- Parts List
- Getting Started
- Temporarily Install Upper Fork Brace
- Install Fork Tubes in Bottom Yoke
- Check Front Axle
- Measuring Fork Alignment to Reduce Stiction
- Assemble Upper Brace, Steering Stem Top Nut & Fork Cap Nuts
- Add Fork Brace & Lower Fork Slider, 1st Time
- Tighten Steering Stem Nut and Fork Cap Nuts
- Installing the Fork Boots, Lower Brace and Sliders, 2nd Time
- Install Steering Damper Assembly
- Final Product
Earlier, I rebuilt the front forks, refinished the sliders and replaced the steering head bearings. I also polished the steering damper knob and added a modern BMW emblem to the center of the knob. You can read about how I did that here:
- 31 BMW R75/5 Rebuild Front Forks
- 31 BMW R75/5 Remove & Install New Steering Head Bearings
- 51 BMW R75/5 Refinish Steering Damper Knob
So, now I’m ready to install the front forks and align them.
Here are the new parts I used.
|31 42 1 230 696||Upper rubber gasket, headlight bracket||2|
|31 42 2 000 385||Lower rubber gasket, headlight bracket||2|
|31 42 1 232 511||Upper Fork Cross Brace||1|
|31 42 2 002 115||Fork boots, 13 rib, pair||1|
I relied on material published by Randy Glass, Duane Ausherman and Robert Fleischer. Duane’s page includes a link to an in-depth article about fork alignment published by Randy Glass. However, the link in Duane’s page to Randy’s article doesn’t work with all browsers. The link below works.
- 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
- Robert Fleischer, Front Forks on Your BMW Motorcycle
(See in particular the section on stiction and the working link to Randy’s Material)
I use an 8 inch vernier caliper with 0.001 accuracy for the measurements.
I also use a glass plate, 10 inch x 12 inch, and a 0.002 inch feeler gauge to align the forks.
NOTE: I added this section after I first posted this write-up. Although all of this information is available in the references I mention above, I think providing this overview helps you understand why I did things in the order I did.
Before I go into how I did the work and the problems I encountered along the way, let me summarize how the fork construction affects alignment.
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 upper fork brace (upper brace), the lower fork yoke (yoke), the lower fork brace that goes on the 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 rotation of the tubes when clamped only in the lower yoke is equal so one tube is not forward of the other.
Each of the four horizontal connections between the tubes is an opportunity for misalignment in both planes. And, the tubes themselves can be bent, but I didn’t check them as this bike has not had any front end collisions. So, I need to measure the tube alignment in the X and Y planes as I add each horizontal connection and then correct any misalignment I find.
The top of the tubes touch, but don’t penetrate, the holes in the upper brace. A large fork cap nut goes through the hole in the upper brace, threads into the top of the fork tube and connects it to the upper brace. The upper brace also connects to the steering stem using a large acorn shaped steering stem nut. The steering stem nut has a boss that goes through the hole in the upper brace and presses on the slotted nut used to adjust the pre-load on the steering stem bearings. I show a picture of this later on. Therefore, the upper brace is connected at three points. Any differences between the height of the fork tubes and the top of the slotted nut will bend the top brace affecting fork tube alignment.
NOTE: Bob Fleischer told me that there have been instances where the top triple plate has been worn, and the hole the steering stem nut boss goes into is too large, which you find out after the fact as there is no official dimension published for that hole. Any wear or elongation of the three holes in the upper brace will result in unstable alignment. That is one reason I bought a new upper brace. You may need a new steering stem acorn nut as well. For these reasons, some folks install beefier third party upper braces machined to closer tolerances. I choose not to do that on this project.
The lower U-shaped fork brace that mounts on the lower sliders can also be a problem as the brace can be too wide or too narrow, one of the legs can be bent ahead of the other, and the face that mounts on the lower slider can be uneven. Both Duane and Bob talk about how to handle this. Mine was fine.
