- Fork Design Overview
- Summary of Fork Installation and Alignment Procedure
- Install Fork Tubes In Lower Yoke
- Test Fit Toaster Tan Top Brace
- Check Forks Are Parallel In The X-Plane
- Check Forks Are Parallel In The Y-Plane
- Install Toaster Tan Top Brace
- Recheck Fork Alignment
- Check Fork Stiction
- Install Fork Sliders and Fork Brace
This 1983 R80ST “project bike” had the front forks and the BMW top plate when I acquired it. I rebuilt the front forks and replaced the steering head bearings and you can read about how I do that work here.
Now it’s time to install and align the front forks. I replace the stock BMW top plate that fits over the steering stem and on top of the fork tubes with a Toaster Tan Top Brace and the Toaster Tan acorn nut that secures the top brace to the steering stem. This top plate is much stronger than the stock thin steel top plate BMW uses. This improves handling as the top plate doesn’t bend and flex like the BMW top plate does.
Bob Fleischer, Duane Ausherman 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 Ausherman: The Stiction Test for BMW Motorcycle Telescopic Forks
- Duane Ausherman: BMW Motorcycle Fork Alignment Tool
- Duane Ausherman: 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 when I rebuilt my 1977 R100RS on which I installed the Toaster Tan fork top brace and my 1973 R75/5 which has the original BMW top plate. There are some differences in tools and techniques depending on the bike so these are worth reviewing. Here is how I do that work.
- 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
I previously installed a Toaster Tan top plate on the 1977 R100RS and 1983 R100RS rebuild projects. But the design of the front suspension for the R100RS bikes includes an instrument bracket which is not used on the R80ST. It’s much easier to install the Toaster Tan top plate on the R80ST. You can read how I installed the Toaster Tan top brace and aligned the forks on the 1983 R100 RS (which references the work I did on the 1977 R100RS) here.
- 31 BMW 1983 R100RS Install and Align Front Forks
- 31 BMW 1983 R100RS Install Toaster Tan Top Fork Brace
There is a major difference in the Toaster Tan top brace installation on the 1983 R80ST compared to the R100RS projects. The R100RS model has a bracket for the instruments, but the R80ST does not. So the stock handlebar risers for the R80ST mount to the Toaster Tan top brace without any modification. You have to modify the handlebar risers on the R100RS bikes by replacing the studs with bolts.
I will borrow some of the relevant material from the 1977 and 1983 R100RS projects in this document as appropriate. But I did the work on the R80ST in a slightly different way than on the two R100RS bikes.
When I install the fork tubes I verify that the tubes are parallel in the X and the Y planes and adjust them as required. Getting them parallel in both planes greatly reduces fork stiction. High fork stiction 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 the same top to bottom. Being parallel in the Y-plane means one fork is not ahead of the other.
These are the new parts I install. I replace the stock fork top plate and acorn nut with a Toaster Tan fork top brace and acorn nut.
|31 42 1 242 26||UPPER FORK CROSS BRACE;
Toaster Tan, Part# TT-104
|31 42 1 235 429||CAP NUT;
Toaster Tan, Part# TT-301
|07 11 9 963 073||GASKET RING||2|
|07 11 9 963 130||GASKET RING||2|
|07 11 9 963 037||GASKET RING||2|
|07 11 9 963 037||GASKET RING – A6,5X9,5||2|
|31 42 1 237 213||FELT, Fork Seal Dust Cap||2|
|31 42 1 238 909||CAP, Fork Tube Bolts||2|
|31 42 1 237 205||CUP, Fork Seal Dust Cap||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 also use the feeler gauge.
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. I use a 2×4 with rope to move the forks forward and backward in the Y-plane to align them.
VIDEO: 1983 BMW R80ST Install-Align Front Forks & Toaster Tan Top Brace
Fork Design Overview
Duane’s material in the References section 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 is the same.
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, 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 go through the holes in the fork 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 fork tube cap ring that fits on top of the fork tubes inside the hole in brace. These rings are pressed down on top of the fork tubes by the fork cap bolts 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. Therefore, the torque for the cork cap bolts is much less than the torque BMW uses with their top plate
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 are 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 and bolts 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 Procedure
Here is a summary of my procedure for checking the fork tube alignment while I install the forks. It is slightly modified from the procedure I used with R100RS bikes. This procedure applies when using the Toaster Tan top brace.
- Install the fork tubes in the lower yoke so they are 160 mm above the lower shoulder of the lower yoke. Test fit the Toaster Tan top brace. Hand tighten the Toaster Tan acorn nut. Then check that the Toaster Tan fork tube rings are high enough that the fork tube rings are 0.7 mm proud of the top of the Toaster Tan fork top brace.
The rings are sized so they usually fit correctly 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. Then remove the Toaster Tan top brace.
