wheel bearings

BMW motorcycle wheels, bearings, remove, replace, stack, tapered roller, preload, shims, wedding band, spun, hub

 

BMW motorcycle wheel bearings, 1956 to 84

by Duane Ausherman

This page is about how front and rear wheel bearings work in the BMW motorcycle models R26, R27, R50, R60, R69, R50/2, R60/2, R50S, R69S, R50/US, R60/US, R69US, R50/5, R60/5, R75/5, R50/6, R60/6, R75/6, R90/6, R90S, R60/7, R75/7, R80/7, R100/7, R100S, R100RS.

In 1985 BMW stopped using tapered bearings in the wheels.  While this info is mostly about the /2 and /5 wheels, the principles apply to the later wheels too.  The non drum front wheels don’t have a stack that comes out one side.  Each bearing goes in from each side.  It still must be preloaded correctly.  In about 1981, BMW started installing a steel sleeve in the hub and that stopped the race from spinning.  Click here for info on rear wheel splines.

Bearing basics for BMW motorcycle wheels

The post war (mid 1955 on) BMW was designed to withstand the forces of a sidecar, the same as a car.  To properly support a car or truck wheel, the bearings must withstand great side loads.  Unlike a car, a solo motorcycle only needs to support forces through the vertical axis of the motorcycle.  A BMW, in solo operation, has at least 1000 times the capability that it needs.  The system costs a bit more and requires some special maintenance.  The system can easily outlast any other moving part on the motorcycle.  BMW has the best wheel bearing system of any motorcycle.   See the engineering calculations at the bottom of this page.

The wheel hub 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 tapered parts are angled or pointed in towards each other.  Tapered bearings need some small preload, or pressure, to obtain maximum life.  Cars have some type of externally adjustable nut to set them.

I can’t recall ever seeing a BMW wheel bearing fail from wearing out.  The most common failure of the bearings is caused by foreign material contaminating the lubricant.  It is usually water and/or soap from high pressure coin car washes.  One can reduce the chance of failures by keeping the direct spray of hot soapy water off of bearings in the wheels, steering and swing arms.

This photo shows how water can rust the bearing and make it rough. It will soon fail.

Rusty bearings are usually the result of high pressure washing.


The real issue of wheel bearing preload

If you read the Timken numbers, found at the end, you will find that we use some very good bearings.  The calculated life is 2.6 billion miles for the lighter /5 bearings and far longer for the heavier /2 wheel bearings.  The Timken website has a curve showing the preload vs. the bearing life.  It is cluttered with info that we don’t need.  I have just made a crude representation of the info for our purposes.

This graph is to show the relationship between preload and the life of a taperd roller bearing.

Thanks to Chris for sending me this much improved graph.  I think he took pity on my very poor hand drawn one.  The horizontal line shows the left end with some looseness and the right end with preload.  The vertical line is neutral preload or no play at all and no looseness.  Going up the vertical line is the miles, or life of the bearings.  Anything on the left side is going to have a loose wheel and we know that will give poor handling.

1.  With this bearing set up, the wheel would be loose and the bike would handle poorly, however the bearings would still last 2 billion miles.

2.  The wheel would be held solidly and not be the source of any poor handling.  The bearings would last slightly longer.

3.  The point of maximum bearing life, with some preload and they are good for 2.6 billion miles.

4.  Half way down the failure curve, but still lasting a billion miles.

5.  WAY down the curve of bearing life, but still probably 100 million miles.  Just how far do you plan to ride it?

All of this is to show that one really has to mess up to make the bearings fail.  The adjustment is nearly unrelated to practical life.  Basically, keep the bearings clean by lubing often and they will last.  So what is the big deal?  Why is so much fuss made about this issue?

The big issue is one that is only “related” to the bearing preload.  BMW did not know of this, or deal with it in any way during most of the years of the use of tapered roller bearings in the wheels.  A higher bearing preload will cause it to produce heat.  The excessive heat will cause the aluminum hub to expand.  Once it expands enough, the bearings won’t be held in place.  The aluminum hub will expand faster than the steel bearings.  They will rotate with the axle and the outer race will spin in the hub.  In time, any amount of foreign material in the grease will slowly grind away at the softer hub material.  Soon, the hub won’t even hold the bearing when cold.  It will get loose and allow the wheel to move around.  Usually it is first noticed during any wheel bearing maintenance or a rider notices funny handling.  A quick check and the wheel is found to be loose.  Further inspection shows a spun bearing.

BMW installed ALL of the /5 and later tapered wheel bearings with way too much preload.  The shake the wheel test described below showed that many times in the showroom.  I never found a single /5, or later, with proper preload in either the front or rear wheel.  The front wheel is in “clean” air (less turbulence) and the only source of heat is the brakes.  The brakes will heat up the inner portion of the hub first.  That means the inner bearing, or the hard one to get to.  I have seen the inner bearings spin on only a few wheels of the /5 and later.  It is harder to fix with glue.  How does one get it past the first hub wheel bearing mount while hot?  We just replace the hub or complete wheel.

The rear wheel is in a completely different environment.  It has far less air and what air gets in has been slightly heated by the engine and transmission.  It is surrounded by the final drive and it gets really hot.  On a day of 75 F.  it will get to about 155-160 F.  in the oil alone.  The rear wheel is easily heated up during normal operation.  Wheel bearings that are too tight (too much preload) in the first place are prone to spinning in the hub.

The reason to be concerned with bearing preload is to save the hub, not the bearings.  We want to operate at the graph’s vertical line or slightly to the right, but not far.  Why risk the hub to get another piddly 100 million miles.

A war story

One of our regular customers came in one afternoon with a complaint of “The bike feels funny.” We quickly found the loose rear wheel.  The mechanic was almost unable to get the axle nut off of the axle.  He had to use a cheater and estimated that it was over 75 lbs of torque.  That was the “alarm bell” that we looked for.  I showed the owner that his rear wheel hub was shot due to someone over tightening the axle nut.  We knew better and I asked him what he had had done, or did himself.  He confessed that he had a flat tire and took it to the local BMW dealer (my competitor) to get it repaired.  We were 20 miles away and too much hassle, so he took the easy way out.  Those mechanics there were all BSA and Kawasaki mechanics.  They only knew chain drive and one must really tighten the axle nut to secure the chain tension.

We didn’t have a new wheel in stock, so we ordered a new one for him.  The bill came to almost $300 and that was in 73.  A few months later he comes back with the same “The bike feels funny” complaint.  We found exactly the same result………….again.  He admitted that he had another flat and took it back to the same shop for repair.  “Didn’t you learn the first time?” His only answer was “How hard can it be to repair a flat?” “It is not very hard, but it should be obvious to you that they can’t do it.” He paid for another wheel.  Two wheels in less than a year.  I felt bad for his first experience, but certainly not for the second one.  Someone said, “A fool and his money are soon parted.” This is worse, as it is also a safety issue.  A man named Darwin wrote something about that.

