Very slight wobble when shut off the throttle at 100mph or more?

[Guest]

1981 gs750, 1100 engine, 1166 kit. Bike lowered in rear 1in shorter
shocks. Stock front end, fresh oil, newish neck bearings. I even added
A frame to fork damper last summer( helped a lot but still happens, just not
As bad). It's not really a wobble, more of a 1 time drift.
Would a fork brace be the next step? Or is it possible to tune it out with rear
Wheel alignment? I've lined up with the string method, hands off bars, bike tracts
Straight. Now their is quiet a bit of dive when u shut the throttle off at 100mph.
All ideas welcome.

[gsrick]

1981 gs750, 1100 engine, 1166 kit. Bike lowered in rear 1in shorter
shocks. Stock front end, fresh oil, newish neck bearings. I even added
A frame to fork damper last summer( helped a lot but still happens, just not
As bad). It's not really a wobble, more of a 1 time drift.
Would a fork brace be the next step? Or is it possible to tune it out with rear
Wheel alignment? I've lined up with the string method, hands off bars, bike tracts
Straight. Now their is quiet a bit of dive when u shut the throttle off at 100mph.
All ideas welcome.

You might want to talk to Rich at Sonicsprings. He's a member here and can hook you up with the new fork springs.

[steve murdoch]

Certainly wheel alignment could be a cause.
Swingarm bearings good?
How new are the tires? Tire pressure?
On my '82 GS1100E one fork leg would lose air pressure and it would affect the handling.
Just a couple other things to check.

[81gs7501166]

Certainly wheel alignment could be a cause.
Swingarm bearings good?
How new are the tires? Tire pressure?
On my '82 GS1100E one fork leg would lose air pressure and it would affect the handling.
Just a couple other things to check.

swinger bearings, newish, I replaced when I built the bike, also installed the alum arm,off a different bike, 2300 miles on clock.
New tires last summer. Tire psi 30. Fork air 5psi, and they leak slowly. I've tried air up and down, it changes the dive rate but still have the drift when it falls. I have the factory under fender fork brace under my custom fender. I really like the soft ride with the stock springs. I weigh 180lbs. But stiffer springs might be needed. I messed with the rear wheel alignment again yesterday, haven't rode yet, thunder storms all week.
If it always drifts left, then tightening the sprocket side would be the correct move???
Is this correct? 20171122_190734.jpg

[81gs7501166]

Had a break in the rain, got 15 miles in. It's some better. Going to realign with a different method I read bout on the net. You use 2 8ft flor light tubes, straped tight to rear tire, instead of string. Much less likely to screw up? I have it rigged in shop now. It's off bout 1/8-3/16 in. I'll have it right by days end. Got some garden work to do while sun out, may be tomorrow before test ride!
question bout damper? I had it to the max stiff to try to eliminate drift. I noticed it's a tad nervous to maneuver when going slow and trying to turn, is this normal? This is my first experience with a damper?
Thanks

[81gs7501166]

More pics.

[bonanzadave]

Everything you've done Ive done twice. I have a weave at 100mph that is very un-nerving. DO NOT want that to go to a full on tank slapper. Like I said.....tried everything. The only thing that works for me....don't go 100mph !

[81gs7501166]

I'm not sure I understand, don't go 100 mph, just exactly what does that mean? I don't hear you? Lol. I'm not giving up yet! The dive I can stand, it's the drift that's new e racking?

[Rob S.]

High speed wobble? I owned the original Z1, the father of the high speed wobble. It may have been what killed the PO, it was definitely what caused me to drop it at high speed.

I installed a steering dampener, had about five settings. I used all the way on on highways, all off for streets.

One thing I found out one day at about 110 mph: hitting the brake made it worse. So I kept the throttle on and gently rested my foot on the brake.

On my KAW, people blamed the skinny-tube swing arm, others welded braces near the fork head. I don't know anyone ever completely solved it.

But yours is a different bike; keep trying and good luck. Mine was more than a "very slight" wobble, but at the buck, any wobble is scary. One thing we both might have learned: when that wobble rears it's head, sudden or hard deceleration can turn a scare into a catastrophe.

Keep us informed.

[Steve]

You have installede shorter shocks. That has changed the steering geometry. That will change the way the bike handles. Usually, dropping the rear will increase rake and trail, making it more stable, so there is likely something else wrong.

You say the swingarm bearings are "newish". That's nice, but are they ADJUSTED? Loose bearings can easily induce some handling quirks.

