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Instability. Weaving, Wobbling, Wallowing.
Tank slappers?
.....the REAL information.

Copyright 2018, R. Fleischer
section 54, sub-section 10C

Over the many decades of my riding life I have heard & read of "tank-slappers"; weaving, wobbling, ...& various ills involving bike instability ...of one sort or other.   I have experienced these things.    There is a tremendous amount of JUST PLAIN BLARNEY and OLD WIVES TALES involved with these subjects well as a lot of misunderstanding. Unfortunately, bad information has continued to be passed around, and likely always will be.  There are other stories in common use in the motorcycling world.  One such is "...he was going to hit me so I laid it down"...or, some-such.  Possibly true, usually NOT, and often the rider just froze. The bald truth is that few motorcyclists practice emergency swerving and braking; and, it is amazing how many do not know nor practice proper swerving...and, especially, braking, particularly being afraid of strong use of the front brake. In an emergency, most will do what their ingrained habits have them do.  This article is not about rider training.

Tank slappers are actually rare; ...certainly much rarer than the various 'stories' passed around about them.   Riders seem to do a rather substantial amount of story embellishment.  A true tank slapper is almost always quick happening, scary, frightening, and dangerous.   A real tank slapper is an generally uncontrollable (some exceptions) sudden movement of the front fork from one mechanical stop to the other, and rapidly continuing back and forth.   To be a real tank-slapper, the fork must move fully between the actual mechanical stops.  The divergence can start small to modestly and then increase to the limits, or it can be violent and then die out with smaller and smaller divergences.   A tank slapper is NOT some sudden one or two jerky movements that dies down very quickly.   Jerky movements, such as from striking something in the road, is a normal response to hitting something.  What is not normal (or, should not be) is having that jerk increase in violence and rapidity.

Wallowing and weaving are different from each other, although both can happen at the same time ....and neither is very likely to turn into a tank-slapper.  Wallowing on a motorcycle is a rolling sensation, usually the motorcycle is leaned-over some in a sweeping & not sharp turn. It typically involves an oscillation of modest increases and decreases in the rolling sensation ....and, in fact, that is exactly what is happening. While there are a number of accumulative causes for wallowing on Airheads, a cause  ...perhaps easiest to visualize to think about a twin-rear-shock/spring type of Airhead, in which the arms to the spring/shock units are particularly bendable & flimsy. Thus, as weight and pressure is loading one arm, spring, and shock side; the other side is unloading such forces. As the wallow continues, the rear suspension works back and forth, together with the normal suspension up and down. I hope my attempt at describing a wallow is descriptive enough for you here.

Weaving is broadly defined by me here as the front tire is possibly tracking reasonably well, but the entire rear of the motorcycle (if you were following it) is seen to be dancing back and forth across a modest distance on the road. That is, the rear moves sideways, to and fro. There are several causes, but poor rear shock units are a prime cause. Weaving can also be caused by such as saddlebags, where a bit of side-force (as the motorcycle moves through the air, or there is wind, etc) on one side causes the motorcycle rear to move sideways, and then move back. This can set up an oscillation. The oscillation is generally controllable from speed reduction.  Weaving can also be caused by IMPROPER adjustments. The most common weaving is a LOW SPEED weaving, and may occur from approximately 8 mph to about 40 mph, but usually it is from 12 to 25. Those are the range of speeds where the weaving is felt, and due to centripetal forces, the weaving usually totally disappears above those speeds.  This particular type of weaving is extremely common to sidecar rigs that do not have steering dampers, but any 2-wheeler can also experience weaving.  The most common miss-adjustment is a too tight steering head preload on your Airhead. ANY Airhead can experience such low speed weaving with a too-tight steering head adjustment. If the motorcycle is on its center-stand, front wheel off the ground, a nudge on the bars hardly moves the front fork, and the fork certainly will NOT fall to one side. Once in awhile a weaving, perhaps with a bit of wallowing, is caused by a bad steering head bearing (or, hardened grease), where it has a center-ahead notch you can easily feel. The preload need not be overly tight for that.