Mechanical details

The /2 BMW motorcycle wheels are interchangeable from the introduction in late 55 to the end of production in 1969.  However, the wheels came with a variety of details, often being very minor.

A list of variations

  1. Rims could be alloy or steel.
  2. Allow rims could be low lip or high lip.
  3. There were two spoke patterns with two spoke sizes.
  4. Solo and sidecar rims were different width.
  5. Two types of seals were used.

The wheel bearing stack was identical from front to rear wheel in any given year.  A sidecar rig had 4 wheels.  With three wheels on the ground and a spare on the “trunk” of the hack, all wheels could be used in any position.  They were totally interchangeable.

The /5 front and rear wheels are different.  The rear wheel has a drive spline and a 18″ rim that is wide.  The front has a narrower 19″ rim and no drive spline.  The bearing stacks are the same for the front and rear wheel.  The only real difference is that the front axle is smaller in diameter than the rear.  A spacer is inserted into the front wheel hub to hold the smaller axle.  The front wheel bearings are treated exactly the same as the rear.  The wheel bearings are the same size as the rear swing arm bearings.  They are # 30203 and can be purchased at any bearing house.

The combination of the two bearings and spacers are called a “stack.” It is removed as a unit.  The spacing system for the wheel bearings is internal and part of the stack.  This spacer system is in two parts.  The first, the inner spacer, is in two parts and holds the cones (inner race with rollers) apart.  The second, the outer spacer, is one part and holds the cups (outer race) apart.  Changing the length of either will change the spacing or bearing preload.  BMW provides for this adjustment of the spacing, or preload, by changing the length of the inner spacer.  The smaller of the two inner spacer parts, comes in many sizes and it resembles a wedding band, so it is called that.  These “wedding bands” are made in small increments, so it would seem that a box full is needed to adjust the spacing.  It’s actually not as hopeless at it might seem.  The /2 wedding bands are in increments of .1 mm or .004 of an inch.  The /5 are made in .05 mm or .002 of an inch.

Checking the spacing of the /5 and the /2

The BMW method requires removal of the stack to check or change the spacing.  The BMW procedure is not only time consuming, but allows for an inaccurate result, especially important with the /5.  A common failure (mentioned above) of the /5 is the bearings becoming loose in the hub.  The wheel is then loose on the axle and allows for funny handling.  It is easy to test for this fault and prevent a failure.

Warning, use only the very short 22 mm wrench from the tool kit, or a torque wrench.  Using the short wrench makes it very hard to over tighten the nut.

I have a very easy test that you can perform to determine if the spacing is in the ball park, or not.  I call it the “shake the wheel” test.  This test is no secret, as it is basically what is done on car wheel bearings.  I just stole it for use on our motorcycles.  This takes two persons.  Have your helper, on the right side, with the 22 mm wrench at the axle nut.  Do not have the swingarm pinch bolt loose.

This shows the BMW motorcycle rear axle nut being tightened up for the shake the wheel test.

“Tightener”

My helper is taking the picture, so I must do both for purposes of demonstration.  I am in position to adjust the axle nut for testing.

This shows how one holds the BMW motorcycle wheel for the shake the wheel test on a slash 2 model.

“Shaker”

Now I am on the left side with the wheel in one hand and the frame in the other hand.  Try to feel side play, you should feel nothing with the axle nut tight.  Have your assistant to loosen the axle nut a turn, or two while you feel for play.  Once the “shaker” fells the play, the “tightener” slowly tightens the axle.  The shaker can feel the play go away as the tightener slowly tightens up the axle nut.  The shaker just orders “more” until the play is gone.  Do the test again for accuracy until the “tightener” has a feel for where the wrench ends up.  Now loosen the axle nut with a beam type torque wrench to see how tight the axle nut was.  BMW recommended the axle nut be tightened to 25 ft. pounds.  If it happens under 15-20 lbs, then the preload is too tight when the nut is at 25 lbs..  If it happens over 25 lbs. then the preload is too loose.

That is just a standard so that everyone will tighten the axle nut to the same torque.  BMW could have picked another number and it wouldn’t matter as long as we all use it.

The easy fix for the /5

To change the spacing means changing the thickness of one of the two parts of the inner spacer, the wedding band.  By installing a thicker wedding band, it will space the cones farther apart.  This will fix the problem.  This can be done by one of two methods.  I will first describe a method that doesn’t even require removal of the wheel from the bike.

Pull the axle.  Remove the 5 bolts holding the chrome hub cap onto the wheel.  Carefully remove the cap and the seal holder.  Be careful, the cone of the outer bearing can fall on the ground, or it may stick to the seal holder.  The wedding band may even stick to the bearing too.  Reach in and remove the wedding band.  Now you are in a position to see if the bearing cup has spun in the hub.  Try to pull the outer bearing race, or cup, from the hub with your fingers.  If it comes out, then it has already spun in the hub and you have a problem.  It can often be felt during the shake the wheel test, as play that didn’t go away.  The bearing will probably still be ok, but the hub is not.  See “fixing or replacing the hub” below.

Replace the wedding band with a thicker one, but you probably don’t have a box full of assorted sizes.  If you have a shim kit, add one of them to the well under the wedding band.  Now it is trapped in place.  Shims are available from Scottie Sharp, see below for details, for cheap and wedding bands are available from your BMW dealer.  Which size of wedding band do you need to buy?   You probably don’t even have a micrometer, much less a metric one.  With shims it doesn’t matter.  More on shims below.

For a quick test, reinstall everything, but the chrome hub cap.  Don’t forget to put the top hat spacer into the seal holder.  Those 5 bolts don’t really do anything in this test so just use one to keep the parts from falling on the ground.  Reinstall the axle and perform the “shake the wheel” test again.  Be sure to reinstall the axle correctly.  Keep changing spacers or shims until the play just goes away when the nut is about 15-20 lbs of torque.  It is better to be too loose than to be too tight.  If one wedding band produces a spacing that is 10 lbs and another one produces 25 lbs, go for the 25 lbs.  You could lap the wedding ring down a bit and get 15-20 lbs too.  Shims make the spacing easy.  Install all 5 bolts and go riding.

This “easy fix” only fixes the spacing.  This method assumes that both bearings, seals and lubrication are ok.  If your bike has junk bearings, no lube or shot seals, then you are wasting time with this type of procedure.  You really should remove the stack, clean and inspect all parts.

