- Fork Design Overview
- Summary of Fork Installation and Alignment Measurement Procedure
- Install Fork Tubes In Lower Yoke
- Install Fork Top Plate or Toaster Tan Fork Top Brace
- Check Forks Are Parallel In The X-Plane
- Check Forks Are Parallel In The Y-Plane
- Check Front Axle Sliding Fit In Fork Lower Sliders
- Torque Steering Stem and Fork Cap Nuts
- Check If Forks Are Still Parallel In X And Y Plane
- Check Fork Lower Slider “Stiction”
- Complete Fork Installation
- Toaster Tan Top Brace Final Assembly
- Fork Slider Final Assembly
When I install the fork tubes I verify that the tubes are parallel in the X and the Y planes. Getting them parallel greatly reduces stiction which results in a harsh suspension.
I refer to the X-plane and Y-plane of the forks throughout this procedure. Being parallel in the X-plane means the distance between the forks stays same top to bottom. Being parallel in the Y-plane means one fork is not ahead of the other.
I replace the stock fork top plate with a Toaster Tan fork top brace. Since the top brace is part of the front fork system you will want to read about how I do that here:
Bob Fleischer, Duane Auscherman and Randy Glass published articles on fork alignment, stiction and suspension caused instability.
- Bob Fleischer: Front Forks
- Bob Fleischer: Instability. Weaving, Wobbling, Wallowing. Tank Slappers
- Duane Auscherman: The Stiction Test for BMW Motorcycle Telescopic Forks
- Duane Auscherman: BMW Motorcycle Fork Alignment Tool
- Duane Auscherman: Tests for BMW Motorcycle High Speed Wobble or Weave
- Randy Glass: BMW Fork Alignment Procedure
I also published material about installing and aligning the front forks from my rebuild projects for my 1977 R100RS on which I installed the Toaster Tan fork top brace and 1973 R75/5 which has the original BMW top plate in these articles. There are some differences in tools and techniques depending on the bike so these are worth reviewing.
- 31 BMW 1977 R100RS Install Steering Stem, Front Forks, Telefix Fork Brace and Toaster Tan Top Brace
- 31 BMW 1973 R75/5 Install and Align Front Forks
The parts include the lower fork slider mounting and drain bolt washers and the fork gaiters instead of the fork dust cap (part# 31 42 1 234 396) originally installed on the bike.
|07 11 9 963 151||GASKET RING-A12X17-CU (from 09/80)||2|
|07 11 9 963 010||GASKET RING-A5X7,5 (from 09/80)||2|
|31 42 1 234 908||RUBBER BOOT (to 09/80)||2|
To get the forks parallel in the X-plane, I use the Cycle Works jig with dial indicator that reads in 0.0005 inch increments (five ten-thousandths).
Unfortunately, this tool is currently not available from Cycle Works. 🙁 Maybe it will be available again in the future.
I use an 8 inch vernier caliper with 0.001 accuracy for the measurements of fork tube height. This can be used to get the forks parallel in the X-plane if you don’t have the Cycle works tool as I show on the 1973 R75/5 project.
I use a glass plate, 10 inch x 12 inch, to align the forks in the Y-plane.
I use a wood clamp to clamp the forks together or spread them apart when I need to make adjustments to the X-plane alignment.
VIDEO: 1983 BMW R100RS/RT Install and Align Front Forks
Fork Design Overview
If you look at Duane’s material, he has a very nice conceptual diagram of the front fork assembly; two vertical steel tubes with four horizontal members that keep the tubes parallel. From the top, the horizontal members are the fork top plate (in my case, the Toaster Tan fork top brace), the lower fork yoke, the fender brace that mounts between the fork lower sliders, and the front wheel axle.
If you look at the fork tubes, you notice that there are two planes in which they have to be parallel; the X-Plane in which the distance between the tubes is equal from top to bottom of the tubes, and the Y-Plane in which the forward/rearward alignment of the tubes.
Each of the four horizontal connections between the tubes is an opportunity for misalignment in both planes. And, the tubes themselves can be bent; mine have a minor kink in both tubes toward the bottom, but not enough to worry about. I measure the tube alignment in the X and Y planes after I tighten the lower yoke Allen bolts, after I tighten the steering stem and fork top cap nuts, and after I tighten the fender brace and correct any misalignment in either plane that I find as I go.
