The R80/100GS was equipped with 40mm Marzocchi forks. While the fork tubes and sliders are functionally the same from left to right, the damper unit in each side is different. They can be swapped from side to side, but originally the left leg handled compression and the right leg handled rebound.
Here's how the fork is constructed. The damper rod is bolted to the bottom of the fork slider (that's the bolt on the very bottom) and the valve is clipped into the end of the fork tube. When the forks compress, the damper rod slides up through the valve. Likewise, when they extend, the damper rods extends through the valve.
Moving on, to focus on the damper rod assemblies themselves:
In both cases the valve (the aluminum body and the disc) are retained in the bottom of the fork tube. The damper rod is fastened to the bottom of the slider, so when the fork extends the rod moves through the valve, to the left in the picture above, ultimately compressing the top-out springs between the valve and the upper end of the damper rod. When the forks are compressed, the rod moves to the right until the bottom of it is nearly flush with the bottom of the valve.
The extra bit on the left side is a sleeve that fits over the end of the rebound damper rod. At the upper right is the spring seat for the main spring on the rebound leg. The seat for the spring on the compression leg is part of the damper rod.
All of these parts are immersed in oil, and the fork damping is determined by the way the oil is handled during these movements. There are two principle movements of oil: across the valve, and in and out of the damper assembly. I'll call these primary and secondary flow. The total damping is basically the combined effect of all fluid friction.
Primary flow: Oil moving across the valve is easy to visualize in the picture above, by simply imagining the valve sliding back and forth with an imaginary fork tube providing walls on each side. When the forks are compressed, the valve is to the left and all of the fluid is to the right. Whe they are extended, the valve is to the right and the oil is all to the left. Since the valve body is fitted tightly into the tube, all of that oil has to flow between the valve and the damper rod as the fork moves. This is between 5 and 6.5cc of oil per centimeter of fork travel.
Secondary flow: Some oil has to leave the damper assembly because as the fork compresses the volume that's taken up by the wall of the fork tube displaces oil, pushing it above the damper and into the area where the main fork spring is (to the right of the illustration). When the fork extends, it withdraws oil from the spring area to refill the damper. This amount of oil doesn't represent the entire volume of the fork during that compression, only the volume that the walls of the fork tube require. On these forks, that's about 3.5cc of oil per centimeter of fork travel.
The total damping effect is controlled by the speed of the oil being forced through various gaps and the size of the opening. While every movement of oil within the fork contributes to the damping, it's my conclusion that the vast majority of the rebound damping happens in the primary flow of the right leg, and the majority of the compression damping happens in the primary and secondary flow of the left leg. Most of the pertinent dimensions are given so that you can form your own conclusions; a better answer can only come from a shock dynamometer.
The valve body contains a check valve which lets oil flow easily from bottom to top, but less easily from top to bottom. The working part of the oil is trapped between the valve and the white plastic ring at the top. As the forks extend, oil is forced between the damper rod and the inner diameter of the closed valve. When the forks compress, the force of oil from the bottom lifts the valve and allows relatively free flow back into the damper area.
Assembled rebound valve
Disassembled rebound valve
|The spring holds the valve disc (in the middle of the right-hand picture) against the seat, which is the step in the aluminum body. Oil flowing from top to bottom must go between the damper rod and the inside of the valve disc. When the fork compresses, the oil lifts the disc off the seat and flows around the outside., The three prongs on the upper ring limit the movement of the disc and maintain the oil flow path when the valve is open.|
Secondary flow is handled by the hollow damper rod. The holes at the bottom and the opening at the top allow unimpeded flow in both directions.
Additionally, there's a simple anti-bottoming feature at work. As the fork bottoms out, the damper rod hole is closed off and oil is trapped in the bottom of the fork between the bottom of the valve and the end of the slider. This creates a cushioning effect as the oil forces its way through the very small gap between the valve and the end of the rod. In the picture to the right, the space around the steel cap on the end of the damper rod (look at the first picture on the page to better visualize this) represents the trapped oil volume.