Full service of the /5 bearings

For both the /2 and /5, BMW recommends about the same maintenance schedule for wheel bearings.  Here you will learn how to get more reliability and with less service.  BMW wants us to remove the “stack” every 8 or 16 k miles and fully lube and check the spacing.  This is not needed and on the /5, and it can harm the hub.  Each removal of the stack runs the risk that you might damage the hub.

The reason for renewing the lube is twofold.  One, it assures lubrication.  Two, it cleans out dirt.  A type of greasing tool can be bought, or made, that will lube both bearings without removing anything but the axle.  With that tool, one can give it a shot of grease every time a tire is changed.  Not a lot of grease, but a pump or two.  The first I knew of this tool was when my ace mechanic, Brian Hilton, suggested it.  He made one and it worked beyond our expectations.  It just slides into the axle hole and has O-rings to seal off the bearings.  A hole through the center allows new grease to be pumped into each bearing from one side.  The old grease is pushed out the other side.  The hub isn’t threatened by being heated, or hammering on the bearing stack.

If one rides in the rain for a few days, then it is a good idea to insure that there is no water in the wheel bearings by adding some grease to shove out any water.  The water would only get in the outside bearing, so here is the simple way.  Just remove the axle and you will have room to insert this tool and give it a shot or two of grease.  If the bike was riding through water so deep that it temporarily got water in the rear drum, then it is possible that water could have gotten in both bearings.  I would completely remove the wheel and give both bearings a shot or two.

One can see the tool made by Ed Korn to grease the wheel bearings in a BMW motorcycle wheel. This one is for a slash 5 model.

This tool can be purchased from Cycle Works at http://www.cycleworks.net/ very cheaply.  The shot is so close that it is distorted a bit.  It is exactly 2″ long.

To remove the /5 bearing stack

1.  Remove the wheel.

2.  Remove the 5 bolts, seal holder and hub cap.

3.  Insert an axle into the brake drum side.  This is backwards, or the “wrong” side.

4.  Install your handmade tool.  Get a 3/4″ X 4″ plumbing nipple from any hardware store.  Depending upon the model of BMW you may find that 4″ is slightly too long or too short.  You can use one of the top hat spacers to make up for a short one.

5.  Put the washer and nut on the axle threads.  The nut should tighten up on the plumbing nipple, which puts pressure on the bearings.  This holds the “stack” together.

6.  Heat the hub up to about 250 F degrees.  Set the wheel on the floor and hold it up between your legs.  Hold the axle in one hand and use a plastic hammer to gently tap the stack out of the hub with the other hand.  If it doesn’t come out easily, then heat it up more.  A drop of water should dance on the hub casting.  Keep the axle totally straight so that the bearings stay straight.  If they get cocked off to one side, they will try to gouge the inside of the hub.  You can see an example of a “cocked” bearing stack below on the /2 portion.  Don’t try to drive it out cold, you will ruin the hub.  If it doesn’t come out easily, then heat it more.

7.  Disassemble the stack, noting the position of the top hat spacers on each end of the stack.  The left top hat spacer will have stayed in the 5 hole seal holder when you removed it.  You may have used it on the axle to take up space.  That spacer resembles the other one, but is wider (see photo).  The bearing cone often sticks to the seal holder.  These top hat spacers are for one purpose, to transfer pressure from the axle to the bearing stack, through the seal.  The top hats ride in the seals so they need to have a smooth surface.

This photo shows an exploded view of the rear wheel bearing stack of a slash 6 BMW motorcycle. This is almost identical to a slash 5 BMW motorcycle.

From Left to Right; the top hat thrust spacer, seal holder (this one is from a /6) bearing cone, bearing cup, wedding band spacer, Inner spacer, outer spacer, bearing cup, bearing cone and the smaller top hat spacer.

The top hat spacer is easily lost due to the fact that it can come out of the seal holder and fall on the floor.  It is also easily confused with other thrust spacers.  Here is what it looks like with dimensions.

This is the left side top hat spacer from a rear wheel of a BMW motorcycle.

This is the one that gets lost easily.

The BMW motorcycle right and left top hat spacers are shown against a ruller.

Here they are together with the one on the left as the one shown above.  The one on the right is the one shown on the right of the stack above.  It is trapped by the seal and can’t fall out.

8.  Clean the bearings and inside spacers of all grease.

9.  Inspect the bearings, especially the cup, for pitting and stains from water.  Replace as needed.

Time out.  You aren’t really ready to do the spacing just yet.  This part of the procedure is my addition, but it is worth the trouble.  I have had lots of experience with the /2 and later the /5.  The axle nut tightness thing was found to be a bit “spongy” on the /5.  Not at all solid like the /2.  I like to partially fix this.  BMW didn’t machine the spacers very well.  They had two defects.

First, inspect the the outside spacer, larger part of the inside spacer, and the wedding band spacer with a good eye or magnifying glass.  The machined ends are poorly cut.  They are rough.  Cleaned up faces offer more surface mating area.  This reduces some of the spongy feeling.

This photo shows the inner and outer spacers from a slash 5 , or later stack BMW motorcycle.

Second, the ends should be parallel with each other and square with respect to the length.  About all you can do is to get them parallel.  Square is nice, but not nearly as important as parallel.  The /5 and later inner spacer (middle part) can be modified to make life easier in the future (see below).  The two large centering ridges won’t go through the bearing cup.  You can file, grind or turn them down to just pass through the bearing cone, or outer bearing race.  Now you will be able to remove the two cone spacers while the stack is in place and sort of get to the inside (deeper) bearing without removing the seal.  You can even give it some grease if you don’t have the proper tool.

I lap them to be smooth and as parallel as I can measure.  I use a glass plate as my “flat surface”.  It is 9″ X 15″ and useful for fork work too.  I use # 240 – # 400 grit black paper for the abrasive surface.  Add solvent to the paper to reduce it’s tendency to clog up.  Rub them (lap) in a circular pattern for about 5 rotations.  Change the place you hold it by 30 degrees.  Then lap another 5 rotations.  This will insure more accurate lapping.  As they start to clean up, check for parallelness if you can.  I have seen them .001″ (.025 mm) out of parallel and that is way too much.

This shows the set up for lapping spacers for the BMW motorcycle wheel bearings.

Spacers on # 240 – # 400 grit water proof paper on a glass plate.

10.  With only a drop or two of oil on each bearing, reassemble the stack and the tool (plumbing nipple) on the axle.  Put the washer and nut on and only finger tighten it.  If the bearings still have solvent in them, that is enough lubricant.