If the steering head bearing preload is TOO LOOSE, then secondary oscillation effects can occur, and the result can be high speed instability ...this can be quite dangerous, particularly if the rear tire is also squared-off.

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 continued on since it was controllable.

I have rarely experienced real tank slappers on customer's bikes ...I have on a few old English and quite a few 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 adjustments at several places in the suspension, wheels, swing arm, & steering head.   Squared off rear tires 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 tank slapper, also tends to be rapid).  Most were on downhill sweepers, on bikes that were of the twin-shock type, and many had saddlebags or tall backrests ...and MOST had squared-off rear tires, many had worn-out rear shocks.  

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 shocks and squared-off rear tire, together with 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.

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 loose 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, 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.    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, 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 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.  Many shock 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 Koni motorcycle shock absorber business) 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 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.

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, was the lengthening of the wheelbase.  That eliminated most dealership and owner 'fiddling' problems.  Not all.

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 folks crashed 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, & scillatory 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 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.  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.

I have hundreds of thousands of miles on my own Airheads, and many more in my own 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.

With the initial broad-based maturing of the Internet, I built this website beginning at the end of the nineties.  I then began doing formal tire testing again.   I wrote an article on the results (I still try to keep that article current, as best I can).  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 mid-1900s era bike instabilities (even on very current & modern bikes), 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 a very long time.   Finally I purchased a 1995 R100RT in good condition, went through it to be sure it was....and...managed to get that low mileage, excellent condition 1995 R100RT, to do damped oscillations.  Just like it was likely designed to do.  Initially, I also had fastened the 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, decaying oscillation. I could induce a 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 fall to the side. That, unsurprisingly, corresponded to BMW's adjustment criteria!!

But, the steering was not even close to being 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 usually somewhat difficult to induce oscillations, no matter what, 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 managed to get more of a reaction by putting a quite light weight, but decent sized load on the rear rack: My biggest sleeping bag.  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 due to more modern tires), while making the bikes far more competent in bumpy twisties.  I borrowed a wheel, and put a 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 would go into increasingly bigger (diverging) oscillations if a goodly nudge was done. I repeated it, slower, at 50 mph, just by a nudge this time; was not well-damped, not hardly, but not scary. That did not happen with normal preload, just a long, 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.

I have experienced mild to modest high speed WEAVING on some bikes, but it was always brought under control by slowing down or making a less sharp curve. The MOST effective QUICK fix was to lean/lay forward, head & upper body well down, as flat as possible.

The first of the Airheads, the 1970 to early 1973 /5 models, had a shorter wheelbase than all the later models except the early R65 (and R45, ??).  The /5 was modified for a 50 mm longer driveshaft housing in mid-1973.  Thus, it is known as a LWB, or a 1973-1/2.   There are reasons to change, and reasons NOT to.    The SWB handles a bit 'quicker'.  It won't easily accommodate a physically larger battery.  The SWB has a somewhat more torque-effects feeling if the throttle is suddenly snapped off, and this is more noticeable in turns.  Things are a lot worse by use of FORK MOUNTED windscreens or fairings.  The installation of modest-sized windscreens or fairings on the LWB are less noticeable.  If the fairing is frame-mounted, it may exhibit a mixed response, but usually no serious instabilities.   Early Airheads had LESS STIFF frames (including the rear subframe) than later models.  The SWB is more susceptible to high speed wobbles from installation of accessories, particularly, as noted, fork mounted windshields (even modest-sized) and luggage and tall backrests and tall padded or blocked sissybars.  I experimented on a /5 that had really good shocks, really good front forks, all suspension bearings, etc., good.  I rode it on flat roads, canted roads, and a lot in mountain twisties (up and downwards).  I really got myself an education!   That education helped when I began to race my own R75/5.  BTW ...I made very few front fork modifications; one was brazing up one or two holes in the fork internal rod, but if done to a street bike, I'd not recommend but one hole brazed, but keep the oil viscosity fairly low, as it was originally, or, not over 7-1/2 effective weight.

