Tank-slapping?

[Dog80]

What is tank-slapping and how can I avoid it?

[Edward_The_Head]

@Dog80

What is tank-slapping and how can I avoid it?

Well if your talking about motorcycle riding it’s when the handle bars start jerking back and forth. I’m not sure how to really avoid it as I’ve only ever had it happen once in 100,000 miles on the bike. I do know that the best thing to do is generally start slowing down gradually.

The time I had it happen to me I was accelerating pretty quickly and the front wheel started coming off the ground. I was going much quicker then I should have.

If you’re talking about something other then motorcycling then I can’t help you.

[Gary_T]

@Dog80

What is tank-slapping and how can I avoid it?

If you don’t know what it is, how do you know you want to avoid it?

This has some info for you. For more, Google on “tank slapping”.

[Uncommon_Sense]

You definitely want to avoid it. (from Gary’s link)
Click on the photo for the movie clip. :eek:
Don’t know how the hell to avoid it 'cept to drive reasonable like.

[vetbridge]

The Sherman Tank was an awesome machine for its time. It’s one weakpoint had to do with percussive waves that would be set up in response to someone slapping the exterior of the turret (tank-slapping). This would result in pain, hearing loss, and possibly insanity for the tank crew.

http://www.2worldwar2.com/sherman.htm

[WhiteyFoo]

@Edward The Head

I do know that the best thing to do is generally start slowing down gradually.

As it has been said a tank slapper is when the front wheel of a motorcycle goes lock to lock very quickly. It is most common on high steering angle bikes (sportbikes) and can be almost eliminated with the use of a quality steering damper.
However, should you get into a tank slapper, the worst thing you can do is slow down. It’s counterintuative, and you often don’t have time to think about it, but if you find yourself in a tank slapper you need to get on the gas to unweight the front end, and relax your arms. The shaking will hopefully end and you can then beging slowing down and leving streaks in your pants.

[Mangetout]

Looks like one of those constructive resonant feedback things - where your instinctive reaction tends to overcorrect/amplify the problem - not unlike (although clearly rather more dangerous than) ‘kangarooing’ in a car - stepping on the gas causes the car to lurch forward, causing the driver to flop backward, relieving the pressure on the pedal, causing the car to slow abruptly, causing the driver to slump forward, increasing the pressure on the pedal etc.

[CynicalGabe]

@vetbridge

The Sherman Tank was an awesome machine for its time. It’s one weakpoint had to do with percussive waves that would be set up in response to someone slapping the exterior of the turret (tank-slapping). This would result in pain, hearing loss, and possibly insanity for the tank crew.

M4 Sherman Tank

I actually clicked the link, thinking you might be serious.

You’re not, right? :dubious:

[Fridgemagnet]

@Mangetout

Looks like one of those constructive resonant feedback things - where your instinctive reaction tends to overcorrect/amplify the problem

It is if the motorcycle rider isn’t competent.

Tank-slappers on motorcycles are purely down to the machine hitting a crucial natural resonance frequency at a particular speed, and require no input from the rider to create them. I’ve seen a particular fine demonstration of this by a motorcyclist deliberately setting up a mild tank-slapper, and riding no-handed. But it is possible for a novice rider to make them worse.

Bikes that are prone to tank-slapping are by definition inherently a little unstable, but the upside to this is that they can be turned faster (with a good rider).

I’m always a little suspicious of steering dampers on a bike. It smacks of a frig to make a bad design more stable.

[Mangetout]

Another way to look at the dynamics of it is to consider a person skiing the slalom - it’s quite possible to get into a rythm that is difficult to stop; indeed in the case of skiing, it is often the case that the person continues to swerve back and forth a few times after losing control, but before falling over completely.

[vetbridge]

@CynicalGabe

I actually clicked the link, thinking you might be serious.

You’re not, right? :dubious:

Sorry.

I shoulda had one o’ these:D

[Mr.Goob]

@Fridgemagnet

I’m always a little suspicious of steering dampers on a bike. It smacks of a frig to make a bad design more stable.

A steering damper is required on every professional road race bike.

That aside, personally I’ve had it happen once. I was on a friends TL1000R, an insane bike. It was a rippled pavement with a slight curve and I was accelerating very hard. It happened so fast that I didn’t have time to react. I hung on and after an eternity (a few seconds) it straighted out and I was on my way. To pull over a mile down the road shaking like a leaf and asking to trade back to my nice Triumph.

I don’t care for it to ever happen again. Unless you are on the track or riding a very agressive handling bike there is little to worry about. The guy who owned that bike had the same thing happen once, of all my other friends with their tens of thousands of miles of riding nobody else has had it happen.

