- Inspect Fork Tubes
- Remove Damper Rods
- Modify Damper Rods
- Setup Cartridge Emulators
- Assemble Damper Rod & Install In Fork Tube
- Prepare Race Tech Fork Springs
- Repaint Fork Lower Slider
I’m rebuilding the forks and I’m going to install the Race Tech Gold Gold Valve Cartridge Emulators. This is the first time I have used them. I’ve heard that they make a significant improvement in handling and compliance compared to a damper rod design
I already removed the front forks when I disassembled the bike down to the frame and you can read about how to remove the forks here:
You can not use the stock BMW springs with the Race Tech cartridge emulators. I ordered the cartridge emulator kit and fork springs by filling in information about the specific model/year in the specification tool which generates an order form. The link below shows the Race Tech order form for a 1983 BMW R100RT, which is what I will end up with as I’m converting the RS into an RT configuration.
The Race Tech web site has a number of technical documents. That said, I found it not so easy to find all the materials that apply. Here are links to what I used to install and configure the cartridge emulators, springs and to understand how the cartridge emulators work.
- Race Tech: Gold Valve Cartridge Emulator Instructions
- Race Tech: Damping Rods and Fork Emulators
- Race Tech: Type 2-4 1980-92 BMW R65/80/100 Flat-Top Brembo 36 and 38mm
- Race Tech: Emulator Tuning Guide
- Race Tech: High Performance Fork Springs, Street-Installation
In the cartridge emulator box a card with a one time only access code to the Race Tech setting tool. There is a particular location on Race Tech’s web site you go to for downloading the settings for your cartridge emulators. Entering this code opens a configuration tool where you enter information about the make, model and year bike; the primary riding style. e.g. racing, street, touring, etc.; how experienced a rider you are; and your weight. The tool creates a setup sheet showing how to configure the cartridge emulators, springs and weight fork oil to use for your specifications.
You can not go back and change your mind after you enter the specifications. If you make a mistake, you have to contact technical support to get your code reactivated. This lock & key approach is to protect their intellectual property from theft.
This the parts list for my bike.
|FEGV S3301||Race Tech: Gold Valve Cartridge Emulator||1|
|FPEV AD3301 P||Race Tech: Fork Emulator Adapters (pair)||1|
|FRSP S2643080||Race Tech: Springs [0.8 Kg/mm]||1|
|31 42 2 301 724||GUIDE SUPPORT (from 09/80)||4|
|31 42 1 241 737||INTERMEDIATE RING (09/80 to 05/81)||2|
|FOH-2||Rubber Chicken Racing Garage Kit
– 07119963073 GASKET RING-A10X13,5-AL, (2)
– 07119963151 GASKET RING-A12X17-CU, (2)
– 07119963010 GASKET RING-A5X7,5, (2)
– 07119934645 LOCK RING-30X1,2, (2)
– 31421237213 FELT, (2)
– 31421238909 CAP, (2)
– 31421237205 CUP, (2)
– 31421232655 GASKET RING FORK SEAL (2)
I use the Race Tech web page to specify my bike model and year. It produces an order form for the parts required to fit my specifications.I also provide information about my weight and how I will ride the bike into the Race Tech tool to select Race Tech fork springs since the stock springs will not work with the Race Tech cartridge emulators. Your springs may differ from mine based on your weight, riding style and bike model & year.
My bike had an after market fork spring kit installed and the stock fork spring supports are missing, [Part#: 31 42 2 301 724], so I got new ones. And, the spacers at the bottom of the damper rod that fit into the bottom of the fork slider were different thickness, so I replaced them with the stock part [Part#: 31 42 1 241 737].
I got a fork rebuild kit from Tom Cutter at Rubber Chicken Racing Garage that has all the parts for a 1983 R100RS/RT.
I use a pair of snap ring pliers to remove the snap ring that secures the damper rod inside the fork tube.
I make a draw bar from a piece of all thread rod, washers and nuts to compress the two springs on the end of the damper rod enough to expose the snap ring groove at the bottom of the fork tubes so I can install it. The snap ring retains the damper rod inside the fork tube.
I use tubing cutters to cut the PVC pipe used to set the fork spring preload.
Here is a short video showing how I did this work.
VIDEO: 1983 BMW R100RS Rebuild Forks with Race Tech Cartridge Emulators
Inspect Fork Tubes
Before I removed the damper rods, I checked the fork tubes to see how straight they are. One face of each fork tube shows some wear at the bottom. This is where the lower fork slider has rubbed on the front of the fork tube. This happens when the wheel hits a pot hole or bump.
