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Instability; weaving, wobbling, tank slappers
.....the REAL information.
 

http://bmwmotorcycletech.info/instability.htm
section 54, sub-section 10C

Copyright, 2016, R. Fleischer

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.     There is a tremendous amount of JUST PLAIN BLARNEY and OLD WIVES TALES involved with these subjects....as well as a lot of misunderstanding.   

I have experienced REAL tank slappers on customer's bikes....a few old English and quite a few old Japanese bikes in particular. It was particularly bad on the earliest 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.  On one bike it was due to a combination of quite excessive wear & wrong adjustments at several places in the suspension, wheels, swing arm, & steering head.  On most of the others, some may have called what happened a tank-slapper, but they were NOT, instead, the actions were hard oscillations, but not mechanical stop to mechanical stop movements (which, on a tank slapper, also tends to be rapid), and were controllable.

On MOST these were short wheelbase (SWB) /5 that had been fitted with a HANDLEBAR or FORK-MOUNTED FAIRING & I was at fairly high speed, ....and most of these bikes had saddlebags.  One also had a substantial-sized TRUNK & BACKREST.    I repeated the tests several times after proper adjustments, and testing included removing things, returning things.  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 oscillations by having the steering head bearings too loose....on many bikes.  MOST of these also had saddlebags, some had trunks or backrests.  Bikes with those things, but especially with larger FORK MOUNTED windscreens, were much easier to get serious instabilities from.

Decades later!.... I managed to get my own low mileage, excellent condition 1995 R100RT, to do light to moderate oscillations.  Bike had near-perfect tires...NO squaring off of the rear tire. Tires were front and rear matched brand and model.   The phenomena was NOT a real tank slapper. I induced oscillations by having the steering head a bit loose....that is, the steering would fall rather easily to the right side if nudged, front wheel off the ground, on the center-stand.  Testing showed the oscillations, mild as they were, to only occur with one saddlebag (but could get the bike to do it with two if pushed to do so, up to about 90 mph, as high as I tested for it), hands off the bars, and some road irregularities or bars nudging.  The tendency disappeared if the steering head was tightened to the point the front end would not fall to the right side .... just move somewhat.  But, the steering was NOT QUITE 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.  It was difficult to induce oscillations without taking my hands off the bars...that is because one's hands on the bars acts like a steering damper!  Note that the late model RT's, like my 95, don't even come from the factory with steering dampers, like the early 80's RT's did. 

I have experienced mild to modest high speed WEAVING on bikes, but that was always brought under control by slowing down or making a less sharp curve; but the MOST effective cure was to lean/lay forward, head & upper body well down, as flat as possible.  NOTE that I don't weigh very much, and the lighter the rider, the more any high speed weaving is likely at the same speed, as opposed to with a heavier rider.  Said differently, if the rider is light, the high speed weaving is going to occur at a lower speed, than if he was heavier.

The first of the Airheads, the /5 models, had a shorter wheelbase than the later models.  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.   The swap is simple.  There are reasons to change, and plenty of reasons NOT to.    The SWB handles a bit 'quicker'.  It won't accommodate a physically larger battery.  The SWB has a somewhat excessive torque feeling if the throttle is suddenly snapped off, and this is more noticeable in turns.  The SWB is more susceptible to high speed wobbles from installation of FORK MOUNTED windscreens or fairings.  The installation of such on the LWB is less noticeable, and hardly noticeable as far as instabilities, if they are FRAME mounted.  Early Airheads had LESS STIFF frames (including the rear subframe) than later models.

Many tests on both a 1983 and a 1984 R100RT have shown that typically high speed weaving happens earliest on THOSE bikes, 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 Rubber Cow effect on the Airheads, and that milder effect can show up at lower speeds too.  A 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; and, same for saddlebags and weight in them, except that the onset speed was slightly lower.

Weaving/wallowing seems to be a function of many things, but the rather 'not very stiff' connection of the rear frame to the main frame, plus other things, is what gives the twin-shock Airhead motorcycles that 'rubber cow in the middle' feeling. Mostly it is due to the rear frame bolted-on section; as opposed to a full wrapped frame, such as on a /2.  However, making things stiffer will still produce the uncomfortable and possibly dangerous weaving, just at a higher speed.    BMW moved the weaving speed higher and higher in later models, with its frame stiffening changes, etc.

Here is an old video, with some useful information...but hardly complete.  This 10 minute video gets into low speed wobble, high speed weaving, showing effects of rider weight, lying flat, rear tire wear, versus speed, etc:
http://www.youtube.com/watch?v=z3OQTU-kE2s&sns=em

Quality shock absorbers in the rear suspension will help considerably; as will good UNworn 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.

In general, the earliest years Airheads have more problems with weaving/wallowing & instabilities of various sorts, with the weaving beginning at lower speeds.  The later Airheads have stiffer frame backbones, and other changes, sometimes small, that add up.  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 bikes 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.  In fact, a squared-off rear tire is the MAJOR contributing factor in high speed weaving on ALL motorcycles.   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.

