- Snowflake Wheel Design and Bearing Preload
- Remove Wheels
- Remove Front Disk Rotors
- Remove Wheel Bearings
- Powder Coat Wheels
- Install Outer Bearing Races
- Set Bearing Preload
- Install Grease Seal
This bike does not have the original spoke wheels. Instead it has the cast aluminum alloy “snowflake” wheels that came latter. The rear snowflake has a drum brake while later versions of the snowflake rear wheel have a rear disk brake on the left side.
It seems that airhead wheels are a component that BMW changed a lot over the life of the airhead bikes. Further, parts fiche diagrams are notoriously confusing and frequently show incorrect parts. As I tried to get solid information about my wheels, one sage, long time wrench advised me to just inspect the parts that are there and replace what’s worn.
That said, these wheels had parts that took me some time to figure out. First, the snowflake drum rear wheels were only available with an aluminum hub in 1978. Later versions had a steel sleeve in the hub that the rear wheel bearing outer races fit into and this design continued through the disk brake rear wheels. This is an improvement as the aluminum hub to steel race interference fit was prone to allowing the race to spin in the aluminum hub which leads to a mess not to mention it can lock up the rear of the bike if the bearing seizes to the axle. A steel sleeve is not prone to spinning in the hub.
My drum snowflake rear hub has a steel sleeve as shown below. The inner edge of the hub is magnetic and you can see the line between the inner sleeve and the outer aluminum of the hub.
The rust on the end of the steel sleeve is evident on the left side of the rear hub.
It’s important to be sure you know what material the hub is made of, aluminum or steel, as the method for removing the outer bearing race and the internal hub components is different. In particular, if you don’t heat an aluminum hub to about 250 F before removing the hub internal components as a complete stack, you will damage the aluminum hub leading to a spun outer bearing race in the future.
On the rear drum snowflake wheel, I found the right top hat to be shorter (9.2 mm) than the left (13 mm) which is the same height as the two top hats on the front wheel. It turns out this is normal for the rear snowflake wheel. There is also a taller top hat for the right side of the rear, a 10.7 mm one (part# 36 31 2 301 737). It is used to move the wheel to the left a bit when fitting a larger rear tire,–a 110 or 120 metric tire–so it will not rub on the swing arm.
My bike had a 120 rear tire that I had to deflate before I could remove the wheel as the profile was too wide to clear the left swing arm tube and the brake housing. Since I’m going to mount a 4.00 x 18 rear tire, the standard right (9.2 mm) and left (13 mm) top hats are what I need.
All the top hats have a groove worn into them by the lip of the grease seal and several had some rust on the end that is proud of the grease seal. I replace all of them which will help keep water & dirt out of the wheel bearings.
The wheels are discolored, there is aluminum corrosion and the paint is missing in a number of places. There are no deep gouges or dents in the rim so they don’t look like they have been abused, just neglected.
I decided to have them powder coated rather than painted. Opinion about using powder coat vs. paint is strongly divided. I decided to try the powder coating rather than paint due to the design of the cast wheel with the many corners and edges that invite drips and sags when spaying paint. Time will tell if this will be a decision I regret. It wouldn’t be the first time 🙂
Bob Fleischer has a number of sections of content about wheels, hubs, top hats, bearings and how to adjust the preload on the tapered roller bearings used in the wheels.
- Bob Fleischer: WHEELS, Wheel Spacers, Seals + Hints
- Bob Fleischer: Airhead Wheel Bearing Clinic, BMWMOA National Rally, July 2005
- Bob Fleischer: Wheel Bearings; preload & servicing
Duane Auscherman also has a good page about wheel bearings.
- Duane Auscherman: BMW motorcycle wheel bearings, 1956 to late 70s
The graph he shows of the tapered roller bearing life vs. preload setting was instructive. I conclude that lower preload values affect bearing life less than too high a value. And, the miles available from a preloaded bearing are “large” compared to the typical mileage expected for a BMW motorcycle, except when the preload is too high. Bob Fleischer points out that BMW’s recommendation of 21 – 42 inch-ounces of torque puts too high a value on the range. He recommends no more than 30 inch-ounces unless you have a side car attached. He is concerned with a bearing overheating if it has too much preload and that can lead to welding a bearing to the axle. I’m aware of one such incident, but I suspect the owner never lubricated the wheel bearings which certainly leads to bearing failure.