One other adjustment to perform is the steering stem bearing pre-load. This too is an opportunity to tweak the upper brace and/or compress the tubes creating misalignment. The slotted nut puts pre-load on the bearings and the steering stem acorn nut presses on the top of the slotted nut and also adds some pre-load to the bearings. My bike is 1973 and 1/2 with long wheel base. It has the newer /6 series steering stem slotted nut for adjusting the bearing pre-load. Duane and Bob have details on how to adjust the pre-load with the earlier split ring. I’m glad this bike has the newer slotted nut.
So we have two fork tubes, four horizontal members and various nuts all interacting and potentially creating misalignment of the fork tubes.
Summary of Alignment Measurement Steps
Here is a summary of my procedure for checking the fork tube alignment.
- Install the fork tubes in the lower yoke so they are flush with the bottom of the upper brace.
- Tighten the yoke nuts to 25 FOOT/pounds.
- Check the axle for smooth fit in the holes in the lower fork sliders.
- Check the lower yoke for X and Y plane alignment and correct as needed.
- Check the upper brace for X and Y plane alignment and correct as needed.
- Check the axle for X and Y plane alignment and correct as needed.
- Check the lower fork brace for X and Y plane alignment and correct as needed.
Summary of Final Assembly Steps
Now it’s time to put everything together starting with the fork tubes in the lower yoke and the lower yoke nuts torqued to 25 FOOT/pounds.
- Install the headlight ears with turn signal wiring (I didn’t install the turn signal wires as I’m using a Windjammer Fairing).
- Install the upper fork brace and finger tighten the fork cap nuts and steering stem nut.
- Loosen the lower yoke nuts so they are finger tight.
- Set the pre-load on the steering stem bearings with the slotted nut and then torque the steering stem nut and the fork cap nuts.
- Check X and Y plane alignment.
- Torque the lower yoke nuts to 25 FOOT/pounds.
- Check X and Y plane alignment
- Install the lower sliders on the forks.
- Install the axle and tighten axle clamp bolt
- Check X and Y plane alignment.
- Install lower fork brace
- Check X and Y plane alignment.
At this point, the forks are installed and parallel in both planes and it’s time to install the front brake assembly and front wheel. I’ll cover that in a separate write up.
Things I Learned Along the Way
- It’s important to keep checking X and Y plane alignment after you make any change to the fork assembly. That way, when things change, you can start with what you just did to find out what caused the change.
- The upper headlight ear gasket caused the upper brace to be above the top of the slotted steering stem nut. I added a washer on top of the slotted nut to correct this. I learned from Bob Fleischer that at one time BMW offered different thickness top gaskets for the headlight ears. I didn’t see any of the other part numbers when I ordered the one I show in the table above. I suggest that you check X and Y plane alignment of the upper brace with the headlight ears and gaskets installed and then remove the top gasket and see what happens.
- BMW upper braces are notorious for being out of tolerance. Many folks use third party products for the upper brace. I installed a brand new polished brace from BMW and did not find it out of tolerance.
- Patience is very important. It took me two weeks to complete this process. I had to wait for two days for some of the adjustments to the lower yoke to become permanent and then had to do some of the adjustments again.
Here is where I started. I have the front steering stem installed in the new bearings in the steering head.
I have the lower sliders mounted on the fork tubes.
Note there is a left and right lower as shown here: top is right and bottom is left side slider.
Temporarily Install Upper Fork Brace
Here is the new upper fork cross brace. The dark areas in the cross brace are reflections in the polished surface of me taking the picture, not discoloration.
This is the lock nut and washer on the stud in the lower yoke. This secures the fork tube in the yoke.
The steering stem top nut has a boss on the bottom that fits inside the center hole in the upper cross brace centering it on the steering stem.
Here is the upper fork brace mounted on the steering stem using the stem top nut. I tightened the top nut just enough (not fully finger tight) to be sure the boss on the bottom is inside the hole in the top brace and the top nut is flush with the top of the brace.
Install Fork Tubes in Bottom Yoke
The top of the fork tube is threaded for the large fork cap nut that holds the fork spring inside the fork tube. The fork cap nut sits on top of a large steel washer and the top unthreaded section gets centered inside the holes in the upper cross brace.
I want the top of the fork tubes to touch the bottom of the upper cross brace as I slide them through the holes in the lower yoke, but not enough to deflect the cross brace.