The published torque for this year/model is 29.5 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.
- Install the Toaster Tan top brace using the steering stem acorn nut 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.
- Install the fork sliders on the fork tubes and finger tighten the large Allen bolts that secure the sliders to the damper rods. Check the axle 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.
- Remove the Allen bolts from the fork sliders. With the axle in the fork sliders check that the fork stiction s low.
- Install the fender brace and torque the fender brace Allen bolts on one side to 15 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. If not, you may need to straighten the fork brace or use some shims to get the fork brace to fit correctly between the fork sliders.
- Remove the fork lower sliders with the fender brace attached and install the fork slider fork seal dust caps.
- Install the fork slider with fender brace assembly on the fork tubes. Holding the fork brace 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 test fit the Toaster Tan fork top brace to ensure the fork tube height is correct.
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.
Test Fit Toaster Tan Top Brace
I’m using the Toaster Tan fork top brace on this project and here is the procedure for test fitting the top brace and being sure the fork tube height is correct for the Toaster Tan 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. I had to adjust the R80ST tubes so they were a bit higher than 160 mm.
Here are pictures showing the test fitting of the R80ST Toaster Tan top brace which is a bit different than the the R100RS top brace, and the Toaster Tan acorn nut that threads onto the steering stem. Both the top plate and the acorn nut went on without any difficulty.
Then I insert the Toaster Tan fork tube ring on top of the fork tubes and measure how high it is above the top brace. There is a bevel on the ring that is 0.7 mm thick and the ring should be 0.7 mm proud of the top plate. I use a 0.7 mm feeler gauge to check the ring height. I move my finger over the blade and the top of the ring and if I don’t feel a bump or valley, then the ring is at the correct height.
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 25 Ft-Lbs before I check if the fork tubes are parallel.
I torque the bolts to 25 Ft-Lbs which is about 15% less than the published specification of 28.5 Ft-Lbs because I am using a stainless steel bolt and I use anti-seize to keep the bolt from galling in the aluminum threads. Anti-seize is a lubricant so I reduce the torque so I don’t strip the aluminum threads in the lower yoke.
Check Forks Are Parallel In The X-Plane
The fork tubes need to align in two planes; the X-plane alignment checks if the fork tubes are bent inward or outward, the Y-plane alignment checks if one of the tubes is ahead of the other.
I refer to left and right in this material from the perspective of facing the front of the bike, not from the standard reference for left and right when you are sitting on the bike,
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.
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 put the half-circle against the right fork tube and carefully rotate it against the inside of the left fork 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 every 30 minutes or so. The forks are now parallel within 0.00025 inches (two ten-thousandths of an inch), 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
As shown in the picture below, 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 fork 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 left fork tube is behind the right fork tube and they are not aligned in the Y-plane. I got ringing when tapping the left edge of the glass plate so the left fork tube is behind the right one.
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. There is no ringing.
I use a 2×4, a piece of rope and a hammer to adjust the forks in the Y-plane. Since the left fork is behind the right, I put the 2×4 behind the left fork tube and in front of the right one. I make a loop of rope around the 2×4 and over the cylinder. I insert the handle of the hammer inside the loop and then twist it until the rope is pretty tight. I put the handle of the hammer against the frame to keep the tension on the rope. I check the progress of the forks about every 30 minutes. After about 2 hours, the fork tubes are aligned in the Y-plane.
After I get the fork tubes aligned in both the X and Y planes, I wait overnight and check the alignment again to be sure the forks are stable.
Install Toaster Tan Top Brace
Now that the forks are aligned in the X-plane and Y-plane, I install the Toaster Tan top brace. I start by temporarily installing the handlebar risers in the top plate keeping the riser nuts a bit loose.
I insert a handlebar into one of the risers before torqueing the Toaster Tan acorn nut to 40 Ft-Lbs so I can snug the top plate onto the fork tubes. I don’t want the forks to butt up against the stops on the lower yoke when I tighten the acorn nut since that will put enough force on the lower yoke to alter the fork tube alignment.
I install the fork springs inside the fork tubes. Then I partially tighten the fork tube cap bolts to 15 Ft-Lbs. I insert the handlebar in one riser and torque the opposite fork tube cap nut and then move the handlebar to the other riser and torque the other fork tube cap nut.
I tighten the Toaster Tan top brace pinch bolts that face to the rear on the R80ST top plate to 18 Ft-Lbs to secure the rings on top of the fork tubes.
Now I torque the acorn nut to 85 Ft-Lbs using the handlebar in the right riser and putting it behind my back so I can tighten the acorn nut easily. I take care not to turn the nut under the chrome cover that sets the steering stem roller bearing pre-load as I had adjusted it and I don’t want it to change.
Then I loosen the fork tube cap bolts and torque them to the final torque of 35 Ft-Lbs using the handlebar in the alternate fork riser to keep the forks centered.