11.  Put the stack in a vise, vertically, and tighten it on the nut.

Here one can see the way that BMW intended the stack to be tested while being held in a vise. This works to get the spacing close, but not exactly to spec.

12.  This is the very inaccurate procedure shown in the BMW shop manual.  This is just to get in the ball park.  Use the tommy bar, or a screwdriver, in the axle hole to tighten and loosen the axle.  As you slowly tighten the axle, continue to rotate the bearings by hand.  The large outside spacer should be easy to move sideways easily.  That’s the visible part between the bearings.

13.  Expect the spacer to tighten up at a higher torque than you found by the “shake the wheel” test.  If the spacer gets tight at a torque lower than 25 lbs.  the spacing is too tight.  If the spacer is still loose at 25 lbs.  the spacing is too loose.  BMW wants you to change the wedding band spacer until you reach the correct spacing.  With shims, you can just add them until the correct spacing is reached.  The shims are available, by order, near the bottom of this page.  Don’t bother trying to add shims at this time, as the preload will change when the stack is inserted into the hub.  Adding a thin shim, or thicker wedding band makes the total inner spacer wider.  That is exactly the same thing as lapping the outer spacer down.  Either will work to change the relative length.

If you don’t have shims and don’t want to wait for an order to arrive, then follow this procedure.  Instead of adding a thin shim or changing the wedding band to a slightly thicker one, just lap the large outer spacer down a bit.  This really takes some time because it is so large.  In a few minutes it will be ready to try.

If you find the stack to be too loose then you need a thinner wedding band spacer.  You can lap it down quickly, as it is small.  Check it often as you could go past the correct amount.  Then you would have to lap the outer one down.  Don’t get into lapping them too much.

14.  When the outside spacer moves, with medium thumb pressure, at 25 lbs of torque, BMW is happy.  Don’t you be, because that is wrong.  What BMW didn’t know is that when the heated hub cools, it contracts and shrinks down on the stack and tightens it up considerably.  At this point I go for the stack to be slightly loose at 25 lbs.  Don’t worry you will add or subtract shims to perfection later.  It’s time to reassemble.

BMW /5 wheel bearing modification

At this point you have the perfect opportunity to modify the inner spacer so that it may be removed without removing the whole stack.  I highly recommend this.  You will need a lathe, grinder or a file to do it.  You only need to remove a bit of metal from the centering ridges of the inner spacer.  Here is what it looks like.

This inner spacer of the slash five model of the BMW motorcycle may be trimmed down a little bit to allow for it to be removed from the hub.

The inside of the cup is 1.250″ (31.76 mm) and you only need to remove about .010″ (.25 mm) to get it to fit through the cup.

15.  Lube both bearings and assemble everything on the axle.  You can’t over lube it.  Any excessive lube will get squished out and harm nothing.    Tighten it up to about 10-15 lbs.  I like to put the stack in the freezer to cause it to shrink, making it smaller, which allows it to go into the hub easily..

16.  Heat the hub as before and slide the cold stack in quickly.  It will go easily as the hub has expanded and the cool steel stack has contracted.  BMW did it by freezing the stack in dry ice for an hour and putting the stack into a room temperature hub.

17.  Perform the “shake the wheel” test again.  If it tightens up at a lower torque than 15-20lbs, add one shim at a time until it passes the test.  The shim goes under the “wedding ring” spacer.  Now it is “captured” in place.  To get shims, skip to the bottom for full information.

18.  Install the hub cap and 5 bolts.  Tighten them only when the wheel and axle are installed.  This causes the seal to “center” itself.  If the seal holder is tightened up “off center,” the seal will rub on one side and fail quickly.  This “worn space” may allow water and/or dirt into the stack.

From now on, you only need to give the bearings a shot of grease at each tire change and perform the “shake the wheel” test.  You won’t need to do a full service for a long time.  I would want to look at it again in 50,000 miles.  By not removing the bearings every 8-16 k, the hub is less stressed.  Buy the greasing tool and use it.

The ultimate test is to ride 100 miles on a day of 100 degrees and then perform the “shake the wheel” test.  It should still tighten up at slightly less than 25 lbs.

Balance the wheel at every tire change.  Static balance is good enough up to any speed that a stock /5 will reach.

Fixing or replacing the hub

A hub with a spun bearing is evidenced by you being able to lift the outer race out of the hub, without the use of heat.  The usual bearing to spin out is the one on the left side, but the inside one can spin out too.  If the race just barely slides out then one can get away with Locktite to hold it.  The only disadvantage of a glued in bearing race is that it is hard to remove for servicing the bearings in the future.  Proper spacing and it should never again need to come apart.

If the race is loose and wobbly then replace the hub.  A really excellent machinist can bore it out and press in a sleeve.  It is very hard to maintain “centers” between the stack and the brake drum.  You may trade one problem for another one.  Finding a good used hub is very difficult.  Most of them have already been bought to fix this common problem.  A seller may not be able to determine if the hub has been spun out.

The /2 BMW motorcycle wheels

The front and rear wheels are exactly the same on the Earles fork twins and the same with the singles.  The twins have 40 spokes and the singles have 36.  The wheels are interchangeable between front and rear.  In 68 and 69, BMW produced a model we usually call the US model.  It has telescopic forks and has different front and rear wheels.  The wheel bearing system is exactly the same as the others.  This article calls the entire production of all models, twins and singles, from 1956 through 1969, the /2.

The bearings are the same for the front and rear wheel of any one model.  The /2 twin uses the 30204 and the single, a smaller bearing, the 30203.  The only real difference is that the front axle is smaller in diameter than the rear.  A spacer is inserted into the front wheel hub to allow for the smaller axle.

The wedding bands are in increments of .1 mm, or .004.” The shims are in smaller increments and that allows you to get more accuracy.  In spacing one stack, it isn’t a big deal to lap the spacers to get it perfect.  If you will be doing this on several wheels, consider the time savings and accuracy of using the shims.

The hubs are steel so the expansion, from heat, of the bearings and hub is identical.  The amount of interference fit can be much less.  This virtually eliminates the chance of spinning the bearings in the hub.  I have only seen one /2 hub fail from spinning bearings.  Before 1965 the seals were made of felt and not very effective.  In 65, BMW changed to a modern knife edge seal and they worked much better.

The “shake the wheel” test works well on the /2.  The change in spacing from the “axle in the vise” to the fully assembled wheel is much less and almost insignificant.  Because of this, we assumed that it would work the same way in the /5.  It didn’t.  I suspect that BMW was also lulled into this fallacy.

