How to handle a tank slapper

I just got hit by a tank slapper on my way home and boy was it fun.
Seriously. It reminded me that I've but hit by quite a few in my years and
never went down and maybe I should share the wisdom with any new riders yet
to experience the joy of your motorcycle taking control.

There's lots of videos out there of tank slappers. Most show the rider
getting tossed off while the bike rights itself and keeps going in a
straight line for a while. I even saw one that won a race, or at least the
bike did. This is where to draw insight into surviving a tank slapper. The
key is to not DO anything. Just grab the tank as tight as you can with your
legs, LET GO of the handlebars and be ready to grab them again when the
shockwave subsides (it will as long as you don't add to it by trying to
regain control). Do not try to regain control, you WILL ALWAYS make the
situation worse. Just ride it out, it will diminish and will you will
regain control. That is unless you're about to hit something, then you're
screwed.

 

Hands off the bars will chop the throttle which can make things a lot
worse. There is an article on this and weave in the current Sport Rider.

 

I think the key is really just to loosen up on the bars - tension is alwys
the enemy. Actually, I come off the gas - I have never tried applying
throttle. I have always managed to stay upright.

 

wrote in

You're right. Here's an article I wrote some years ago on the same
thing:
===========================================

The Human Steering Damper


Many people have come to understand that being tight on the bike has the
undesirable consequence of amplifying handling problems. Fast riding
has a way of creating tension in riders, as taut muscles are part of the
fight or flight response. The more anxious we become, the more we tend
to tense up. You'd think that taut limbs might be useful for
controlling unwanted oscillations in the steering. The opposite is
usually true; we tend to make these oscillations worse, the tighter we
hold on.

On a recent ride, a few of us were out playing on a particularly bumpy
road. Apart from the bumps, it's a pretty fast road in places.
Headshake and even tankslappers are common lunchtime topics out there.

Over lunch on this day, one of my friends commented on being pretty
tired from controlling the bike over bumps. The remark got my attention
and I asked what "controlling" meant. This rider was trying to control
the headshake by gripping the bars. I asked if it was helping. The
answer: It didn't seem to be.

About a year before, this same rider had had an epiphany of the kind one
usually gets from direct, frightening experience. Exiting a turn at
Thunderhill Raceway, my friend was startled to see a person (me,
actually) standing on the track waving oncoming riders to slow down.
Another rider had highsided coming out of this turn and was lying across
the track, just out of sight at the crest of a hill. The turn workers
hadn't gotten a yellow flag out yet, so the first indication of trouble
my friend had was while still leaned over, exiting this turn.

For an instant, my friend grabbed the front brake, which locked the
wheel. It looked like a certain lowside. Realizing the mistake in
time, my buddy got off the brake. The wheel had begun to tuck and now
with the brake off, it snapped straight. My friend was gripping the
bars like there was no tomorrow and the bike went into an incredible
tankslapper.

You could tell the moment my friend gave up and prepared to crash. Body
went limp, feet came off the pegs and hands relaxed. The front end
suddenly stopped whapping back and forth and straightened right out.
Amazed, my friend brought the bike to a gentle stop as if the whole
thing had been planned.

Back in the present, over lunch, my friend recalled this event. I
asked, "So, do you suppose you can get your bike to stop wiggling over
the bumps today if you just hold on a little tighter?"

"Hmmm," came the thoughtful answer. "Probably not."

We then started talking about why it is that a rider can't seem to be an
effective steering damper. Not only are we lousy dampers, we tend to
amplify wiggles when we clamp onto the bars. There may be other
reasons, but I've latched onto a pet theory.

Background
----------------
There are little sensory organs in your tendons that react to sudden
changes in tension. They are known as Golgi Tendon Organs, also
commonly called stretch receptors. They cause the attached muscle to
twitch in response to forceful inputs to the tendon. They do things
like allow you to compensate in real time for accidentally stepping in a
small hole or preventing joint hyperextension by causing the supporting
muscles to tighten. You have witnessed the Golgi Tendon Organ in action
if you've ever had a doctor tap the great tendon below your kneecap in a
checkup. When he does that, your leg jumps.