When using the stock BMW top plate the top of the tubes touch the bottom of the top plate, but don’t penetrate the holes. But, in the Toaster Tan top brace, the fork tubes go inside the holes in the top brace so they are aligned at the top more precisely. The fork cap nuts goes through the holes in the upper brace and thread into the top of the fork tubes. In the case of the stock top plate the fork cap nuts pull the top plate down on top of the fork tube, so any gap between the stock top plate and the top of the fork tubes will cause the tubes to go out of alignment. But, with the Toaster Tan fork top brace, this can’t happen.
The Toaster Tan brace includes a cap ring that fits on top of the fork tubes inside the hole in brace. This rings are pressed down on top of the fork tubes by the fork cap nuts and secured by two forward facing pinch bolts on the Toaster Tan top brace. They seal the fork tubes so the fork oil doesn’t leak out of the top of the tubes.
The stock top plate and the Toaster tan top brace also connect to the steering stem using a large acorn shaped steering stem nut. The steering stem nut has a skirt that goes through the hole in the upper brace and presses on the slotted nut used to adjust the preload on the steering stem bearings. The Toaster Tan steering stem nut skirt is twice as deep as the stock nut and the hole in the top plate fits closer to the nut improving the fork rigidity. Both the stock top plate and Toaster Tan top connected at three points: the steering stem and each fork tube.
The lower U-shaped fender brace that mounts on the fork lower sliders can also be a problem as the brace can be too wide or too narrow, one of the legs can be bent, and/or the face that mounts on the lower slider can be distorted. Both Duane and Bob talk about how to handle this. I use small shims (wave washers and flat washers) to remove the gap between the fender brace and the fork lower sliders.
So we have two fork tubes, four horizontal members and various nuts all interacting and potentially creating misalignment of the fork tubes. It helps if you follow a procedure when installing and aligning the front forks to avoid rework.
Summary of Fork Installation and Alignment Measurement Procedure
Here is a summary of my procedure for checking the fork tube alignment while I install the forks. This procedure applies when using the Toaster Tan top brace.
- Install the fork tubes in the lower yoke so they are high enough that the fork tube rings are 0.7 mm proud of the top of the Toaster Tan fork top brace (160 mm is the specification).
The rings are sized so they usually fit without changing the fork tube height. I had to make a small adjustment to the height of the fork tubes so the rings were proud of the top of the top brace.
- Tighten the yoke Allen bolts to 25 FOOT/pounds.
The published torque for this year/model is 29 FT-Lbs. I’m using stainless steel yoke bolts with anti-seize so I lower the value to 25 FT-Lbs (13 %) since anti-seize is a lubricant and the published torque will increase the force on the threads and I don’t want to strip them.
- Check the X and Y plane alignment and correct as needed.
- Check the axle bolt for smooth movement in the holes in the two fork lower sliders and correct the problem if they won’t slide easily in the holes.
- Install the steering stem and fork tube cap nuts. Use a handlebar in one of the handlebar risers to keep the forks centered and the stops on the bottom fork yoke from contacting the stops on the steering stem and torque the nuts: Steering Stem Nut, 80-88 FT-Lbs; Fork Cap Nuts, 35 FT-Lbs
The published torque is 80 FT-Lbs for the fork cap nuts, but that can damage the Toaster Tan top plate so a lower value is used.
- Check the fork tube X and Y plane alignment and correct as needed.
- Temporarily install the fork lower sliders which requires installing all the fork internal components and finger tightening the fork cap nuts. This provides enough pressure on the damper rod to keep it from spinning while I hand tighten the large Allen bolt in the bottom of the fork lower slider.
- Remove the fork cap nuts and the springs and preload spacers so the fork lowers slide easily on the fork tubes.
- Check the fork stiction and verify it is constant and low.
- Install the fender brace and torque the fender brace Allen bolts on one side to 18 Ft-Lbs.
- Check for any gap between the fender brace and the fork lower slider on the other side of the brace and shim as required.