/2 bearing stack removal and spacing

The /2 has a seal holder, called a /2 hub nut, with 4 holes, that requires a special tool to remove it.  Buy or make one.  Never use a hammer and punch to remove the BMW /2 hub nut.  It is very important to mention that this seal holder does not need to be tight.  Tightness only makes trouble.  I only put it on about hand tight.  That means hand tight using the tool below, but not with a breaker bar in it, just by hand.  This dust cover has nothing to do with the pre-load on the bearings in any way.  In the center of the dust cover is a tension bushing.  If you removed that bushing and installed it on the axle, you could leave the dust cover off entirely.  The bearings would get dirty, but the bearings system would be fine.  Tightening the dust cover only makes trouble.

Directly under the hub cap is a wavy washer that keeps tension on the hub cap so that it doesn’t vibrate.  The seal holder and seal are only needed to keep foreign material out.  The bearings don’t need them or the wavy washer and hub cap for spacing purposes.

1.  Remove the wheel.

2.  Remove the seal holder (bearing retainer) and hub cap.  This can be a problem.  I will go into some detail about removing the seal holder and felt seals.  My examples are of the older type seal holder used up to 1965.  The 1965 and later have a far better seal system.  If you are not restoring for show, then convert over to the later style modern seal.  It is far better at sealing out the water.  You will need to buy a hub nut and seal.

This shows typical damage done to the dust cover of a slash 2 BMW motorcycle wheel.

A typical BMW /2 hub nut (seal holder) that was mangled by a PO.  The tool may not work as it won’t seat into the 4 holes.

The tool for removing a stuck hub nut of a wheel on a slash 2 BMW motorcycle. It is made by Ed Korn and is a very good tool to have.

I have used a homemade tool, the factory one and this one.  This is Ed Korn’s tool, Cycle Works, and it is the best one I have used.  It not only fits the original holes perfectly, but has that bolt that holds it in place from the other side so that it doesn’t try to jump out of the holes.  It works on both types of hub nut.  Get it, as it works and is bargain priced.

This shows drilling out the 4 holes in the hub nut of a slash 2 BMW motorcycle.

This is just in case that you can’t get the tool into the 4 holes.  I hate doing this procedure, but sometimes you have no choice.  Drill out the holes with a numbered bit, #15, which is .178″.  Only go to the bottom of the old hole.

The hub and dust cover may have to be heated by a torch to break it loose.

In the rare cases where the tool fails to get the seal holder loose, you do have another option.  Use a butane/propane torch to heat the seal holder up.  Since they are both of the same steel there is no thermal expansion difference to exploit.  What happens, I believe, is that the parts are forced by heat to so some moving and the old dry grease is possibly able to lubricate the threads.  When I do this I allow it to cool completely again and then try the hammer again.

This is what you will see as you remove the dust cover.

This is what you will see as the dust cover is removed.  Barely seen is my old tool, not the “superior” one by Cycle Works.

Removing and replacing the old felt seals

This shows the removal of the felt seal cap from the hub nut of the pre 1965 BMW motorcycle wheel.

This shows the axle installed in the seal holder exactly the way it would be on the bike.  Use your other hand to tap the bushing and seals out of the hub nut.  Do this with the axle only a couple of inches above a soft surface.  It comes out easily and the axle is going to fall all of the way through the hub nut.  You don’t want it hitting the concrete from 3 feet and bunging up the axle threads.

This shows the felt seal parts as they are removed from the hub nut of a pre 1965 Earles fork BMW motorcycle.

After it falls out and you take it apart, the pieces look like this.  From l.  to r.  hub nut, smaller felt seal, thrust bushing or top hat spacer, flat washer, larger felt seal and it’s holder.

This is another view of the same hub nut parts.

Here is another view of the parts and in the same order as above.  I doubt that in 2017 one can get those felt seals.  If you wish to reuse them, clean them very well, dry them out and liberally regrease them.

This shows the parts assembled and ready to install into the hub nut.

Here they are all scrunched together again and ready to press or tap back into the dust cover.  They should go into place easily, but if not, find a suitable sized socket to use as a pressing tool.

3.  You are now ready to remove the stack.  Insert an axle into the brake drum side.  This is backwards, or the “wrong” side.

4.  Install the “BMW special tool.  The tool is nothing more than a spacer, needed because the axle is too long.  I use a 3/4 X 4 plumbing nipple, available from any hardware store.  Depending upon the model of BMW, you may find that 4” is slightly too short.  You need to fill that empty space with something.  You can use the outside top hat spacer for that spacer.

5.  Put the washer and nut on the axle threads.  The nut should tighten up on the nipple, which puts pressure on the bearings.  This holds the “stack” together.  Only a few lbs of torque are needed.

The hub with the axle and tool installed to remove the stack of a slash 2 BMW motorcycle.

This is what it looks like with the axle installed “backwards” with the “tool” in place, the nut holding it all together and ready to hammer out.  I am using a wheel hub because I didn’t have a /2 wheel for the photographs.

6.  One does not need heat on the /2 hub.  Use a dead blow hammer to tap/pound the stack out of the hub.  If it doesn’t come out easily then it can need some serious hits.  A “serious hit” is what you would use to pound a 16 penny nail into wood in only 4 hits.  Keep the axle totally straight so that the bearings stay straight.  If they get cocked off to one side, they will jam against the inside of the hub and stop moving or take out some of the material.  The stack may move as little as 1/8″ with each hit.

You may use a hydraulic press for this, if you have access to one large enough to take the entire wheel.  I have several times seen a hub be ruined by someone using a press on the hub alone.  There is nothing wrong with the basic procedure, but if one fails to notice that something is jammed up, the hydraulics may have enough power to press it out anyway.  I prefer a hammer.

Some are quite nervous about hitting a good bearing.  That makes sense.  I was also unhappy about doing it at first.  The BMW service school used that procedure.  I was expecting premature bearing failures due to hammering, but they failed to show up.  It is far better to hit it more times with less force.  After you remove the stack, you will be cleaning and inspecting the parts carefully.  Pull the cone and clean the outer race really well and examine it for telltale “dents” in the polished surface.  You should find none.  I never have found any damage from the hammering.

This shows the stack with the first bearing out of the hub on a slash 2 BMW motorcycle wheel.

This is what it looks like with the stack 1/2 way out.  See the “tool” on the left?  This one isn’t a pipe nipple and so therefore has no threads.

This shows the axle aligned perfectly in the hub.

This photo shows the axle centered in the hub.  The stack is 1/2 way out and the extra space around the axle must be kept equal all of the way around to be sure that the stack is coming out straight.  If it cocks to one side then it is digging into the hub and may make a mess of the hub bore.