Many tests on both a 1983 and a 1984 R100RT (I have owned and ridden quite a few) have shown that typically high speed weaving AND wallowing happens earlier with bags attached, at ~85 mph, in a downhill sweeper ....but the weaving/wallowing was easily controlled in the beginning.    The weaving/wallowing is what is meant by the Airheads Rubber Cow handling, and that milder effect can show up at lower speeds too.   A quite large tank bag, with a goodly amount of weight in it and again without that weight, and again with bag removed, had almost no effect.  This was no so for saddlebags and with or without weight in them, and the onset of instability speed was lower, as expected.  Poor shocks would also cause the weaving to begin earlier.

The 'not very stiff' connection of the rear frame to the main frame is primary in giving the twin-shock Airhead motorcycles much of the 'rubber cow in the middle' weaving/wallowing.  Re-said, it is the rear frame bolted-on section is the cause, as opposed to a full wrapped and stiff frame, such as on a /2.  Making things stiffer, as BMW did over the years, still allowed production of the uncomfortable and possibly dangerous weaving, just at a higher speed.  The monolever with its frame change made a considerable improvement.   BTW ...BMW warned, in its Rider's Handbook, not to overload the saddlebags (usually 11 pounds per saddlebag), and not to exceed certain speeds, usually 81 or 82 mph with the luggage installed.  BMW said not exceed 11 pounds on the stock rear rack. That means 11 pounds including the available trunk!

Here is an old but very useful video. I suggest you watch it fully. It also has BMW Airheads in it.  This 10 minute video gets into low speed wobble, high speed weaving/wobbling, showing effects of rider weight, lying flat, rear tire wear, versus speed, etc:

Quality shock absorbers in good condition in the rear suspension will help considerably; as will good not much worn tires (especially, perhaps surprising to you, the REAR tire), proper wheel bearing adjustments, ETC.   A heavier rider or rider with a passenger was less affected.

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 problem until speeds are higher.  Neither do the Paralever Airheads.   One can get them to act up if the rear shock is quite 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, particularly if downhill.  This is particularly so with tail trunk, or saddlebags, or backrest, or fork mounted fairing structures, or some combination of these things.

While one can delve into Tony Foale's technical articles & publications on chassis design, etc., those will not tell you easily what you might really want to know,  below, is some REAL information ...which I hope suffices.  Some is repeated from earlier in this article, but most is covered differently, or in more depth, etc, and new ideas are introduced, and explained.

Usually, when one hears of someone having some sort of weaving or wobbling, one tends to immediately think of the following:
1.  Tire pressures and tire condition, and maybe wheel bearing condition.
2.  Steering head bearing condition and adjustment.
3.  Swing arm bearing condition and adjustment.
4.  BMW's rubber cow effect caused by the weak attachment & thin tubing of the upper rear frame to the main frame (that attachment is on either side of the battery, essentially), together with the twin shock arms.
5.  Other contributions by the suspension; even loose engine mounts, worn shocks, etc.

There is truth to the above items, but they are not hardly the entire story; and, maybe not all that much of the story.

The information below is from my memories, and notes I made during multiple tests over a long period of time; generally and mostly using the same basic criteria as the famous Gordon Jennings (see link to his article, well below).   It also comes from an old movie by one of the major bike tire manufacturer's, which shows quite clearly the weaving effects and what changes make a difference.  I could not find it on the Internet, but did find this oldie, which is pretty good.  This link is to the same video as earlier in this article:

A very good treatment of the subject, with a slightly different slant, is in the February 2003 issue of AIRMAIL.