I slide a fork tube into the lower yoke so it touches the bottom side of the cross brace, I tighten the nut on the lower yoke stud to hold it in place. Then I insert the fork cap nuts into the fork tubes finger tight to settle the cross brace on top of the fork tube. I make sure the fork cap nuts pass through the hole in the cross brace so the bottom of the nut rests on the washer with no gap. I keep both the steering stem top nut and each of the fork cap nuts loose until I get the fork cap nuts aligned and then finger tighten all three nuts.
I measure the distance from the machined land on the lower yoke to the bottom of the cross brace. It should be close to 160 mm.
Check Front Axle
With the forks assembled in the lower yoke, I torque the lower yoke lock nuts on the studs to 25 FOOT/pounds.
The front axle should slide through the holes in the lower sliders without any resistance. If there are any nicks or burrs on the axle that prevent this, they should be cleaned up removing the minimum amount of steel necessary for smooth sliding.
My front axle slides very smoothly through the holes and I can push it completely through the hole in the left fork slider. It can not go all the way through the right slider hole as that hole is a smaller diameter.
I inspect the fork tubes for any nicks or gouges (there are none) and clean the outside of the fork tubes to get any dirt and grit off them. Then I use the axle to push the lower sliders up the fork tubes to feel how much effort this takes. I want to compare the effort before and after I align the fork tubes.
I remove the lock nut on the damper rod at the bottom of the fork slider using a 13 mm socket clamped in my vice grips and an Allen head socket. I keep the axle installed in the lower sliders and put them aside.
Measuring Fork Alignment to Reduce Stiction
Here is where I started with the fork tubes mounted in the lower yoke with the lock nuts on the lower fork yoke torqued to 25 FOOT/pounds.
I removed the the steering stem top nut, the top brace and the two fork cap nuts so the fork tubes are only clamped by the lower yoke.
A primary cause of stiction, which is resistance that hinders smooth movement of the slider up and down the fork tubes, is the fork tubes not being parallel.
There are two planes, the X and Y, in which the fork tubes have to be parallel. The Y-Alignment is measured with a piece of glass placed across the front of the fork tubes and a 0.002 inch feeler gauge to ensure one tube is not in front of the other. The X-Alignment is measured by the distance between the fork tubes at the top and the bottom of the tubes with the vernier caliper. The distances should be the same if the tubes are parallel in the X-Plane.
Y-Plane Measurement and Alignment
I bought a 10 in X 12 in piece of glass from the local Ace Hardware store. The edges and corners are very sharp so I used some 600 grit wet/dry paper with a little water and rounded all the edges. Pay attention to the corners to avoid chipping them but make sure all the sharp edges of the corners have been smoothed.
Put some blankets or soft padding under the fork tubes to cushion the glass plate should it slip and fall. Then, with clean hands so the plate won’t slip out of your fingers, place the plate flat on the fork tubes with the 12 inch side along the tubes and the 10 inch side across the tubes. Slide the plate over so you can hold the left edge against the left fork tube (as you face the bike from the front) with your thumb in the middle of the glass on the front side and the fingers behind the fork tube.
It’s tight against the left tube but loose against the right tube. Now, with the index finger of your right hand, gently tap the top right corner of the glass and then the bottom right corner of the glass and listen for any “click”ing. You will likely hear a click either when tapping the top edge or the bottom edge, but not both. The click indicates the tubes are not parallel. No click and you are aligned, but check with the feeler gauge to be sure.
In my case, the lower right edge clicked. To verify there is a gap and estimate about how much it is, I use a 0.002 inch feeler gauge and very lightly slide it behind the glass and on top of the tube. At the top right edge, with very little pressure, it stops and does not go between the glass plate and the fork tube. I slide the feeler down the tube with the same light inward pressure and sure enough as I get lower down the right tube, the feeler starts to move further over the top of the tube until toward the bottom, it slides between the tube and the glass.
Now I slide the glass over so the right edge is along the right tube and repeat the tapping test on the upper left and lower left corners of the glass. This time the click is at the top left edge but not the lower left edge. I use the 0.002 inch feeler gauge and can see it slip between the glass and fork tube at the top of the left tube.
