wobling at high speed

[bonanzadave]

When the handlebars on my "80" GS 1100 "E" started slaping both sides of the tank, at appx. 115mph. Was that a (low speed) wobble or a (high speed) weave. Never figured out which it was, cause there wasn't enough left of the bike to try to find what caused the problem. Believe me, it was a very very serious condition.

Ahhh.......Not to be a smart ass or any thing but are you posting this from heaven ?

[Boondocks]

When the handlebars on my "80" GS 1100 "E" started slaping both sides of the tank, at appx. 115mph. Was that a (low speed) wobble or a (high speed) weave. Never figured out which it was, cause there wasn't enough left of the bike to try to find what caused the problem. Believe me, it was a very very serious condition.

That's the classic "tank slapper". It's the most dangerous and worst instability of all, but it's usually treated as a special case when motorcycle instability is discussed. It's like a low speed wobble in its behavior, except it can be uncontrollably violent at higher speeds. It isn't common on bikes like yours, which are more prone to a high speed weave (rear wheel steering). It's more likely to found on a high powered motorcycle like a racing bike which doesn't have adequate steering damping. Sometimes on bikes prone to this behavior, a certain type of road surface can induce an uncontrollable high amplitude steering wobble. There are no tell-tale signs that a road surface might cause this, it is an individual interaction between the motorcycle, its tires and the road.

One of the most common occurences of a tank slapper would be a race bike hitting a sizeable bump or lofting the front tire when accelerating out of a turn. If the front tire doesn't land perfectly true to the direction of travel, the self-centering action of the front end will whip it to the other side. The bump or off center landing can create the tank slapper oscillation. The very steep steering geometry of race bikes contribute to this tendency. That's why they typically have steering dampers, and some have reduced the steering lock to a small arc to prevent head shake from gaining momentum. It would be hard to turn one of these bikes around due to the lack of front wheel turning arc, but it is not missed in high speed racing and contributes to stability.

See the section on Headshaking, tankslapping and steering dampers for a good explanation.

[rphillips]

Well Boondocks, again you are right. The bike I wrecked had been having a slight wooble at high speed. but nothing serious, it did get your attention, but nothing I couldn't handle. The day of the wreck, I was drag racing a GS 1150, & was in front of him. The front end was light, as I was accelerating, & I did hit a rough place in the pavement.Since then, I've had a few other GS's that had that same wobble (weave) that I never got straightened out, but now it no longer matters, I'm not planning on going that fast anymore.

[Boondocks]

Well Boondocks, again you are right. The bike I wrecked had been having a slight wooble at high speed. but nothing serious, it did get your attention, but nothing I couldn't handle. The day of the wreck, I was drag racing a GS 1150, & was in front of him. The front end was light, as I was accelerating, & I did hit a rough place in the pavement.Since then, I've had a few other GS's that had that same wobble (weave) that I never got straightened out, but now it no longer matters, I'm not planning on going that fast anymore.

I think that a lot of high speed weave problems are not solved because a rider may not realize that it is a rear end oscillation and focuses his attention on the front end. It's usually sort of counterintuitive to think of the rear wheel having steering input without tire slippage. A rear tire with excessive lateral runout (had one of those) can cause the weave, as can loose swingarm bearings, wrong/worn tire, and improperly adjusted shocks among other things.

My next set of tires is H rated, not V, so I don't plan on going over 130 mph.:-D