A terrific series of Airheads List postings, with me participating to a goodly degree ...covering weaves and wobbles in great depth, and highly recommended is at:
Check Technical Tips, Section TT-32.  Eventually, some form of the article you are presently reading will be added to TT-32, and Section TT-32 possibly edited

I do not agree with some information Duane Ausherman has, or did have, on his website, ...essentially that he had some sort of secret idea, details not mentioned! (why not?) about how he & his shop, years ago, 'fixed' some sort of wrong machining by BMW, on the /5 bikes.  I know of NO such common production miss-machining of the bikes.  Duane is not in business now, & he should release the information ...if he has it; or, ever did!
UPDATE:  I think some, or much of the information on his new site!  W6REC.COM

To adjust the steering head bearings, see article 10A:
See also Anton Largiader's site, for the R100GS type:

I have ridden several hundred bikes, literally, including a dozen+ of my own, but most were customer bikes. Testing was done on customer's bikes after we repaired/serviced them ...and I have a fair amount of experience 'curing' ill handling customer bikes. My most extensive early BMW testing was on a /2, & then a /5 SWB with handlebar/fork mounted fairing, believed a worst case for any BMW Airhead.  I also owned a SWB /5 with a full Avon frame-mounted fairing, and put a lot of miles on it, single and 2-up, lightly and heavily loaded, all sorts of combinations, including tire types, inflation variables, and road conditions.

Everything having to do with the stock suspension, etc., was good & proper on the two /5 test bikes just mentioned....both was almost brand-new too (and, one other, that I purchased new), and I continued to test now and then until I sold them at high mileage.  I also did extensive testing on a very stock late model RS; and, my own R100RT bikes. Tests on a ST and other models were also conducted.   I updated this article with my own testing on my low mileage 1995 R100RT, which was not obtained until early 2015, and was not even properly tested until I completed my work on it in 2016. It was stock.    I have also done testing with very heavy added weights on the seat, etc.

I refer you here to a truly good article, if incomplete, by Gordon Jennings, entitled "Shake, Rattle & Yaw", published in Motorcyclist in July of 1995.    Gordon did a LOT of testing, purposely doing some crazy things such as adding weights to handlebars, and also to just one end of the handlebars; deliberately messing-up the suspension, deliberately putting huge run-outs on wheels, ....deliberately doing things to motorcycles to try to find out what causes what sort of instability.  His findings generally go along with what I have personally found ....particularly with the flat-worn rear tire (squared off).  He certainly went further in some testing than I did.

For clarity, it is necessary to re-define some terms, as these terms are often not used properly.  While I have tried much earlier in this article to explain tank-slappers, wobbles, and weaves somewhat differently, below I will use the more common understandings.

Wobbles (and shimmy or flutter ....and full Tank-Slapper):
Wobbles happen at two speed areas, which is why, perhaps, there is confusion.  Wobbles of the sudden violent type that are often called a Tank Slapper are where the bars start oscillating back & forth some... and...if a tank slapper, the oscillation happens very quickly and does it full-left to full-right, back and forth.  This is a VERY scary situation, & should really be called flutter...or, in car terms, full-shimmy.   This full-shimmy (aka true tank-slapper) has an oscillatory frequency of perhaps 6 to 10 cycles per second.    Some have compared the fast shimmy to what happens, sometimes, with grocery cart wheels (usually the front wheels). There are some types where the frequency is considerably lower.  Note that in general, motorcycles with comparatively short "trail" measurements will have more tendency to do these various things, by whatever name.  Sidecarists, who usually shorten the trail to reduce steering effort, are only too well aware of such instabilities, and have various fixes.

A quite slowly occurring wobble might better be called a WEAVE.  A WEAVE is very unlikely to be full steering lock to lock at slow speeds.  As Gordon pointed out, there is also a super slow weave that involves a degree or less of handlebar movement, you probably won't feel that type of weave ....that is the type keeping your motorcycle upright, and I am not going to discuss it further here, it is a nerdy engineer-type thing.

