BMW wobbles

By Duane Ausherman

    I want to describe my experience with testing and correcting wobbles on the BMW motorcycle.  What is a wobble?  A wobble is an oscillation, or side to side shake, in the front end of a motorcycle.  The cause can be from almost anywhere on the motorcycle, rider or road conditions.  The problem can be found and corrected with some tests and repairs, or changes.  A wobble should be corrected immediately as it is can cause a serious accident and even death.

    Wobbles are of two types, low and high speed.  A low speed wobble is usually in the 30-50 mph range.  A high speed wobble is usually in the 65 mph and up range.  They are different but can have one source, or they can have several causes.  My approach to a wobble is to carefully examine the motorcycle.  A visual inspection can often find the most likely causes.  Correction of these is to be done before riding it again.  A special test ride regimen was followed to reduce the chance of an accident.  First, find and correct the low speed wobbles and then test for a high speed wobble.

    If all of the standard mechanical possibilities have been checked, it's time for the low speed test.  Testing for low speed wobbles isn't very dangerous.  I have not had any scary experiences with them.  I look for them first, as they can show the likelihood of a high speed problem.  My test ride is much more inclusive than I will describe.  Here we are only investigating wobbles.  First, I test for straight tracking.  If the bike doesn't track straight, I want to find out what's crooked.  My test is to get up to about 20-25 and let loose of the bars and see if it goes straight.  The amount of lean I must use to keep it straight is my measure.  I estimate how far off center my head is with the bike.  If it takes 6" of lean, that's too much. I find out why.  Some lean isn't going to insure a wobble, but it needs investigating.  I have seen a 6" lean BMW that handles very well at all speeds.  However, over several hours of riding, the riders shoulders will get sore from holding it up.

Testing for wobbles

    Some caution first.  I am describing these tests to acquaint the reader with our methods.  I can't over emphasize the danger.  Here is one story to show the danger.

    My cousin's wife rode a Yamaha 350 and had felt a bit of a twitch, but nothing really scary.  My cousin had ridden it and had found nothing.  Different riders, especially amateurs, find different results.  At 65 mph the bike went into such a wobble that the bars were immediately wrenched out of her hands.  The speed and force of the bars was such that the knuckles on both hands were broken by the bars swinging back and hitting them. She had no time to get her hands out of the way, it was instant.  Then the bike flipped over.  When someone tells you that you should speed up or slow down, remember, there may be no time.  Her injuries were not limited to broken fingers.

    The most likely condition for a wobble is in slowing down, while in a downhill gentle curve with hands off.  I take it up to 30 mph and slow down gently.  Then up to 40 mph and slow down.  Then I take it up to 35-40 mph and shut the gas off and let go of the bars.  This one will get a lot of failures, maybe 25 % will wobble on this one.  Then up to 40 and let go of the bars and bang them quite hard and see if the shake dampens out quickly or slowly or not at all.  At any time the wobble can be controlled by grabbing the bars.  The last test, and the strictest, is to post or stand up a bit and take weight off the seat and put it on the pegs.  My legs aren't gripping the tank, but sort of bowlegged to clear the tank.  I want to reduce the biomass dampening to the bike. All the while I am slowing down from 40 mph with hands off the bars and then I wrap the bars.  This position shifts the weight forwards as I must balance on my feet.  It is very hard to shift the weight forwards with a fairing and easy with low bars.  No BMW I have ever tested, with a handle bar fairing, will pass this last test.

Possible causes of the low speed wobble.  A low speed wobble has few aerodynamic factors.  Some of the factors are hard or impossible to isolate to only one thing.  A good example is the handle bar fairing or windshield.  Is the problem one of raising the center of gravity or of the pivoting front end?  A handle bar fairing will always alter results, remove it for the tests.  Here are some things to check.

1. Correct tires and pressure . Twenty five years ago this meant Conti or Metzler at about 30-34 lbs.

2. Tires balanced correctly.  Are you sure you have really balanced them?  Can your repair shop really do it?

3. Tires in very good condition, especially the front?  We found that when the tread is down to 1/2, replace it, balance it and many wobbles will disappear.

4. Tight wheel, swing arm and steering bearings?

6. Saddle bags?  Remove them.

7. A top box is just about the worst thing on a bike.  The problem is both aerodynamic and weight.  Even empty they can cause a wobble.  Remove it for the tests.

8. A BMW /5 (and others too) must be neutral steering.  To test this I prefer to go through a 30-40 mile sweeping curve.  Let go of the bars and it should stay in the same curve.  If it wants to turn tighter (fall) it is too low in front or too high in the rear. Is the bike high enough?  Does it have sagged out springs or a front tire with a low profile?  It can also have forks bent straight back towards the engine.  It may track straight but not turn properly.  If it wants to stand up and go straight, maybe the front tire is too big, the rear too small or the rake too great.