I use a large glass plate to check how flat the tubes are. I roll the tubes on the plate and look for when the gap between the tube and plate is largest. This occurs when the wear area on the bottom of both tubes is facing up, which is what I expected. I use a 0.002 inch feeler gauge to see if it will slide between the tube and glass plate. One small section, about 2-3 inches, of each tube is bent enough for the feeler gauge to slide between the tube an the plate. I try a 0.003 feeler and it won’t slide under these areas. I conclude the tubes are deformed on the order of 0.003 inches maximum. They are serviceable. I also mark the section with the maximum deformation with a Sharpie.
I mark the top of each tube with a Sharpie: “A” or “B” to identify the tube; a line on the side of the tube that faces to the front; and “0.003” to indicate the maximum deformation of the tubes. I’ll install the tubes with the line facing to the front to minimize fork slider stiction since both tubes will slide over an equal amount of deformed tube at the same time.
Remove Damper Rods
I previously removed the front forks when I stripped the bike to the frame. You can see how I do that here.
Inside the fork slider that holds the axle is a thick aluminum spacer that fits in the bottom of the slider. My forks have spacers with different thickness, so I will replace them with the correct ones.
I remove the snap ring on the bottom of the fork tube. This secures the damper rod inside the fork tube. I use a pair of snap ring pliers. The damper rod has a fairly strong spring that pushes a damping valve body against the snap ring. So the snap ring may come loose only on one side. If it does, use a small blade screw driver to force it completely out of the groove on the inside of the fork tube.
I pull the damper rod out of the bottom of the fork tube. At the top of the damper rod is a ring with a black plastic bushing that acts as an oil seal. The ring screws onto the end of the damper rod. On one rod, the ring is not screwed down all the way. I unscrew the ring to remove the parts on the damper rod.
I carefully remove them and place them in order on the work bench so I preserve not only the order, but which face of each part faces its companion parts.
BMW parts fiche for fork parts are notoriously confusing, if not incorrect. This is due to the number of different front fork designs BMW used on the airhead bikes. I’ll identify the parts in the pictures below with their part numbers.The snap ring is [Part# 07 11 9 934 645 LOCK RING – 30X1,2 (from 11/82)]. The top valve housing has a shoulder inside. One side is deeper than the other and fits around the lower spring. The bottom spring fits between the two valve housings. The white washer fits into the shallow side of the upper valve housing.
The plate with holes fits on top of the upper valve housing capturing the white washer inside the upper valve housing.
An upper spring, exactly like the lower spring, comes next.The ring that screws on the top of the damper rod, shown in the picture below, is not sold separately but is included with the damper rod, [Part#: 31 42 1 241 654 ABSORBER (from 09/80)]. The black guide ring is a separate part-[Part# 31 42 1 241 649 GUIDE RING (from 09/80)]. In order to install the damper rod easily, as I show later, the damper rod support [Part#: 31 42 2 301 015] needs to be removed.
At first I couldn’t get the damper rod support [Part#: 31 42 2 301 015] off the end of the damper rod. It is not threaded onto the rod, it is a slip fit but it didn’t want to come off. Later I learned twisting it does the trick .
The bottom of the damper rod has a support that is a press fit. I used a small Phillips head screwdriver mounted in my vice and put it through a set of damper rod holes. I used vice grips to grab the face of the support and twisted it. These were on fairly tight and I learned that over tightening the large Allen bolt that secures the lower fork slider to the damper rod can deform the aluminum end of the damper rod enough to make it hard to get them off.
Modify Damper Rods
The Race Tech cartridge emulators require modifications to the damper rods. At the bottom of the rods are two holes 180 degrees apart as shown in the red circle of the picture below.
These two holes provide compression damping when the forks compress. Fork oil in the bottom of the fork slider is compressed as the slider moves up the fork tube forcing the oil through the two damper holes and up the inside of the damper rod to the top of the fork tube.
As explained in the Race Tech publication, “Damping Rods and Fork Emulators“, in the Resources section, the cartridge emulator provides two stages of compression damping; the first stage is for small bumps and the 2nd stage is for large sharp bumps. The stock damper rod only has one range of compression damping which is why the cartridge emulators should provide a smoother ride.
Therefore, the damper rod compression damping has to be turned off so the cartridge emulator controls compression damping. This is accomplished by have six, 6 mm holes in each damper rod. This many holes effectively turns off compression damping since there is very little resistance to the fork oil flowing up the inside of the damper rod.
The existing two holes are about 4 mm, so I enlarge them. However, the top of the damper rod support [Part#: 31 42 2 301 015] is very close to the hole. So I can only enlarge them to 5.5 mm, or 7/32 inch.
The other four holes I have to add are spaced up the damper rod with each set of two holes 10 mm on center from each other. The holes in each set are spaced opposite each other, or 180 degrees apart. Since the damper rod is aluminum, to maintain it’s strength, the set of two holes above the original two holes are located 90 degrees around the rod so they don’t line up on top of each other. The final set of two holes is located another 90 degrees from around the rod, so they end up lined up with the original set of two holes.