Theory
---------
You have these little gizmos in your arms too (fact). If you're clamped
onto the bike's handlebars like an abalone on a rock and the bike hits a
bump, the stretch receptors in your elbow will make your triceps twitch.
A twitching tricep, when connected to an arm that is firmly attached to
a handlebar, will make that handlebar move. Chances are, that movement
will not be the perfect antidote to the jolt that triggered it. Bigtime
headshake will develop if you persist in hanging on.

We finished our ride for that day and all of us had a little more focus
on staying light on the bars over the bumps. My friend noticed the
difference in stability and definitely in reduced effort.

It's probably not vital to the business of riding a motorcycle for us to
know all about this, but it does shed some light on why we tend to make
wiggles worse by hanging on tight, despite our most resolute intentions
to the contrary.


ab

 

I don't know about letting go, every V-Max I've ridden would go into a
scary front-tire skidmark leaving tankslapper with very little
provocation. 3-5 oscillations and it would be to the stops. From my
racing experience you just relax (boy oh boy that's hard to do) and
apply a wee bit of throttle. And wait. I've never crashed due to a
tankslapper, though I've had racebikes that HAD to have a good
steering damper. It's all about the bike's natural wiggle damping,
which can be affected by tires, setup, etc.

Modern bikes with radials are fairly pro-stability, but there's
usually enough power to make things twitchy on the gas hard over bumps
while leaned over. All in good fun.

-Anton

 

rider getting tossed off while the bike rights itself and keeps going
in a straight line for a while. I even saw one that won a race, or at
least the bike did.

Those were NOT "tank slappers", Richard, those were "speed weaves" that
got out of hand...

Been there, done that, still around to tell the tale...

with your legs, LET GO of the handlebars and be ready to grab them
again when the shockwave subsides (it will as long as you don't add to
it by trying to regain control). Do not try to regain control, you WILL
ALWAYS make the situation worse. Just ride it out, it will diminish and
will you will regain control. That is unless you're about to hit
something, then you're screwed.

BULLSHIT, Richard...

You don't know the difference between a "tank slapper" and a "speed
weave", Richard. There are two troublesome oscillations that a
motorcycle chassis can do. The first is the dreaded "tank slapper", a
TRUE *speed wobble* where the front forks and wheel and fender and
EVERYTHING THAT PIVOTS AROUND THE STEERING STEM move side to side at a
very high frequency and the motorcycle does NOT yaw side to side. This
is caused by unweighting the front tire contact patch over railroad
tracks, choppy pavement, or applying too much throttle. The tank
slapper starts and finishes so quickly, there just isn't anything you
CAN do about it.

The tank slapper may be so violent that it knocks your hands off the
bars. You don't have TIME to think about BULLSHIT like letting go of
the bars...

A tank slapper can be so violent that it breaks your thumbs. If the
front tire sets back down on the pavement at a severe angle, you can be
spit off the bike. But this usually doesn't happen, the tankslapper is
over, it's time to check your underwear for damage...

The other motorcycle oscillation, which is often erroneously described
as a "tank slapper" or a "speed wobble" is actually a "speed weave".
ALL MOTORCYCLES WILL SPEED WEAVE, at some critical
speed where the front tire contact patch cannot stabilize the
motorcycle by making corrections to the chassis trying to follow the
rear tire as it darts off to the right of the left...

Since motorcycles steer by leaning, and steering is the same as yawing,
the motorcycle chassis is not only yawing, it's rolling and it's even
pitching up and down a bit, all around the motorcycle's CENTER OF MASS,
at a much lower frequency than that of the dreaded tank slapper. There
are things that you can do about a speed weave, just undo some of the
things you did to get into the speed weave, like roll off the throttle,
lay your chest on the gas tank, scoot your weight forward if you can,
drag the rear brake lightly to transfer weight to the front, and QUIT
TRYING TO USE SO MUCH POWER IN THE TURNS on track day...

I started seeing speed weaves getting out of control back around 1986,
when riders of bikes like GSXR-750's and GSXR-1100's would
try to fit larger-than-standard rear tires on their bikes. They were
fitting tires that were up to an inch wider on the back, while there
were no wider front tires available...

The work of the front tire contact patch became harder than it should
have been. The riders felt that they had tons of rear tire grip, and
they kept applying more and more throttle, while trying to lean the
bike more and more to tighten the radius of the turn...