- With only the bolts on one side of the brace torqued, test the fork stiction with the axle installed (but the axle nut loose) and verify it is low.
- Torque the Allen bolts on the other side of the fender brace and test fork stiction. It should be just about the same.
- Remove the fork lower sliders with the fender brace attached and install the fork gaiters.
- Install all the fork internal components and the fork cap nuts and torque the cap nuts to 35 Ft-lbs using the handlebar to prevent contact between the lower fork yoke stops and the steering head stop.
- Install the fork lower slider with fender brace assembly on the fork tubes.
- Holding the axle with one hand, torque the large Allen bolts on the bottom of the fork sliders to 25 FT-lbs.
Install Fork Tubes In Lower Yoke
As part of this procedure, I also install the Toaster Tan fork top brace. I’ll provide links to relevant sections in that document where appropriate.
Set Fork Tube Height
The specification for the fork tube height is 160 mm from the top of the shoulder of the lower fork yoke and the top of the fork tubes. I use a large blade flat screwdriver to spread the holes in the yoke, slide the tubes up through the holes and set the height.
Install Fork Top Plate or Toaster Tan Fork Top Brace
I’m using the Toaster Tan fork top brace on this project and here is the procedure for installing the fork top brace.
- 31 BMW 1983 R100RS/RT Install Toaster Tan Top Fork Brace
You may have to adjust the fork tube height to get the rings that fit on top of the fork tubes to be 0.7 mm proud of the top of the fork top plate.
If you are using the stock BMW top plate, you can see how I do that installation on the 1975 R75/5 project.
Check Forks Are Parallel In The X-Plane
I check if the forks are parallel in the X-plane and Y-plane at several points during the assemble procedure. That way, if what were parallel forks go out of alignment, I know what I did that caused that to happen so I’m not guessing about what maybe going on.
After I get the Toaster Tan fork top brace (or the stock fork top plate) temporarily installed, I torque the lower fork yoke pinch bolts to the published torque value.
The specification for the lower fork yoke pinch bolts for this bike is 28 FT-Lbs. But I’m using stainless steel Allen bolts from the bolt kit I got from “The Bolt Guy”. This requires anti-seize on the threads to prevent galling in the aluminum threaded hole in the lower fork yoke. Anti-seize is a lubricant so I reduce the torque about 15% to protect against stripping the threads.
Now I check if the forks are parallel in the X-plane using the Cycle Works tool. You have to be careful that you don’t spread the tubes apart as you take the measurement, particularly when taking the bottom measurement. As shown in the picture below, I balance the jig in my fingers and carefully rotate it against the inside of the left tube until I get near the narrowest point and then very gently rock the jig forward and back across the narrow point watching the dial indicator for the lowest reading. I do this several times until I get consistent readings as the dial indicator moves. It’s takes some patience, practice and a light touch to get good readings.
I measure them in three places, top, middle and as close to the bottom of the fork tubes as I can to see if they are parallel.
I define being parallel if the maximum difference between the measurements is 0.001 inches or less.
I found the bottom of the forks were wide about by about 0.002-0.0025 inches. I use the wood clamp to squeeze the forks together. I let the clamp stay on for about 1.5-2 hours and then take new measurements. The forks are now parallel, but I always wait over night and check them again in case the aluminum in the fork lower yoke relaxed, before I move on.
What is going on here? The fork lower yoke is made of aluminum which when subjected to a prolonged force will take a permanent set. So the clamping force is deforming the hole in the fork lower yoke enough that the forks become parallel in the X-plane.
Check Forks Are Parallel In The Y-Plane
I use the glass plate and place it across the front of both forks with the bottom of the sheet even with the bottom of the fork tubes. I hold it tight to the right fork tube with my thumb on top of the plate and my fingers behind the fork tube. I press the plate firmly against the right tube being careful I’m not pushing the tube forward or backward. I tap the left edge of the plate from the top to the bottom listening for any ringing caused by the plate not being in full contact with the front of the tube. It the plate rings, the part of the left tube where it rings is behind the right tube and they are not aligned in the Y-plane.
Then I repeat using my left hand to hold the plate against the left fork tube and tap along the right edge along the right fork tube. I hear no ringing so the tubes are parallel in the Y-plane.
