Wheel alignment

Tuesday 15 May 2012

Tankslappers, top boxes and the trembles

Excuse the alliteration in the title but it pretty much sums up an exchange of emails this week with my riding mates Dr Andy West and fellow Kiwi blogger extraordinaire Roger Fleming

Now please bear with me as it's a bit of a detective story!

It started with an email from Andy who in addition to his wicked Triumph Daytona 675, bought a BMW800 GS in more recent times. No embarrassment to Andy intended (he's an extremely accomplished rider by the way) and the full details won't be publicly disclosed but he dropped his Beemer following a massive tankslapper at open highway speeds on a public sealed road.  Fortunately, the outcome could have been a lot worse than it was, although I'm sure that he didn't see it that way at the time.

What's a tankslapper?
At this stage, a few riders might be unsure with respect to what a tankslapper actually is so I'll elaborate.

One definition (source: Triumph Rat Forum - Tbirdnz)
The “tankslapper” is a very frightening experience. Usually occurring when accelerating hard over bumpy pavement, a tankslapper ensues when the front tyre becomes airborne, then regains traction outside the rear tyre’s alignment. The resulting deflection bounces the tyre off to one side, followed by another bounce in the opposite direction as it contacts the pavement again. Unless the bike’s steering geometry is able to damp out the deflections quickly, the resulting oscillations from the front tyre as it bounces back and forth will swiftly gain in strength, causing the bars to swap from side to side with increasing ferocity. The oscillations can be violent enough to rip the bars out of your hands, and fling your feet off the pegs. You can guess what happens next.

Here's a video of a tankslapping nightmare, source You Tube:

Tankslappers/headshakes are at the extreme end of a set of conditions which upset the stability of a bike.  They can be felt as a gentle weave at the other end of the scale and more often than not, there's little or no warning.

From personal experience, I don't think that the definition above quite covers the whole set of circumstances as it can occur under under deceleration too but it's pretty comprehensive - especially in terms of the first sentence and the last but one!

My first major tankslapper was on the Honda Blackbird, accelerating hard past a car towing a boat and trailer. During the passing manoeuvre, I hit a patch of bare tar, the rear tyre broke traction and next second, a tankslapper had started.  I had both time and presence of mind to open the throttle right up and it straightened up.  It rattled me but was over so quickly that there was no soiled underwear!

The second one was far worse and at much lower speed.  Again on the Blackbird, I'd accelerated away from a crossroads, not particularly hard and was still in a low gear when the tankslapper occurred.  This time it was so violent that all I could do was hang on loosely and not try and fight it, which might well have saved me.  When it had finished its shakes, I pulled over and got a dose of the shakes myself!  It was a really close call and I had a huge welt on one leg from a foot coming off the peg and the inside of my leg smacking the frame. 

I never want a repeat of those experiences.  There is something which goes a long way to alleviating the problem and that's fitting a steering damper but let's do a bit more digging.....

Some further thoughts
Andy West's email has prompted a bit more musing.  Both incidents of mine occurred after I'd raised the rear ride height of the 'bird to quicken the steering.  It's pretty certain that a steeper or more sporting geometry raises the risk and is one significant reason why drag bikes have heavily raked steering.  The sharpened steering on the Blackbird may well have been one contributing factor and the fact that it's a heavy mother is another.  Once that mass starts swinging, it's like an oversized pendulum which is hard to stop.  As Andy's 800 GS has relatively conservative steering however, there might be other considerations too which predispose a bike to tankslappers or weaving at certain speeds and conditions.

It was another comment from Andy which caught my eye - he mentioned in passing that he had a top box fitted and he was riding solo.  Whilst I didn't have a top box hanging off the rear of the bike, I did have a high-mounted pack on both occasions - see picture below.

Blackbird with double pack

Whilst the Ventura zip-together pack shown above raises the centre of gravity and therefore lessens stability, it's saving grace is that it's reasonably aerodynamic and sits well forward.  Contrast this with the BMW below where the top box is both high and aft of the rear axle

Top box a long way aft

You don't need to be a physicist to appreciate that a load high up and well to the rear of the bike will shift the centre of effort aft (and upwards).  Now add the effect of wind pressure at speed on the top box and the dynamic centre of effort will move even further aft potentially causing stability problems.  Of course, the severity will heavily depend upon the type of bike you ride - suspension travel, level of wind deflection, weight distribution and so on plus the speed you're travelling at and maybe even wind direction.  Bikes ridden two-up appear to be less affected, probably due in part to mass centralisation and also shielding the top box from an unstable slipstream.