Whilst one can delve into Tony Foale's articles & publications on chassis design, etc., those will not tell you easily what you might really want to know ...so,  below is some REAL information...which I hope suffices.


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.  PARTICULARLY 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).
5.  Contributions by suspension problems, and other things.... loose engine mounts, worn shocks, etc.

There is truth to the above items, but they are NOT HARDLY the entire story; and, may not be all that much of the story either.

NOTE:   The information presented below is from my memories, my own very specific testing ....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 is the same link as earlier in this article.
http://www.youtube.com/watch?v=z3OQTU-kE2s

A very good treatment of this 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....and covering weaves and wobbles in great depth, and highly recommended is at:  http://www.airheads.org
Check the Technical Tips section for the article....hopefully still there.


I do not agree with some information Duane Ausherman has, or did have, on his website, ...essentially that he has 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!


To adjust the steering head bearings, see article 10A:   http://bmwmotorcycletech.info/frontforks.htm


I have ridden several hundreds of bikes, literally, including a dozen+ of my own, but MOST were 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 fairing, and put a lot of miles on it, single, 2-up, loaded, lightly loaded, all sorts of combinations, including tire and road conditions.  Testing was also done on customer's bikes after we repaired/serviced them...and I have a fair amount of experience 'curing' ill handling of customer bikes.

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 I continued to test now and then until I sold them at quite 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.

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 bars, and one end of bars; deliberately screwing up suspension, deliberately putting huge runouts on wheels, deliberately doing plenty 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.
 

 

It is necessary to define some terms, as these terms are often NOT used properly:

Wobbles:   Wobbles happen at two speed areas, which is why, perhaps, the confusion.  Wobbles of the sudden violent type that are often called a Tank Slapper are where the bars start oscillating back & forth some... and...very quickly and suddenly start doing it full-left to full-right.  This is a VERY scary situation, should really be called flutter...or, in car terms, full-shimmy.   This shimmy has an oscillatory frequency of perhaps 6 to 10 cycles per second.   There are some types where the frequency is considerably lower.

Weave:  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 one....that is the one keeping your motorcycle upright, and I am not going to discuss it, it being more a nerdy engineer-type thing. 

Normal weaves can GAIN strength (amplitude & frequency or speed of movement) with 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 weave at slow speeds....it makes the bike hard to stay in one place in your lane, and makes it take much more steering effort to make a change. This type of weaving has also been described as very heavy handling.  This type of weaving is particularly noticed when you try to lean the bike to turn it at all. However, weaving due to an overly-tight steering head adjustment is not likely to be anything but an annoyance and tiring..... at low or high speeds. 

High speed weave 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 speed of the wallowing can vary from modestly slow to modestly fast.  Quite often when it occurs it is in a downhill sweeper of a turn, and you have a fairing and saddle bags, maybe backrest and trunk. It is noticeable, controllable (slow down, move weight forward...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 approaching; that is, the pressure is lowering in the tires.   If the rider does not slow down, but increases his speed, the weaving/wallowing will almost always get worse.    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 (tail wagging).   If seen from the front, the same thing is happening but SOMETIMES harder to see.  This type of wallowing CAN be relatively constant for long stretches of road, usually does not freak you out much at all...but the bike feels unsettled.  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 some manufacturers might state in their literature not to exceed something like 80 or 85 mph with saddlebags on the bike; and many will put a weight loading limit on saddlebag use specifications....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.  NOTE that almost any motorcycle can exhibit high speed weaving, but most very modern bikes do not exhibit it, because the effect has been moved to a VERY high speed, by many small changes in design. STILL, if the rear tire on any bike is worn to flat (squared-off), such weaving could happen at a much lower speed, and, suddenly, and greatly, SURPRISE the rider!

The more rubber the front tire puts ACTUALLY on the ground (the footprint), the LESS the effect. That is something I know about, that Gordon never mentioned; I believe he never tested for this. 

Another contributing factor, and in many instances this is THE biggest contributor, is a squared-off tread on the REAR tire.   Gordon and I fully agreed on this.  The larger front tire footprint, the more the front end acts like it has some sort of steering damper...which, in effect, IT DOES.  SAME for having hands on the bars!

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.   This is usually normal.   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....this is also quite common. It is certainly a signal to you to adjust the bearings, as you do NOT want violent instability at high speeds...the next stage problem.

