Tank slappers

Accelerating hard out of a corner this afternoon, noticed a definate "oscillation" on the bars. Never had this before.

Now it was not a tankslapper by any stretch of the imagination but as I'm a bit of a geek can someone explain the physics of whats going here ?

Your front wheel hopped off the ground, then was not pointing in the same way the bike was going, so when it touched the floor again it kicked back into line, and overcorrected, then back the other way and so on.

It's quite common - you're accelerating hard, the bike is lifting the front, not enough to wheelie, but enough to make it go light, and induce some 'chatter' into the front wheel - in the same manner that an unbalanced supermarket trolley will wobble a wheel in similar circumstances

feels good doesn't it

Twist of the wrist 2 explained all the physics of biking to me A worthy read

The recomended action for headshakes is to get on the gas, not off it until the bike straightens up or you get past the bumps. If you get off the gas it loads up the front wheel and makes it worse My ZX6 is a bit 'flighty' and have had headshakes at a speed that I couldn't get out of my SV so can tell you from experience

[northwind]

He says that like it's a good thing

Yer like good for ****ting you pants

Last year, when my forks were too soft compared to the rear I could get a headshake on one particular bend with a ripple running across just at the exit where I put the power on. It's most likely to be something similar for you.

Imagine the front wheel leaned over on a left bend. It hits a bump and is forced up, making the bars turn to the right. As you crest the bump the wheel "falls" over to the left again and the bars go to the left. As it levels out they return to the natural angle for the bend but often you'll get smaller and smaller repeat oscillations until it dies out.



If this is not what's happening, can someone explain what caused it?

ive had a few tank slappers but there not what your describing

[Sid Squid]

Originally Posted by I previously - on a related subject but the principal is the same -

As the tyre wears flat in the middle the contact patch gets wider, depending on the specifics of the tyre's interaction with the road; camber, load, carcass flex etc, the exact point of drive, (either forward or reverse: braking/engine braking), may not be exactly centred along the axis of the bike, as the steering tries to self centre - a function of the castor and trail - the contact patch centre moves around, also the fact that the steering apparatus, (forks, wheels, 'bars etc), has mass and therefore, if moving at sufficient speed, momentum, causes the steering to pass through the centre position, and the process repeats itself in the other direction, thus the oscillation of the front end.
In this circumstance, that is; off the throttle or just light a throttle, the front tyre will often damp out the oscillation by scrubbing on the road, sometimes just the weight of your hands on the bars will be enough to change the resonant frequency and still it.
Generally it will only turn into a 'tankslapper' if there is relatively little contact of the front tyre, for instance when hard on the throttle, or when the front end is lightened over a bump, but the reasons are similar; the front wheel is trying to line up with the rear.

Generally this occurs when the front tyre only has a light load on it, accelerating hard for instance, in the circumstance that the bike is off the vertical the effect of off centre forces on the tyre exacerbate the problem.