When I had my two incidents, I searched the international Blackbird internet forums and found quite a few cases of everything from an alarming weave through to a full tankslapper, almost always with top boxes being used.  Mentioning this to Roger yesterday, he was pretty certain that he'd seen similar reports for other bikes.  I've just jumped on the 'net to check for myself and yep, there are plenty of cases there for all manner of bikes!

So there you are.... if you didn't know about the potential for weaving or tankslappers before, particularly with top boxes; you do now and that's half the battle.  You also know that a steering damper will help enormously, at least in mitigating the more violent effects.  Let's hope you never have one as it's a truly terrifying and dangerous experience!!!

Regular readers will remember that it was an email from eminent motorcycle journalist David Hough that set me on the path to raise my riding skills with IAM and we've stayed in touch.  David's knowledge of motorcycling in general and safe riding in particular is legendary.  I floated the subject of tankslappers past him and as always, got a comprehensive and enlightening response.  Here's what David had to say:

Tankslappers are a reality. Dynamically speaking, the front end wants to balance itself (drag on the tire off-center steers the front wheel to the side--which countersteers itself back the other way, restoring balance) but outside forces can interfere. For example, if the tire loses traction momentarily and then regains traction, and the wheel isn't pointed exactly in the direction of the bike, tire traction attempts to steer back into a (more or less) straight line. But there's a risk the tire gains traction so quickly it over-centers, then reacts by steering back the other way, etc. etc. In time it should self correct, but a rider tends to panic and attempt to force the bars to settle down. Problem: the front end left-right frequency is typically faster than a human can respond to, and the rider imparts steering pressure in the wrong direction.
Momentary loss of traction (or reduction in traction) can also occur when the front suspension can't react quickly enough for a dip, bump, or groove. A painted line can be slippery in the rain. A dip can hold grease. A groove can momentarily steer the tire. Loose wheel bearings or swing arm bearings can encourage oscillation. The rider imparts a sudden steering input (say when recovering from a lane change) Wind drag on luggage or fairing can cause instability as turbulence pushes side-to-side. It's worth noting that the California Highway Patrol required special radio box mounts on their former Kawasaki bikes. The radio box (on the back of the machine) had sliders to allow it to move around laterally without imparting undesirable forces to the back of the bike. 
In the scenario of a rider passing another vehicle on wet pavement at speed, I'm not amazed a tank slapper occurred.
Attempts to hold the bars steady rarely do anything other than to accentuate the wobble. Changing speed can change the bike's reaction. For example, applying the rear brake, or rolling on the throttle can help. But the speed change needs to be quick, before the bars are oscillating at a frequency that makes it difficult to hold on.


  1. That looks and sounds pretty terrifying. When I picked up my bike, it came with a top box and it's rack removed. The local BMW airhead guru told me not to put it on as he considered them dangerous on that bike. He didn't elaborate but it has been sitting in storage ever since.

  2. Hi Richard
    Your experience is really interesting. Well, I can't think of any other reason so maybe it's connected to the subject of the post. It's certainly food for thought and thanks for the feedback!

  3. Horrifying! I have never heard of, seen or experienced this phenomenon before. I somewhat get it that it might happen on a sports bike on higher speed, but on an F800GS? I hope your friend has recovered completely and didn't loose his interest in riding over this. I am not sure I could have hold on to the handlebars like you did if that happened to me.

    1. Sonja:
      Certainly didn't mean to alarm you but it's worth knowing that these things can happen. It was on a wet road and it would seem that slight loss of traction through either wet or other slippery conditions is one of the factors which might trigger it, although not necessarily always. Andy is still a little sore but he's riding and it certainly hasn't put him off. I was detuned for a day or so but got back in the groove fairly quickly as I didn't come off (more by pure luck than any skill on my part).

  4. Geoff this sounds terrifying and I hope I never experience. Oh dear!

    1. Hi Dar - long time no hear, hope all is well.

      It's a pretty rare phenomenon, I've only experienced 2 in the last 3 decades albeit close together so the chances are low. Much as you might have preferred not to know about it, the knowledge may well be useful to someone! A good time to wish you safe travelling I guess!

  5. Perhaps a comment for the newer riders - I had my one and only full blown tank slapper while still a noob on my XS250. It happened when I pulled over on the highway to let cars past (stupid NZ rule that limits learners to 70km/h - just damn dangerous in my opinion) and ran into a really nasty tar snake. The bars shook like mad and I still don't know how I got out the other end...

