Snowbum's BMW Motorcycle Repair & Information Website

It is relatively common to see a motorcycle exhibiting weaving and wallowing at the same time, and this can be amusing to someone following you, perhaps going down a moderately steep road, perhaps at 30 mph. I've ridden bikes that did it, and I have  continued on since it was easily controllable, and does not feel dangerous to ME, and generally is not, not at all.

I have rarely experienced real tank slappers on customer's bikes ...I have, on a few old English and quite a few very old Japanese bikes in particular.  It was particularly bad on the earliest, fifties and sixties, of the Japanese bikes shipped to the USA, as frames & suspensions were very weak.  I have only a few times managed to get some BMW Airheads to display true tank-slappers.  It can happen due to a combination of excessive wear & wrong adjustment at several places in the suspension, wheels, swing arm, & steering head.   Squared off rear tires can greatly contribute to the problem.  On many of the others, some may have called what happened to them a tank-slapper, but it was NOT; instead, the actions were just oscillations, but not mechanical stop to mechanical stop movements (which, on a real tank slapper, tends to be quite rapid).  While some were on downhill sweepers, on bikes that were of the twin-shock type, and many had saddlebags or tall backrests ...MOST had squared-off rear tires, many had worn-out rear shocks; and most were on flat level surfaces or going slightly uphill.  Sitting back in the seat, and not having passengers, and possibly having saddlebags and backrests, make things worse.   An "unloading" of the front tire is common.

Many riders seem to be unaware of the effects of luggage, backrests, & squared-off rear tires.  While loose wheel bearings can contribute, usually this is not a factor.  Many have complained about Airhead motorcycles, particularly the older models, as having a rubber-cow feeling, as if there was a soft rubbery hinge in the horizontal frame members (fore-aft).  In general, the complaints greatly lessened with the Monolever and Paralever bikes.  While it is true that worn shock absorbers can bring about instabilities at a lesser speed, they were seldom the prime cause.  However, bad REAR shocks and a squared-off rear tire, a windshield and saddlebags and a tall upholstered backrest, ....together ....almost always would lead to serious instabilities, and they could be quite dangerous, as they might very suddenly appear.  Excessive speed can add to the fun.   The bad rear shock I have mentioned is typically going to have a dead zone, it need not be excessive.  You may find it a bit difficult to determine if the shock is bad, without removing the spring!

I want to make clear what a common bad rear shock absorber situation is.  It is certainly true that a shock absorber section of a rear shock/spring unit can wear and lose its good shock characteristics.  It is also true that, in general, you get what you pay for in a rear shock.  However, all shocks wear, some wear fast, and the worst problem wear, as far as its effect on stability, which is what this article is all about, .... is not easy for the novice, or even average to better rider to determine.  While there are various modes of failure, the two that come to mind involve one internal, one external.  The external one is relatively easy to see.  The mounting eyes of some shock absorber units (the spring-shock assembly) wear at the bushings, whether rubber or metal or both.  In many shocks, there is no visible wear after very large mileages at these end 'eyes', and they are OK.  In some, there is a noticeable movement, and even with the unit still mounted in/on the motorcycle, you can lift the rear wheel up and down and see/feel this eyes looseness.  The effect is similar, if usually on a smaller scale, than the internal fault I will describe; but, rather often, the lifting and up down tests will show a banging noise, this is due to worn-out rubber in the eyes.

The internal fault is usually due to a slow loss of oil, over a long period of time.  In some shock units it is a loss of nitrogen pressurization, or worn OR BROKEN internal parts, etc.  In all instances, the result is a shock absorber that has DEAD ZONES.  It is not easy to determine if you have this problem, because the relatively stiff exterior spring tends to mask it.  If the spring is removed, you can easily feel the problem, by testing the shock with your hands, as you go from piston fully in to piston fully out, while simultaneously trying for short in-out movements.  ANY dead zone is BAD.

Most shock-spring units are rebuildable.  Your shock-spring unit may or may not match your riding habits, your weight, loading, etc.  Different springs are usually available.  Many shock units are adjustable.  Many folks STILL do not know that many Koni and Ikon (who took over the Koni motorcycle shock absorber business) units are actually adjustable for the shock portion; rather, they know only of the spring perch adjustment, which is to accommodate heavy riders and heavy loads, by setting the SUSPENSION HEIGHT.  On some, you must remove the shock unit and adjust the end by rotation.  On others, there is a rubber cover at the top, and peeling it back will show a graduated wheel (do look all-around, as it appears in its own window area) to adjust.