Tank Slapper anecdotes almost always refer to 'the old days', when frames were not so strong nor stiff, forks not so strong and stiff, and so on.  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.  However, RIBBED front tires can ADD to the effect, particularly starting up earlier on rain-grooved highways.    Today, TRUE tank slappers on bikes made since the late 60's, or maybe even into the mid-70's, are rare indeed.   What HAS happened, is that with stiffer frames, better shocks, and other 'improvements', the speed at which instabilities are noted, has simply moved to higher speeds, and typically any instability is much milder. Sport bikes with very small frontal drag and with fairings designed in wind-tunnels, can be very stable to super high speeds.  BTW...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 answer to totally eliminating the phenomena 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 quick WEAVING; AND, NOT lock to lock oscillations.   They tend to grossly exaggerate what happened, typically because they were FREAKED-OUT.  They may report having a Tank Slapper, at almost any speed.   This is false reporting.

 

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.....but any instability will 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 like trail, angle, and centrifugal forces from wheel weight and diameter; and the 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 condition.  ESPECIALLY, if mild, and only with hands-off the bars, 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.  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 applied to present day bikes...  "one should speed up to get out of a wobble".  >>>>DON'T!!!!

Once the Japanese improved the suspension components, their particular cornering problems magically went 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....is 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.  WORSE:  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 really for 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, another effect, it makes the 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.

Super stiff frames and suspensions:
These do not always work out for racing, they won't 'hook up' properly.   On the other hand, the early Airheads were "considerably too loose/flexible".  BMW DID beef up the frames on the Airheads, depends on year and model as to what the bike has for changes.

Tire pressures:  If low enough, bike will wallow and handle mushily at low speeds, but pressures don't seem to effect high speed instability. You won't like the handling though.

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 as before.

Stiffer top triple clamp: Often does wonders, often does nothing.   When it helps, it usually helps REAL handling more than a fork brace does (by quite a bit, actually).   Has very little to do with wobbles and weaves though.



Bikes with larger diameter fork tubes
tend to be more stable. That is because they are stiffer, and any small bending from road irregularities is minimized, and does not easily 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....what else can be a problem?...:

1.  Saddlebags, rear tour trunks (scoot boots), large fairings.

2.  Every additional pound in the saddle bags will lower the speed at which fun stuff begins....even fairly small weight increases.  Yes, this conflicts with the information about RIDER weight, which is opposite, and helps.

3.  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.

4.  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 the first place, & in SOME instances they do help; but usually with vibration, not stability.

5.  Handlebar or front fork mounted fairings, especially large ones (tiny fly screens are vastly less of a problem) can be a HUGE problem.  Such fork-mounted windshields & fairings were THE big problem with the short wheelbase /5 bikes; and, when, worse yet, you coupled with a squared off rear tire, & maybe a trunk or whatever on a rear rack, and saddlebags, you are surely asking for serious problems.  You COULD have a diverging high speed wobble.  The biggest problems are handlebar/fork mounted fairings, heavy bags, and having a big backrest or touring trunk, and squared off rear tire.

6.  What was the biggest bad effect, if one was careful to have one's suspension in reasonably good condition, bearings adjusted reasonably well, and not excessive aft weight ????.....

You may be surprised to find out that a well-worn, flatted area rear tire was a HUGE cause for instability.....and high speed scary results are possible.  Yes, even in just about a straight line, or slow curves.  This was exactly what Gordon Jennings found out....and I duplicated his testing and I TOTALLY AGREE.  I can take this a bit further, perhaps you won't like this interpretation!  If you tend to make very mild turns, and ride straighter roads a lot, that rear tire will square off...guess what happens then, in, for example, a downhill sweeper at medium to higher speed?Guess what happens in an uphill hot-dogging and curved top hill?


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. Hopefully do all that is in this paragraph at essentially the same time.

Non-Believer?....see the Motorcycle Safety Foundation's Basic Rider Course, Unit IV, page 43.
http://www.msf-usa.org/curriculummaterials/brc_handbook_vs71_noprint.pdf

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 uses 4 inch ounces):
Check with motorcycle on center-stand, and front wheel slightly off the ground...most Airheads with stock center-stands will attain that position with the rear of the motorcycle push down a bit.  Adjustments require loosening the lower triple allen screws a bit, and checking the effects after the top nut is tightened.  DO NOT forget to re-tighten the lower triple allen screws.

Naked Airheads (no fairing, no windscreen):  Adjust so steering WILL, if nudged a bit, fall fully to either side, close to the mechanical stop, but not quite reach 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....absolutely will NOT reach mechanical stops.

Faired Airheads, such as RS, RT, Windjammer, or other fairings:  Even more tight.  Steering will NOT fall fully to one side, but a nudge will move it near or a bit more than half-way to the mechanical stop.

TEST ride:  Should be no weaving (or, just the very slightest weaving) at low speeds, perhaps 10 to 25 mph.  At ~75 mph on a smooth, relatively flat, paved road, hands off the bars should show NO instability.  If a bar end is nudged sharply but not hard, there should be NO instability, NO building oscillation; but one MILD instantly corrected SHORT oscillation, just 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.


rev.
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.  

 

Copyright, 2016, R. Fleischer

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Last check/edit: Monday, August 08, 2016