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Old 03-01-2009, 09:00 AM   #1
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what causes a "tank slapper"

just wondering..

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Old 03-01-2009, 10:01 AM   #2
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Multiple "causes" and one final nail in the coffin, so to speak

Assume I know no more than what my curiousity has led me to, and you'll know how much credibility this post contains. Having said that...

Condition of tire, tire tread, headset bearings, wheel bearings, frame misalignment, road surface, surface condition, surface slope, debris of any type on the road surface, rake and trail of the forks, wheel out of true, wheel out of round, wind, front/rear load on each wheel whether from acceleration/deceleration or the way the bike was loaded and/or it's "normal" weight distribution, loose stuff and PHYSICS ALL OVER THE PLACE; all that exacerbated by rider input once it occurs.

Someone pointed out to me the other day that he had seen several tankslappers occur on roadracing and dirt track bikes, and in every case where the rider was thrown off or just bailed out, the bike immediately straightened out and cruised on until it hit something. IIRC, the book A Twist Of The Wrist has a lot about the physics of motorcyles that is informative and interesting.

I sure as hell hope I never find out firsthand. I can certainly understand the instinct to try to control it and get the bars centered again, and I can also imagine the way that would just make things much worse.

On a related note: I had a problem with the dreaded "death wobble" on my Suzuki Samurai, and the only thing that would stop it was to accelerate until it stopped and then slow down oh so carefully. FWIW, a stock steering stabilizer lasted about a month and the problem came back. I then installed a heavy-duty steering stabilizer and the problem vanished. It's been about three years now and all is copasetic. I'll be interested to hear from more knowledgeable inmates.

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Old 03-01-2009, 04:04 PM   #3
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Here's a simpler explanation. The wobble occurs when the front tire becomes airborne, then regains traction again outside the rear wheel's alignment. Imagine a small rock sliding under your tire raising the wheel off the pavement and physically turning your front wheel ever so sligtly before it regains traction.

Watch the very beginning of the video slowly, he's likely accelerating coming out the turn which most likely brought his front wheel off the ground and the pressure he's placing on the bars cocked his steering out of alignment with the rear.

Here's another one that you wheelie fans can appeciate. Pop a wheelie at a moderate rate of speed then bring the front wheel down out of alignment with the rear. The force of the front wheel trying to correct itself to put it back inline results in it "overshooting" the alignment...and back and forth it goes.

Best way to correct it? Enough acceleration to take the pressure of the front wheel so it can regain it's line of traction, or apply the rear brake gently, which relieves some of the forward force to the front wheel. Usually happens so fast, most folks don't have time to react, or are too busy shitting themselves to think straight.
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opium89 screwed with this post 03-01-2009 at 04:09 PM
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Old 03-01-2009, 04:28 PM   #4
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Wobble is a steering oscillation that is basically the same as the wobble of a grocery cart wheel. Weave is a fish-tailing type motion involving roll and yaw. Every situation is different, of course, but wobbles are a slightly higher frequency than weave. Wobble is pretty much always caused by something in the front end, and weave is always caused by something in the rear. Sportbikes are much more prone to wobble because of the very small amount of rake. Steering head dampers can help with this, but they exacerbate the weave.

In my years of repairing motorcycles, the vast number of wobbles was caused by faulty steering head bearing, or incorrect adjustment of steering bearings, while weave was almost always caused by loose swingarm bearings.

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Old 03-01-2009, 09:00 PM   #5
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I hit a rock on my KDX 220 which sent it into a tank slapper so violent it ripped the bars out of my hands ended up with a broken collarbone . This is the second time that bike has done this, somehow I managed to hang on the first time... time for a steering damper for this bike!
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Old 03-03-2009, 10:10 AM   #6
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I crashed due to a tank slapper a little over five years ago. You can read my analysis of it that I wrote on the five year anniversary of the event here:

I've become much more interested in the subject after experiencing it firsthand. For those who have experienced a little bit of headshake, it's hard to convey how violent a real tank slapper is. And those who recommend recovery by giving it more throttle or other such actions aren't taking into account how impossible that is with the bars swinging back and forth so quickly.

It's possible to have a tank slapper on a bike with no mechanical issues. I'm pretty sure that mine was in that category. From reading about lots of tank slapper experiences, plus my own observations, the most likely condition leading to a tank slapper on a mechanically sound motorcycle is accelerating on a bumpy surface while in a slight turn. That's the one common thing in most of the tank slapper crashes that I've studied.

In many cases a good steering damper would have prevented the tank slapper and I'm quite certain that mine wouldn't have happened if I had had a steering damper. I had been warned too, but didn't listen to the advice.

I originally thought that the steeper the steering (my Monster was 23 degrees), the more likely a tank slapper would be, but I know of one case of one happening on a Harley. And I still don't have steering dampers on any of my bikes. I'm hoping that my modified riding behavior can prevent the next one.