I not only tested my own bikes, but I often test rode customer's bikes, whether or not they had complained of instabilities. It always appeared to me that, OVERALL, the more common egregious problems were seen on the short wheelbase (SWB) /5 that had been fitted with a HANDLEBAR or FORK-MOUNTED FAIRINGS & ridden at fairly high speed, ....and many of these bikes had saddlebags, with trunks and backrests making things much worse.    I repeated tests several times after proper adjustments and testing, and I would test with and without such as saddlebags, and same when possible, for fork-mounted windshields, a particularly bad thing to have. I also changed a few rear tires for final tests.  I got a good feel for what was happening.  NOTE that I also managed to get the LWB early models to have serious instabilities with the same sort of equipment I just mentioned...and a flatted rear tire wear.  Those carrying heavy loads aft of the rear axle are particularly likely to have problems.

I've managed to induce high speed oscillations by having the steering head bearings too loose on many bikes, not hardly just Airheads.  Most of these bikes had saddlebags, some had trunks or closed cavity backrests.  Bikes with those things, but especially with larger fork-mounted windscreens, were much easier to get into serious instabilities.  In my opinion, the BMW 'fix' for the initial /5 series, which was the lengthening of the wheelbase, also improved a few other things, such as the ability to install a larger battery.  That change to LWB eliminated much dealership and owner 'fiddling' problems.  Not all.  Certainly the /5 had a soft rubbery feeling to the bike, and high speed weaving was possible and probable.

I've read ..and listened-to ....many anecdotal 'reports' on high speed wobbles, ...and crashes, ...and not one person has ever documented the exact details of how their bike was really aligned & adjusted. Trying to pin down the reasons for the crash was always difficult, with the rider being overly defensive of his abilities and actions; some blamed causes that could not have been the reason.    SO ....I think many (maybe most) folks who crashed, was for other than stated reasons, and sometimes even BMW's design was the easy way out of admitting their incompetence, including poor braking, poor swerving, etc.  BMW had the ability to cause and analyze and read out on instrumentation such as frame twisting, vibrations, & oscillatory and flexure effects, etc., under riding loads.  We don't have the instrumentation, but we can feel the effects.   BMW lengthened the wheelbase for, yes, improvements in stability, but ALSO (IMO) did so to more easily install a larger battery, and a few other things.   You CAN get almost any motorcycle to have stability problems, sometimes serious ones.    It is way beyond this article, which is lengthy enough, to analyze every possible way there can be instability on a motorcycle.    Many a racer has found what helps, and what does not, or even makes things worse.    Many a biker has found that adding something like saddlebags (which have effects from the size, presenting to the oncoming wind) with too much weight in them is dangerous.  Some found stability WITH a passenger, where before, sans passenger, the motorcycle might have been somewhat squirrelly.  Some found backrests to cause problems.    In MY opinion, a properly adjusted and properly equipped & ridden SWB /5 bike is nice to ride, and does not have problems unless pushed hard, and then the effect is usually only high speed weaving.  It isn't difficult to CAUSE them to be poor-handling.   Duane Ausherman had a lot to say about /2 era and /5 era wobbling, etc.  https://w6rec.com/.  I strongly suggest not wearing the rear tire to flattened tread, NOR having the steering too loose; and for sure, clean and grease the bearings, and if still notchy feeling (that is almost always felt, if any, straight ahead), replace the bearings.  I suggest that the SWB /5 (the /5, both SWB and LWB, had a more flexible frame, too) have its steering stem preload adjustment towards the tighter side of its setting.  A wee bit of quite low speed (10mph?) weaving is what I am suggesting for the adjustment.  If you must have a fork mounted windshield, bags, backrest, etc...my suggestion is strong.  In fact, in all my SWB /5 miles, I found that a bit of extra tightness at the steering stem, and with excellent rear rubber, made for a nicer handling bike in the twisties.    Be sure the rear shocks are fully functional!  Don't push the bike in tight twisties and high speeds, particularly if you have a fork mounted fairing (of almost any size), saddlebags, rear trunk, etc...one or more.

I have hundreds of thousands of miles on my own Airheads, and plenty of miles in my testing of customer bikes. Except when purposely trying for instabilities to duplicate customer complaints, I had few problems.  I had no problems with my own BMW Airhead motorcycles.

Tires are seldom responsible for stability problems, EXCEPT that a squared off rear tire is ONE OF THE MAIN CAUSES for the /5 and other seventies era bike instabilities (even on some very current & modern bikes), even once the steering head adjustment and stiction, etc.,  are attended to. 

There were major tests done by a motorcycle magazine (I quote some of the details, the testers name, etc. in my articles, including this one).  We all came to the exact same conclusion, ever so long ago.  Tony Foale also had things to say.   Gordon Jennings and Tony Foale are not lightweights in knowledge and experience!

I tested my own motorcycles and customer motorcycles.  I never owned an Airhead beyond a 1984 model R100RT for most of my riding years.   Finally I purchased a 1995 R100RT in good condition, went through it to be sure it was good....and...managed to get that low mileage, excellent condition 1995 R100RT, to do damped but quite modest-sized oscillations.  Just like it was likely designed to do.  Initially, I also had fastened the way large BMW rear trunk, that came with my purchase, back onto that motorcycle. I expected stronger oscillations, and so it was.  I removed that trunk, decided to never use it, and sold it and the bracketry.