If you have to adjust the forks in the Y-plane, I show how to do that in the 1973 R75/5 procedure:
Check Front Axle Sliding Fit In Fork Lower Sliders
I temporarily install the lower fork sliders. I insert the spacer in the bottom of the lower fork slider and then slide it over the foot on the bottom of the damper rod then tighten the large Allen bolt that secures the lower fork slider to the damper rod. I insert the front axle from the right side to see if it will slide easily through both holes. I find I can’t get the axle to slide easily through the hole in the right fork lower.
I inspect the hole and I see a ridge cut into it. When I check the shoulder on the right end of the axle it has a ridge.
I remove the ridge (NOT THE SHOULDER) with a flat file and polish the shoulder with 600 wet/dry sandpaper. I wrap the sandpaper on my finger and burnish the inside of the axle hole in the right fork lower. When I insert the axle it slides smoothly through the hole in both fork lowers. This confirms the forks are aligned in the Y-axis and are the same distance from the top brace.
I suspect someone over tightened the front axle nut as they were not aware that BMW uses tapered roller bearings in the wheels. Other brands of motorcycles use ball bearings and the axle nuts are tightened very tight. (I sometimes call this the “Harley” axle nut tightening technique.). I also found the the front wheel bearings were damaged with grooves cut into the outer races by the tapered rollers in the inner race, which is exactly what would happen to the wheel bearings if the front axle nut is over tightened.
I remove the fork lower sliders so after I torque the steering stem and fork tube cap nuts I can recheck the X and Y plane alignment of the fork tubes because sometimes it will change after tightening the three cap nuts.
Torque Steering Stem and Fork Cap Nuts
Torquing the steering stem and fork cap nuts can cause the forks to go out of alignment. I usually torque them together and then measure the X and Y plane alignment. The reason I torque all three nuts is that any distortion to the top plate caused by the steering stem cap nut is frequently offset by tightening the two fork cap nuts.
If the forks are out of alignment after I torque the three nuts, I loosen one fork cap nut and check the alignment. If it has gone back in alignment there is a problem with that cap nut. If it’s still out of alignment, I loosen the other fork cap nut and if the forks are back in alignment than it’s that cap nut. Finally if the forks are still out of alignment, I loosen the steering stem cap nut and if the forks are back in alignment, it’s a problem with the steering stem nut.
Usually, the problem is that the nut causing the problem is distorting the top plate and bending it. If the nut is not flush with the plate it will bend it as it’s tightened. This is usually not a problem with the Toaster Tan fork top brace due to its design.
Install Handlebar Risers
Before I torque any of these cap nuts I install the instrument bracket and handle bar risers on the Toaster Tan fork top brace as described here:
- 31 BMW 1983 R100RS/RT Install Toaster Tan Top Fork Brace
The reason I install the handlebar risers is that I don’t want to have the forks to turn all the way to the stops while I torque the cap nuts because the twisting force on the stops will deform the aluminum fork lower yoke and the forks will not be parallel in the X and Y planes any more.
Use A Handlebar To Provide An Anti-Torque Arm
I use an old handlebar in the right handlebar riser so I can use it to push in the opposite direction as I torque the cap nut which prevents deforming the lower fork yoke.
I don’t want to tighten the steering stem bearing preload nut since the amount of bearing preload is small so I use the hook spanner in the tool kit to hold it in place while I torque the cap nut.
I place the handlebar against my waist, hold the hook spanner on the preload nut with one hand and the torque wrench in the other and torque the cap nut in two stages: 40 FT-lbs and then 80 FT-lbs. After the first stage, I rotate the handlebars to check that the steering head bearing preload didn’t change.
I install all the parts in the fork tube if they aren’t inside already. Then I torque the fork cap nuts to 35 FT-Lbs using the handlebar in the opposite handlebar riser to prevent the forks from turning against the steering stops on the lower fork yoke.
BMW specifications call for 80 FT-Lbs for the fork cap nuts. But Toaster Tan fork top brace specifications call for a lower 35 FT-Lbs so you don’t damage the top brace. When using the stock BMW top plate, the higher torque keeps the top plate tight against the top of the fork tube to prevent the top plate from flexing. But the Toaster Tan design is different with the fork tubes inside the holes in the top brace and a ring on top of the tube to seal the oil in the tubes.