Normal weaves can GAIN strength (amplitude & frequency or speed of movement) with road speed.   That is why if one happens at very high speeds it can be very scary and very dangerous.

If the steering head bearings are adjusted too tight, you will feel weaving at slow speeds makes the bike hard to stay in one place in your lane, and it takes much more steering effort to make a change. This type of weaving has been described as very heavy handling.  This type of weaving is particularly noticeable when you try to lean the bike to turn. However, weaving due to an overly-tight steering head adjustment is not likely to be anything but an annoyance and tiring ..... at any road speed.

High speed weave:
This is rather common.   On our twin-shock Airheads, and to much less extent on other models of Airheads, it tends to occur as a 'wallowing about', usually above 75 mph; and the rapidity of the wallowing can vary from modestly or quite slow to modestly fast.  Quite often when it occurs it is in a downhill sweeper turn, and you have a fairing and saddle bags, maybe backrest and trunk. It is noticeable, controllable (slow down, move weight forward ...and LAY DOWN), but may freak you out, particularly if you entered it via a rapid speed increase. The ONSET of this sort of instability, this sort of slower weaving rate, is a feeling of WALLOWING.  This can feel SOMEWHAT like a flat tire is beginning to happen.  If the rider does not slow down, but increases his speed, the weaving/wallowing will often get worse, but sometimes you 'ride through it'.   On BMW Airhead motorcycles, this type of weaving is what is typically meant by the Rubber Cow description.  The bike feels like it has a hinge of sorts in the middle, and someone following you will usually see the rear end moving back and forth across some of the lane and perhaps call it tail wagging.   If seen from the front it may be difficult to see.  This type of wallowing CAN be relatively constant for long stretches of road, usually does not freak you out much  ...but the bike feels 'unsettled', somewhat weavey, and you may decide to slow down.  The front and rear movements happen in opposite directions, hence the wallowing-weaving appearance.

Certain things can make weaving much worse.   These are not often spoken about enough.   Light weight riders will find it happens at a lower speed, or happens for them, and not heavier riders on the same bike.  With a passenger, the effect may disappear, or only show up at much higher speeds.   Large fairings and saddlebags are big offenders, and manufacturers might state in their literature not to exceed a certain speed (BMW says 81 or 82 mph) with saddlebags on the bike; and many will put a weight loading limit on saddlebag use specifications (BMW says 11 or 22 pounds) ....and, contrary to some beliefs, this is often due to instabilities and not bag frames breakability, etc.   So, full dressers are more likely to have handling problems.  One item that contributes greatly to this phenomena is a tail trunk &/or excessive weight behind the rear axle.  Almost any motorcycle can exhibit high speed weaving, but most very modern bikes do not exhibit it noticeably, because the effect has been moved to VERY high speed, by many small changes in design.  If the rear tire on any bike is worn to flat; or, starting to get squared-off, such weaving could happen very suddenly at a much lower speed, and GREATLY SURPRISE the rider!

The more rubber the front tire puts ACTUALLY on the ground (the real footprint), the LESS the effect, and, probably, a small amount less likely it will happen at all.  I believe it is due to the dampening effect.  The dampening effect is increased by you keeping your hands on the handlebars!  All these things are something I knew about, that Gordon never mentioned.  I believe he never tested for this.

The major contributing factor, and in many instances THE biggest contributor, is a squared-off tread on the REAR tire. Gordon Jennings and I fully agreed on this. 

More on weaving...but adding Tank Slappers & fast high speed instabilities in general:
Many bikes have some non-violent instability as one slows down, particularly with trailing throttle ....& most particularly if your hands are not on the bars.   Changing handlebar length, adding weights to the bar ends, etc., are not going to do much for you in this regard.   Sometimes it is due to a too-loose adjustment on the steering head bearings.  There may be a secondary problem of high speed instability.  Sometimes there is a clicking sound upon using the front brake. One or more of these things is relatively common. It is certainly a signal to you to check into the situation, perhaps clean, lubricate, and adjust the bearings, as you do NOT want violent instability at high speeds ...the next problem ...nor, do you want any other problem.