As I said earlier, this list of parts is specific to what I found for these wheels when I took them apart. YOUR WHEELS ARE LIKELY DIFFERENT. So, take note of the parts you find, which side they are on and the order they are assembled from the outside to the inside. I also replaced the front and rear axles with good used ones which you likely won’t need to do. Mine were badly rusted and I sourced some nice used ones that shined up nicely.
|Cycle Works||Wheel Bearing Preload Adjusting Kit
|07 11 9 985 005||TAPERED ROLLER BEARING – 40X17X14 (to 01/81)||4|
|36 31 4 038 142||SLEEVE – 9,2MM; (Top Hat, RIGHT REAR)||1|
|36 31 1 230 322||COMPRESSION RING (Top Hat Stnd 13 mm)||3|
|36 31 1 240 238||SHAFT SEAL – 22X40X7||3|
|36 31 1 235 833||SHAFT SEAL – 40X22X5||1|
|34 11 1 233 490||QUICK-RELEASE AXLE – D=17MM/L=253MM, Front||1|
|33 41 1 230 303||PLUG TYPE AXLE DRUM BRAKE – D=17MM/L=296MM||1|
I need a blind bearing puller to remove the outer bearing races from the steel sleeve. I have a Cycle Works tool that removes and installs the swing arm roller bearing which is the same bearing used for the wheels.
Only use the Cycle Works bearing puller, or an equivalent blind bearing puller, IF YOU HAVE THE STEEL SLEEVE VERSION OF THE SNOWFLAKE WHEEL. Using it on an all aluminum hub will ruin the hub.
I use the Cycle Works wheel bearing extractor and preload tool to preload the rear snowflake wheel bearings. At 113 mm, it is too long for the front wheel so I use a 1 inch diameter piece of steel tube about 3-1/8 inch (80 mm) long. I had a friend mill the ends flat and parallel.
Snowflake Wheel Design and Bearing Preload
BMW used tapered roller bearings for the wheels, not ball bearings. Duane Auscherman has good information on the affect of bearing preload on bearing life. As I show later, the “wedding band” spacer is the component used to adjust the bearing preload.
The spoke wheels are designed differently from the snowflake wheels I have. The spoke wheels use a larger diameter center pipe that butts up against the outer bearing races as there is no shoulder inside the hub for the outer races to sit on. In the snowflake wheels with the steel inserts such as mine, there is a shoulder that supports the outer race. Therefore, my wheels do not need the large diameter center pipe.
The spoke wheels use a smaller pipe that goes inside the larger one to press on the center ring of the inner bearing race. The wedding band spacer adjusts the length of the smaller pipe which in turn adjusts how much preload is applied to the bearing race and the rollers. So, for spoke wheels there are two center pipes, a larger diameter one pressing on the outer bearing race and the smaller one inside it that presses on the center ring of the inner bearing race.
In my snowflake wheels with the steel inserts, I only have one center pipe that presses on the center ring of the inner race. It uses the same wedding bands as the spoke wheels to adjust the preload.
My rear wheel is not the standard snowflake rear wheel BMW used with the drum brake. It has a steel sleeve which was NOT used in the standard rear wheel. I show how to determine if you have steel sleeves in your snowflake hubs and it is important to know this as the technique for removing the bearing races and center pipes depends on if you have aluminum or steel sleeve hubs.
Remove the front and rear wheels by removing the axles and then removing the wheels. It helps when removing the rear wheel if you can put the bike’s center stand on a 1″ or thicker board so the wheel has a bit more clearance for removal.
I removed my tires and tubes since I am replacing them and having the wheels powder coated. If your snowflake wheels have all aluminum hubs, you will need to remove the tires and tubes since you will be heating the hub to 250 F to remove the bearing stack. I didn’t need to heat my wheels since both have steel sleeves in the hub and there is no benefit since the steel sleeve and the steel outer bearing race will expand at the same rate.
