Airhead Tech Pages


BMW Wheel Bearings, 1956 On -

Duane Ausherman  -  w6rec@mail.softcom.net


Wheel Bearing Basics

BMW has the best wheel bearing system of any motorcycle. To properly support a sidecar rig the wheel must withstand large side thrust, the same as a car. Unlike a car, the requirements of a solo motorcycle wheel is to withstand only forces through the vertical axis. A BMW, in solo operation, has ten times the capability that a solo bike needs. The system is slightly more expensive and requires some special maintenance, but the bearings can easily outlast any other moving part of the motorcycle.

Let's talk about what is involved. The wheel has a bearing on each side to hold it up and allow it to  rotate. These bearings are tapered and have rollers, not balls. The two tapered bearings are opposed to each other. That means the taper parts are angled or pointed in towards each other. This part of the system is the same for any car or truck. BMW Wheel bearings  Inside the wheel and between the bearings is a spacing system. This  system is basically in two parts. The first, an outer spacer, that keeps the two outer races, or cups, apart.  Second, the inner spacer, that keeps the inner races, or cones, apart. The relative length difference of the two spacers determines the spacing, or pressure, applied to the rollers. Without a spacing  system to keep the tapered bearings the correct distance apart, they would just jam together and not turn. Regular ball bearings do not need this protection or spacing. The outside spacer keeps the outer races, or cones, apart a certain  distance. On a car the outside spacer is permanent and is machined in to the axle/spindle. The inside space is adjusted by the tightness of the nut that holds it all together. On the BMW, the axle is completely removable and no permanently machined  place exists for the spacing. If the outside spacer is too long, then the inner race, or cones, would   be too tight and the bearings would overheat and fail. If the outer spacer it too short, then the inner races would never get close enough to the cups to tighten up and the wheel would be loose. With a loose fit the bearings will still fail. The bearings must be held together and with a certain amount of pressure. This pressure is called prelude. With the correct amount of preload and lubrication and it will roll for a long time.  So how do we check or adjust this spacing system? This adjustment can  occur by changing either the length of the inner or outer spacer. BMW provides for this adjustment in spacing with the inner race. The total length of the inner spacer is easily adjustable. This is because it is made in two parts, one long one, and a tiny insert. I call it a wedding band and it is made in many sizes. Change the wedding band size and the total length of the inside spacer changes. The wedding bands are made in .1 mm, or .004" sizes. So it would seem that you will need a box full of these wedding bands to adjust your wheel bearings. It isn't so hopeless, as you will see.

Bearing Removal

I think that it is time to talk about removing the wheel bearings. I need to divide the various BMWs into two styles of removable bearings. This system was first introduced with the advent of what we erroneously call the /2. My first /2 was a 1956 R69 from the earliest ones brought into the USA. It had old style ball bearings of the /3 days. I didn't know enough to realize that I had an odd one or that somebody had changed over to the old system. In any case, somewhere in 56, BMW changed to the tapered roller wheel bearings for the Earles fork sidecar type front end. In error, we call this series the /2 even though the /2 wasn't introduced by BMW until 61. I will treat the /2 first and then later the /5 - /6. Read and understand the /2 first, don't skip to the /5.