As the rider added power, the rear swingarm would start jacking up the
rear end of the bike, and the front tire would start tucking under.
Eventually, the front contact patch would break away, and we would see
what you described, the rider getting spit off the motorcycle, usually
a highside get off...

 

Relax and ride it out but make all your inputs VERY slowly. IOW, let
the bike correct itself.

 

this post gets my vote as best summary of this topic

 

I once had a BMW R60 with Earles forks, which are notorious for inducing
tank slappers. Mine would shake and shiver all the time. Coming out of a
turn, going into a turn, accelerating, decelerating, rain, sunshine, etc. It
was really quite annoying.

I finally acquired the knack of kinda letting go of the
bars. I did this by just loosening my grasp on the grips. I'd open my hands
a bit while retaining the shape of an encircling loop. The bars would
rattle around inside my C-shaped hand and quickly calm down. I think this
approach did more for me than the bike. It's almost impossible to get
yourself to release control when your overpowering instinct is to grasp
harder to regain control. My approach gives the brain the illusion of
retaining control while actually releasing my grip, thereby allowing the
front end to straighten out.

nb

 

Get wider handle bars.

Regards
Mike

 

Same here and it was on an old CB360T. The bike came with a rather
large dorky fairing and couldn't go past 80 mph in 6th gear. One day
I discovered if I kept it in 5th I could hit 100. I upshifted to 6th
and the front end started to wiggle. As I slowed down and fought it
it kept getting worse. At about 50 mph I made it to the shoulder in
order to get out of the path of the semi's behind me and was almost
ready to bail since I was now heading off road...started to let off
the handle bars and at that point it smoothed out like nothing
happened. I was probably 18 at the time and had no clue as to what
happened or that it was even had a name. I just assumed the wind
caught the fairing at an odd angle.

Four bikes since then and I've never experienced it again and
hopefully I never will.
Any rhyme or reason as to what starts them? Most I've seen on squid
videos are from wheelies that were set down hard. Don't think I've
ever heard of a cruiser style bike going into a tank slapper either?

George
CX500 TURBO
CX650 TURBO
'04 Hayabusa

 

the telelever R11 bikes I've ridden are nearly impossible to destabilize,
unless you do something really silly and put a really heavy cargo load in
a jumbo topcase or on a rack so its high and far back. even then, they
dampen any wobbles pretty quickly, even at very high speeds on very bumpy
paved or dirt roads. The single sided swingarms are very rigid, there's
no flex in the bike chassis at all.

 

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If you buy into this crap-o-la, I've got a bridge in NYC for sale real
cheap. You R-GS riders are just too pussy to ever have to worry about
a "tank-slapper". I call it BMW "bullshit", nothing more. If either of
you ever rode a real sport bike you wouldn't know how to handle them;
front end feel and all

 

No, my point is not based upon what something is called. A speed weave
and a speed wobble are two completely different situations and Richard
doesn't know the difference, so he gives examples of an out of control
speed weave and calls it a "tank slapper" and his utterly stupid advice
is to *let go of the handlebars*...

So, as a community service, I spent *my valuable time* explaining the
difference between speed weaves and speed wobbles, and where is
Richard? He's not calling me six kinds of @$$hole for calling bullshit
on his bad advice which could conceivably get a newbie *killed* if he
lets go of the handlebars when a speed weave or speed wobble occurs...

it, but go with it. That is still true in all cases.

The speed wobble (tankslapper) begins and ends so quickly the rider has
no time to react to it. The frequency of a speed wobble may be 10
oscillations per second around the steering axis...

The frequency of the *speed weave* is much lower, it may be 1 to 3
cycles per second and the rider *can* react to that, it's about the
same frequency as tail working, the momentary breaking and regaining of
traction that the rear tire contact patch routinely does as it
sacrifices its rubber to turn the motorcycle under brisk riding...

The rider can react to a speed weave by rolling off the throttle,
leaning forward, moving his butt forward, or tapping the rear brake to
weight the front tire contact patch...