Check If Forks Are Still Parallel In X And Y Plane
I repeat the X and Y plane measurements to determine if the fork tubes are still parallel in both planes. The X-Plane was within 0.00025 – 0.0005 inches (2.5 to 5 ten-thousandths) of being parallel which is no problem.
I use the glass plate to recheck the Y-plane alignment and there is no ringing of the glass plate so the Y-plane is in alignment.
Check Fork Lower Slider “Stiction”
Now that I have the fork tubes assembled and parallel in both the X and Y planes, I want to see how much stiction I have in the fork lowers. I install the lowers inserting the damper rod spacers into the bottom of the fork lower sliders. I attach the fork sliders to the damper rods with the large Allen bolt finger tight for now.
The fork spring pressure ensures I can finger tighten the Allen bolts on the bottom of the fork sliders. Now I remove the fork tube cap nuts using the anti-torque handlebar and remove the fork springs and preload spacers so there will be no pressure on the damper rod and lower fork sliders.
I put a little fork oil on the outside of the fork tubes to lubricate the fork seals. Then I pull up on the axle and see how fast they fall to the bottom. The fork lowers slide pretty smoothly and don’t hang up confirming that the tubes are still parallel and that the amount of stiction is pretty low.
Install Fender Brace & Verify Fit
Then I install the fender brace on the lowers using the Allen bolt, wave washer and locking nut with the wave washer under the locking nut. With the fender brace bolts loose, I apply a bit more fork oil to the fork tubes and retest how fast the forks fall after I pull them all the way up. They fall a little faster due to the increased weight.
I torque the two bolts on the left (as I face the forks) to 18 FT-Lbs and inspect the right side to see if there is a gap between the fender brace and the fork lower slider. There is a gap, about the same size, next to the front and rear bolts. A flattened wave washer is just about the right thickness. I insert one between the fender brace and fork lower yoke front and rear bolt holes, then torque the right side bolts to 18 FT-Lbs.
Test Fork Stiction with Fender Brace Installed
I repeat the drop test of the fork lower sliders and it looks like the fork lowers fall just a little bit slower than before, but the difference is negligible.The forks are aligned and the stiction is very low so they should work smoothly.
Complete Fork Installation
I install fork gaiters instead of the rubber dust cap that was originally installed on the top of the fork lower slider over the fork seal. The gaiters protect the fork tubes from being nicked by gravel and debris so the seals will last much longer. This year bike uses the 11 rib version of the fork gaiters while the older bikes used the 13 rib version.
I remove the two fork lower sliders as a unit with the fender brace attached so I don’t disrupt them. I slide the gaiters onto the fork tubes and slide on the fork lower sliders with the fender brace. The I install the hose clamps on the top and bottom of the gaiters to secure them to the top of the yoke and the top of the fork sliders.
I install the fork springs, washers and preload spacers and finger tighten the fork cap nuts. I loosen the two forward facing Allen pinch bolts on the Toaster Tan fork top brace so when I torque the fork cap bolts, the Toaster Tan fork tube sealing rings will compress fully on top of the fork tubes.
Toaster Tan Top Brace Final Assembly
I torque the fork cap bolts to 35 FT-Lbs using the anti-torque handlebar in the opposite handlebar riser so the fork lower yoke doesn’t bump against the steering stops and twist the forks out of alignment. Then I torque the front facing Allen bolts to 18 FT-Lbs to secure the cap rings on top of the fork tubes.
Fork Slider Final Assembly
Now I torque the large Allen bolts that secure the fork lower sliders to the damper rods to 25 FT-Lbs. The spring pressure keeps the damper rods from turning which is why I do this after I have the springs installed.
I install the fork gaiter hose clamps to secure them. Finally, I put the black plastic caps on the top of the fork cap nuts.
The forks and Toaster Tan fork top brace are installed.
At this point, the forks are installed, parallel in both planes and have low fork stiction. I will install the front brake assembly and front wheel and will cover that work in a separate write up.