Remove Front Disk Rotors
I use my 20 volt cordless electric impact drive to remove the Nylok nuts on the left side of the front wheel from the five bolts that go through the hub securing the rotors to the wheel.
I mark the rotors “L” and “R” for left and right in several locations so I can install them on the side they came from. Here is picture of the rotors and matching side of the wheel.
Each rotor has a shim under the bolt heads and nuts that I mark “L” and “R”. I reinstall them on the bolts to avoid getting things mixed up.
The rear wheel has an arrow showing the direction of rotation so you won’t install the wheel backwards. But you can only mount the rear wheel one way due to the drum brake hole on the right side. The front wheel does not have an arrow, but there are differences in the hub design on each side as shown below.
Remove Wheel Bearings
I decided to start with the rear wheel. This pictures below show the right side that has the brake drum and left side of the rear wheel.
Rear Wheel Top Hat, Oil Seal, Wedding Band and Inner Bearing Race
First, I remove the “top hat” on the right side from the oil seal using a screw drive to pop the top hat out of the seal.
In the process, the oil seal came out as well since the rubber in the seal is hardened.
I lift out the inner tapered roller bearing race.
Next I pull out the “wedding band”. This is the part that adjusts the bearing preload.
Here are the right rear bearing parts from outside to inside, left to the right: oil seal, top hat, inner bearing race and the wedding band.
I can now see the center pipe, one of the centering ring and the outer race secured in the steel sleeve.
The wedding band comes in different thickness to adjust the bearing preload. Mine is approximately 7 mm thick. These are available in increments of 0.05 mm (0.002 inches) from 6.300 mm to 7.700 mm. I’ll need to use my good vernier caliper or micrometer to make a correct measurement of the thickness of wedding bands.
I inspect the splines in the rear hub for excessive wear. The teeth appear to be in good condition and are not rounded off nor sharpened at the top of the teeth.
I repeat the procedure to remove the left side top hat, use the screw driver to pry the oil seal out and remove the inner bearing race. There is no wedding band on this side. I’ve been told it doesn’t matter which side you install the wedding band as there is one per wheel and it will adjust the bearing preload whichever side it is installed on.
The spring separated from the left oil seal when I removed it.
Different Rear Wheel Oil Seals
The rear wheel uses different oil seals on the right and left. The right one is not as thick (5 mm) as the left one (7 mm). The thicker left one is used on both sides of the front wheel.
This is the left side seal mold mark which is one more than the last three digits of the original part number (36 31 1 235 836). This part number has been superseded by part # (36 31 1 235 838).
The dimensions of the seal are shown here (22 x 40 x 7).
I measure the depth of the top of the outer bearing race on the left and right side of the hub with a vernier caliper. The left is 14 mm deep and the right is 9 mm deep.
The outer bearing races show no sign of brinneling (shaded vertical bands) nor scoring. When I clean the inner bearing races of grease, they rotate smoothly and are not notchy. I will reuse the rear bearings.
Front Wheel Top Hat, Oil Seal, Wedding Band and Inner Bearing
The procedure for the front wheel is the same as for the rear wheel. I found the front wheel wedding band on the left side. The centering ring on the center pipe is positioned to hold the wedding band, by accident I suspect.
Again, when I inspected the outer bearing race there was no sign of brinneling nor damage and the cleaned front bearing spun freely so I will reuse them.
Remove Outer Bearing Races
If your cast wheels are the earlier ones WITHOUT THE STEEL INSERTS this procedure will ruin your wheels.
I assemble the Cycle Works swing arm bearing puller and insert the puller body into the hub.
I tighten the center bolt to expand the bottom of the puller body until I can’t pull the body out of the hub.
I assemble the larger outer sleeve with the washer and nut and tighten the nut with a wrench to extract the outer bearing race from the hub.