A 15/64 inch drill bit is 6 mm so I drill the four new holes with it.
I put the rod in the rubber jaws of my vice, center punch the center of the hole so the drill bit won’t wander and drill the hole using my variable speed drill at a low speed so the drill bit won’t grab the soft aluminum of the damper rod.
I need to chamfer the edge of the holes so they are rounded. I use a larger drill bit to start. I run the drill very slowly and carefully and cut a larger hole into the rod a couple millimeters deep. Then I use a small round file to smooth the transition from the larger hole to the original 6 mm hole.
Next, I wrap a drill bit with some 600 wet/dry paper and sand the sides of the chamfer to get it smooth.
I need to remove any burrs on the inside of the damper rod. I use a large diameter round file to do that. Then I run a brass tube brush inside the damper rod while flooding the inside of the rod with my parts washer. When I inspect the inside of the rod, there is no debris and the surface is shiny.
Setup Cartridge Emulators
Race Tech has an online tool you use to get the settings for your cartridge emulators, fork springs and fork oil amount. When they ship the parts, there is a business card inside the box with a special access code to use to access their database of suspension settings.
This code can be used for only one configuration. Therefore, be sure you enter all the required specifications correctly. You can not go back to change any specification. Also be sure to print out your specification sheet. If you do make a mistake, call Race Tech technical support and they will reset the access code for you so you can redo your specifications. I was told they do this to protect their proprietary configuration data from being used by other companies that copied their design.
The way cartridge emulator operation differs from damping rods is explained in Race Tech’s document “Damping Rods and Fork Emulators“. The cartridge emulator has small ports that provide compression damping when the fork is traveling at a low speed as happens of small bumps and irregularities. The cartridge emulator uses a spring to keep a plate closed on the high speed compression damping ports of the emulator. By adjusting the spring pressure, you adjust when the slow speed port is bypassed by the high speed damping ports. When the forks velocity is high, as happens when you hit a pot hole or pavement irregularity, the high speed plate opens reducing the damping rate so the forks can move fast without a harsh impact to the bike.
The kit comes with three pair of different color coded springs; the one I need (Blue) was installed on the cartridge body.
As shown in the picture above, the Brembo 36 mm forks on the 1983 R100RS/RT require an adapter (on the right) the fits on top of the damper rod and the bottom of the cartridge emulator. It’s a separate part that the Race Tech tool adds to the order sheet automatically.
The Allen screw on top of the spring adjusts the fork speed the high speed plate opens on the fork compression stroke. According to my setup sheet, the high speed compression damper spring setting for my configuration is 2.5 turns of the Allen screw.
The Allen head on the screw used to adjust the spring pressure is a non-standard size as none of my metric or SAE Allen wrenches fit it. I believe it maybe a 3.5 mm Allan. That said, a T20 Torx wrench fit the Allen bolt head.
There is lock nut on the other end of the screw thread. I loosen the lock nut and back the screw out until the bottom of the Allen head just touches the top of the washer under it.
Then I twist the brass body of the cartridge emulator 2-1/2 turns while holding the Allen screw with the T20 Torx wrench. I tighten the lock nut until it just touches the bottom of the brass cylinder.
The lock nut is pretty tight on the screw. If you aren’t careful when you tighten it you will spin the brass body of the cartridge emulator screwing up (pun intended) your spring setting. Be careful.
When I’m done setting the spring tension on the cartridge emulators, I mark the bodies with a sharpie pen “2.5 T” to indicate what I set them to. I’m curious if the ink will stay on the bodies when exposed to shock oil.
Assemble Damper Rod & Install In Fork Tube
Installing the damper rod requires compressing the two 33 mm springs enough to expose the groove for the snap ring on the bottom of the fork tube. I use a long piece of all thread rod–about 26 inches long–some washers and nuts to make a draw bar to compress the springs.
On the end that goes against the face of the lower damper rod spring retainer, I use a flat washer that is about 23 mm outer diameter with a smaller washer that will fit under the face of the nut.
The snap ring has ears and I need to have enough space between them when they are compressed and the edge of the flat washer and enough space between the ears and the flat washer so I can remove the washer.
The other end of the draw bar goes over the the end of the fork tube. I use a large fender washer and a second smaller diameter flat washer for the nut.
Before I install the damper rod inside the fork tube, I use the all thread and a shop towel to clean out the inside of the fork tube. I don’t want any debris inside. The inside of the fork tube should have a mirror like finish so the damper rod will move inside it with minimal friction.
I install damper rod “A” inside fork tube “A”. I use a new plastic seal in the sleeve that screws on the top of the damper rod.
