motorcycle tank slappers, weaving, wobbling, instabilities

Instability; weaving, wobbling, tank slappers.....the whole story, in a way seldom presented

instability.htm-76B

© Copyright, 2011, R. Fleischer

Over many decades of my riding life, I have heard and read of tank-slappers; weaving, wobbling, and various ills involving bike instability of one sort or the other. There is a tremendous amount of just plain blarney involved with these subjects....as well as a lot of misunderstanding. I have experienced REAL tank slappers on customer's bikes....old English and old Japanese bikes in particular. I have only a few times managed to get a BMW Airhead to display anything even vaguely approaching a true tank-slapper. On one bike it was due to a combination of quite excessive wear and miss-adjustment at several places in the suspension, wheels, swing arm, steering head. On the others it was a short wheelbase (SWB) /5 that had been fitted with a HANDLEBAR/FORK-mounted fairing and was at high speed, with saddlebags, substantial-sized TRUNK and BACKREST. I have experienced high speed WEAVING, but that was always brought under control by slowing down or making a less sharp curve. Typically this happens on a RT with the bags attached, at 85 mph, downhill sweeper ....but it is a wallowing, easily controlled. Quality shock absorbers in the rear suspension will help a LOT; as will good tires, proper wheel bearing adjustments, ETC.
In general, the earliest Airheads have slightly more problems in this area, with weaving beginning at somewhat lower speeds, than the later ones that have stiffer frame backbones, and other small changes made by BMW. All these changes were fully incorporated by the early 1980's. The MONOLEVER bikes do not exhibit the problem hardly at all. Neither do the Paralever Airheads. One CAN get them to act up if the rear shock is quite bad. The weaving seems to be a function of many things, but certainly the rather 'not very stiff' connection of the rear frame to the main frame, is what gives the twin-shock Airhead motorcycles that 'rubber cow, in the middle' feeling.

Whilst one can delve into Tony Foale's articles and publications on chassis design, etc., these will hardly tell you easily what you might really want to know ...so here 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 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....folks will mention loose engine mounts, for instance.

There is a lot of truth to the above items, but they are NOT the entire story...so, if you are really interested....read on....

NOTE: The information presented below is from my memories, my own very specific testing ....using the SAME criteria as Gordon Jennings. I have ridden hundreds of bikes, literally, and my extensive early testing was on a /5 SWB with handlebar/fork mounted fairing, believed a worst case for any BMW airhead. EVERYthing having to do with suspension, etc., was good and proper on that test bike. I also did extensive testing on a very stock late model RS. 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. 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 found in my riding and wrenching lifetime.

I want to caution you that the input and responsibility for what follows below is MINE.

First, it is necessary to define the terms, as these terms are NOT used properly by most folks:
Wobble: 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 (the bars start oscillating back and forth and very quickly and suddenly start doing it full-left to full-right, a VERY scary situation), should really be called flutter...or, in car terms, shimmy. This shimmy has an oscillatory frequency of perhaps 6 to 10 cycles per second.

A very slowly occurring wobble might better be called a WEAVE, and these always happen at very much slower rates of oscillation. 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 here, it being more a nerdy engineer-type thing.

Weaves can GAIN strength (amplitude) with speed. That is why if one happens at very high speeds it can be scary. 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 is very particularly noticed when you try to lean the bike to turn it at all. High speed weave is rather common. On our Airheads, it tends to occur as a 'wallowing about', and usually above 80 mph.....and 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, but may freak you out. On BMW Airhead motorcycles, that 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.

Certain things can make weaving much worse. These are not often spoken about (or, enough). 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 those bags on the bike. So, full dressers are more likely to have handling problems. The ONSET of this sort of instability, this sort of slower weaving rate, is a feeling of WALLOWING. If the rider does not slow down, but increases his speed, the weaving/wallowing WILL get worse. One item that contributes greatly to this phenomena is a tail trunk or excessive weight behind the rear axle. Note ALSO, that the more rubber the front tire puts ACTUALLY on the ground (the footprint), the LESS the effect. This last thing is something I know about, that Gordon never mentioned...amongst some other things in this article you are reading.

Many bikes will have some instability, not violent, as one slows down, particularly with trailing throttle and most particularly if you hands are not on the bars. This is normal, usually. 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 too-loose adjustment on the steering head bearings.

Tank Slapper anecdotes almost always refer to the 'old days', when frames were not so strong, forks not so strong and stiff, and so on. Modern tires are much less conducive to helping to start such instability. Today, tank slappers on bikes made since the 60's are rare indeed. What HAS happened, is that with stiffer frames, and so on, as noted in the first sentence of this paragraph, the speed at which any instability is noted (NOT a tank slapper), has moved from the tank-slapper speed area, to an area more like 40-60 mph.....and the instability is mild. Increasing frame and suspension stiffness is NOT the answer to trying to totally eliminate 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 WEAVING....NOT lock to lock oscillations. They tend to grossly exaggerate what happened.

It is not uncommon to be able to induce some sort of mild instability, perhaps up to maybe 60 mph or so, by taking one's hands off the bars and giving a bar end a bit of a hit.....but any instability will be 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 centripetal force from wheel weight and diameter. If the instability continues to INcrease, that is not good.

Back in the early days when the Japanese bikes were first coming to the U.S., the suspensions were simply awful, particularly the shock absorbers, and 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 that if the rider shut off the throttle to slow down, the wobble GOT WORSE. It was from THAT, on these old badly suspended Japanese bikes, 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".

Once the Japanese improved the suspension components, the cornering weave magically went away.

Some additional points:
1. That a heavy...OK, overweight....rider can supposedly destroy a small bike's handling....is not true in the weave/wobble sense here.
2. Tire run-out won't cause those serious types of wobbles...even if pretty darn extreme.
3. Loose wheel bearings, loose suspension bearing points, dead shock absorbers, loose steering head adjustments: yes, can cause problems.

Fork braces: These were originally really for old-time race bikes when 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 another effect, it makes the WEAVING a BIT WORSE!   Seems wrong, but, remember, a bike and its suspension must be taken as a whole.

Super stiff frames and suspensions:   These do not work out for racing, won't 'hook up' properly.   On the other hand, the early Airheads were a bit too loose. BMW DID beef up the frames on the Airheads.

Tire pressures:   low enough, bike will wallow and handle mushily at low speeds, but pressures don't seem to effect high speed instability.

Large tire/wheel imbalance: no great effect from this has been noted....even if fairly extreme.

Stiffer top triple clamp: Often does wonders, often does nothing. When it helps, it usually helps 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.

So, while the bearings need to be adjusted correctly, and the shocks and springs 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.
3. Weight in tour trunks is especially vicious in its effects. Just HAVING a backrest or tour trunk can greatly influence tendencies towards instabilities.
4. REMOVE 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.
5. Handlebar mounted fairings, especially large ones (tiny fly screens are vastly less of a problem) can be a HUGE problem.
The biggest problems are handlebar/fork mounted fairings, heavy bags, and having a big backrest or touring trunk.

What was the biggest 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 very slow curves.
This was exactly what Gorden Jennings found out....and I duplicated his testing and 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!

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

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