If the rider is approaching a crested turn, such as Monroe Ridge (Turn
6 at Willow Springs) the strategy is to roll off the throttle
momentarily to load the front tire contact patch before attacking the
crested turn while leaned over. That avoids having a merely frightening
speed weave turn into an *oh shit!" moment...

 

Speed wobbles are caused by the forces of action vs. reaction and
inertia...

The rear wheel of a motorcycle has no inherent stability, it's trying
to fall over to the right or to the left constantly and that cause the
steering head to tilt slightly from side to side...

The front wheel, attached to the chassis by a fork that has a rather
severe castor angle and therefore has lots of trail is forced to follow
the motions of the rear wheel when it tries to fall over to the right
or left...

The trail in the front end causes the front tire contact patch to
produce a thrusting force that returns the chassis to the upright
position...

Then the rear wheel tries to fall over toward that side and the front
tire has to provide a correcting thrust to move the bike to the upright
position again...

This correcting thrust has to REACT AGAINST THE PAVEMENT in order to
push the chassis back to the upright position...

And so we go down the road, weaving almost imperceptibly from left to
right and back at a very low frequency...

Suppose we go over a railroad crossing as the front tire is trying to
correct for a lean to the right or left. The front end of the bike
becomes very light, and the force of the front wheel's correction has
NOTHING TO REACT AGAINST EXCEPT AIR...

The front wheel has mass and so do the forks. The weight of the
handlebar mounted fairing or windshield ahead of the steering axis adds
its inertia to the mass that is about to oscillate from side to side...

The front wheel snaps violently to the right or the left, inertia
alternately tries to damp or enhance the oscillating motion and
gyroscopic precession begins to work, adding its two cents worth
by torquing the front wheel at 90 degrees to the motion it's going...

And the front tire contact patch momentarily kisses the ground and rake
and trail try to restore stability, but the front tire is at an angle
to the direction of travel so it gets kicked of in some other direction
and the rear wheel is still trying to fall over to the right or the
left and it's moving the angle of the steering head around too...

The front wheel does its little dance back and forth on the pavement at
around 10 cycles per second, too fast for the rider to react...

 

Great info, thanks for taking the time to write all that!

Now if only Discovery Channel would do a show on motorcycle physics
with slow motion cameras and 3D cgi models.

George
CX500 TURBO
CX650 TURBO
'04 Hayabusa

 

He just knew the road much better-

Seeing the picture makes me even more convinced it was road knowledge
+ skill. Some guy used to call me the Ghost up at Canyon Lake until I
had breakfast with him and told him there were lots of faster guys in
our group. IOW, there are no myths when you get the facts down.

 

Sounds like the bikes I learned to ride on. My old GS1000 nearly
killed me a number of times. Once on the Dumbarton bridge when it was
still a low two laner with a lift in the middle, I got into a speed
wobble @ between 110-120. Thank God no one was coming in the opposite
direction-

 

Sorry for top posting but ... Damn, Krusty you be da man!
That is one of the most coherent explanations of single-track "physics"
I have read.

Your description of how the forks react to a 'falling head' to set the
bike back up (sorry about my paraphrasing)
is exactly the concept I've read in published papers... but much clearer!

BTW: this weaving you are talking about is imperceptible 'at speed', but
when we slow way down (eg. trying to
avoid stopping completely at a stoplight) we have to help out the
weaving process to keep the bike upright. The
alert rider notices that at 7 or 8 MPH the bikes helps out a lot more
than at 1 or 2 MPH. The bike does all the
work > 15 MPH and we hardly notice.

Based on this: I trust your description of the speed weave and speed wobble!

John Beck
(Of course I'm one o' dem pussy GS riders who never goes fast enough to
have to worry about it!)

 

His advice wasn't *that* far off. If I remember one of your previous
posts, even you observed that in a tank slapper, the rider's hands may
end up off the bars whether he wants them to or not. I think you even
said something about broken thumbs. Sometimes the wobble settles down
and sometimes it doesn't, but an attempt by the rider to hang onto the
bars does not usually have a productive outcome.

I have personally seen riders in either a weave or outright slapper give
up, let go of the bars (willingly or not) and recover, much to their
amazement. It's certainly easier and better to get out of the throttle at
the onset of a wiggle, before it magnifies into a slapper. You do
typically have a moment to do this, but it's a short moment...

ab