1985 K100RS RaceTech gold valve cartridge emulator upgrade.

I disassembled the one fork today to start this upgrade and I noticed something interesting, or perhaps it isn't! My bike has the sport suspension spacer tube, but a single, not dual springs. The spacer tube is white and 105mm long.

Are the springs progressive or straight wound?

98lee said:

Are the springs progressive or straight wound?

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The latter.

Check the diameter on the fort tube, the early ones up to late 86, early 87 where Fichtel & Sachs and where 41.3 mm. The later ones where 41 mm. It is not uncommon to find stuff different from stock on these old K bikes.

You could have a look at Bob's BMW or another site and see what the contents of the tubes should be. It sounds like somebody installed linear springs because with these you set the pre load based on the length of the plastic tube. Or maybe somebody just did their own thing in there.

My tubes are 41.3mm. Who knows what was going on before my ownership. It doesn't really matter I suppose as I am now upgrading with cartridge emulators.

Sounds like yours are linear ( straight wound ). I actually prefer linear springs over progressive springs if they are the right weight for you and your bikes weight/load. Much more consistent throughout their travel range. They can make a big difference in dive with a big fairing on the bike. They really firm up the front end of the bike.

I believe the standard 1985 K100RS fork was not the Sport fork. It came with a fork preload spacer that was about 108mm.

03 31 42 1 450 529 SPACER TUBE 0.12 2 $6.76


Later bikes came standard with the Sport forks with a different (but similar spacer).


01 31 42 1 457 596 SPACER TUBE 0.08 2 $9.06

I thought you guys might be interested in a couple of photos of what this upgrade looks like. As soon as I find replacement drain screws (one hole was stripped half way and had an incorrect screw, so I re-tapped them both to make them the same) I will be putting in the fork oil and reinstalling. The fork oil level is 140mm from the top of the stanchion, but I will endeavor to keep track of what the actual volume used is.


This is the damper rod drilled out. Damper bottom is to the right. The bottom four were already there and opened up to 5/16". Two more were added for the requisite six. I also installed new guide rings on the dampers while I was at it, BMW part number 31421450517. I also elected to install new fork seals although there was really no reason to.



Below is a layout of components in the order in which they will go into the stanchion. From the left: stanchion, emulator spacer (required between top of damper and emulator), cartridge emulator with blue springs set to 2.5 turns pre-load, RaceTech .85kg spring with OEM plastic spring end caps, spacer washer, PVC spacer cut for 20mm pre-load, spacer washer, stanchion end cap. The two spacer washers are RaceTech supplied items, too.

A couple of things to mention. The two RaceTech springs had the following numbers on them: 651228-80 and 643449-90. As per Mike Crow at RaceTech Tech Support, ".85kg spring is a combination of a .9 and a .8kg spring. This produces the average of .85 once the axle and clamps are tightened."

I've also chosen to use the BMW plastic end caps from the OEM springs. Mike Crow told me that, "if you like how they fit you can use the oem spring ends." You don't need to but I liked how the spring sat in the tube, especially the fit over the cartridge emulator. Obviously using the end caps makes the spring longer, sitting higher in the stanchion and requiring a shorter PVC spacer as a result. The RaceTech springs are ground flat on either end and will sit flat naturally.

This was a very straight forward upgrade with no surprises, other than my stripped drain hole, which had nothing to do with the upgrade. It goes without saying that tuning your emulator will be a PITA as it will require popping off the stanchion end cap, pulling everything out, and sticking a magnetic tool in the stanchion to extract the emulator. I hope that my initial setup is okay so I don't have to involve myself in that process. I won't know until March or April whether this was worth the investment or not as it is winter here in The Great White North, but come Spring........


Tom

The last time I installed linear springs on a bike, when measuring the fork oil level, it is without the springs in and the fork tube compressed down as far as it will go. Then I just measured and bent a piece of wire to the length I needed and let it sit into the fork tube, and fill until it just touched. I would assume that this is the way Race Tech does it as well. I just mention this for others that may wonder how you get that measurement.

cycleman2 said:

The last time I installed linear springs on a bike, when measuring the fork oil level, it is without the springs in and the fork tube compressed down as far as it will go. Then I just measured and bent a piece of wire to the length I needed and let it sit into the fork tube, and fill until it just touched. I would assume that this is the way Race Tech does it as well. I just mention this for others that may wonder how you get that measurement.

Click to expand...

RaceTech has you install the emulator, and the spacer that sits between the damper and emulator, when measuring the fork oil level, but no spring, spacer or spacer washers. I worked the stanchion up and down a few times and then let it sit for a bit to make sure all bubbles were out. When pouring in the final amount for the 140mm the stanchion just rests all the way down in the slider. Below is what I used - a piece of white plastic card cut in a T-shape so that it rested across the top of the stanchion. The length extending into the stanchion was about 160mm with a line drawn across at 140mm so that I could see it.