Okay, the tubes are not parallel in the Y-Direction. The fix for this is to put pressure on the tubes so the lower yoke, which is aluminum, deforms enough to make the tubes parallel.
I use a four foot piece of 2 x 4 and put it between the tubes and then pull on the long end to move one tube forward and the other backward. Based on Randy’s description of how to interpret the clicking, I conclude that the left tube is in front of the right tube, as viewed from the front of the bike. I need to push the left tube back and the right tube forward, so this is how I inserted the 2 x 4 between the fork tubes. I used a rag between the wood and the tube to protect the tubes from being scratched.
I loop a piece of 1/2 inch rope around the rear foot brake lever and the 2 x 4 and knot it. I use the handle of a hammer between the ropes and twist them until they are pretty tight and rest the end of the hammer against the engine so it won’t unwind.
I estimate the tubes are not far out of alignment as the 0.002 inch feeler pushed the plate up a bit when it slide between the glass and the fork tube. So I leave the pressure on the tubes for an hour and then check the alignment with the glass plate. I can’t hear any more clicking and the feeler gauge doesn’t go between the glass and the tubes. I will measure the Y-Plane alignment again after I adjust the X-Plane to be sure the stretching in the yoke is stable.
X-Plane Measurement and Alignment
To measure the X-Plane alignment, I use an 8 inch vernier caliper with 0.001 inch resolution. I take two measurements, just under the bottom of the lower yoke aligning the knives of the caliper using the fork boot pins as a guide. It’s very hard to ensure I am holding the calipers perpendicular to the tubes. So I removed the lock screw on the right side sliding jaw of the caliper, and adjust them to fit between the tubes with the fixed knife edge on the left tube and the slider end of the caliper on the right just touching the tube and angled a little up hill of the fixed knife edge. This measurement is too large since the distance isn’t the shortest distance between the tubes as the caliper is not perpendicular to both tubes. Then very slowly and gently I rotate the right side slider knife edge down the tube and then back up the tube until I get a constant reading. This is the shortest distance between the tubes with the sliding knife edge very lightly touching the tubes so they aren’t being pushed apart by the caliper. I take multiple measurements and get good repeatability. I find a difference of +0.013 inches, meaning the tubes are wider at the bottom than the top.
To align the tubes in the X-Plane, I clamp then with a bar clamp under the lower yoke. I need to close the tubes under the yoke to decrease the distance between the bottom of the tubes since the bottom is wider than the top. If the bottom was closer than the top, I would clamp the tubes above the yoke to increase the distance at the bottom of the tubes.
The objective is to deform the aluminum yoke which is softer than the steel fork tubes, not to bend the fork tubes. You don’t want bent fork tubes and it would be hard to do that with the clamp anyway.
I measure the top and bottom distances after applying the clamp and now have a negative difference of -0.027 inches, so the bottom of the tube is narrower than the top. I leave them this way for a day, remove the clamp and measure the distance between the tubes again. The bottom has closed and is now +0.006 inches wider at the bottom. I have removed just about half the original difference.
At this point I add a second clamp to spread the tubes above the lower yoke while compressing them below the yoke.
This increased the bottom distance to a negative -0.256 inches closer at the bottom than the top. I leave the clamps on for another 24 hours, remove them and measure the distance between the forks again. This time they are within about 0.001 inch. I let them sit for another 24 hours and then measure them again. I find that both the Y and X alignment are still within 0.001 inch.
Assemble Upper Brace, Steering Stem Top Nut & Fork Cap Nuts
I add the upper cross brace, steering stem nut and fork cap nuts. I fit each nut finger tight so the upper cross brace is aligned with the fork tubes. The nuts should tighten until flush with the cross brace without any resistance and the brace should be flush with the top of the tubes and the top of the slotted steering stem nut under the upper brace.
Then I measure the Y-Plane and X-Plane to be sure they were still parallel. The Y-Plane was but the X-Plane wasn’t. The tubes moved about +0.006 inches closer at the bottom than the top. 🙁 The tube cap nuts went in very smooth with no binding so I don’t think there was any pressure on the tubes. Curious.
I install my wood clamp as a spreader between the tubes below the yolk for 24 hours. I measured the X-Plane and Y-Plane again. Both are parallel again.