[Edward_The_Head]

@WhiteyFoo

However, should you get into a tank slapper, the worst thing you can do is slow down. It’s counterintuative, and you often don’t have time to think about it, but if you find yourself in a tank slapper you need to get on the gas to unweight the front end, and relax your arms. The shaking will hopefully end and you can then beging slowing down and leving streaks in your pants.

I don’t know if the best thing to do is to speed up. When this happened to me I talked to some riders who have hundreds of thousands of miles under thier belts and they almost all said that you don’t know which is the best thing to do so you could either slow down or speed up. Though by slowing down I don’t mean just dropping off the throttle, but a gradual slow down. I can see not grabbing the breaks as that will screw you.

I can have my Kawasaki Concours do this all the time at around 40-45mph if I take my hands off the controls, and depending on the tire it happens to most Concours as well at that speed.

[Inigo_Montoya]

@vetbridge

Sorry.

I shoulda had one o’ these:D

Not at all! That was masterfully done! The quote is an essential part of the Mischief-Woosh. I saw it for what it was but nevertheless felt compelled to read the entire link…just in case. 8 points out of 10 for that one.

[WhiteyFoo]

@Edward The Head

I don’t know if the best thing to do is to speed up. When this happened to me I talked to some riders who have hundreds of thousands of miles under thier belts and they almost all said that you don’t know which is the best thing to do so you could either slow down or speed up. Though by slowing down I don’t mean just dropping off the throttle, but a gradual slow down. I can see not grabbing the breaks as that will screw you.

I can have my Kawasaki Concours do this all the time at around 40-45mph if I take my hands off the controls, and depending on the tire it happens to most Concours as well at that speed.

I doubt your Concours is experiencing a tank slapper. A front end wobble due to poor pavement and worn or crappy tires is way different. A tank slapper is a very violent event. It happens wether your hands are on or off the controls. It’s an experience that will turn any man onto Jesus rather quickly.

The problem with slowing down, even a little bit, is that it weights the front end of the bike. When the front end is already unstable and shaking violently, additional weight will exacerbate the problem. Accelerating will remove weight from the front end, allowing the tank slapper to end.
However the reality of the situation is that usually the event is over even before you can react. Either it will smooth itself out, or you will go down. But for those few times where the tank slapper just keeps going, it’s handy to know what to do.

[CynicalGabe]

@Inigo Montoya

Not at all! That was masterfully done! The quote is an essential part of the Mischief-Woosh. I saw it for what it was but nevertheless felt compelled to read the entire link…just in case. 8 points out of 10 for that one.

Agreed. Good work.

[Schuyler]

From an older unpublished report of mine - in this context, a “tank-slapper” is a wobble-mode oscillation:

Single-track vehicles, including motorcycles and bicycles, commonly exhibit three principal lateral-directional modes with possibly divergent behavior: 1) capsize, a stable or slightly divergent motion related to vehicle roll and balance, 2) weave, a coupled roll/yaw/steer oscillation, and 3) wobble, a high-frequency oscillation associated with front fork motion about the steering axis. In general, motorcycle or bicycle models contain all the motions associated with a steered vehicle confined to a plane (i.e. yaw, steering, and forward motion), as well as other free-body dynamics (i.e. pitch and roll). In addition, there are complicating constraints on the allowable motions arising from the rolling contact of the front and rear tires with the ground surface, possible compliance within the vehicle structure, and motions of the rider relative to the vehicle. The challenge involves capturing the relevant dynamic behavior, identifying stability regions, and understanding unstable modes, without adding unnecessary complexity to the analysis.

Past studies have elucidated the dynamic behavior of motorcycles, including the identification of weave, capsize, and wobble instabilities [1-3]. The influence of various model parameters on the speed at onset and the damping and frequency of unstable modes has also been documented, including the effects of motorcycle steering geometry [1, 3-6], tire deformation and side-slip properties [1, 3, 5, 6], and mass distribution and inertia [1, 4, 6, 7]. Particular emphasis has been given to the effects of frame flexibility on high-speed wobble stability [4, 6-10]. Frame flexibility is most commonly modeled as rotational compliance of the steering axis about a horizontal axis relative to the vehicle frame.

Relatively fewer researchers have investigated bicycle dynamics or identified stable speed ranges and parameter sensitivities [11-13]. There has been no quantitative study of bicycle frame flexibility and little accounting for tire/roadway interactions or rider dynamics. In fact, since the rider so dominates the mass and inertia of the rider/bicycle system (mass ratios of 4-10:1 are common), the bicycle cannot properly be treated as a lighter, thinner, and slower motorcycle, but instead should be modeled with an allowance for relative motion between the rider and bicycle during free travel. For these reasons, we believe the development of bicycle dynamic modeling, stability analysis, and parameter sensitivity lags behind the corresponding understanding of motorcycle dynamics, and therefore merits additional analytical and numerical study.

