Tank Slapper

[kopfjäger]

[koko64]

Had three. One I don't remember.

[S21FOLGORE]

I'm simply not familiar with this type of steering behavior and the different associated terms.

Typically, it happens something like this.
(On the street, anyway.)

You are riding (relatively) quick steering geometry sport bike.
Or, big ADV bike with hard pannier cases mounted.

Accelerating quickly on not so perfectly smooth road surface.
The front tire is just barely touching on the ground.
The front tire hits very small gap that normally doesn’t matter. (because roughly 50% of the rider+bike weight on the front end.)
But because now there’s almost zero front end load, the front end gets kicked to one side very quickly, hit the steering stop and bounces back to the other side, hit the steering stop again, swing back to the other side ...
Yes, it happens quickly.

Some bikes are (were) more prone to massive tank slapper than the others.

First generation TL1000s, first gen. SV650, for example.

My friend, Dawn, crushed it because she did EXACTLY OPPOSITE of what she should have done.

chopping the throttle

holding on to the bar tight, went totally rigid on her upper body

Moments later, she was thrown off of the bike. (She flew quite a bit of distance before landing on the ground.)


The SV was totaled because the steering stop tub on the head pipe was ripped off.

[Howie]

Simple explanation:

Your bike wants to stay vertical and go straight.  Something happens, like the scenarios mentioned by S21FOLGORE or what you saw in the videos posted by kopfjäger happens and the handlebar(s)
will go into a high speed oscillation from side to side (headshake) or, if bad enough from steering stop to steering stop very quickly (tankslapper).  Sometimes even violent enough to break the steering stops.  The best you can do is maintain throttle and hope the bike stabilizes and throw you to the ground.  Your steering input will not be quick and accurate enough to correct a tank slapper and will  probably make it worse.  A steering damper will help slow down the oscillations.

Serious headshake happened to me twice.  Once, a pot hole I couldn't avoid The pothole itself was not the problem , but the large rock in it that deflected the front wheel.  The other time it was self induced.  I was riding home too tired to be riding.  I target fixated on a car wandering towards me.  So I started wandering towards the car.  My quick, sloppy correction caused it.

[diamonddog-2]

Okay, it's clearer now. I watched the video clips with my roommate, who use to race, and he pointed out the difference between a high side and low side crash. He once went off the track at 120 mph and into the weeds. He said it was all he could do just to keep the bike upright, stay off the throttle and downshift to get slowed down. Finally got back in the race after missing 2 turns worth of real estate.

howie:  it sounds like a headshake is, more or less,  a less violent version of tankslapper?.  So maintain speed and don't stiffen the arms? and hope I'm not going down right in front of/next to a 16 wheeler.  *splat*

[diamonddog-2]

I also noticed that in1 of the videos a rider had a steering damper mounted on his bike and still experienced the oscillation. Maybe it wasn't as bad as it could've been? or maybe it was a crappy steering damper? 

[Howie]

<clip>
howie:  it sounds like a headshake is, more or less,  a less violent version of tankslapper?.  So maintain speed and don't stiffen the arms? and hope I'm not going down right in front of/next to a 16 wheeler.  *splat*

Yes.

[DuciD03]

 

okee thanks for the video and further analysis Good analysis / point re: the ft wheel not having much contact to start a tank slapper.

 tanks slapper = yee - haw! ouch sometimes

high side = YEEEE Hawww!  Ouch all the time; but more fun to watch; would think most were caused by a patch of slippery stuff;(oil, antifreeze, water) loose traction in the rear; slide; then full traction when sideways and catapult into a "fwwwwing!"

Wouldn't wish that on anyone; also other observations form vid analysis; tank slappers ... can be the lead into a high side... with cartwheels; and once your at that point there's no lol-ing only wtf-ing. In either event theres a total loss of control and if you ride out of one or the other without crashing its as much luck as good management.  When you slid under a semi your just soooo dam luckey that some or all the dieties were looking and arguing; and saw that guy through.

One though, if you had the presence of mind, would be to hit the rear break? Help straighten out and load the front end?
Further research suggests to "relax and hold on roll off acceleration"; back breaking (any breaking) would load the front tire and depending on where the front tire is pointed in the oscillation; might shoot one off in an unanticipated direction???

and another thought; note to self make sure suspension is tight and steering head bearings are snug!