Add Fork Brace & Lower Fork Slider, 1st Time
I fit the lower sliders on the fork tubes so I can mount the lower fork brace on the sliders to be sure the tubes remain parallel in the X-Plane. I have to fish the damper rods out of the holes in the bottom of the fork tubes with a small screw driver.
I insert the axle and made sure it still slides smoothly in the holes and then slide the lowers up and down the fork tubes to help center the lowers on the axle and eliminate any X-plane binding of the lower sliders. I measure the X-plane distance between the tubes to be sure they are still parallel, and they are.
Then I add the fork brace to lower sliders and finger tighten the four lock nuts then torque them in a cross wise pattern to 12 FOOT/pounds. I kept the forks centered with my hand on the axle while I torque the lock nuts. I check the X-Plane distance between the fork tubes again and the measurement hasn’t changed.
Tighten Steering Stem Nut and Fork Cap Nuts
It’s time to put the headlight ears and turn signal stalks on the forks and tighten the steering stem nut and fork cap nuts to the final torque. I remove the finger tightened steering stem nut and fork tube cap nuts along with the upper brace so I can slide the turn signal stalks and headlight ears on the top of the fork tubes. Here are the parts for the headlight ears, turn signal stalks and the chrome decorative caps that go on top of the fork tube cap nuts.
The turn signal stalk mounts over the locating pin on the lower fork yoke. These stalks are cut down to fit inside the Windjammer fairing so they don’t bind on the fairing.
I install the rubber gaskets on the headlight ears. and then install the ears over the top of the fork tubes. The bottom gasket has a hole that fits over the same locating pin on the lower yolk.
The top rubber gasket is a little bit proud of the top of the fork tube, so it will be compressed by the upper brace when the tube cap nut is tightened.
NOTE: This turns out to be a the cause of a problem later on, but I didn’t know that yet.
I install the upper brace on the steering stem, insert the new fork springs in the top of the fork tubes and finger tighten the steering stem nut and the fork cap nuts being sure they go down flush on top of the upper brace.
I install the fork sliders on the tubes with the lower fork brace and measure the X-Plane with the vernier caliper to be sure the tubes are still parallel.
I loosen the lower yoke nuts to finger tight. I adjust the steering stem bearings with the hook spanner so the bearings are tight enough that the forks don’t fall to one side when nudged from the center, but just start to fall when they are very close to the stops. Then I torque them to 25 FOOT/pounds.
I plan to use the old handlebar as an “anti-torque” tool to prevent tweaking the forks when I tighten the steering stem and fork cap nuts. I install the handlebar clamps with the pull backs my wife prefers and then lock one end of the handlebar in the clamps.
I torque the steering stem nut to 80 FOOT/pounds with a 36 mm socket using the anti-torque tool to keep the forks centered. I use the dog-bone wrench in the tool kit to tighten the fork cap nuts again using the handlebar to keep the forks centered. I snug the cap nuts up using a plastic hammer on the end of the dog bone wrench.
I pull the fork lowers off the tubes and measure the X-Plane and Y-Plane alignment. They both have moved. The X-Plane is too close at the bottom by about 0.013 inch. I am a bit bummed out by this. 🙁
I’m not sure if tightening the fork cap nuts and steering stem nut caused the tubes to walk or not. Dennis said that the /5 tubes are prone to walking when everything is tightened and Duane’s material talks about the problems shops had with brand new /5 bikes from Munich with forks made with out of spec parts.
So Why Did The Forks Suddenly Go Out of Alignment?
I remove the upper brace, steering stem nut and fork cap nuts. I loosen the nuts on the lower yolk studs. Then I install the upper brace, steering stem nut and tube cap nuts and torque them using the anti-torque handlebar to prevent tweaking them. I install the fork sliders with the lower fork brace and then I measure the X-Plane distance with the vernier caliper and find the tubes are within 0.001 inch. Next, I evenly and alternately tighten the lower yolk nuts and measure the X-Plane distance between the tubes as I go. As I get them close to tight, the X-Plane goes out of parallel by about 0.011 inch. Interesting
Now I loosen the steering stem nut and fork cap nuts and the forks move back to being close to parallel. Hmm. Something is pushing the tubes outward at the top and when I tighten the nuts on the yoke and that pushes the tubes inward at the bottom. When the yoke nuts are loose, the bowing at the top is absorbed by the slop in the yoke.