Earles forks on the /2

    BMW introduced the Earles forks in 1956 to appeal to the sidecar driver.  They have the characteristic of being very soft and comfortable.  For the sidecar the allow braking while maintaining a constant wheel base length, a good thing for sidecars.  Earles forks are heavy and swing slowly.  The unsprung weight is, however, very light.  For solo sport riding, the Earles forks are a poor choice.

    This configuration has unfairly been criticized for being susceptible to wobbles.  This is simply untrue.  Any motorcycle can wobble, it is the nature of the beast.  There is nothing wrong with the design if the Earles forks and frame to make it wobble.  At this point, only a bare bike as delivered by the factory is being discussed.  If one decides that a wobble is dangerous, therefore undesirable, then it follows that it is a fault that should be corrected.  Since most of them didn't wobble, the ones that do, must have something different about them.  It is logical to say that that particular bike has a problem that is very dangerous.  We should find that "difference" and correct it.  To alter the design is not the solution, finding and correcting the fault is.

    The manufacturing tolerances of the /2 were not very close in some respects.  One can easily find small variations in the forks and frame.  I know of one case where a frame was out of spec and it didn't track straight.  It was a European delivery and was taken back to the factory to get the frame straightened.  I was returned working very well.  That is an unusual case, but serves to show that these bikes were made by humans.

    I started riding BMW's in 1962 and had no knowledge of motorcycles.  I loved riding so much that it was easy to become enthralled by BMW.  Soon I owned several and my social circle centered around my new hobby.  By 1965 I was living on the east side of Cleveland and no dealer was closer that 15 miles.  They had no interest or knowledge of BMW.  Penton Bros. in Lorain, was 40 miles away and they were very good.  I sort of became the default "fixer" of minor BMW problems on the East side of Cleveland.  Back to wobbles.

    The few that wobbled were usually fixed by standard repair practices.  My sample wasn't large enough to see real problems.  In 1966 I cut up a perfectly good 1962 R60/2 and installed a VW engine.  It required cutting the frame in half and lengthening it by about 2."  Knowing little about frame geometry, I just welded it up in alignment.  It was stable, it should be, as it was now longer.  It also has neutral steering and that was an accident.

    I opened up a repair shop in 1967, in San Francisco, and started learning how little I knew about them.  Wobbles fascinated me and I did all I could to learn more about them.  Here are a few factors and how to check them.

Front wheel, swing arm and steering bearings

1.  Testing the front wheel and swing arm bearings.  Grab the front wheel in one hand and the fender brace in the other.  Shake the wheel sideways.  If any "play" is felt it must be in the wheel or swing arm bearings.  By looking at the gap at the wheel or the swing arm pivot, one can see which is loose.  Grab the front fender with one hand and pick it up till the wheel is off of the ground.  Spin the wheel, with your other hand, and "feel" any vibration through your fender hand.  It should feel completely smooth.  If you feel vibration, it is from a failed wheel bearing and you are noticing the rough surfaces of rollers against the races.  Replace them.  Read how on my page "All about BMW wheel bearings."

    If the swing arm has some play, it must be adjusted to fix it.  If no play is felt in the swing arm, that doesn't mean that the bearings are good, only that they haven't yet been proven to be bad.  To make the next check: block up the front end and remove the wheel and shocks.  The swing arm is now loose and will swing up and down freely.  If it is more than a few years old, or hasn't been lubricated regularly, or both, it probably has notched bearings.  Do you feel it swing smoothly or with some "tight spots?"  Replace them.  Before 1965 the front swing arm couldn't be lubricated.  It is easy to add a grease zerk for lubing the bearings.  For more info on this go to "Adjusting the /2 front swingarm."

With the front wheel in the air, the steering damper off, gently and slowly swing the forks back and forth.  Does it swing completely smoothly, or with a notch in the dead ahead position?  A notch can be felt while riding at low speeds by the rider noticing that the bike won't go straight.  It wants to constantly curve.  Notched bearings must be replaced.  Loose bearings can be checked in a few ways.  1. When applying the front brakes lightly, at low speeds, did you ever feel uneven braking or a kind of resistance that was related to wheel speed?  This could be an out of round brake drum or loose steering bearings.  2.  With engine off, push the bike and apply a bit of front brake.  Do you feel a "click" or movement in the front end?  3.  With the front wheel off of the ground, grab the front end by the lower shock legs, just above the swing arm, and pull gently.  Don't pull if off of the center stand.  A bit of "play" or a "click" can be felt if the bearings are loose.  Tighten the steering bearings, as needed, and retest it for a notch or proper tightness.

    It is possible for Earles forks to be bent backwards and could cause a wobble.  It will certainly increase it's tendency to wobble.  Can you see black paint on the front engine cover?  It could have been replaced.  The older frames didn't have strengthening gussets at the head stock and easily bent.  There is no proper distance between the engine and front cover.  Two types of covers and three types of cross braces were used so takes lots of experience to know if it is correct.  I used to be able to judge it by sticking so many fingers in that space, as my measure.

Last modified 27 Dec 2001  Visit our web site at http://www.softcom.net/users/w6rec/index.html