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Discussion of 'Tank Slappers' Part 1

From the Airheads Mailing List, July 2002

Compiled by This email address is being protected from spambots. You need JavaScript enabled to view it. #6223

Note: individual messages are separated with "==="

From: Bill Harris
To: BMW Airhead Boxer mailing list
Subject: Surviving a Tank-slapper
Date: Tue, 30 Jul 2002

As you know, if the front-end bearings are not adjusted properly, the front wheel can get into a mode of uncontrolled-- and uncontrollable-- oscillations, commonly known as a "tank slapper" in a severe case.

I've only had this happen once, and I held on for dear life and braked as best I could. Still have pucker marks on the seat from that incident...

However, I'm not sure what is the proper or correct way to deal with steering oscillations at highway speeds, and I surely don't know of a way to practice this. What approach would you advise taking in this situation?

--Bill Harris
ABC#6223
'73 R60/5 Monza Blue Toaster: "Edril"
Jasper, Alabama

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From: R. Fleischer
Subject: Surviving a Tank-slapper
Date: Tue, 30 Jul 2002

While this is NOT 100% positive for finding things, this is what I do: 1. I tighten the steering head, during my cleaning and regreasing maintenance, to JUST the faintest touch of weaving at around 20-30 mph. I have the wheel bearings set the way I have posted. 2. I go for a ride with decent tires, not scallop worn, etc. At various speeds, in 5 or 10 mph increases, I take my hands off the bars...with NO steering damper engaged...and give a little hit to the end of the bar...first one side, then the other. Should be nothing at any speed other than perhaps a very small...really small...wiggle, that self-corrects. If ANY tendency to start a real oscillation, ESPECIALLY a diverging one (increases size per each oscillation), it is time for a very serious look at things. Curing an on-highway high speed oscillation, that can very QUICKLY lead into a tank slapper is usually done by throttle and brake use, and there are all sorts of 'rules' and they do NOT all work! Sometimes one works, and the opposite does not! GENERALLY I have found, during testing of two bikes over the years that did this sort of thing on demand, was that UPhill I cured by rear braking and down throttle, downhill by light front braking and throttle up. I wish that this would always work. riiiight

da Bum

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From: Carol Baker
Subject: Re: Surviving a Tank-slapper
Date: Tue, 30 Jul 2002

Bill,

I have not had this happen to me directly yet, but I did get to see my husband Charlie practice this a few times on our last ride to Trenton. It was most disconcerting to being behind him when all of a sudden his whole bike would develop a wobble that resembled a dog's body in a tail wagging frenzy. This seemed only to be occurring when we were at speeds over 85 mph. Which only added some extra spice to the thrill factor.

I can tell you that I never saw Charlie use his brake, but instead I suspect he ever so gently backed off on his throttle and did his best to hold the bike steady till it regained it's composure and smoothed out. He did an excellent job at regaining control of his bike.

We are not really sure WHY this was happening, but we do suspect that his RT fairing and being fully loaded with camping gear were both factors involved. Maybe the weight distribution was off just enough to kick it into a wobble at speed. Dunno?

I sincerely hope that none of you ever have to experience a tank slapper or be behind someone else who is. I personally didn't enjoy it.

Best,

Carol Baker #5442

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From: R. Fleischer
Subject: wobbles, weaves, tank slappers
Date: Tue, 30 Jul 2002

I would like to emphasize that a high speed wobble or weave is NOT a 'tank slapper'.

At low speeds, overly tight steering head bearings will cause the bike to feel heavy handling, require a lot more muscling around, and does not steer well at all by leaning. At high speeds an overly tight steering head bearing adjustment results in about the same feeling...leaning needs to be rather aggressive to work at all. This type of problem is called WEAVING. The steering head is USUALLY adjusted just about right with the faintest amount of weave at low speeds...perhaps 20-40 mph test range. This usually leaves the bearings in the right area, to avoid the more dangerous high speed problems of a too loose bearing.

There are TWO types of effects one might call wobble. The low speed one often occurs at quite low speeds, you bump a smallish rock or glance off the edge of a pothole, or some such, and the front end does a quick wobble, often a goodly fork movement. That one is generally NOT an indication of anything being wrong. In the dirt, if equipped, one can turn on some steering damper (those are NOT for hard street paving use...if you need it there, something is WRONG).

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