If you tighten the center bolt too far, you can jam the center bolt assembly of the puller against or even push it past the heads of the 6 machine screws. If the center bolt is jammed, it will be too deep and when you put the large outer sleeve on the hub, you won’t have enough of the center bolt thread to attach the nut. The solution is to continue tightening the center bolt until it goes past the heads of the 6 machine screws so you can remove the puller body. Disassemble the puller body and reinstall the center nut assembly and try again.
Now I can remove the center pipe with the plastic centering rings from the inside of the hub. I mark the plastic sleeves with “L” and “R” to be sure I install the center pipe and sleeves in the same orientation.
The two sleeves are not installed on either center pipe symmetrically. It doesn’t matter since they only help to keep the center pipe centered inside the hub. In the picture below, one of the centering rings, the left, on the front wheel is proud of the center pipe. It was done this way to hold the wedding band on that side. The wedding band is against the center pipe and the inner race of the left wheel bearing so there is metal-to-metal contact.
The front center pipe is longer than the rear one.
Like the rear bearings, the front outer races show no signs of damage and the inner bearing races spin freely when cleaned so I will reuse the front wheel bearings.
Powder Coat Wheels
I took the wheels for powder coating to a local firm. They had a good match to the original paint color used on the wheels. I could have them put on a powder clear coat on top of the color powder coat, but I decided not to do that. The clear coat is very shiny and the original wheels were not so I think this is a better match.
Install Outer Bearing Races
I use the Cycle Works tool to install the outer races in the hubs. The rectangular aluminum plate short side is wide enough to span the edges of the outer race. I put the outer races in the freezer for 30 minutes. I heated the steel sleeve in the hub with a heat gun. I start the new outer race using the old one to start the race into the hub.
The new race starts to become uneven in the hub so I tap the high side of the old bearing race to straighten it. When the new race is even with the hub, I use the Cycle Works aluminum plate to drive the outer race until it’s solid against the shoulder in the hub.
I strike the aluminum plate, rotate it 90 degrees for the next hit and continue this way to help keep the bearing square in the hub. The sound of the hammer changes when the outer bearing race butts up against the sleeve–a ringing sound like striking a hammer on an anvil.
I insert the inner pipe with the ends pointing to the side of the wheels I found when I removed it.
Then I drive in the other outer bearing race. I repeat this procedure for the rear hub.
Set Bearing Preload
There must be 10,000 words written about this procedure and more than one technique to determine if the preload is correct. For that reason, I’ve put off setting the preload for many years. So, now’s the time.
I decide to use a spring scale to measure the amount of preload. This technique uses a steel pipe as a spacer to compress the inner roller bearings into the outer races. The axle goes through the bearings, wedding band and spacer and the axle nut and washer are tightened to the recommended torque, for this bike, 25 FT-Lbs. A string is wrapped evenly and tightly around the spacer for about 2 inches. I put a loop in one end and hook it onto the hook on the scale. I pull on the scale until the wheel is rotating at a steady rate and read the force required. This is pretty straight forward.
The complicated part is figuring out how to convert the torque setting BMW recommends to a force on the spring scale using the diameter of your spacer. This is explained in Bob Fleischer’s material and I can summarize it here.
- I measure the diameter of the spacer, mine is 1.1 inch.
- I divide this by 2 to get the radius, so mine is 0.55 inch.
- What torque range should I use? I decide to use Bob Fleischer’s recommendation. He suggests a range of 20 – 35 inch-ounces is ok, but the upper end, 30-35 inch-ounce is more suitable for a sidecar rig. I decide to use the 20 – 30 inch-ounce range.
- My scale measures grams, but the torque range is in inch-ounces. There are 28.35 grams in 1 ounce. I convert the lower and higher torque values of the range to inch-grams by multiplying the values by 28.35: 567 inch-grams to 850 inch-grams
- Divide the lower torque value by the radius: 567/0.55 = 1031 grams
- Divide the higher torque value by the radius: 850/0.55 = 1546 grams
I clean the new inner bearing races with carburetor cleaner and dry them. I use 3-in-one oil and lightly lube the rollers.