Tank Slapper anecdotes almost always refer to 'the good old days', ....when frames were not so strong nor stiff, forks not so strong and stiff, shock absorbers poor ...etc.  There is an effect by the tires, even properly inflated, but modern tires are less likely to start such instabilities, until the REAR is squared-off; and, multi-compound modern tires do not usually square off so quickly, some hardly do at all.   However, RIBBED front tires can ADD to the effect, particularly with instabilities starting up earlier on rain-grooved highways.    Today, TRUE tank slappers on modern bikes, are rare.   What HAS happened, is that with stiffer frames, better shocks, and other 'improvements', the speed at which instabilities are noted, have often simply moved to very high speeds, and typically any instability is much milder, due to centripetal forces at high speeds. Sport bikes with very small frontal drag and with fairings designed in wind-tunnels, can be very stable to quite high speeds.  Those sport bikes, if equipped with even a small tail trunk and bags that they were not designed for; OR, they are being ridden at speeds specifically NOT OK, per their owner's booklets, can exhibit NASTINESS at speed, and very surprisingly suddenly, with little warning!

Increasing frame and suspension stiffness is NOT the total answer towards eliminating the instabilities either ....that is a story in itself.

Some folks will report having a Tank Slapper, when all they have really experienced might be a sudden jerk and correction, perhaps from a rock on the road, ...or a mild to modest amount of a short incidence of WEAVING; AND NOT lock to lock oscillations.   These folks tend to grossly exaggerate what happened, typically because they were FREAKED-OUT.  They also may report having a Tank Slapper, at almost any speed.   This is false reporting....and, almost never do they ever report the exact conditions, exactly what they did, etc.

It is not uncommon to be able to induce some sort of mild instability, perhaps from 10 mph up to maybe 60 mph or so, by taking one's hands off the bars and giving a bar end a bit of a mild hit or having it start from an irregularity or bump in the road surface, etc.      ....but any instability will usually be quickly reversed on its own; or, instantly reversed by placing a hand or two on the bars.  This is normal, and varies with bike, model, and many small things; and some larger things trail amount (typically larger on cruisers), angle, and centrifugal forces from wheel weight and diameter; and rear tire wear.   If the instability continues to INcrease, that is not good.  If the effect is pronounced, simply lay forward on the tank.  Be SURE, if this sort of thing happens to YOU and YOUR BIKE, that you carefully check the steering head adjustment, the swing arm adjustment, and rear tire wear.  ESPECIALLY, even if mild, and it happens only with hands-off the bars, then do check the steering head bearing adjustment.

Back in the early days when the Japanese bikes were first coming to the U.S., the suspensions AND FRAMES were simply awful & the shock absorbers were also lousy.  These bikes would have a tendency to wobble in cornering ...perhaps around 60 mph.  The speed at which this happened was a combination of speed and lean angle, and often DURING the transition from one corner to another.  Big speed, less angle needed.   This was a real problem with these bikes, and an even bigger problem was when the rider shut off the throttle to slow down ....the wobble GOT WORSE.  If the rear tire was squared off, things could get "interesting", mighty fast.    It was from these old badly suspended Japanese bikes and maybe with squared off rear tires too, that came about the old-wives tale (with a modicum of truth)that is still being applied even to present day bikes (from any Country)...  "one should speed up to get out of a wobble".  >>>> I strongly suggest you do not do that!!

Once the Japanese improved the frames & suspensions, their cornering problems magically went mostly away, except for those that added windscreens, bags, trunks ....and had squared-off rear tires ....and a few instances with ribbed front tires.

