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Harmonics At Work

By: James R. Davis

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!

Tank Slapper Movie

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 aggressively using your 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?

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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

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