That motorcycle had near-perfect low mileage tires ...and NO squaring off of the rear tire. Tires were front and rear matched brand and model. Proper pressures were used, and testing showed that BMW's Riders Handbook recommendations for tire pressures to be good.  BMW's large stock touring saddlebags were on the bike.   The oscillation phenomena was not anything more than a quick, short movement, a proper decaying oscillation. I could induce a somewhat worse oscillation by having the steering head too loose ....that is, when setting the steering head preload the steering would fall very easily to either side if nudged from the center-ahead position, front wheel off the ground, on the center-stand.  Road testing showed the damped oscillation to occur with one saddlebag (but could get the bike to do it with two saddlebags if pushed to do so, up to about 90 mph, as high as I tested for it), hands off the bars, and some goodly road irregularities or goodly bars nudging.  The tendency disappeared if the steering head was tightened to the point the front end would barely fully (or near fully) fall to the side if nudged, wheel off ground. That, NOT surprisingly, corresponded to BMW's adjustment criteria!!  Of course the 1995 RT is stiffer than early non-twin-shock RT's, so tends to be pretty stable. 

But, the steering was not so tight as to cause noticeable low speed weaving.   I played with tire pressures, swing arm adjustments, even played with a fairly dead rear shock absorber, borrowed from my hoard of old worn parts.  BTW ...that worn shock made the bike nasty in bumpy turns.  It was somewhat difficult to induce oscillations without taking my hands off the bars ...that is because one's hands, even lightly on the bars, acts like a steering damper. This motorcycle was more stable than much earlier models, as I had expected. BMW knew that, and did NOT install any sort of steering damper, and in my testing, it did not need one.  I borrowed a good wheel/tire from someone that had a high mileage, flattish worn tire, and redid the tests.  I did not like the effect.  On a much earlier Airhead, the oscillation would likely be intense; not so this last model.

Back to the 95RT, but the NOT flat worn wheel and tire:
I managed to get more of a reaction by putting a light weight, but large sized load on the rear rack: My biggest sleeping bag, inside a large bag, with air pillows inside, to maintain a large shape.  As noted above, late model RT's, like my 95, don't come from the factory with steering dampers, like the early 80's RT's did.  Many tests on many bikes showed me that BMW's frame and suspension stiffening (Monoshock) had some good effect, as did a lot of other small changes over the years.  BMW managed to retain most of its classic ride and comfort (the 95RT is a bit stiffer riding, some of that is due to modern tires), while making the bikes far more competent in bumpy twisties.   The stability of the bike suffered from the large, but light, loading.

I did one final series of tests with the borrowed wheel/tire this time, and put a customer-takeoff quite BADLY squared-off rear tire on the wheel, and went for another ride, this time with the preload on the steering again quite light (and, later, repeated, with it set normally). With the squared-off tire the bike was squirrely with the preload low; particularly so since a fairly good sized oscillation came on by itself from a mild road bump....it would go into slightly increasingly bigger (diverging) oscillations if a goodly nudge was done. I repeated it, slower, at 50 mph, just by a nudge this time;...it was not well-damped, but not scary. That did not happen with normal preload, just a slightly longer and slightly largish damped oscillation. My prior experiences were certainly confirmed as to cause, and also my testing certainly confirmed what Gordon Jennings found in his testing. If I had taken my hands off the bars, and had been at, say, 90+ mph, things might have not ended so well. Conclusion: Start by being sure the steering head adjustment is correct, and get rid of worn-flattish rear tires.    We all know that many of you will not heed my advice.  You've been warned.

The earliest years Airheads have more problems with weaving/wallowing & instabilities of various sorts, with the weaving beginning at lower speeds than later years models.  The later Airheads have stiffer frame backbones, and other changes, sometimes small, that add up.  Most all these changes (except eliminating the rear section & going to the Monolever & later the Paralever), were fully incorporated by ~1981 (the R80ST and G/S had the Monolever earlier).   The Monolever Airheads do not exhibit the problems until speeds are higher.  Neither do the Paralever Airheads.   One can get them to act up if the rear shock is bad; and, if the rear tire is squared off in the middle.  A squared-off rear tire is the major contributing factor in high speed weaving on all motorcycles, not just Airheads, not just BMW's.   In tight twisties this 'high speed' can be lower, maybe as low as down into the 40 mph area.  This is particularly so with tail trunk, or saddlebags, or backrest, or fork mounted fairing structures, or some combination of these things.   The last of the RT's had the usual large RT windshield, tour type saddlebags, and, if ordered,  large rear trunk.  It is more stable than earlier RT's, but I recommend removing the rear trunk (and NOT using a tall backrest), and being careful about high speed riding.  Few seem to know that BMW set a limit, with saddlebags, generally at ~80 mph.

Instabilities tend to happen, on the same bike, differently, if going uphill or downhill, other things being unchanged.   This is due to the loading on the front tire, well, primarily.