This shows the axle cocked to one side. This must be corrected before driving the stack any farther out. If this happened while the hub was in a hydraulic press, it would ruin the hub.

This is what it will look like when it is off center.  Just tap the axle sideways to center it.

The stack and axle of a BMW motorcycle wheel after it has been removed from the hub.

 This is what the stack looks like after it has been removed and wiped off.

7.  Disassemble the stack, noting the position of the “inside” top hat spacer.  There are two of them, one on each end of the bearing stack.  They are not identical.  The “outside” top hat spacer will have stayed in the seal holder when you removed it.  These top hat spacers are for one purpose, to transfer pressure from the axle to the bearing stack, through the seal.  Each top hat spacer rides in the seal, so it needs to have a smooth surface.

It is not unusual to remove a stack and find that the grease is so old that it has hardened up and useless.

Typical inner spacer with old dried grease.  This shows that the grease that is packed up around the inner spacer never gets hot enough to “flow” to the bearings.  I use the softest grease I can find just to try to allow it to flow if it ever gets hot.  Properly spaced, they will never get hot.  Don’t obsess over the grease, just don’t use the old fashioned stringy type used on cars.

The old dried up grease has been broken off of the inner spacer of a BMW motorcycle wheel bearing stack.

 This is the dried grease that has just been “chipped off “of that inner spacer.  This is a commonly found situation.

This is what will happen if you don’t remove the bearings as a stack. 

This shows typical damage to a top hat when the mechanic has no idea of what to do.

As I pulled the stack apart I found this disaster.  This is the spacer (bushing) on one end, often called the top hat.  This was just luck that my only wheel to photograph happened to have signs of earlier abuse.  This is caused by someone removing the stack without information.  The person just hammered them in or out without holding the stack together with the axle and 4″ spacer.

This shows the other side of the same damaged spacer, or top hat of the photo above.

The other side of the same spacer.

This shows a badly damaged outer spacer from the stack of a slash 2 BMW motorcycle wheel.

The outer spacer that has been hammered badly.  This is one of the worst that I remember.  In our shop we would see this a couple of times a year.  Makes one want to use the hammer on the guilty idiot.

More evidence of damage caused by using the wrong procedure to remove BMW motorcycle wheel bearings from the hub.

The damaged outer spacer shows exactly where the person used a punch and missed the actual edge.  This is proof that the person hammered it out in pieces.  Had it been held in place as a stack, this area wouldn’t ever be available for hammering.

If you find such a disaster, all isn’t lost.  You can dress the parts down with a file and get them close again.  Then finish the job by lapping them on the paper and glass plate.

8.  Clean the bearings and inside spacers of all grease.

9.  Inspect the bearings for pitting and stains from water.  Replace as needed.  I don’t currently have any bad bearings to show, sorry.  If I find one or someone emails me a photo, I will add it here later.

Time out.  You aren’t really ready to do the spacing just yet.  This part of the procedure is my addition, but it is worth the trouble.

First, inspect the wedding band spacer, the large outside spacer and the larger part of the inside spacer with a good eye or magnifying glass.  The machined off ends are usually poorly cut.  They are rough.  Cleaned up faces offer more surface mating area.  This will give a more solid feel to the preloaded stack.

Second, the ends should be parallel with each other and square with respect to the length.  About all you can do is to get them parallel.

I lap them to be smooth and as parallel as I can measure.  I use a glass plate as my surface and #400 grit black paper for the abrasive.  Photos of this procedure are above in the /5 section.  Add solvent to the paper to reduce it’s tendency to clog up.  As they start to clean up, check for parallelness if you can.  The /2 stack tightens up really well and feels solid, compared to the /5.

10.  With only a drop or two of oil on each bearing, reassemble the stack on the axle.  Put the washer and nut on and finger tighten it.

11.  Put the axle, with the stack, in a vise, vertically, and tighten it on the nut.

12.  Use the tommy bar, or a screwdriver, in the axle hole to tighten and loosen the axle.  As you slowly tighten the axle, continue to rotate the bearings by hand.  The large outside spacer should be easy to move sideways with medium thumb pressure.  That’s the visible part between the bearings.

13.  BMW wants you to change the wedding band spacer until you reach the correct spacing.  With shims, you can just add them until the correct spacing is reached.  When the outside spacer moves with medium thumb pressure, at 25 lbs of torque, you are finished.  See below for ordering the shims.  You don’t have to have the shims, just go to # 13 in the /5 section above.

14.  Lube both bearings and assemble everything on the axle.  You will be unable to over lube it.  Tighten it up to about 10-15 lbs.

15.  Drive the correctly spaced and lubed stack back into the hub.  It will go in with about the same hit as removing them.  If you really like to go to extra work, heat up the hub a bit and freeze the stack.  The stack will go in very easily that way, it may even fall into place with no hammering.

16.  Install the seal holder hand tight, but don’t tighten it down with force.  Tightness plays no part in the bearing or sealing function.  Making it tight now only insures that it will be hard to get off later.

17.  Install the wheel and axle.  To reinstall the axle, be sure that you know how to do it or use this article to see the correct and incorrect way.  Go down to the picture of a /2rear axle.  I will describe what to look for.  See the right side of the hole in the axle for the tommy bar?  Notice the distance from it to the swing arm.  Now, go to the other side of the swing arm and notice the distance to the ridge on the axle.  Those two measurements should be the same.  When an error is found, it is always the right space is less than the left space.  That is because some idiot tightened up the axle nut without the axle being fully seated in place.  That actually pulls the left swing arm in towards the wheel.  The installer should have rotated the axle with the tommy bar and that would allow the swing arm to “pop” back to where it should be.  After a few months, or years, the swing arm takes a new set and is now bent.  Just pull it back out to the proper place and tighten the swing arm pinch bolt.  It will take a new correct set.

18.  Perform the “shake the wheel” test to check your work.  It may tighten up at a slightly less axle nut torque than when originally spacing it.  If it is way off, a shim can be added to correct it.

The disc brake wheels, /6 and later

The theory of the taper bearings and preload is exactly the same as the /2 and /5 as mentioned above.  The advent of the disc brake on the front wheel of the /6 models is a slightly different situation in practice.  To make it even more confusing is the front wheel in 74 uses a 14 mm axle.  That axle proved to be too light and easily subject to damage in a minor accident.  In 75 BMW upgraded it with a 17 mm axle which is the same size as the former front axles.  They kept this arrangement for several years.

The R60/6 wheels with the drum brakes are basically the same as the /5 wheels.  The /5 use a chromed hub cap to cover up the hub castings.  The /6 did not have the hub cap.  Treat the R60/6 and R60/7 as a /5.