Removing the /2 bearings To remove the bearings you will need the axle. I prefer to use the rear axle because then I don't have to pound on the front wheel spacer. The front and rear wheels have different diameter axles. The wheel bearings are the same, so the front gets a thin spacer inserted so that the smaller axle is tight.   Remove the threaded cap, or dust cover, that holds the bearings into the hub. It has four holes that are for a special tool. Get the tool or make one. Two pin punches can be inserted into two opposite holes and a lever put in the middle and turned. You must find a way to hold the punches from splaying outwards. This dust cover can also be removed with a punch and hammer, but you won't make any friends that way, only a screwed up dust cover. It isn't usually tight, but it can be. The inner race, or cup, can now fall out and get dirty, guard it. Lift it out with your finger and look at it. Is it pitted or water spotted? Does it have grease or is it dry? Put it back in for the nest step.  Insert the axle into the wheel in the wrong direction, the side that is   the drum. Now to buy a BMW special tool. The axle sticks out of the exposed inner bearing about 4 or 5 inches. Measure the distance from the inner bearing to where the end of the threads for the axle nut starts. This measurement is about 4 inches. Go to the hardware department of your favorite hardware store and buy a galvanized 3/4X4" nipple. Slide it over the axle and it should cover the bare part of the axle and leave only about 1/2" of threads. Put the nut on and tighten it with only about 5-10 ft lbs. Now that you know that the nipple is the correct length, write "BMW tool" on it. Now it is official.  Rotate the axle and "feel" the bearings. Are they rough and gritty? Move the axle sideways, is it loose? This is just for experience and feel. It should be completely tight and smooth. The bearings come out with another special tool called a hammer. I prefer to use a medium weight ballpeen type. The plastic reboundless will usually work. I also have sacrificed one axle as this "tool." I prefer not to pound on my good axle. I have one hand holding the axle straight and the other driving it out with the hammer. If the axle tries to get cockeyed, I can feel it and straighten it out. This method keeps the whole thing straight and in tension while being pounded upon. It isn't a pretty feel, but BMW has blessed this method. It usually only moves about 1/8th inch or so each whack. Now you have the wheel bearing system in your hands.   Tighten the nut into a vise. I also have a sacrificial nut to squeeze in the vise jaws. You didn't use your "good" one for this did you? Remove the axle and pull it out of the bearing parts. You have these things in your hand. Two tapered bearings that consists of two parts too. The spacer system in the middle of the bearings is in three parts. The large outer hollow spacer, the inner and the wedding band. The wedding band is fitted into the inner spacer to complete it. You also have a "top hat" spacer that rides in the grease seal on the inside. The same "top hat" spacer on the other end of the bearings is in the dust cover.  A quick examination of the running surface of the outer races will  usually tell the story. Clean all parts of dirt and grease. If the bearings are badly pitted it is usually the one under the dust cover. It is the one that is exposed to dirt and soapy water from the coin operated car washes. This outer bearing fails ten times more often than the inner one. Replace as necessary. Now you are ready for the fun part. Testing the spacing  Grease up the races of the clean bearings. Put the bearings and spacers onto the axle just as it came off. Tighten it a bit. This axle is standing vertically in the vise, just as your BMW manual shows. Spin the bearings and they will loosen up and get sloppy. Tighten it a bit more and spin it. If the bearings get tight, and won't turn, before you get to 25 ft lbs. then it is too tight. If you get to 25 ft. lbs. and it is still floppy then it is too loose. Here is where you see a picture of the German
technician pushing it sideways with his thumb to see if it is adjusted correctly. If you can move the outer spacer sideways with medium thumb pressure, then it is about correct. It will never come out correctly the first time, so don't even hope to get away with it. OK, so you have a spacing problem, now what?

Adjusting the spacing We must discuss the variety of ways to remedy this, and there are a few.  The BMW standard is to change wedding bands. If you have a box full, that might work. Use your micrometer to measure the width of the old wedding
band and find another one a bit bigger. Plan A didn't work?  OK, plan B is to have a lot of Flanders shims. They quit making them I think. Earl Flanders was the west coast importer of BMW until Butler and Smith took it away from him in 71. He was a clever guy and  saw this problem and made thin shims to fix it. He had two sizes, think and thin. They were about .001 and .004 or so. They were cheap and fast. You don't have any, so on to plan C.

Lapping the spacers If you need a thicker inner spacer, as we have  been discussing, you could shorten the outer, and have the same result. Or, you could also shorten the wedding band if you needed the inner spacer to be shorter. Examine the ends of the outer spacer. See the rough lathe marks? You are going to use them to your advantage. Get your BMW surface plate and 220 grit wet-n-dry black paper. Oh, you don't have a surface plate? Go to your glass shop and buy a 9 X 15 plate glass. That size will be useful later as another BMW tool. You can also buy mirror glass and use as another tool. Whenever I have a really big BMW problem, I look into the mirror to see the source of my problem. The glass shop will probably have "used" glass at a cheaper price. You only want the flatness of it and "used" is OK. Scratches don't matter here. Use some solvent on the grinding paper, so it doesn't load up with grit, and lap the outer race, or the wedding band, depending on what is needed. Grinding the outer is slow and would be even slower if BMW had a good surface on the ends of the spacer. Grind in a pattern and change the place you hold the spacer. Every 4 or 5 laps grab the spacer about 30 degrees around  and lap 4 or   5 more. Changing your grip tends to make the lap even. The tolerance of BMW isn't anything to write home about. Remember, they were glad to get it within one wedding band or .004 thousandths. This process can achieve a much better fit than BMW ever did. Lap a bit and try it. It should get closer each time you check. If not, maybe you are lapping the wrong spacer. Go back and reread this. Here is where the rough lathe marks are to your advantage. It the surface was "clean" then you would have to lap much longer, as you would have to remove more metal to get the same amount of shortening. I always prefer to need to lap the wedding band, as it is thinner and therefore faster to grind down. So I select a wedding band that' s too thick and the setup in the vise is loose and wobbly.  You have been lapping for an hour and finally it fits. Your thumb can easily push the outer spacer sideways, with the axle at 25 ft. lbs. Now you must forever use the same 25 ft. lbs. on the axle. On the /2, the spacer system is stable and the torque isn't very critical. Not so on the /5 system, more on that later.