[Moronic]

On a sweet handling bike like a Monster, with all bearings in good condition and proper tyre pressures, the most likely reason for a tank slapper is either poor suspension or a bottoming out of the front suspension, IMO.

I believe the root cause is usually a powerful twisting force exerted by the front tyre. Typically this will happen if you hit a sharp bump or hole slightly off-centre. If the tyre can move freely up and away from the impact, it will compress only a little bit and the torque it exerts on the steering is mild. You feel a nudge through the 'bars but they self-centre quickly.

If the compression damping prevents the tyre from moving away much at the high compression speed generated by the bump, then the tyre compresses more and puts lots more torque into the steering system. You get a sharp deflection of the steering, and the self-centering effect may over-correct.

If you immediately hit a second bump, it may catch the tyre when the steering is already deflected and add to the problem.

Hit several bumps that arrive with the right frequency and you have lost control - the deflections are much too powerful and rapid to be modulated with your arms. You are a passenger until the bike settles down - by which time you may be out of your lane or off the road.

The most powerful steering deflections occur if the suspension bottoms. The entire momentum of the bike is then compressing the tyre, and the reactive torque it generates through the steering system may be very high.

I have read on here that many stock Monster front suspensions have too much high-speed compression damping. In which case a steering damper could be a good idea.

The Ohlins fork on my S4Rs has nice, soft high-speed compression damping and I don't use a steering damper. I can recall two instances of steering oscillation in about 55,000 miles.

The first took place when the fork fluid had mistakenly been replaced with higher viscosity fluid. All of a sudden the bike was at risk of steering oscillation in even moderately bumpy corners. The instance that sticks in my mind was when taking a rural left-hander (we ride on the left here) at a good but not spirited pace with oncoming traffic. The oscillation over a series of quite gentle mid-corner bumps was not severe, but enough that I was momentarily concerned I might cross into the oncoming lane.

That sort of behaviour disappeared immediately the fork fluid was replaced with the proper stuff.

The second instance happened with correct fork fluid, in a sweeping left-hander at roughly the open-road speed limit. The corner was quite open and could have been taken very much faster but I was in no hurry. However I failed to notice a big, sharp lump on the surface - it felt almost as though I had hit a brick, but it wasn't a brick. The front suspension bottomed, the 'bars did a dramatic side-to-side, the rear wheel left the ground, and my line moved a couple of feet right. Approaching me in the other lane was a large truck. I was grateful the bike sorted itself out so quickly, as I was a passenger momentarily.

The Ohlins fork might benefit from a bit more very-high speed compression damping, to cushion such instances. But they have been very rare in my experience.

Not all bikes are as stable as the Monster.

At one point in the early 90s I had a 600cc Yamaha sport bike on extended loan and on bumpy roads it was appalling for tank slappers - even in a straight line. Wherever the roads were not smooth, I would be riding scared. The problem turned out to be the very softly sprung, underdamped fork - assisted in its evil effects by a wimpy rear shock. Stiffer fork springs, slightly heavier damping fluid and an Ohlins shock cured the issue completely. You could attack any road with complete confidence. In this case the more viscous fork fluid helped, because the stock fluid offered almost no damping on compression at all and even a moderate bump could bottom the suspension.

[koko64]

Thanks for the interesting contributions fellas.

[the_Journeyman]

I don't have the technical background on this, but do have some FHE w/ before and after info.

My bike is an M750 Dark, back in the day with the Dark seemed to be a bit of a budget version.  It had the non-adjustable Marzoochis (sp?) with were pretty soft, and I got into one.  Managed to ride it out, but it was one hell of one.  Had a couple more issues with headshake/speedwobble whatever you want to call it. 

I ordered new springs suited for me, and used a little extra on the pre-load spacing.  Problem solved without a damper.  I've had a couple gnarly "could've been" issues on roads including saving a rear slide with my front wheel and knee after sailing around a curve onto some nasty pavement.  However, no more issues with slappers/headshake/wobble.

JM

[RottDude]

[Speedbag]

That video makes the ol' sphincter pucker up tight enough to shear head bolts every time I watch it.