Like former BMW’s, the all of the wheel bearings were set up a bit too tight by the factory.  Test it by the “shake the wheel” test.  If you find it way too tight, you may want to do something about it.  At this point, I must emphasize that the likelihood of experiencing a hub failure is very remote.  With the heat from braking dissipated in the disc, no braking heat threatens the hub.  Only the heat from bearing friction is applied to the hub.  Too much preload results in more drag by the bearings, but that is minor compared to the always dragging disc.  The front wheels on the /6 just don’t spin freely like the former models.  Unless something were really amiss, I would ignore the bearing spacing.

Since the axle is an “odd size” Cycle Works doesn’t seem to make a tool for removing the end caps.  The end caps should not be very tight.  “Tight” serves no purpose at all.  I used a very simple scheme to get them off.  I just happen to have two punches that are a good tight fit in the holes.  If I didn’t have these to use as tools, I would select drill bits that are a good fit.  A “good fit” means that they don’t need to be pounded into the holes and aren’t sloppy either.  I would not use good fitting nails, except as a last resort because they bend too easily.  Use some type of long lever to twist the cap off.

This shows using a pry bar between to punches to remove the slash 6 hub nut on a BMW motorcycle.

This is the unofficial tool

I grab the bar with both hands equally spaced from the hub nut.  This places about equal pressure on the hub nut on each side.  Most come off easily by this method.

For a really stuck hub nut, one may have to use a punch to tap on the nail, or punch that is in the hole.

Second option

If the bar fails to get the hub nut loose, then one must apply some real force.  I use one of my punches in a hole and use a pin punch to hit it.  See how the punch is horizontal rather than downwards?  This applies the pressure directly to the punch in the hole and all energy is rotational.  After one hit, move the punch to the opposite side and give that a hit, then back to the first one.  As soon as you see it move a little, try the bar method again.

This shows the most common way to try to remove the hub nut. It is the wrong way and will damage the holes in the hub nut. Do not do this to your BMW motorcycle.

Do not do this

Do not, under any circumstances. stick a punch directly in the hole, at an angle, and hit it.  That will damage the hub nut quickly.  Then the proper tool won’t work well, or at all.  It also fails to get the hub nut off easily, as some energy is downwards and not rotational.  Under extreme circumstances, use some heat.

The 1974 BMW 14 mm axle front wheel bearing spacing on the R75/6, R90/6 and R90S

The bearing parts of a 1974 BMW motorcycle front wheel layed out for inspection.

A view of the parts

1.  On the left and right ends are the fancy hub nuts.  Basically, they just hold the seals in place.
2.  The distance spacers are identical.  The assembly on the left of center has had the bearing pressed off to show how it looks.  These parts are symmetrical, even though it doesn’t appear so here.
3.  My shim is shown where it has been added between the bearing inner race (cone) and the shoulder of the distance spacer.
4.  What is effectively the “wedding band” is in the very center of the hub.  It doesn’t look that way due to the “expanded parts” on the left side.

How to remove and install the bearing cone from the distance spacer.

This shows one way to press apart the wheel parts of a 1974 BMW motorcycle wheel.

I just use a socket in a vise to remove the bearing or install it again.  Just make sure that the shoulder on the spacer doesn’t hit the bottom of the socket.

BMW intends you to have a whole box of the wedding bands, as they come in 13 sizes.  How many shops actually have a full set, not to mention how many owners like you have this set?  They are about $10 each.

The idea is to add in a thicker wedding ring if you want to reduce the preload.  The affect on the “shake the wheel” test would be to make the torque required to get to neutral play go up a bit.  One would just try a thicker and thicker one until one reaches 15-25 ft. lbs of torque.

On the opposite side, if the test shows that the wheel is still loose at 25 ft. lbs of torque, then use a smaller wedding ring until you reach your goal.  One could just lap it down, but don’t go past the amount needed, as one can’t put the metal back.  This set-up doesn’t have an outer spacer (as in the /2, /5 and the rear wheel of this same bike) to lap down, as it is the hub casting itself.

This is a custom made shim for going between the distance spacers of a 1974 front BMW motorcycle wheel with a disc brake.

A custom made shim

On the left is a custom made shim and on the right is the wedding band spacer.  This one was carefully hand made by a reader to solve this problem.  This is a solution, but far from ideal.  One would have to very carefully assemble the parts to make sure that the shim is fully aligned with the wedding band.  Otherwise when the axle is inserted it could grab the fragile shim and tear it out of its place.  Since so many forks are poorly aligned, owners are often forcing the axle in already and may not notice the “new” obstruction.

There are two ways to go with a 1974 system

If we replace the wedding ring spacer with the thickest one by BMW, then the wheel will be almost guaranteed to be loose.

Now you must make a choice

1.  One can come back in with the /5 type shim and add it in on the distance spacer and under the inner bearing as shown in the photo.  This way the very thin shim is captured and can’t make any trouble.  As you add shims the “looseness” goes away.  It is a slightly greater job to change the shim a few times until you get it exactly where you want it, but certainly cheaper than having the whole set of wedding rings in your stock of spares.  Besides, with proper bearing maintenance, you will only do this once in the life of the bike.

You would need to have a /5 shim kit and the thickest wedding band.  You could buy the shim kit from Scottie (see below) and then order a thick wedding band from your dealer.  The BMW part # is 36 31 1 231 140.  Or, would you just prefer to convert over to the 17 mm axle front end and be done with the “weaker axle” issues?  It is your time/money, you decide.

2.  One could choose to lap the new wide wedding band down to the exact thickness needed.  With either method, one will probably have it apart a few times in order to “sneak up” on the correct size.  With the shim kit, one can reverse it by removing the needed amount.  With the lapping method, the metal is gone and there is no going back.

The 17 mm axle, 1975 and 76 system

The parts of a 75 and 76 front wheel bearing setup.

This shows the parts of the non-symmetrical stack.  The end caps are identical to each other and to the ones used in 1974.  The end cap on the right side is assembled and the one on the left has the top hat pulled out so that you can see it.  Instead of having the wedding ring in the center, it is off to one side.  I prefer to have it on the side of the hub with the brake disc, as that way I can have the disc up when changing the spacing.  That helps to prevent a warped disc due to it being on the bottom, or having to find an old tire to support the disc in the air.  The parts book calls the tube a “rube” in my parts book, but that is just one more of the hundreds of mistakes in just about any publication.  It is a typo, as the T and R are next to each other on the keyboard. The photo shows where I should have put a shim, but I forgot.  It goes just under the wedding ring with grease to keep it in place.  If you really want to be sure it doesn’t get shoved aside, then insert the axle into the end cap, insert the axle into the hub and screw the end cap into place.  When you remove the axle, the shim should stay in place with the grease.

