 100mm air gap was a bit too much... the fork will never get full travel. I think 120mm may work better, some GSXR people say the same thing. I will have to remove my fork from the bike to do this... or do volume calculation, unload front forks, and remove oil volume via syringe.

RaceTech recommends leaving the physical stop in but disabling it by drilling hole at bottom of "bucket". I compromised and drilled a <3mm hole 1/2 way down.

Finally, you are right -- on the K5, I can screw in the compression screw, but as long as there is even a LITTLE bit of rebound open, the fork will not lock up compression. If I screw rebound in, the fork WILL lock up compression.

There is something going on with that bleed hole at the top. I think the K5 is indeed a hybrid internal-external bleed, and this "flaw" you mention is actually a forced-minimum value for compression/bypass.

What has your investigation revealed?
 

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Discussion Starter · #11 · (Edited)
Just rode 100km of bumpy countryside twisties. Now when I have the fork settings dialed in, I feel the first time that I have a fork setup that is doing the same things as the öhlins shock in the rear.

My settings are comp 4 clicks out, rebound 1 turn out.

An oil level of 100mm is very high. Race tech recommends 120mm with the ducati forks. I'm using 0.85 kg/mm 900 ss springs from race tech.

I would not say that the showa pistons don't have enough "flow". The port size is large enough to absorb fast bumps without harshness. The port size just has an effect of the deflection of the shim stack with a given pressure, so the shim stack needs to be matched to the valve port size and number, you can't just copy a shim stack and use it with a different valve with the same results.

If you want to match the C34/R18 stack stiffness (and k2 comp stack), remove 4 17x.1 and one 16x.1 shim from the k5 comp stack. I suggest to leave the rebound stack as is. I made an error with the k5 rebound stack in the previous calculations I pm'd you. The K5 rebound stack is pretty stiff as is and very close to the R18.

Keep in mind that the port dimensions are measured from the pics on Peter Verdone's site, I don't have gold valves to measure. And the port dimension inputs in Restackor don't take into account different port shapes (sector/round) so this is really an approximation.

I guess I'll take apart a K5 cartridge now I have them lying around to find out how the rebound circuit on them really works. I'll post pics when I have the time to do it.
 

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Discussion Starter · #12 ·
Here's the shims I had in my K4-5 cartridges, check that you have the same before modifications

k4-5 750 showa comp
17x0.10x5
16x0.10x4
15x0.10x2
14x0.15
13x0.15
12x0.15
11x0.15
8.5x0.20x2
11.5x0.40

rebound
17x0.10x5
17x0.15
16x0.15x2
15x0.15
14x0.15
13x0.15
9.5x0.20x2
11.5x0.40x2
 

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Hm.... mine seem to be different. This is really concerning.


Do K5 GSXR-600/750 have different setups? I need to speak with the person I bought my forks from.

What shim stack did you use in your calculation?

Code:
k4-5 750 showa comp

[Yours]		[Mine]

17x0.10x5	17x0.10x10
16x0.10x4	
15x0.10x2	15x0.10
14x0.15		14x0.15
13x0.15		13x0.15
12x0.15		12x0.15
11x0.15		11x0.15
8.5x0.20x2	8.5x0.20
11.5x0.40	11.5x.04
 

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Discussion Starter · #14 · (Edited)
Your original stack calculates about 12% stiffer than mine with restackor.

I can not be sure that the one I had wasn't already modified but I think it's unlikely. Maybe there are differences between the year models or 600/750 models.
 

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Discussion Starter · #15 · (Edited)
https://picasaweb.google.com/102766334876883285574/GsxR750K5Cartridges

The K5 cartridges have a loose fit of the rebound adjuster needle shaft to the rebound valve holder bore. The bore diameter is 4.49 mm and the needle shaft is 4.32 mm. This leaves about 1.2 square millimeters of gap for the oil to leak out of the cartridge during rebound and compression strokes. Compare that to the 1.3 mm compression bleed hole (1.5mm2) race tech specifies for gold valve applications for some non-adjustable forks. This explains why I needed 0 to 1 click open comp setting on these cartridges to get enough compression damping.

The K2 cartridge is otherwise similar in design but it has a non-existent gap between those parts, the fit is air-tight.

Plugging the "weep hole" in the fork cap would prevent the leak but leave some air trapped inside the damper rod. I'm not sure if that would have a noticeable effect on the damping function.
 

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Valving

We use to install Showa cartridges in mid 90's 900/750's all the time. The trick for those was machining the lowers for compression adjusters...lol

Please also remember that the washer below the clamping shim is not always flat, and this causes abnormal damping. One area which you might check is the preload on the check washer, and also to ensure the washer is not grooved. It is important to make sure it seals.

100mm will not work. I do not know your weight, bu .85's are in the ballpark for a lighter rider, however a fluid height of 100mm will not give the proper feel at the transition of trail braking to full lean. However, this appears to be a street ridden unit, so you may not feel it...

It is common to have one leg slightly longer than the other due to machining and also the different cartridges. It is then important to set the ride height with one leg and alter the other slightly higher or lower to ensure the axle slides through smoothly. Then before tightening the pinch bolts, bounce it up and down to be certain everything settles in. Since you seem to understand the rest of it, I would imagine you already know these simple items.

Good Luck,


Thor
 

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100mm will not work. I do not know your weight, bu .85's are in the ballpark for a lighter rider, however a fluid height of 100mm will not give the proper feel at the transition of trail braking to full lean. However, this appears to be a street ridden unit, so you may not feel it...
Thank you , Thor. Are your recommendations for "spring in" or "spring out" when doing oil level?

The 2005 K5 GSXR 600 shop manual (PDF) gives bad instructions (100mm spring out)... or perhaps that only applies for the (longer) GSXR stock upper.

I weigh 150lbs/68kilo without gear. In order to remove 20mm of oil without removing the forks from the bike, I need to do some math:

Code:
GSXR damper rod OD:	12mm
Showa damper rod OD:	10mm

			Marzocchi SC	Showa 748
Lower ID:		40 mm		(40mm??)

Lower OD:		43 mm		43mm
Bushing notch:		42 mm		(42mm??)
Bushing thick (x2):	1 mm		1.5mm

Upper ID:		44 mm		(45mm??)
pi * [ (44 mm)^2 - (43 mm) ^2 + (40 mm)^2 - (12 mm)^2] / 4 * -20mm in cc = 24.24 cc

Assuming when fully compressed the fluid fills the gaps between (the upper and the lower) and (the lower and the rod), I need to remove 24.24 cc using a syringe and PVC tube. This will save me having to remove the forks from the bike.
(I will need to measure and account for the inner volume of however long PVC tube I use.)

Bradblack did something similar and MEASURED 27cc per 20mm for Showas.
Likely scenario: pi * [ (45 mm)^2 - (43 mm) ^2 + (40 mm)^2 - (10 mm)^2] / 4 * -20mm in cc = 26.33 cc
Works out perfectly.

I have a photo of the compressed cartridge showing the fluid will not expose the top of the cartridge proper even at 120mm.


Your original stack calculates about 12% stiffer than mine with restackor.

I can not be sure that the one I had wasn't already modified but I think it's unlikely. Maybe there are differences between the year models or 600/750 models.
I think you are correct. I contacted my seller. Mine came off a 2004 GSXR 600. They were likely not modified... my stack holders showed no signs of marring/tampering (until I did :p).
 
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