Tests for BMW motorcycle high speed wobble or weave |
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This is a sure fire treatment to cure a wobble. This article also applies to the /2 models made in 1968-69 that we call "US" models. They have the same telescopic fork as the later /5 thru /7 motorcycles. A few ancillary parts are different, but from a construction standpoint, they are identical forks. They too will all wobble if a handlebar fairing is mounted. Many riders have assured me that their bike does not wobble with the handlebar fairing. On all that I have been allowed, I have ridden it in front of the owner and demonstrated a low speed wobble. No exceptions on any telescopic fork BMW from the first ones in 68 up through the SWB models of the /5. I avoided writing this page for years due to the seriousness and how knowing how difficult it is to describe the testing. A couple of "stories" are included, but that is in an attempt to communicate how serious this issue actually is. 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 can cause a serious accident and even death. If you want more information on wobbles, use Google to find some highly technical information. 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, we would 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 want to 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, after several hours of riding, the riders shoulders will get sore from holding it up. Yamaha motorcycle wobble storyMy 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. BMW motorcycle wobble storyI asked Jim for his permission to use his story and here is his answer copied directly. duane, after all the help i have recieved from your site, you are welcome to anything you need from me. as i said, i'm no brain child when it comes to the fine details you guys get into on that forum, but i have had an earles fork under me since around 1972. i went down twice over the years for the same reason. . even though it can be passed off as some anomoly, bearings, etc., it comes down to proper maintenance on the rider's part. a false sence of security because i have this "tank" under me, just does'nt cut it. this is my first ground up restore and i have not much to go on except a parts catalog, a clymer manual, slabon's book and the most helpful so far, you guys. i ask some dumb questions at times but i do learn from the answers. thanks again for all the help. jim His story not being the engineer type i can only say you beat the reaper, this time. i agree with duane, you never want to experience the excitement of flight twice. last year a high speed wobble broke my bike and my bones. i have since replaced every bearing, wheel, spoke and tire on the bike. it is still not done but getting back on it will be a slow process. not being able to determine the cause of these events is the scariest part of the deal. i have also changed out my u s bars for lower euro bars. they are longer ( sidecar bars). i have no idea how this will effect handling but i guess we will see. i regularly rode my bike at 70 to 75 mph. all i remember was an uneven bit of road between lanes. as i crossed that ridge, all hell broke loose. things happen so fast that you only have time to realize you are a goner. the bike spent an eternity in the air, hitting all four corners before coming to rest right behind me. the brunt of the damage to the bike was sustained by the crash bars, handle bars, front fender and headlight bucket, rear solo seat and hard bags. all new. i have also installed new shocks, the rears being nitrogen filled replacements by works performance in california. i figured i had to start at the beginning anyway so why not replace everything that could have gone wrong. keeping everything in proper adjustment is next. you can get to a point where you feel these old beemers are indestructable and snicker at hog riders who have to wrench their bikes daily to keep all of their parts aboard. i have spent nearly 12,000 bucks to repair the bike. the hospital bills were much higher. consider yourself ahead of the game if no doctors were involved. jim Testing for wobblesSome caution first. I am describing these tests to acquaint the reader with our methods. I can't over emphasize the danger. A wobble can kill or maim. The most likely condition for a wobble is in slowing down, while in a downhill gentle curve with hands off of the handlebars with no weight on the seat, sort of posting while riding a horse. This testing is progressive, in that I perform a test under unlikely conditions for a wobble. If that passes, then I make it a bit more likely to wobble. From experience, I know when to stop increasing the risk and quit. This is not for your average owner/rider. I take it up to 30 mph and slow down gently and feel for any sign of instability. Then up to 40 mph and slow down. Then I take it up to 35-40 mph and shut the throttle 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. I actually try to induce a wobble. 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 give the bars a "hit" with my hand. This position shifts my body forwards as I must balance on my feet. It is very hard to shift the weight forwards with a fairing, but easy with low bars. No BMW I have ever tested, with a handle bar fairing, will pass this last test. I have had many owners tell me that "My BMW doesn't wobble." If allowed, I have gotten on the "stable bike," gone down the street and come back at 35 mph with the bike in a wobble. Point made. 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. It is usually a combination of several things. Often one can get a false sense of security. The owner will remove or correct something and the bike now shows no wobble. Is the bike fixed? Maybe not. What if the wobble goes away with some other aspect removed? What if the wobble will go away with any of several things changed? I contend that just removing one of the factors is only a start. 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? Is the greater weight on the forks compressing the fork enough to change the trail? A handle bar fairing will always alter results, remove it for the tests. Here are some things to check. 1. Correct tires and air pressure. Twenty five years ago this meant Conti or Metzler at about 30-34 lbs. Today it is higher. 