Discussion of 'Tank Slappers' Part 2
Subject: Wobbles, Weaves, Tank Slappers
Date: Wed, 31 Jul 2002
The past several days on this topic has resulted in a host of opinions ranging from almost agreement to diametric opposition. It needs some clarification. There is a lot of flesh at stake... The causes and cures have been published in AIRMAIL tech pages more than once and many years ago in BMW News in the late 1970's in feature articles on the subject. But for you neophytes and latecomers here it is in a nutshell......
- First and foremost, never let the problem manifest into a worsening condition. The progress from normal handling to dangerous deterioration is usually slow. Don't ignore the warning signs and get it fixed before it fixes you so nobody can fix you. You will usually get some early warning signs before the big event of the tank slapper happens.
- The tank slapper is of course the worse, almost always preceeded in miles and time with a lesser degree wobble mishandling. Weaving is an entirely separate phenomenon.( discussion forthcoming)....
- The most dangerous combination for the wobbling and tank slapper is a handlebar mounted wind screen arrangement, a solo lightweight rider, a significant load at the rear of the machine rear of the rear axle, diagonal headwinds, and of course, steering head bearings too loose or worn and notched. The more of these ingredients in the act, the greater the chance of disaster-and the event may happen without warning.
The primary instigation of wobble is a physical resonance set up in the frame and steering geometry that once starts, feeds the accumulated resonant energy back into itself to accentuate the problem. This is what makes it so difficult to squelch once commenced. A rigid frame and steering coupling (tight steering head preload) will avoid the resonance,by absorbing the energy needed to create and manifest the problem, but a small amount of liberty in movement of the steering is needed for continuous self correction of tracking versus road aberrations as the machine moves along. The proper preloading of the bearings is a compromise-to allow enough movement for corrective needs and not so much as to allow resonance to initiate. Kind of a tightrope act.
If you incur the tank slapper, the name of the game is to make every move you can to inhibit the resonant energy feeding the monster. Bodily moving forward (shifting your weight) as much as you can may help. Do NOT speed up. Avoid the front brakes while the wobble is in effect. Suppressing the resonance is the name of the game. If you are strong enough and can get hold of the bars without having them fling you and you can hold them still and squelch the resonant energy that will likely help. If you get the machine under control and can come to a stop, go clean out your pants.!!
Randy G. posted a memo quoting his motorcycle mechanic saying the way to negotiate a wobble is to speed up lighten the weight on the front wheel. WRONG. That is exactly what you do NOT want to do. It has been reported and makes valid sense that machines that wobble solo won't usually do it with 2 up. That is because the extra weight and force downward on the front wheel has a slight effect of more preload on the steeringhead bearings and also increasing the mass that must be slung around during the tankslapping event. The resonance doesn't like that and sees it as a damping mechanism thus may refuse to support and may suppress an existing oscillatory motion.
Loading heavily rear of the rear axle is dangerous in that is lightens the load on the front wheel and steering preload thus enhancing the conditions for resonance and oscillatory motion of the front end. The handlebar wind screen is taboo in that too will enhance the tendency for front end resonance especially with gusting winds hitting the screen at an angle. There are ample reports in my files to corroborate that from years gone by where some of the riders crashed.
With all that being said, you know now what NOT to do. Prepare your machine for maximum protection via the laws of physics. Then make sure your steering bearings are not notched and properly adusted. Actually, if installed and adjusted properly initially, they NEVER