    Since then I've only had the odd head shake when punting more sporty bikes (RF900 and VTR1000) hard out of corners. The front tends to get light and if you hit a wee bump you might get a bit of a shake going on - something is telling you to ease up a tad. Push any harder and you might get into a proper slapper...The narrower handle bars on these sorts of bike also mean you need more effort to hand on.

    Never had any issues on my bikes with Top boxes

  6. Hi Andrew and thanks so much for another perspective on the issue. I think that the Blackbird can be a bit sensitive with a heavy pack (altered ride height notwithstanding)because the fairing blade doesn't deflect the air as high as some other sport tourers. I've heard that Bandits are not immune from it either.


  7. Excellent post Geoff. It looks like there are a few riders amongst us that don't know what a tankslapper is.

    I still remember my first big tankslapper vividly and it made me price a steering damper the next day. My bike being a R1 sportsbike is probably not as rare as other type of bikes for this type of thing.
    My brother and I were on our way to Tassie for the week and we were riding up Brown Mountain in southern NSW. We had our gear racks and bags on just like in your pic of the blackbird.

    Its a tight twisting mountain road and I was following my brother on a slight uphill incline just coming out of a tight right hander that had a bumpy surface.
    There was a car and caravan coming the other direction, I just started accelerating out of the bend(about 50-60kmh) leant over when the front started to slap. Slightly at first then it quickly developed into a major slapper where I came up out of the seat and was looking over the screen until it actually ripped the bars out of my hands at which point the bike decelerated and came back under control by itself.

    Needless to say that scared the crapper out of me for the next couple of bends and I took it fairly easily.


  8. Hi Steve!
    So you're a member of the brown trouser brigade too! Yeah, a short wheelbase like the R1 with a decent load and under acceleration is like poking the devil with a stick! Thanks very much for the experience-sharing, real instances are valuable learning.

  9. Hi Geoff ...good to see you here again after your "holiday"

    Tankslappers !!! Now there's a subject we could talk about for years with no outcome / solution at the end

    Tyres , suspension , wheel bearings etc etc ..TOP boxes and the likes ?

    Having experienced one bad one..way back in 1970's on my then new T160 Triumph , NOT something you wish for everyday for sure ....it turned out to be a front wheel bearing ..completely SHOT.

    My Honda X11 ..I have also raised the rear suspension for that quicker turn in , and so far ( 35,000 miles since installation ) no issues

    Honda Pan 1300 ..after years of weaving , Police Bikes withdrawn.. Nico Bakker from Holland has finally cured it , with the installation of two small metal strips into the screen set up.

    So , we just need to be sure we try to remain calm and "in control" of the bike should we ever experience this awful event

    Keep up the great posts Geoff


  10. Geoff, the early 1970's Z900 Kawasaki's were (in)famous for it. One of my mates totaled one and thereafter always referred to his main injury as his "Kawasaki" elbow. Fortunately I've never experienced one - hope it stays that way.

    If I recall correctly, there was an issue with the Honda ST1300 in Police configuration developing a nasty weave at around 100 mph.

  11. Hi Geoff,

    Great post!! That video is amazing, scary, but the rider did so well to hang on for as long as he did! I've never had a tank slapper, (and hope to never experience it!) I have a top box on my V-Strom, but really prefer to ride without it, and use my Kriega tail-bag strapped to the seat instead. The bike handles better without the top box.

    Got to see the new Harley today!! It's soooo beautiful and I've posted some pics on my blog. Enjoy the week.

    cheers mate.

  12. Geoff:

    I hope I never have a tankslapper, BUT when Sonja and I were coming back from Oregon on I-5 just north of Longview, WA we were caught between an army of Truck Tractor Units. The highway was 3 or possibly 4 lanes, very busy road. They were going slightly faster than the speed limit and we were caught in their wind draft, making our bikes shudder in their wake. Trucks on both sides we attempted to pass them so we could be in front of their airstream. V-stroms with side cases are known to have the "sailboat" effect and catches the wind. It was a real struggle with the buffeting. Not really a tankslapper, but really close with all the shaking going on. Once we were in front it was smooth sailing

    Riding the Wet Coast
    My Flickr // My YouTube

  13. Yikes. Haven't had one and don't want one, thank you.

    Brad and I were just talking about this the other day. I saw on a Gladius forum where people were talking about steering dampeners to use on a Gladius and I asked him why they would want one. To which he replied - tank slappers.

    I didn't know about the top box potentially being an issue. For some reason I never though of them changing the center of gravity.

    Great information you have there Geoff. Thank you.