Reassembly It is time to put it all back into the hub. Grease the cones and load the inside spacer with grease too. Put all back onto the axle. The spacer for riding in the grease seal must be put on the right way. The seals seldom need to be replaced as the problem was water. Remember, don't use coin washers, and repack the outside bearing each time the wheel is off. The bearing system is now fitted onto the axle the other way around. The reason for this is that you always want to pound on the flat end of the axle, not the nut end. The nut is on with 5-10 ft. lbs. or more. Oh yes, clean out the wheel. Slide it all back in place and now hammer it into place.  You can heat up the hub for assembly if you want, it helps some. You must act quickly in that case or the bearings absorb heat and swell up and you still must hammer it in. Heat won't help getting them out, don't even try it on the /2.  Remove the axle and guard the outside bearing cone. I like to use an old cone, for the insertion, to reduce the chance of hurting the good cone while hammering it. I hate hammering a BMW. I have no evidence of damaging any bearing with this hammering. I have serviced /2s for over 100 k and had the bearings out several times and they live on. Grease the outside of the cone. Put the dust cover on and tighten it only a little. Remember it is a BMW. If the dust cover comes loose while riding, the bearings might get dirty, that' s all. The axle, bearings and all dimensional parts would stay in place and all would be OK. I have never seen one come loose.  Follow BMW directions on installing the axle, VERY important.

Slash 5 wheels

The /5 bearing system is the same, but some parts changes make some important differences. First, the bearings are one size smaller. The axles are smaller in diameter. The spacer parts are smaller. The parts are in an aluminum hub and that is the big one. BMW also didn't set the wheel bearings up according to their own specifications. They blew this one big time.

Test your /5 wheel bearings  I have a test for you to perform on your /5 rear wheel. This takes two persons. Have your buddy on the right side of the bike with the axle nut wrench in hand. Have the pinch bolt loose. You are on the left side of the BMW with your hands on the wheel. Have your hands on opposite sides of the wheel. Top and bottom is best, as then you aren't tweaking the swing arm bearings as much and getting a false feeling.You try to feel side play.  You should feel none with the axle tight. Now have the helper loosen the axle nut. Now you can feel the play. If not, change places, as you are comatose and would sleep through a 8.0 earthquake. As you feel the side play, ask the helper to slowly tighten the nut. You should feel the wheel play reduce to nothing. Yell, stop! you might do this several times to see if your "feel" always arrives at the same torque on the nut. At the next BMW club meeting, go around and do a bunch of them. Have everybody try it and "learn" the feel.  Now go around and feel how tight the nut is. It should be 25 ft. lbs. I usually found that BMW shipped them at around 5 ft. lbs. The highest I ever found on a new /5 bike  was 10 ft. lbs. If it is set up for 5 ft.  lbs. and you tighten it to 25 ft. lbs then you run a risk of over heating the bearings and hub and causing the aluminum to expand and the bearings to spin. Then when you go to remove them, they will fall out. You will eventually need to buy a new wheel. Heat is your enemy. For bearing life, it is better to have it a bit loose than too tight. To feel a slight play in the wheel may be OK. The short wrench in the tool kit is a blessing as it is very hard to get to the 25 lbs.  We all know that a motorcycle has it's parts fall off and the rule of thumb is: Tighten it as much as possible, then give it another half turn. On one of those chain drive bikes, the axle nut keeps the chain tight. BMW doesn't have a chain on the rear wheel. If you were to ride around with the nut loose, what would happen? Maybe the nut would fall off, then what would happen? Remember the pinch bolt? The wheel wouldn't fall off. A hot rider might feel the slightly loose wheel. The average rider would feel nothing. When the bearing spins out, the wheel is really loose. You might have to buy a new nut and washer. The price for over tightening the nut is a new wheel. You decide what's best. The /5 has a very spongy spacer system and so it requires better control  of the bearing spacing and nut torque. If you compare a /2 with a /5 in the "tighten the nut while feeling" test you will discover that the /2 is not very susceptible to axle nut torque. The /5 is very sensitive to torque. The /5 bearings are in aluminum and the /2 are in a steel casting riveted into the hub. I have only seen one /2 with a spun out bearing and dozens of /5s with that problem.