So this obviously a bicycle-focused article, but the following discussion of wobble speed ranges may still be of interest:

The final eigenvalues associated with typically recognizable bicycle motion are the wobble mode pair, which are highly damped over much of the forward velocity spectrum encountered in practical bicycle riding. As the velocity increases, however, the frequency of the wobble oscillations increases while their damping decreases. Finally, at a sufficiently high forward velocity, this eigenvalue pair becomes unstable; this instability occurs at approximately 100 kph (60 mph) for the nominal set of bicycle parameters. The wobble mode is primarily characterized by steering motions of the front wheel and is recognized as wheel shimmy. Its frequency increase and damping decrease are generally proportional to forward velocity. The frequency of the bicycle wobble oscillations is generally comparable to those found for motorcycles, i.e. from 6 to 9 Hz above approximately 60 kph (40 mph) [e.g. 1, 4, 6].

At all speeds the wobble mode has a frequency from two to ten times that of the weave mode. At higher speeds, wobble is characterized as a high-frequency steering oscillation and, includes a steering component whose magnitude dominates that of the roll and yaw components over much of the speed range. In contrast, the weave mode occurs at a lower frequency and contains substantial contributions from each of steer, roll, and yaw. During the lower portion of the speed region in which the weave mode is stable (forward velocities greater than 16 kph [10 mph]), the weave mode steering and roll motions are nearly 180° out of phase; as the bicycle steers in one direction, it simultaneously rolls in the other direction. Then as the speed increases, steering lags roll by about 90°. This is qualitatively different from the phasing of the wobble mode eigenvector components at the same speeds, where the steering motion leads the (roughly) in-phase roll and yaw components by approximately 90°.

As a side note, I’ve induced wobble in a racing bicycle - I had a behind-the-seat water bottle holder and I was sitting up w/ hands off the bars. It’s definitely related to mass distribution, steering geometry, forward velocity, and steering damping. Hope this helps.

[Uncommon_Sense]

@WhiteyFoo

The problem with slowing down, even a little bit, is that it weights the front end of the bike. When the front end is already unstable and shaking violently, additional weight will exacerbate the problem. Accelerating will remove weight from the front end, allowing the tank slapper to end.
However the reality of the situation is that usually the event is over even before you can react. Either it will smooth itself out, or you will go down. But for those few times where the tank slapper just keeps going, it’s handy to know what to do.

How are you supposed to accelerate at a steady enough rate with the handlebars going from lock to lock? (for anyone who doesn’t know, the throttle on a motorcycle in on the right handgrip) I would think any attempt to speed up would result in more jerking of the bike which would worsen the situation.

Check out the link in the third post and tell me if that guy would be able to throttle out of that situation, he can barly hang onto the bike…
Edward The Head, I’m seriously considering ditching my GS700 and getting a Concours. How would you rate the bike overall?

[Edward_The_Head]

@Uncommon Sense

How are you supposed to accelerate at a steady enough rate with the handlebars going from lock to lock?

I don’t know if there’s time really to do anything. The only time it happened to me I was moving at around 100 and still gaining when the bars just started jerking back and forth. I swear I was dead but I slowed it and came out ok, that was the last time I tried to go that fast!

I’m still looking on the web and it seems to be split 50/50 that when it happens to either speed up or slow down. I will have to look through my books tonight and see what they say.

Edward The Head, I’m seriously considering ditching my GS700 and getting a Concours. How would you rate the bike overall?

I love mine, I’ve got over 80,000 miles on my 2000 and it’s still going strong. I like the power, the tank range (around 250), and the protection. I can also change the tires pretty easy and do a lot of my own work. They are not the best made though as if you drop it then you might be looking at some expensive plastic. Plus the plastic isn’t all that sturdy and if you’re missing a bolt here or there then sometimes it all moves around.

Then again I’ve been very hard on the bike, I’ve taken it down some really bad “roads” that I wouldn’t even want to take a car. I’ve also taken it to 47/48 lower states and if I wasn’t expecting soon I would take it to Alaska in the near future. I have no regrets about the bike and fully expect to get over 100,000 on the bike. There’s also a large group of owners called COG that’s pretty good.

[Uncommon_Sense]

Thanks much. I’m looking for some fall deals on the bike as we speak, and it looks like I can get an early 2000 model with less than 10k for around $4,500.
Not a bad deal for that bike.