Tank Slapper

[pntrdave]

OK here goes, 78 GS 1000 I took it apart over the winter painted and put it back together. New progressive springs and new steering stem bearings. I just checked everything is tight and where it should be, no loose bolts or parts falling off. Any way I was sitting in parking lot waiting for traffic to pull out on to a 2 lane highway, 2 cars go past and I pull out behind them and nail the throttle to stay ahead of the next car coming. I hit 2nd then 3rd I've caught up with the car no traffic coming so I whip out into the left lane bang 4th full throttle knock it into 5th glance down a second later and I'm showing over a 100 mph, I'm not sure if the handle bars started wobbling side to side before or after I let off the throttle. I stayed off the throttle put my weight on my feet and just let it wobble, didn't try to man handle the bars. Tapped the rear brake gently finally around 80mph it quit wobbling. I rode another 20 miles or so home, under 70 or 75 it handles great, took it up to 90 or 95 twice on the way home and it was solid as a rock. My butt hasn't unpuckered enough to take it any faster yet.
Any ideas on what caused the wobble, I assume that is what they call a tank slapper a lot of side to side motion on the bars. Could the front forks be uneven, or wheel out of balance, not enough air in the forks or to much air, or uneven amounts of air, I'm at a loss here guys, all I know is I don't ever want it to happen again. It never happened before I put the new springs in.

[KGB]

"Could the front forks be uneven, or wheel out of balance, not enough air in the forks or to much air, or uneven amounts of air, I'm at a loss here guys, all I know is I don't ever want it to happen again"

The short answer..yes to at least one of the points above.

And it's not unusual for GS to do this, not all of them do.
My cousins 850 would do this above 80mph.
My 1000S does it over 100, then settles down about 110.

[Jethro]

My Metzeler wore really poorly, was in balance when I mounted it, but something happened and it wore all chunked up, like an unbalanced tire does. Feel all over the tire, maybe it's got a fault.

But yes, one, all, none or any combination of those could be causing the bad wobble. Steering tube bearings, wheel bearings, dragging brakes (poorly lubricated caliper pins?), bent forks and or frame, poor alignment tires are other issues that come to mind.

[bwringer]

Congratulations on surviving, by the way. Tankslappers are no fun atall.

Check everything everyone else said, plus:

Is the level of fork oil set correctly and is it the same in both fork tubes? The allowable variation is just 1mm. Do not just dump in a certain amount of fork oil -- the level is important, not the amount. Set the preload the correct way, with preload spacers.Air pressure is too unreliable.

You also need to check the rear suspension and tire throughly and also check the frame in general for damage and alignment problems.

If I had to bet, the most likely explanation is a tire or tube problem. Have the tires been replaced?

[jimcor]

No fun! The Concours has a problem with the golly wobbles on deceleration if the steering stem bearings are not set just so. Yeah, I found out the hard way, thankfully at only at about 50mph. Might recheck the bearing preload once again.