The advantage of having the top hats mounted on the inside of the dust cap is that during the process of removing the wheel from the motorcycle there is no chance to have it get knocked out and lost.  The /5 was famous for having owners lose them.  The advantage to the top hat being on the outside is that it serves as additional protection from dust and water getting into the bearing system.

Difference in the 74 and the later BMW disc brake front wheel hubs.

If you find a hub, or wheel and wonder if it is the unique 1974, or the later one that uses a 17 mm axle, then this info is for you.

The inside of a 74 BMW motorcycle wheel hub for identification.

The 14 mm axle hub

OK, its not very clean of grease, but for this purpose it doesn’t need to be clean.  Look at the hole in the center and the area between the outer race (cup) and the hole.  It is smooth.  The diameter of the hole is about 3/4″ (19.5 mm).  The purpose of the small hole is to center the wedding ring spacer.  If the hole were larger, the wedding ring could fall down and keep the axle from going through the hub.  This hub can’t be used with the 17 mm axle system.

This photo shows the post 74 hub inside appearance.

The 17 mm axle hub

The later type hubs have a few ways to identify them.  The hole is about .910″ (23 mm).  The hub has the 4 ribs that hold the tube (long spacer) and wedding ring from getting out of alignment with the axle holes.  I do not know how many more years that BMW used this hub, as my parts book is too old to show.  Can someone tell me??

I see no reason why a 74 hub can’t be bored out a bit to accept the 75 and later axle and wheel bearing spacers.  If you are restoring a 74, you may prefer to keep it all stock.  If you are riding it, then consider swapping out the 74 parts for the later and stronger 75 parts.  You will need fork legs, axle, spacers and a different long sleeve.

Oddity in the 74 hub sleeve, or spacer

The 74 had two different sleeves and I have no idea why.  The ID is the same, but the OD is different.

Other wheel maintenance

When you change a tire, always balance the wheel.  An unbalanced wheel can cause a wobble.  See my page “How to balance a BMW wheel.” There are a few good balancers and many bad ones for sale.  I prefer the one made by Marc Parnes.  Tell him you saw it here.

Check for spoke tension.  This is done by plinking the spokes with a metallic tool.  They should all sound about the same with a rather musical note.  They will rarely be loose, but check them.  If you find one that makes a “thud” when the others make a “plink” then it may be loose and require tightening.  Spokes on the 70-71 models are a special case.

Clean and lube the splines on the rear wheel.  Check that the 2 final drive (/5 & /6) drain holes are clear by using compressed air.  A wire won’t work well because the hole isn’t straight.

This photo shows one of the two holes that must be kept clear.

One is located below the axle.

 This hole is to drain out any oil that collects from a leaking seal on the large bearing of the BMW final drive.

The other hole is next to the drain plug hole.  It goes up into the spline area.


How to obtain shims

In 2016 I sold this service to Scottie Sharp.  This service doesn’t make him a cent, it is just a service.  It does help advertise his business.

Scottie’s Workshop

2102 Walsh Ave.

Santa Clara, CA. 95050

408-475-2696


 

Warning to obsessive/compulsive owners

Recently I have gotten email explaining that the mechanic/owner found that he needed a shim of .0025″.  That is between the 2 and 3 size.  The “shake the wheel test” is actually far more accurate than needed.  Keep in mind that BMW was happy with providing steps in size for the /5 of .002″ (.05 mm) and for the /2 the wedding ring shims were .004″ (.1 mm) apart in size.  While I think both are a bit much, getting down to less than one thou (.025 mm) is just not needed.  Besides, if you really do want it that close, then lapping the wedding slightly will get it.  Remember, it is better to be too loose than too tight, so that owner should have used the .003″ shim.

The thing to keep in mind is to not have the preload tight enough that the play goes away with only a few pounds of torque.  That danger is only to the hub, not the bearings themselves.

Quantities

Ask Scottie about quantity sales


/5 wheel bearing design specs.

By Brian Mehosky, Timken employee and BMW nut

How long will a /5 BMW wheel bearing last?

Well, the question was asked, so I went to ask the pros at Timken, where I work.

The specific question was:

“If the tapered roller bearings in our motorcycles were properly maintained, properly preloaded, properly lubed, and never washed out with a pressure washer, or let sand/dirt/debris get into them, how long *would* they last?”

The quick and dirty life calculation is:

L10 (revolutions) = (9.0 * 10E7) * (C90/P)^(10/3)

Where L10 is the 10 percent failure rate (i.e., 90 percent reliability point) (in revolutions);

C90 is the basic dynamic radial load for an L10 life of 90 million

revolutions (in pounds); and

P is the dynamic radial load (in pounds).

*I* assumed that the total dynamic load on the axles was 600 pounds for the machine, plus 400 pounds for rider(s) and luggage, or 1000 pounds (that may be wrong, since it really is a static load, but you will see that it *just doesn’t matter*).  Since there are four (4) 30203 single row bearings on the two axles, I *also* assumed that the load was shared equally, at 250 lb per bearings (again – *it just doesn’t matter*).  So P = 250 pounds (force)

The C90 load for a 30203 Timken bearing (I’m *certain* that it is significantly lower for other manufacturers, but we should always use the best in our machines [grin]) is 5190 lb (force).

When *I* punched in the numbers, I got an L10 life expectancy of 2.2 **TRILLION** revolutions.  If you assume a tire OD of 2 feet, you need 840 revolutions per mile, so the bearings are only good for 2.6 **BILLION** miles.

Although the dynamic load *may* be greater (probably *is*), it should also be pointed out that a “failure” here in bearing-land is that one or more component of the bearing shows some kind of “damage”, like a scuff or mark or pit.  The bearing will probably continue to function for a long time in that condition, it’s just not recommended.  And the *other* 90 percent will last even longer.

As Duane has pointed out, this system is *massively* over-designed.  I never realized just *how* massively over designed the bearings are.

Brian

PS – the number also correlates well (within 10 percent) with a similar

calculation based upon ISO method ISO 281 (for our Continental friends) B

Duane’s note; These calculations assume the stress of normal side loads.  After all, the only reason for this bearing is to provide stability with side loads that were caused by a sidecar.  In solo use the motorcycle never has side loads.  It can only have side loads in sidecar use.  The life of bearings without any side load is far longer than the engineering calculations done by Timken, not that a longer life is needed.  Think of it this way, if you had little risk of damaging the hub, as in the /2 solo application, then you could be off enough to be way out on the edge of the curve and lose 99% of the bearing life and still have more than 1/4 million miles of life.