Tankslappers explained

[jubilant]

riff raff , cheers for this post and to the admins for it being a sticky, it came in use today as i had a moment on the way home from work, just been cut up by a boy racer in his mums car so to avoid him i pulled into the other lane and hit a man hole cover...tank slapper time ( on my hyo 250 gt ) made me crap for a sec but i relaxed my grip and leaning forward sorted me out....shit my knickers though .....im just sitting down with a nice cup of tea to calm my nerves !!

thanks for this , if i hadnt have read it a few nights ago i wouldnt have had a clue and most likely have been on my way to northshore hospital to die of swine flu or mrsa or just through old age in the waiting room....

[batboy]

Right wondering if anyone has ideas on this...... weighted barends?

Now i have a set of these on the monster and one thing they do well is remove some vibration from the bars, but on the topic of tankslappers, do you think they will accentuate any slap that may occur.
My thought is that adding weight furthest away from the steering head would increase the oscillations in the event of said slap? does this make sense?

/discuss

[motorbyclist]

Right wondering if anyone has ideas on this...... weighted barends?

Now i have a set of these on the monster and one thing they do well is remove some vibration from the bars, but on the topic of tankslappers, do you think they will accentuate any slap that may occur.
My thought is that adding weight furthest away from the steering head would increase the oscillations in the event of said slap? does this make sense?

ok, i'm not in the mood to run the numbers, but it would make it slap slower, and possibly get into a slapper easier.

might be more complex than this, but generally, the bigger the mass the slower the resonant frequency

however, it would make the bars less prone to deflection, but only slightly.

[Geemsee]

Right wondering if anyone has ideas on this...... weighted barends?

Now i have a set of these on the monster and one thing they do well is remove some vibration from the bars, but on the topic of tankslappers, do you think they will accentuate any slap that may occur.
My thought is that adding weight furthest away from the steering head would increase the oscillations in the event of said slap? does this make sense?

/discuss

A bit late in the day, but i thought bar end weights were for damping handlebar vibrations not really for tankslappers?! Best fix is a steering damper kit but that might just be disguising the root of the problem ie I had a Z650 way back when it had Dunlop Endurance tyres on it when I bought it and boy did it used to tankslap, fixed it by putting some decent tyres on it, no damper required!!

[Corse1]

Well luckily managed to survive a massive slapper around the East Coast between Te kaha and Waihau Bay on Wednesday. Leaned over powering out of corner and hit bump. Away she went with both tires chirping almost at the same time side to side about 8 times.....sounds like I did the right thing as I did not bin it only reading this sticky tonight.
I thought I was a gonner though for sure. Radical Speed triple geometry combined with harsh standard rear shock. I have some investing to do as I sure do not want that to happen again!!

[pritch]

I have never felt the need for a steering damper. Until just lately - the thought having recently occurred on a number of occasions. I note that Ohlins list one for the model, and that at least one road test recomends it. Now if only I could figure out how to pay for it...

[rok-the-boat]

I agree with the premise of the original posting, but, it is not only the surface of the road - it can also be the design of the bike. My mate's Suzuki GT750 would tankslap between 80-90mph. It would start with a steady weave and then turn violent. You could ride thru it and come out the other side 90mph+ or, slow down - the usual instinct. If you accelerated fast you would go thru 80-90 so fast you wouldn't notice it, but you knew you have to go back down thru it - very disconcerting, as I remember. It did this on any road, any surface etc, bumpy or smooth.

[FlyingKiwi]

Well, this all makes for some scary reading! Fingers crossed for me I guess

[Metastable]

With a true sudden lock to lock tank slapper there is NOTHING you can do. Like some guys have said, you're on for the ride. I was going over a set of double railway tracks and BAM!... the bars went lock to lock 3 times and the first one ripped my hands straight off the clip ons, which is probably a good thing... she went into a strong weave and then settled down. It happened so fast that I really couldn't do squat.

Suspension set up is HUGE! It can make things a lot less likely to happen and of course a steering damper is a good idea too..... although maybe I'm just stupid, because don't have one on my trackbike yet.

[bkker]

man, sounds bad. glad I haven't had to experience this yet, not looking forward to it. I've been on a few really bumpy corners, but luckily no slapping :P

[Chrislost]

riff raff , cheers for this post and to the admins for it being a sticky, it came in use today as i had a moment on the way home from work, just been cut up by a boy racer in his mums car so to avoid him i pulled into the other lane and hit a man hole cover...tank slapper time ( on my hyo 250 gt ) made me crap for a sec but i relaxed my grip and leaning forward sorted me out....shit my knickers though .....im just sitting down with a nice cup of tea to calm my nerves !!

thanks for this , if i hadnt have read it a few nights ago i wouldnt have had a clue and most likely have been on my way to northshore hospital to die of swine flu or mrsa or just through old age in the waiting room....

One twitch upon hitting a man hole in the wet, doesn't even smell like a tank slapper.
may make ya heart beat a tad faster if your not expecting it tho...

[thepom]

I had a real bad one on the gravel and woodsman was following me,I was doing 100 k,s and it spat me off after a hundred meter slapper,bike landed on my ankle after I face planted myself..........woodsman said it was hilarious...

[Woodman]

I had a real bad one on the gravel and woodsman was following me,I was doing 100 k,s and it spat me off after a hundred meter slapper,bike landed on my ankle after I face planted myself..........woodsman said it was hilarious...