2. Tires balanced correctly. Are you sure you have really balanced them? Can your repair shop really do it? Read about balancing a BMW motorcycle wheel. 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? 5. Good shocks and mounting bushings? 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 50-60 mile sweeping curve. Let go of the bars and it should stay in the same curve. If it wants to turn tighter (fall down) 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 in the curve, maybe the front tire is too big, the rear too small or the rake too great. When the dealer got the BMW it was neutral steering. You may not care about neutral steering, but you better be concerned to find out what has changed. 9. The US forks and the /5 and later have a reputation for wobbling. It is only partly true. If the forks are properly aligned they won't have stiction. This can be a very important factor in wobbles, especially high speed wobbles. The forks may need to go through the alignment procedure to work correctly. Testing for high speed wobblesThis requires a lot of experience, paid up life insurance and a bit of insanity.... maybe more than a bit. I will describe my worst experience. Chris Blum, my parts man at BMW of Marin bought a new 1972 R75/5 in curry color. The only modification was mounting European low bars. By 2000 miles he complained about a wobble and was afraid to take it up in speed. Everything mechanical was perfect and we could find no error. The tires were in balance and only took minimal weight. The rims were quite true. All bearings were tight. I was sort of doubting that it had a problem worth mentioning. Maybe Chris was super sensitive and was complaining about a quiver, not a full wobble. As the owner of the shop, I had the most experience with wobbles. I put my riding stuff on and prepared to take it out for a ride. Chris' last words were, "Watch it, this thing is dangerous." I took it out on a seldom traveled road to perform the tests. It had a minor low speed wobble, but it damped out easily by putting my hands on the bars. Many bikes had worse low speed wobbles and no evidence of high speed instability. I rode it up to about 75-80 and while keeping my hands on the bars, gave it a shake. Each one was solid and normal feeling. I started the high speed tests at 50-55 mph and it showed no instability at all. I repeated it at 60 and it was as solid as a rock. I did the same at 65 mph and the forks went into a full wobble instantly. There was no time to do anything. Had my hands been on the bars, my fingers would have been crushed by the wildly swinging bars. The front tire was skipping and airborne. It would come down at the end of the travel and make a huge squeal. The front wheel would become airborne again and head for the other stop. This happened about 10-12 times in maybe one or two seconds. I wasn't paying close attention to my watch. My only reaction was to try to sit down again. Due to the added biomass, or some unknown factor, the bike snapped out of the wobble. It dampened out as fast as it started. I immediately stopped and got off. I was so scared and weak kneed that I could barely put it on the center stand. I just fell to the ground and was in complete shock. Finally, I recovered enough to get up and ride the bike back to the shop. My employees had a new aspect to think about. We got into the truck and drove back to the scene. This was an hour later and I was sort of OK by this time. We examined the tire tracks. Every 15-20 feet there was a 2 foot black mark and each was about 90 degrees from the previous one. The marks weren't straight, but oddly shaped. You could see where the tire smashed back down on the pavement with a wide part and then the traction and trail forced the wheel to correct. This correction was enough force to whip the bike back into the air with the tire again off of the ground. If one drew a line through all of the "left" tracks and another through the "right" tracks, they were about 3-4 feet apart. The only thing that we found wrong with this new motorcycle was that the forks were not in alignment. It showed very little stiction. The forks were .006" out of alignment in both planes. The BMW spec was .004" and so it was very close. We had been aligning forks for about a year to .002", but we had never suspected that such a small error could cause such a wild wobble. We changed our previous spec to .001" or better. After all, what is wrong with perfect? The alignment procedure fixed the bike and it never gave a problem again. Earles forks on the /2BMW 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. That is simply untrue. Any motorcycle can wobble, it is the nature of the beast. There is nothing wrong with the design of the Earles forks or /2 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 they didn't wobble when new, 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. Do not ever put the Earles forks swing arm in the sidecar position for solo use.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. 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 bearingsTesting 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 my page about BMW wheel bearings If the swing arm has some play, it must be adjusted to fix it. It is more likely that they need to be replaced. 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. Read about adjusting the /2 front swing arm. With the front wheel in the air, the steering damper off, gently and very slowly swing the forks back and forth through the straight ahead position. 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 common for one to tighten up the steering bearings and only then be able to feel a notch in the bearings. It is possible for Earles forks to be bent backwards and cause a wobble. It will certainly increase it's tendency to wobble. Can you see black paint on the front engine cover? If not, the cover could have been replaced. The older frames didn't have strengthening gussets at the head stock and bent easily. There is no "proper" distance between the engine and front cover. It is never less than 3/4" and usually more. Two types of covers and three types of cross braces were used so it 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. Wikipedia information on high speed wobbles.
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This page was last edited:
05/25/2007 - copyright
Duane Ausherman |