Some additional points:
1.  The story that a heavy ...(OK, overweight!) ...rider can supposedly destroy a small bike's handling not true in the weave/wobble sense here ...a heavy rider will almost always find stability, or have it extended to higher speed.
2.  Tire run-out won't cause those serious types of wobbles ...even if rather extreme.
3.  Loose wheel bearings, loose suspension bearing points, dead shock absorbers, loose steering head adjustments: yes, can cause problems.  Squared-off rear tire ...can cause really bad effects.   The reason is a complex feedback loop from the transition between flat squared area and even just a bit of the side of the tire tread, which, due to squirming inherent in rubber, causes the problem.

Fork braces:
These were originally used on old-time race bikes, where the skinny forks of those days had to handle the twisting of large tire forces, together with long handlebars, etc.   Fork braces were used to supposedly remove some of the twisting the fork did under race conditions.   A really good designed brace does help with those things.  BUT, a brace has, or can have, other effects, it usually makes any WEAVING a BIT WORSE (unless the original design included it)!   Seems wrong, but, remember, a bike and its suspension must be taken as a whole.  Many a fork brace (many also used as fender mounting) is not installed correctly, and makes stiction much worse!    A stiff top triple clamp is another story, entirely, and usually improves handling, often quite noticeably.

Super stiff frames and suspensions:
These do not always work well for racing, as they do not 'hook up' properly.   On the other hand, all the early Airheads were "considerably too loose & flexible".  BMW beefed up the frames on the Airheads, depends on year and model as to what the bike has for the changes, and many changes were made over the Airhead production lifetime.

Tire pressures:
If the tire pressure is too low the motorcycle will wallow and handle mushily at low speeds.   Pressures don't seem to effect high speed instability very much won't like the handling though.  Modern tires will not perform well at all if pressures are as shown in old Rider's Handbooks. There is no place nowadays, with modern tires, for front pressures below 31 and rear pressures below 36 (except with off-road tires in soft conditions).

Large tire/wheel imbalance:
No great effect from this has been noted ....even if fairly extreme.  You might feel vibration, but weaving and wobbling remains pretty much as before, unless the tire rim is quite badly bent.

Stiffer top triple clamp:
Often does wonders, occasionally does little.   When it helps, it usually helps REAL handling much more than a fork brace does.   Usually has little to do with wobbles and weaves.

Miscl. stuff:
Motorcycles with larger diameter fork tubes tend to be more stable. That is because the tubes, and usually the mounts for them, are stiffer, and any small bending from road irregularities is minimized, and there is, over-all, less tendency to go into a looping oscillatory mode.  That is, bending, however slight, does not get very oscillatory.

So, while the bearings need to be adjusted correctly, & shocks & springs, ETC., not worn out; and, the rear tire not flat-worn ....what else can be a problem?...:

1.  Saddlebags, just having them!   Every additional pound in the saddle bags will lower the speed at which instabilities begin ....even fairly small total weight increases.  Yes, this conflicts with the information about RIDER weight, which is opposite, and helps.

2.  Rear tour trunks (also types called scoot boots).  Weight in tour trunks is especially vicious in its effects.  Just HAVING a backrest or tour trunk (even empty!) can greatly influence tendencies towards instabilities.

3.  Large fairings, even frame-mounted ones.

4.  Fork mounted windshields of any kind and size.  Especially on SWB /5 bikes.

5.  Try removing handlebar weights if you have installed them.  They might reduce vibration, but INcrease potential instability. Do this only experimentally, as there ARE some bikes that were designed to have them, & in SOME instances they do help; but usually with vibration, not stability.

6.  What had the largest bad effects?
    a.  Well-worn, flatted tread area rear tire was a HUGE cause for instability.
    b.  Poor rear shock absorbers (particularly with dead spots in their movement).
    c.  Fork mounted windscreens/windshields.

So; what DO you do if you have a serious high speed weave/wobble?

Try to keep off the brakes, use the engine to slow you down, & not too abruptly.  DO NOT shift downwards ...that will usually cause a large braking effect.  Lean WAY forward IMMEDIATELY, gripping the bars tightly.  Lay forward on the fuel tank if you can; this helps more if you have no windshield.