"War story" warning, skip ahead one paragraph  I must tell a war story here. I had a customer come in with a complaint of "funny" handling. A quick inspection showed the rear wheel to be very loose. The axle nut was at about 50 ft. lbs. and the bearings were spun out as a result. Since we knew the guy, I asked if he had been fooling around with the wheel. He confessed that he had a flat and he took it to my competitor for the fix. The dealer had no BMW mechanics and a "normal" mechanic fixed the flat. $300 later he was back on the road with a new rear wheel. About 6 months later he came back with the same complaint. The wheel was loose again and the bearing spun out again. The nut was over tightened again. This time we had to almost put him on the "rack" to get truth out of him. He had had another flat and took it back to the same place to get it fixed. It was really hard to have any sympathy. Some of us are SLOW learners.


The /5 wheel responds well to heat. That's good and bad. Heat the wheel around the bearings, and the axle, with bearings, will fall out. How hot is hot? Heat it until spit sizzles. All the same lapping and adjusting applies to the /5. Flanders never made any thin shims for these models. We had a box full of wedding bands and just changed and lapped them as necessary.

Tricks
Now a few tricks on these models. The /2 inside spacer will come out with the outside race in place. The /5 won't make it out. The /2 can be sort of greased by pulling the inner spacer out and putting grease into the far bearing. It isn't really the right way, but it works. Brian Hilton, the best BMW mechanic I have ever seen, made a tool for greasing the /5 wheels without removing anything. Maybe it's common now. A similar tool can be made for the /2 wheels. The spun bearing race on the /5 can be fixed. It can be glued in with Loctite. I don't remember which one will work and they must have new ones by now anyway. If the race just barely slides in, it will probably work. If the race is really loose, don't even waste your time. I have heard of machining it out and putting a sleeve in. If it is the inside one then it is hard to put the large sleeve in past the first race seat. We have put shim material in around the race and glued it all in. It may work, but inspect it often by shaking the wheel.

Inserting the axle  When inserting the axle into the front or rear of any BMW use caution. The axle must go in easily, by hand. Never get a couple of threads showing and then draw it in with the nut. When the axle is drawn in it actually catches on the swing arm (rear) or a fork leg (front) and pulls it in with the axle. I have seen the rear swing arm permanently bent in from this. The final test is to back the nut off about 1/2 turn and put the bar into the axle hole and turn it. You may see the swing arm "jump" out as much as 1/2". Tighten the pinch bolt last. Same on the forks. The common symptom of a pulled in swing arm is: When rolling the bike around in a quiet place, you hear a groan, during part of the revolution of the rear tire. This is caused by the axle is in a bind and is actually in an arc, which tightens up the wheel bearings. A long hot ride can spin the race out. On the front forks it will cause "sticking" of the forks and the resultant lack of the tire to follow the rode. We call this stickion (sp), but then it's a made up word anyway. It is pronounced stick-shun.  More on this later.

Summary 

The BMW wheel bearing system can be trouble free if it gets proper attention. A well greased and spaced bearing set will outlast the crankshaft. No expensive special tools are needed. A simple test will show the tightness of the wheel bearings. Keep the water and soap out of the wheel, steering head and swing arm bearings. When going to a BMW rally, carry a spare setup of bearings and spacers. Have them spaced and greased. You won't need them but somebody else may.

(This and other articles by Duane also can be found on his website: http://w6rec.com/duane/bmw/bmw.html)


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Last Update: 11 September 2010