  14. Hi Andrew!
    Interesting about the Nikko Bakker "fix" as I see that on Pan forums, it's still being discussed! I guess it's worse on police Pans with all their extra gear. Thanks for the feedback mate!

    Didn't know that about the 900 Kwakkers thanks. I guess the ST1300's were really a development of the Pan and may have still carried the problem which Andrew mentioned.

    Hi Anthony!
    I have soft luggage for the Street Triple (from the Blackbird)but always have the biggest weight on the seat. Most times, I don't carry much unless on tour.

    Looking forward to reading your blog in a few minutes! Nice to see you posting again.

    Hi Bob!
    Yep, it's not really surprising how stability is affected by extra luggage. It would seem to get a real tankslapper, you need a set of marginal conditions like accelerating or decelerating hard, possibly in conjunction with a slippery surface, but they can occur without some of those factors being present.

    Hmmm... so it's not unknown on the Gladius either! A top box will certainly change the C of G but it's the centre of effort (think of it as a dynamic C of G) which really causes the damage as the forces change with speed. Rigid top boxes are really handy but I normally don't like their looks so I won't be buying one on those grounds alone!

  15. If it goes a bit far...


    1. Highside or the start of a tankslapper as it has a double kick?

  16. Ah Geoff I had a nasty experience with the Bird giving a violent head shake whilst riding down to the French Alps one up with Givi panniers and top box. It had such a profound effect on me that when I got back home I sold the hard luggage off on the Bird site and went back to soft throw overs. The Suzuki TLR1000 was famous for the tank slap hence it never really sold well (I almost said 'took off')

  17. Dylan me ol'mate!
    Didn't realise that you nearly came to grief on your 'bird too. An underwear-changing experience, isn't it??

    I didn't realise that the TLR was prone - thought it didn't sell because it was so darned hard on the knees :-).

  18. I had a tank slapper once and like Jules mentioned it was on my '76 KZ900. Came out of a turn and accelerated hard not realizing the washboard road and put my heart into my throat. It happened so quickly that I must've have eased up on the throttle and she settled down but I never want to experience that feeling ever again.
    The top box makes sense, more reason to use saddlebags/sidecases for the lower CofG instead. Thanks for the post, very informative.

    1. Brad,
      You too! Yep, no-one who has experienced it ever wants to do so again. Thanks for your contribution!

  19. Great read Geoff, Interesting reading all the comments. I thankfully have never had one, a few head shakes over wash board roads is as bad as it got. I do remember the TL1000 having a bad rep for it, although from experience it was fixed with a steering damper. It does more prevalent in certain models.

    1. Hi Rog,
      A mate had a half-faired TL100S and one of the stability problems that had was caused by the rotary suspension damping at the rear. The full fairing on the R model which Dylan mentioned and it's more aggressive geometry might have made it even worse. There sees little doubt that top boxes can adversely affect bikes which are otherwise fine.

  20. Geoff
    I'm really surprised here at all these experienced bike riders not knowing what a tankslapper is. I thought it would be much more common that riders would know a)what a slapper was and b) what causes are.

    Well i guess its never to late to learn something. The more riders know about their bikes and what can go wrong the better equiped they are to prevent or avoid these things. Good post.

  21. Thanks for a very informative, albiet scary, post Geoff. I'm busy packing as much as I possible can on the shadow, but I'm hoping that the center of gravity thing will be alright with saddlebags and things strapped to the back seat as well, the Shadow has a low center of gravity anyway I think.

  22. Steve,
    I was a little surprised too but as you rightly say, learning makes you a little more prepared to meet all eventualities.

    Pleased that you found it informative. The Shadow won't be totally immune but the long wheelbase and decent fork rake makes it far less likely. You're right about the C of G and as long as you keep any top load from going too far aft of the rear axle, the risk is negligible. Hope you guys have a fantastic trip!

  23. This comment has been removed by a blog administrator.

  24. I do realize this is a 5y old post, however...
    ... you did mention about the lifted rear being a factor; rising the rear does modify the rake/trail, but shouldn't it also move more weight to the front? With the common element of most tank slappers appearing to be too little weight on the front due to top boxes and/or front unloading from acceleration.

  25. Hi Anon, thanks for posting!
    Yep, there will be a slight transfer of weight. However, it's likely to be outweighed (no pun intended) by increasing steering input sensitivity. Whilst tankslappers can indeed be affected by weight redistribution (as per the position of topboxes), it's the dynamic characteristics whilst moving as opposed to pure weight or static position. Complex, innit?

    Many thanks for contributing!


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