BMW uses a much higher fork tube cap bolt torque than 35 Ft-Lbs. But the design of the Toaster Tan top brace is such that a lower torque is all that’s needed to push the fork tube rings tight against the fork tube to prevent fork oil leaks.
Recheck Fork Alignment
I recheck the X-plane and Y-plane alignment. I found the forks X-plane alignment had moved outward almost 0.008 inches. That was unexpected. To resolve what happened, I loosened the top brace acorn nut, fork tube top bolts and the fork ring pinch bolts and remeasured the fork tube alignment. It was back to where it had been before I installed the top brace.
So I methodically tightened each bolt in the top brace measuring the X-plane alignment afterwards to try and isolate what was causing my X-plane alignment problem.
- I tightened the acorn nut to 40 Ft-Lbs and measured the X-plane alignment. No problems.
- I tightened the fork tube pinch bolts in the fork yoke to 25 Ft-Lbs. and measured the X-plane alignment after tightening each bolt. No problems.
- I tightened the fork cap bolts to 15 Ft-Lbs and measured the X-plane alignment after tightening each bolt. No problems.
- I tightened the acorn nut to 85 Ft-Lbs. No problems.
- I tightened the fork ring pinch bolts to 18 Ft-Lbs. Each one caused the forks to move outward. This was the problem.
I thought I might be able to use the clamp on the fork tubes to get them aligned, but after more than 10 hours of trying to eliminate the problem the forks were still out of alignment in the X-plane.
So, what is going on with the fork ring pinch bolts and how am I going to fix this?
After a cup of coffee and sitting next to the bike and quietly thinking, I came up with the following approach. Since the fork ring pinch bolts caused the top of the fork tubes to move inward which in turn causes the bottom of the tubes to move outward, I loosened the fork ring pinch bolts and put the clamp on the bottom of the fork tubes and tightened it up to move the fork tubes tighter by about 0.008 mm. Then I tightened the fork ring pinch bolts to 18 Ft-Lbs. The X-plane alignment was within about 0.0015 inches out of alignment. I used the clamp on the bottom of the fork tubes for about an hour and a half and got the tube back in alignment within 0.00025 inches.
Check Axle And Fork Brace Fit
I temporarily install the fork sliders on the fork tubes. I put a little fork oil on the seals to lubricate them and carefully push the sliders up the fork tubes so I don’t damage the fork seals. I finger tighten the large Allen bolts at the bottom of the fork sliders that attach the sliders to the damper rod.
I test fit the axle. I want it to slide easily through the holes in the fork sliders and be able to spin the axle when its in the holes. That ensures I have the fork tube lengths correct and there is no damage to the axle or the holes in the fork sliders.
I need to repaint the fork brace so I temporarily insert it between the posts in the fork sliders. There is a notch on one end of the fork brace that engages with a ridge facing the rear of the fender so I install the fork brace with the notch facing to the rear.
The fork brace should slide in between the four posts on the fork sliders and not bind but not be too loose. I look at the gap between the fork brace and the posts on the fork sliders and it looks like I have contact and the fork brace isn’t too tight.
I later found out my brace was too loose. I show how to fix that later.
Check Fork Stiction
Fork stiction refers to the friction between the fork tube and fork slider. If the friction is too high the fork slider sticks and won’t move freely causing a harsh ride.
I remove the Allen bolts holding the fork sliders on the damper rods. With the axle loosely attached to the fork sliders I push the fork sliders up and let them fall to see how much stiction there is. They fall easily.
Now I temporarily install the fork brace since I need to repaint it. I attach the brace to the left fork slider and torque the bolts to 15 Ft-Lbs. I retest the fork stiction and the fork sliders fall at the same rate as before.
Now I torque the right bolts to 15 Ft-Lbs and retest the fork stiction. Unfortunately the stiction increased measurably. So I remove the bolts on the right side of the fork brace and insert a thin flat washer between the fork slider post and the fork brace at each bolt hole. I install the bolts and torque them again. This time the fork sliders fall at the same rate they originally did. So, the fork brace was a bit too loose and it bent the forks a bit increasing the stiction.
Install Fork Sliders and Fork Brace
I install the new fork seal dust caps on the fork tubes. There is a felt strip that fits inside next to the top of the dust seal. Mine is a bit too long so I trimmed it. Then I install the fork sliders with the fork brace on the fork tubes.
I push up on the slider and finger tighten the large Allen bolt. Then I install the newly painted fork brace and torque the fork brace bolts to 15 Ft-Lbs. Now I can pull up on the fork brace a bit while I torque the large Allen bolts to 25 Ft-Lbs. Since the springs are pushing on the damper rods, there is enough friction to tighten the Allen bolts without the fork damper rods spinning.
Here is the final assembly of the Toaster Tan top brace and the front forks.