I start with the rear wheel. I measure the thickness of the wedding band in the rear wheel; it’s 6.95 mm and I mark the band. The wheel bearing preload adjusting kit I got from Cycle Works includes (10) 0.05 mm shims and two wedding bands; 6.75 mm and 6.30 mm and I mark those bands too so I won’t get mixed up.
I use the original wedding band to start with. I use the Cycle Works bearing stack removal tool, which is 1 inch diameter steel pipe, for the rear snowflake wheel.
I use the axle and push the inner bearing on it followed by the wedding band and then slide the axle through the wheel.
On the right side I slide the other inner bearing on the axle, the Cycle Works sleeve, the large flat retaining washer and the nut.
I torque the nut to 25 Ft-Lbs. Then I wrap the string around the sleeve and attach it to the spring scale and pull.
The force required is more than the maximum reading on the scale which is 2000 grams so the original wedding band is too short for the new bearings. I need to use the shims to increase the wedding band height to push the inner bearing farther from the outer bearing to reduce the friction on the rollers.
It’s hard to control the pull with the wheel on the ground, so I mount it in the rubber jaws of my vice being careful not to over tighten the vice. This makes it much easier to pull the spring scale at a steady speed. It’s also easier to get a steady reading by wrapping about eight feet of string on the spacer and the walking rapidly so any jerking is overcome.
I use 2 shims adding 0.10 mm so the total wedding band height is 6.95 + 0.10 = 7.05 mm. I install the shims against the inner race on the narrow end of the bearing.
Then I add the wedding band so it will be between the shims and the end of the inner pipe.
Then I slide the bearing, shins and wedding band on the axle with the wide end of the bearing against the shoulder of the axle.
I insert axle with the bearing, shims and two wedding band into the hub, slide the other bearing on the axle, the sleeve, the washer and nut and torque the nut to 25 Ft-Lbs again.
Again the force needed to rotate the bearings is too high, about 1600 grams. I add one more 0.05 shims so now I have a wedding band height of 7.10 mm. This time the force is too low; about 300 grams. I remove one shim so now the wedding band height is 7.05 mm. This time I need about 600-700 grams which is too low. I sand the wedding band on a glass plate with 600 grit wet/dry paper in a figure eight pattern.
I sand it for 10 figure eights on each side and try again and it is about 900-950 grams. Another sanding of 10 figure eights and this time I get 1000-1100 grams of force to spin the bearings. This is on the lower end of my range so I’m all set; that much force is equal to about 20 – 21 inch-ounces of torque.
Here is short video of using the spring scale to measure the rear wheel bearing preload.
For the front wheel, I have a 7.20 mm wedding band that is installed in a plastic sleeve on the left side. I use the shorter sleeve I had made and measure the preload in the same way as I did with the rear wheel. I had to add a larger diameter washer under the axle nut as the stock front wheel washer is not much wider that the sleeve and that made it hard to get the sleeve aligned with the top of the bearing.
I got about 1,000 grams and decided to aim for 1100 to 1200. I used the 600 grit wet/dry paper and sanded the wedding band about 15 figure eights on each side. When I tested the preload, it’s in the range of 1100-1200 grams of force which is the sweet spot for this diameter metal sleeve.
Install Grease Seal
Now that the bearing preload is set, I grease the bearings and install the top hats and grease seals. I start with the rear wheel since it uses different size top hats and grease seals on each side. The rear wheel on the right side uses a shorter top hat and grease seal. The left rear bearing and the two front bearings use the same size top hat and grease seal.
I pack the bearing with NLP red grease. I insert the bearing in the outer race and rotate it several times and then add more grease to the bearing. Then I put a smear of grease on the outer race and insert the inner race.
The top hat has a lip, or brim, that goes on the underside of the grease seal.
The other end of the top hat is proud of the front face of the grease seal.
The right rear seal has a metal shell. I insert the seal as evenly as I can with my fingers and the use a large socket to tap it home so it’s just a bit beneath the surface of the hub.
The other three seals are rubber and I can install them with my fingers. I tap them with the socket to slightly indent them.
Here are the powder coated wheels with new bearings, top hats and grease seals ready to roll.
2018-10-12 Fixed error in grams/oz and updated computations.