Non-Believer?....see the Motorcycle Safety Foundation's Basic Rider Course, Unit IV, page 43.

I mostly agree, except that I think that shifting plenty of body weight forward (move forward, lean forward too) is MUCH more helpful than their text would seem to indicate.

Adjusting your Airhead steering bearing preload:
(yes, I am aware BMW has a specification, 4 inch ounces)

The last Airheads models are adjusted using a sleeve adapter, etc.  Information is in my front forks article, and also Anton Largiader's website. The information, below, still applies, IMO.

Begin with the motorcycle on its center-stand, and the front wheel slightly off the ground ...most Airheads with stock center-stands and tire sizes will attain that position with the rear of the motorcycle pushed down a bit, or, otherwise block the motorcycle as required.  On all the older Airheads, the adjustments require loosening the lower triple allen screws a bit, setting the preload adjuster underneath the top triple plate, ....and checking the effects after the top acorn nut is re-tightened.  DO NOT forget to re-tighten the lower triple allen screws.

Naked Airheads (no fairing, no windscreen):  Adjust so steering WILL, if end of handlebars are nudged a bit, just barely fall fully to either side, either close to the mechanical stop, or just reaching it.  DO NOT have steering looser than what just allows this.

Naked Airheads with a fork-mounted small to large windscreen:   Slightly tighter than above, steering will not fall so easily towards either side ....and WILL NOT reach mechanical stops.

Faired Airheads, such as RS, RT, Windjammer, or other fairings:  Tighter.  Steering will NOT fall fully to one side, but a nudge will move it near half-way to the mechanical stop.  You can feel a small amount of friction if you use your hands in the normal riding position, and move the handlebars/forks back and forth.

Sidecar rigs: Slightly tighter than the above. A modestly sharp nudge will move the forks less than half-way to the mechanical stop.

TEST RIDEALL the normal tests are done with any damper OFF!   The primary test is that there should be no weaving (or, just barely the very slightest weaving) at low speeds, perhaps 10 to 25 mph.  A secondary test that is a safety check, because other things can cause problems, is that at ~50 to ~70 mph on a smooth, relatively flat paved road, which does not have rain grooves, and with hands off the bars, there should be no instability.  If a bar end is nudged sharply but not hard hit, there should be no instability, no building oscillation; but one mild instantly corrected short oscillation, or a faint small jerk really, is OK.  You may want to progressively increase and decrease speeds, and see what happens as you snap the throttle closed.  Bike should be stable, not heavy handling, not loose, no instabilities. If the motorcycle has a steering damper, either the early Airheads type which are friction discs, or, later type with hydraulic damper, a bit of heavy-ness should be felt at 10 to 25 mph or so, if the damper is in lightly-set use.

02/23/2005:  Changed to 76A from 80, slight updating for clarity, and minor changes on 02/26/2005, add 5.
01/27/2008:  Edited for clarity, add some emphasis.
06/14/2011:  Updated entire article.
10/07/2012:  Add QR code & language button; update Google Ad-sense code; slight updating in the article.
11/08/2012:  Was 76B.
08/30/2013:  Add link to youtube video.  Correct the article number, 10C was correct, but that's a subsection of 54.
09/09/2013:  Add note regarding Duane's "secret" fix; and, the link to the MSF brc, at end of article.  At some date in 2013: remove language button, as it interfered with handling of javascripts.
05/23/2015:  Clarify the /5 instabilities of SWB.
03/05/2016:  Update meta-codes, layout, information on my own 1995 bike.
08/08/2016:  Update meta-codes, scripts, H.L., layout, fonts. Update for clarity in the text.
02/05/2018:  Complete go-through. Improve layout.  Add 10 pxl margins.  Reduce redundancies, reduce use of fonts and colors.  Clarify and expand explanations.

Copyright 2018, R. Fleischer

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Last check/edit: Monday, February 05, 2018