K100RT fork tubes with RS cartridges

[tannerman917]

Hi all,

Looking to see if I can get some understanding of the differences between a K100RT fork tube and K100RS tube.

I started a suspension refresh on my 85 K100RT with progressive springs for my front forks and a new progressive rear shock.  I disassembled the forks and everything looks clean, and I would've thrown the shocks back together if I hadn't started tinkering to lower the front end.  I saw a video on Youtube of a gentleman cutting a section out of the stock delrin spring spacer and sandwiching it in the fork cartridge which increased the preload a bit.  In attempting the same thing, I galled up the piston not realizing it was locked on with the red Loctite!  I found some K100RS cartridges that fit the diameter of the tube, but the additional springs and hardware mean that the whole assembly doesn't seat all the way into the fork tube; it looks like the RS internals need another ~10mm or so bored out of the tube.  Could anyone confirm this dimension for me so that I can determine if I can modify my tubes to fit the RS internals?  Otherwise I'll use the bits from my RT and go back stock. 

IMG_0386.jpg (63.51 kB . 768x576 - viewed 465 times)
See the helper spring and additional hardware on the left (RS) unit, rather than the simpler tapered delrin washer and bypass valving on the RT unit.

IMG_0387.jpg (50.37 kB . 768x576 - viewed 365 times)
Looking for the dimension from the end of the fork tube to the diameter where the cartridge bottoms out.  I believe I should be able to bore that to size at work, I just don't know how deep to take that diameter.

[The Mighty Gryphon]

Wish I could help.

All I can tell you is that in any K bike shop manual the chapter on forks is the biggest, most confusing one.  I think you are finding out why that is. 

Outside of putting Race Tech cartridges and springs in my K75RT forks I have kept all of mine stock.

[Laitch]

All I can tell you is that in any K bike shop manual the chapter on forks is the biggest, most confusing one.  I think you are finding out why that is. 

Challenging for sure! In the Special Edition model of the K100RS 2V, the components in the right tube differed from those in the left because Sport forks—like those in many K75S models. Your replacement parts could be from one of those.

[Kaos]

Dont chop in the fork components.
Lowering can simply be done by either putting them higher up in the Triple tree and also -this might cause some disturbance in the force - reduce the amount of fork oil (fresh ofcourse) you fill up with. Max lowering by oil reduction is about 2cm before it starts messing with the handling.

[Kaos]

How many centimeters were you planning to drop?
And on top, why.

[Duckbubbles]

I added a spacer tube to limit the extension travel on my '85.

Frank

[tannerman917]

Challenging for sure! In the Special Edition model of the K100RS 2V, the components in the right tube differed from those in the left because Sport forks—like those in many K75S models. Your replacement parts could be from one of those.

Gryphon and Laitch, thanks for chiming in!  Admittedly I jumped into this with a little more ambition than knowledge, and should definitely get a shop manual for myself at this point.  Thankfully the replacement internals look like a matched pair so I don't have to worry about a difference side to side. 

Dont chop in the fork components.
Lowering can simply be done by either putting them higher up in the Triple tree and also -this might cause some disturbance in the force - reduce the amount of fork oil (fresh ofcourse) you fill up with. Max lowering by oil reduction is about 2cm before it starts messing with the handling.

Kaos, wasn't planning to cut down the tubes, certainly; just boring the ID of the tube so that the cartridge can be installed properly.  Not looking to drop it much more than 25-30mm in the front, as I'm on the shorter side and this brick can be a little heavy to handle at times.  Looking to maintain all the damping I can keep but increase the spring preload for ride responsiveness, but no drastic changes.

[Kaos]

Then id personally go for an oil reduced method.
For the: Fichtel & Sachs forks (identifiable by a "rib" running down the front edge of the fork leg):Oil change: 360 cc ± 10 cc per leg
Dry fill (after rebuild): Approximately 370 cc per leg
Showa forks (no rib, often found on later models or K75 swaps):Oil change: 410 cc per leg
Dry fill: 420 cc per leg
Decrease the dry fill measurement with 10cc to drop about 2.5 cm, but use 10w fork oil to keep a firm fork instead of 7.5

[Laitch]

Thankfully the replacement internals look like a matched pair so I don't have to worry about a difference side to side. 

I think I've missed a beat—not surprising for me lately. If the internals to which you refer are the ones in your opening post, tannerman, they don't appear to be identical. One has some different parts.

[Kaos]

I think I've missed a beat—not surprising for me lately. If the internals to which you refer are the ones in your opening post, tannerman, they don't appear to be identical. One has some different parts.

Tannerman's bike: /
or maybe the other way around: \

[Laitch]

Tannerman's bike: /
or maybe the other way around: \

Use the handrails, Kaos.  I using coffee to steady myself. I'll wait for t's reply.

[tannerman917]

I think I've missed a beat—not surprising for me lately. If the internals to which you refer are the ones in your opening post, tannerman, they don't appear to be identical. One has some different parts.

Yes Laitch, that's the comparison between my RT internals on the right of the image and the RS stuff I purchased on the left.  I have a matched pair of the RS internals, and the RT was identical as well.  I took that image to show the distance I am trying to determine is different between my stock stuff and the RS unit.

A pour over a day keeps the... sleep addled brain cells of many a Motobricker and/or father of littles awake?  I'll have to workshop that, maybe with a shot of espresso