When the fork is assembled and filled with oil, with no spring installed, the rebound damping and anti-bottoming can both be felt when moving the fork by hand.
|Damper rod OD||20mm|
|Valve disc ID||20.5mm|
|Valve body ID||25.2mm|
|Valve disc OD||27.6mm|
|Valve chamber ID||
As the forks compress, the tapered damper rod blocks off more and more of the gap between itself and the valve. The valve is actually just an aluminum cylinder with a slight taper in the inside, facing the bottom. The steel disc is the same size as the aluminum cylinder; it's there to protect the aluminum from the top-out spring.
|As the fork compresses, the gap between the damper rod and the valve gets smaller. This provides progressively increasing damping action throughout the full travel of the fork.|
Since the damper rod has no holes in it, secondary flow happens at the upper end of the damper rod. Oil simply flows in and out of the damper area by flowing around the edge of the blue-colored steel disc, which is smaller than the inner diameter of the fork tube. The white plastic bushing keeps the disc centered in the fork tube while still allowing flow.
It's difficult to get a feel for the compression damping with the fork off the bike. Compression damping is intended to control very fast fork movements, and you're not going to simulate that by hand in your garage. When the fork is assembled, with oil in it but no springs, it will feel like there's almost no compression damping at all compared to the rebound side. This is normal.
|Damper rod max diameter||23mm|
|Damper rod min diameter||18mm|
|Valve & disc ID||23.7mm|
|Upper disc (secondary flow)||32mm|
|Fork tube ID||
The stock springs are about 440mm long with 159mm spacers (169mm for the PD). A spacer/seat disc fits into the bottom of the spring on the rebound side.
In general GS riders seem to be happier with a heavy fork oil in the left leg. This is probably due to the relatively large openings in the compression valving. 15W is a popular number that I've been happy with. For the right leg, 10W is popular. BMW specifies 470ml of oil in each side if the fork has been disassembled. For a regular change, it's 410 left and 440 right.
Each fork tube has a bushing sleeve at the bottom, and each slider has a bushing inserted in the opening underneath the fork seal. Normally these parts don't wear very much, but if the fork has been misaligned you may want new ones. In the photo below, the upper fork tube bushing shows distinct wear and should be replaced.
|Fork tube bushings
||Slider bushing (beneath the fork seal)
|Installed orientation of Gold Valve. The fork spring rests on the slotted surface.||The small disc on the top (with the single hole) deflects under compression as the oil pushes upward.||The bolt and nut are tightened to control the amount that the valve opens.|
The Gold Valve inserts are a popular upgrade for street bikes with conventional damper rods (without valves). They work by directing the secondary oil flow from the top opening in the damper rod through an orifice with a spring-loaded valve. As the fork compresses, the flow gets impeded by this valve. There is a return valve within the Gold Valve with a much lighter spring which allows oil back into the damper rod upon fork extension.
Installing a Gold Valve in a GS fork means that you are adding compression damping to the right leg, where there was none before. Instructions usually call for drilling additional holes in the damper rod; I feel this is a holdover from instructions for forks where each side controls both rebound and compression. To disable damping in the right fork leg I would start by removing the disk from the rebound valve.
The most popular kit is the S3802, which uses a 31mm valve (this is the largest that will fit into the 34mm ID fork tube). The basic valve is the same for all 31mm kits; the different part numbers indicate different aluminum adapters for mounting the valve to the top of the damper rod. This can easily be made with a lathe.
Some links to web resources regarding RaceTech Gold Valve installation:
Another option is the Marzocchi magnum cartridges which replace the entire internals of the fork. These were sold by BMW under the part numbers 31 42 2 317 539/540 and were sold until about 2008 by HPN, but they are no longer available.
Adjusters at the top controlled compression in one leg and rebound in the other.
The cartridge cylinder has a slot which provides variable compression damping (visible in the close-up). Some riders (using similar Marzocchi cartridges) have found this to be too progressive, and have modified the cartridge by closing the slot and drilling a series of holes.
HPN inserts for GS forks (ADVRider thread)
John Chay's BMW fork information page (no link at the moment)
Cartridges for 40mm forks? (ADVRider thread)