I'm looking forward to participating in this discussion.
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Old 03-03-2009, 10:22 AM   #7
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I believe the tank slapper in the video you posted was caused by a malfunctioning steering dampener. I saw an interview with that rider on a Speed Channel show of some sort.
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Old 03-04-2009, 07:27 AM   #8
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those cattle grates can be slippery! a friend had a similar incident on his bike. thinking about other things, went over a cattle grate in a very slight lean, and BAM, his bars went lock to lock in the blink of an eye. he was able to ride it out because the bike stablized as soon as it had something to grip again.

i think crossing one of those with any sort of lateral force acting on the front wheel is a recipe for disaster. i usually try to wheelie them if i can, or at least get the front wheel light.

Originally Posted by scottrnelson
I crashed due to a tank slapper a little over five years ago. You can read my analysis of it that I wrote on the five year anniversary of the event here:
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Old 03-04-2009, 08:15 AM   #9
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Older BMW airheads were known to perform tank slappers if fitted with rear tyres having softer walls than their preferred Metzeler or Coni tyres, apparently the softer walls allowed a little frame flex to build up into something bigger, back in the day the only one I have experienced personally was a beauty, accelerating hard on my old CB750 she started shaking her head at about 80 mph, just when the hardest acceleration started to tail off, the osillation got faster and stronger, I stopped it by giving the rear brake a sharp prod which seemed to sort of straighten the bike out, investigation revealed a number of loose spokes in the rear wheel, so again looseness at the rear was the culprit.

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Old 03-04-2009, 05:37 PM   #10
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only time i have ever experienced a tank slapper was on my speedway bike..but then again you are (at least me being a rookie) on the verge of losing it most the time...i have had few..a few times when i have had to shut the throttle really fast for various reasons at the end of a highspeed straight. it seems to load up the tiny tiny front tire and cause it to shake

and when i have had to dive bomb into a corner (two wheeling it in not a typical speedway "skid") and accelerate out hard to beat another rider out. i belive with out the skid you are throwing all 70hp of the 160lb bike to the ground which in turn will cause the front to lift. but since i was cornering the bars where cocked with the fronT pointed WAY of the rear causing a fairly violent shake in to the straight i have allways corrected this by just gasing it more...but i should also add that this very same senario can also lead to wicked high side (trust me)
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Old 03-04-2009, 06:55 PM   #11
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A tankslapper is caused by a 'perfect storm' or convergence of bike geometry, road surface, speed, tires, etc.

A small displacement in the the bike causes a correction in the front end. This (in the perfect storm setting) causes a bigger displacement... followed by a bigger correction... followed by a bigger displacement (etc) ... all this in a fraction of a second. The classic positive feedback loop.

The only escape is to change one or more of the elements of the equation... to kill the feedback loop. Faster, slower, more weight, less weight, firm input to the handlebars. (yada)

Adding a steering damper alters the motorcycle to make the 'zone' where the positive feedback loop goes out of control harder to enter. Different bikes have different zones... hence the variety nasty (or benign) reputations.

Wikipedia has a pretty good article on the dynamics of 2-wheeled vehicles.
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Old 03-04-2009, 09:20 PM   #12
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Once, in a hurry for work leaving from a stoplight, must have accidenly lofted the front an inch or so above the ground. Wouldn't have been an issue if I hadn't be turning left, so the tire was pointed left when it touched down while the bike was going straight. A startling wobble, but nothing bad.

Other time was at a track. Leaning way over, footpegs grinding, and all of a sudden something just happened. Not sure if the brake lever touched down and levered up a wheel, or if the front just lost traction, but I got a tankslapper. Managed to get out of it just fine, got the bike going straight---just not quite the right direction, and ended up riding on the grass and landing in a ditch.
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Old 03-04-2009, 10:24 PM   #13
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good info. I have had 2 memorable tank slapper events, once on an '83 650 Nighthawk after hitting a baseball sized rock on the highway at speed, made it to the ditch before I went down. Second was wide open in 5th gear on an '89 KX 500 and hit some ruts in an open area where some douche went 4-bying when it was wet. Stopper to stopper at 80 plus mph, thought I was toast, but it healed itself with my hands off the bars ready to eat dirt. Dunno how that worked, but I was shaking like a homo at a weiner roast for 10 minutes after....
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Old 03-05-2009, 05:31 PM   #14
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Is it feasible to retrofit a steering damper on bikes that don't have one? I know every bike is different and so on, but are there aftermarket dampers?
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Old 03-05-2009, 05:39 PM   #15
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Originally Posted by Donster
Is it feasible to retrofit a steering damper on bikes that don't have one? I know every bike is different and so on, but are there aftermarket dampers?
There are two types that can easily be fit onto most Ducati models. The same should be possible for most other brands. One connects to the frame for the main body and the damper rod connects to a bracket attached to the fork leg. The other attaches the body to the top triple clamp and the damper rod connects to a bracket attached to the frame. The second type is often sold for dirt bikes and can be made to work for most, if not all, sport bikes.

Here are photos of the two types:

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