I install the sleeve with the plastic seal on the top of the damper rod. I carefully insert the sleeve end from the bottom of the rod being sure the seal slides past the shoulder on the bottom of the damper rod and isn’t removed from the groove of the sleeve. Then I install the top spring, the plate with the holes, the white plastic washer, the top for spring retainer so that the shallow end is against the white plastic washer, the bottom spring and then the bottom spring retainer with the flat face pointing at me.
I push the springs and parts all the way into the fork tube until they bump against the shoulder on the inside of the fork tube; about half of the bottom spring retainer is exposed.
I insert the all thread through the damper rod and install the two flat washers and the nut on the bottom end. I tighten the two nuts keeping the bottom washers a bit off center. I tighten the nuts until I compress the damper rod springs all the way and expose the snap ring groove. I use my snap ring pliers and install the snap ring. Then I use a small screw driver and a plastic hammer and tap on the snap ring to ensure it is completely seated in the groove. I loosen the nuts on the draw bar and remove it. All done.
The I push the foot on the end of the damper rod and temporarily secure it with the large Allen bolt that holds the fork lower slilder onto the end of the damper rod.
Prepare Race Tech Fork Springs
The fork springs need some preparation before installing them. But first I want to show how the damper rod, cartridge emulator and fork spring go together inside the fork tube.
Cartridge Emulator Location Inside Fork Tube
The cartridge emulator fits on top of the damper rod underneath the the fork spring. The bike had after market springs installed and the BMW fork spring stabilizers were missing so I got new ones. They install on the Race Tech forks springs and will help keep the springs centered in side the fork tube and prevent them from rubbing on the inside of the fork tubes.
The cartridge emulator sits on top of the adapter ring which fits on top of the sleeve with the groove that holds the oil seal which screws on the top of the damper rod. The fork spring stabilizer fits over the top of the Race Tech cartridge emulator.
Calculate Length Of PVC Spacer For Fork Spring Preload
The fork springs need some preload to set the correct amount of suspension sag based on my bike, riding style and weight. The Race Tech spring selection tool calculated I need 20 mm of spacer to achieve the correct preload of the fork springs so the front fork sag when the bike is loaded is in the optimal range. The fork springs come with a piece of 3/4 inch PVC pipe and a method for determining the proper length of the PVC pipe to set my preload.
I start by assembling the parts inside the fork tube. I insert the adapter and then the cartridge emulator so the compression damping adjuster spring points to the top of the fork. Next I add the fork spring and then two of the supplied flat washers. A washer will go on each face of the PVC spacer pipe to protect it.
Then I measure the distance from the top of the fork tube to the top of the top flat washer which is about 61 mm in my case, but may be different for you.
Next I install the fork cap bolt and screw it all the way in. I measure the gap from the bottom of the bolt head to the top of the fork tube which is 5 mm in my case.
Then I measure the distance from the bottom of the bolt head to bottom of the bolt which is 23 mm.
I made the following sketch to clarify how I compute the required length of the PVC spacer for my bike.
On the left is a diagram of the measurement I made from the top of the fork tube to the top of the second flat washer. On the right is the diagram of the fork cap bolt measurement. I need to know how far the bolt projects into the fork tube. That distance is the difference between the distance from the bottom of the bolt head to the bottom of the bolt thread; 23 mm. Since the cap bolt does not go all the way into the fork tube, I subtract how much if the bolt is exposed; 5 mm. The result, 18 mm, is how far the fork cap bolt extends into the fork tube.
The distance from the bottom of the fork cap bolt to the top of the second flat washer is the difference of these measurements: 61 mm – 18 mm = 43 mm. The spacer needs to compress the spring 20 mm, so the spacer has to be 20 mm longer than this: 43 mm + 20 mm = 63 mm.
I cut off two 63 mm sections of the PVC pipe with a pipe cutter so the faces of the sections will be square with each other.
I cleaned off the burrs on the ends using 400 wet/dry paper on a flat plate. Then I mark the spacers with their length and the amount of preload (PL) they provide so if I need to change the spacers at some point, I know what I had installed.
Install Spring and Preload Spacer Into Fork Tube
With the fork spring installed inside the fork tube, I install on flat washer on top of the fork spring plastic retainer, PVC spacer pipe, the second flat washer and then screw in the fork cap bolt.
Repaint Fork Lower Slider
I painted the lower fork sliders as there was a fair amount of rock chips. I sanded the lower sliders with 400 wet/dry paper to remove any loose paint and the rough up the surface. I used matte engine paint so they would only have a minimal amount of gloss to compliment the matte white wheels.
After I align the fork tubes, I will install the lower fork sliders on the fork tubes with the large Allen bolt and a new washer.