Hilarious and scary at the same time.

[R-Soul]

Not sure if you guys have seen it but go it from this site:

http://www.msgroup.org/tip.aspx?num=190


A tank-slapper (wild and rapid swings of the handlebars from hard-stop right to hard-stop left and back again) happens when harmonic feedback is generated following your front tire hitting an imperfection on the road surface such as the slight bump at the end of your driveway.

It usually happens at speeds of around 20 MPH - sometimes a little faster, sometimes a little slower - but it CAN happen at any speed!

I have heard otherwise well informed riders give advice to newbies about how to handle a tank-slapper. Usually it goes something like this: 'Just hit the throttle and lift the front-end off the ground. That will stop the oscillation."

Please! That advise is virtually assured of getting you into serious trouble - more than you were in when the tank-slapper began.

Harmonic feedback means that a small force is being applied EXACTLY in-phase with the end of a counter-force (shock absorber) which results in another 'bounce' of the system AND AN AMPLIFICATION of the previous bounce.

The second time it happens the amplification is huge. The third time and you eat asphalt - maybe the fourth time...

It is exactly the same principal that keeps your old grandfather's clock pendulum swinging - it gets a tiny 'kick' at exactly the right time (in-phase) at the end of each swing.)

Your bike probably has one of two kinds of devices in it designed to prevent these from happening:


A small shock absorber attached to the steering stem

A pressure bearing within the triple-tree
These are called steering head dampers or 'stabilizers'. Their sole legitimate purpose is to disrupt harmonic feedback and, thus, stop tank-slappers before they start. Some vendors (and motorcycle shops) will tell you to get one in order to stop your front-end from 'shaking'. NUTS! If your front-end shakes at any speed there is something wrong with the suspension.

If your steering head damper is weak or maladjusted then it cannot dampen (and, thus, kill harmonic feedback) shocks to the front-end.

As to suggestions about how to handle one if it happens ... it is NEVER appropriate or safe to do a wheelie. Your response should be to hold firmly to both grips and absorb the 'harmonic feedback' into your arms - i.e., dampen them - and grabbing a handful of the front brake. Then go get it fixed!

Your NUMBER ONE RESPONSIBILITY on a bike is to maintain control of it.

For those giving advice ... if your bike is already out of control do you really think it makes sense to increase speed? To do a wheelie? Does it not occur to you that you were traveling at some rate of speed and it was just a minor imperfection in the road surface that started this and that when you come back down from a wheelie, going faster than you were, that front-end is going to get a severe jolt? What do you think the odds are of going into another tank-slapper then?

You must disrupt the harmonic. You do that by stiffening your arms and changing the front-end geometry, if you can. Braking, by the way, both slows you down so that if you do hit the ground it won't hurt as much, and it compresses the front shocks which changes the front-end geometry.

Sound like a better alternative to you than hitting the after-burner?

I agree with Ixion - this sounds like the exact opposite of what you should do. I agree with the definition of what a tankslapper is, but not their proposed rsponse.

First they explain that it is teh small "kick" that gets given to the pendulum in a granfather clock that keeps it swinging in harmonic fashion, and then they say that you should stiffen your arms? Stiffening your arms will GIVE the handlebars that additional kick that counters the DAMPING action of the forks.

I had an experience with a car that is quite a good analogy to this:
A rabbit jumped out onto the highway while I was glancing down. As I looked up, I saw it and jerked the steering wheel by reflex. the car swerved to the side, and I corrected, which made it swerve to the other side more violently, which I corrected again, to make it swerve ebven more violently back again, etc etc until the back stepped out and the car did a 360 on the motorway (luckily at night, so it was empty). I was told by an advanced driver afterwards that the car suspension is designed to self correct to a straight line, and it was my arm inputs on the steering wheel that were causing the over correction. Taking that analogy to the bike - similarly by trying to hold the bars firmly, you are not letting the forks 'self damp'. Holding the bars loosely may be a better option.

From an engineers perspective, you can interfere with the harmonics caused by a regular periodic force (the force of the road on tyre) being applied at a resonant frequency to cause resonance by:
1) removing or reducing the forces being applied (by lightening the front wheel to reduce the force being applied by the road to the tyre and letting natural fork damping do the rest- like Shaun said)
2) damping the actual movement (like with a steering damper)

I cant think of any more although the comment earlier about a bike going sweetly into a turn was interesting. Perhaps applying force to only one side of the bars will interfere destructively with the periodic forces of the road on the tyre? In this case, then perhaps swerving from one side of the road to the other might help? Can somebody try this and let us know if this works?



I can't think why back brake would help (except to slow the bike for it you do get thrown)? Perhaps by braking you are changing the frequency that the force of the road is acting on the tyre. If this is true, then I guess this would apply to both brakes then, except that with using the front brakes, the moving bars can cause damage to your arms. Also, if Shaun was accelerating while braking with the back brakes, then this would negate that effect. Any ideas?



In order to break a harmonic, you dont add force into teh equation at the

[SMOKEU]

I've had a few scary experiences when the Gixxer gets into a mild tankslapper, the worst was accelerating hard down a straight piece of road in 2nd gear and the front end started wobbling quite bad. Wasn't expecting that, especially considering the road was pretty smooth.