I take the upper brace off and the headlight ears. The rubber gasket on the ears is above the top of the fork tube which I noted when I installed the ears.
So, I loosen the yoke nuts and then reset the tubes so they touch the bottom of the upper brace when the steering stem nut and tube cap nuts are finger tight instead of leaving the rubber gasket proud of the top of the fork tubes. Then I tighten the yolk nuts to 25 FOOT/Pounds. Of course, the tubes move in the X-Plane again.
What is going on?
I remove the ears so I can measure what is happening in the X-Plane at the top of the fork tubes. I keep loosening and tightening the fork cap nuts and steering stem nut while measuring the distance between the top of the fork tubes with the caliper, and it looks like tightening the fork cap nuts pushes the forks outward when the steering stem nut is tight. But, the fork tubes stay put if I don’t tighten the steering stem nut. As I look closely under the upper brace, the bottom of the brace is no longer resting on top of the slotted steering stem nut when the steering stem nut is loose. There is a gap. Here is a picture of the gap. The upper brace is polished so there is a mirror image of the steering stem slotted nut on top, but there is only one nut.
It dawns on me this is due to the rubber gasket on top of headlight ears. When I moved the tubes up to touch on the bottom of the upper brace to get them flush with the bottom of the upper brace, the ends of the upper brace are now higher than the end of the upper brace around the steering stem creating the gap under the steering stem nut. When I tighten the steering stem nut, it bends the upper brace downward putting pressure on the top of the fork tubes knocking them out of alignment.
I go over to the Ace Hardware store and find a 14 gauge and 18 gauge washer that fit over the threads on the steering stem. I find that adding the 18 gauge washer between the slotted steering stem nut and the bottom of the upper brace eliminates the gap.
The steering stem nut presses on the top of the slotted nut to load the steering stem bearings. A washer between the slotted nut and the bottom of the upper brace won’t change that since steering stem nut sits fully on the washer.
Now when I finger tighten the steering stem nut and the fork cap nuts, the top brace is flush with the steering stem slotted nut. In the picture below, the polished upper brace makes it appear there is a second washer and slotted nut on top, but it is just a reflection.
The tubes aren’t being pushed outward at the top any more. Next, I loosen the steering stem nut and remove the fork cap nuts to add the headlight ears. Then I torque the steering stem and fork cap nuts using the anti-torque handlebar. What do you know, the tubes are now parallel in the X-Plane and the Y-Plane. It doesn’t take much to prevent the tubes from being parallel.
Installing the Fork Boots, Lower Brace and Sliders, 2nd Time
Now I add the fork boots on the fork tubes. The top of the boots have a hole that locates the top of the boot on the pin in the lower yoke.
Then I add the lowers and the fork brace.
I snug up the nuts finger tight and measure the X-Plane distance. I adjust the brace so the alignment stays the same. Then torque the nuts on the lower fork brace to 12 FOOT/pounds while holding the axle to avoid twisting the fork tubes and measure the X-Plane distance between the tubes. The tubes get out of parallel and I loosen the nuts and get the tubes aligned and then tighten them until everything is within 0.001 inch.
Here is the final assembled and aligned fork tubes with fork boots.
Install Steering Damper Assembly
Here are the damper friction assembly parts.
The two plates mount under the steering stem. The one on the left threads on to the steering damper rod and pushes against the friction plate on the right which is attached to the frame via the Allen head bolt.
Here is the friction plate installed at the bottom of the steering stem with the threaded end of the damper rod in the center.
The damper friction bracket installs on top of the friction plate and the threaded damper rod screws into it.
Here is the condition I started with for the upper fork brace and lower fork tubes.
Earlier, I refinished the steering damper knob to remove the scratches and to add a modern BMW emblem to the center of the knob.
I think the effect is very nice and will pull your eye to this small detail. It certainly is a major improvement over the original state of affairs.
2018-09-07 Corrected Randy Glass article link to go to start of article.