You say this problem only happens at REALLY high speed. Rustybronco gave the ultimate answer for that, as there is no place on public highways where it is legal or safe to go that fast. However, since it happens when you close the throttle, I am wondering if it happens when you close the throttle at lower speeds. Give it some modest acceleration at about 50-60 mph, then cut the throttle. Does it still go left? Changing the tension on the drive chain with loose swingarm bearing will pull the swingarm to one side.

You also say that you run 5 psi in the forks, but they leak. That means that they have 5 psi when you fill them, but after that, you have no idea how much they have. And there is a chance that they are not the same. I don't know if your bike has a balance tube and a single fill valve. It would be far better to replace the fork springs with some proper-rate springs and not have to worry about adding any air. Sonic Springs are a forum favorite. Their calculator says a 0.95 rate would be good for you. If you carry a passenger very often, you might consider a 1.0 spring. Top that off with 10w oil, you will have a GREAT ride.

All of that dive when you shut down at 100 mph is likely caused by aerodynamic resitance on your body, which will lift the front of the bike, all of the engine power that is required to get you to that speed, then removing the power and having your too-soft (and now fully-extended) forks compressing. When the forks compress that much, the steering geometry changes, which will affect handling. Getting your forks in shape with new springs and oil will help a lot.

Finally, don't overlook the possibility of loose steering bearings. Not sure how likely they would be, but let's eliminate as many variables as possible.

.

[81gs7501166]

Neck bearings snug. I've already tried to duplicate the drift at lower speeds, anything below 80, theirs none. Around 80 it's just noticeable, then the faster , the more drift.
never thought bout a loose swingarm bolt, I'll check.
So with the sonic springs, you don't give up much or any of the soft ride? But since their progressive you loose some of the dive?
Any chance spring spacers would help?
Bike is now aligned with the 8ft tubes. Can't test yet, storms all day.
I may make some spacers and give them a try? It is extremely plush on the front, almost dirt bike like.
another mod I forgot to mention, I do have the small chain conversion, don't see how this could effect anything though. Also it's a 82-83 1100 engine on a 81 750E frame. All engine to frame bolts lined up, but I had to modify a couple of the mount brackets. Chain runs straight!

[Steve]

So with the sonic springs, you don't give up much or any of the soft ride? But since their progressive you loose some of the dive?
Any chance spring spacers would help?

Actually, Sonic Springs are stiffer than stock.

And they are not progressively-wound, they are straight-rate springs. In spite of that (or is it because of that?), they ride better than stock springs. Brake dive happens because the springs are not strong enough to prevent it. Having proper-rate springs does a lot for consistent handling and performance.

It also helps to couple proper springs with proper fork oil. Older "wisdom" stated that you increase oil viscosity to prevent brake dive. Looking at the physics of that, the weaker springs still allow the dive, the thicker oil only slows it down. It still happens, but slower. With proper springs, you can use thinner oil, which will allow the wheel to move to follow the road, which is what gives you your "soft ride" and comfort. The spring calculation for my bike said to use a 0.95 or 1.0 spring, but I have a full touring package, including fairing, saddlebags and trunk. My gravitational attraction is about 16 stone, so I installed the heaviest spring available, the 1.1. Dropped in the proper amount of 10w oil and it rides like a Cadillac.

Spring spacers are almost always going to be used. You have some in your bike right now. If you want to experiment a bit, pull your current springs (and spacers) out. Some bikes have progressively-wound springs, others simply have two separate springs with different rates. If you have separate springs, remove the smaller, lighter spring. Drop the longer, heavier-rate spring in the fork tube, then cut a length of PVC pipe to fit from the top of the spring to the top of the (extended) fork tube. When the top of the tube is installed, it will compress the spring just a bit to provide a bit of preload, which should be pretty close to what is needed. If you have one-piece, progressive springs, cut off the tightly-wound section and repeat the procedure above, but be sure to grind the cut end of the spring so it is flat against the washer.

.

[81gs7501166]

Still can't test cause of rain, but I made a couple of 1/2 in spacers, mine had none. I got them installed and waiting on break in weather for ride. Seems like this bike has different than stock springs, according to po. It's benn 15 years ago when I bought the bike. All I did was change oil.

[81gs7501166]

Ok, test ride in, bout 20 mile, OMG, as the younger gen would say, it's the best it's ever benn. Didnt get to 100, bug at 85 theirs no drift , hands off bars, just a slight left pull. Going through flor tube line up 1 more time! Either the 1/2in spacers,(doubt it),or the alignment is working(betting on this). 1 more tweak, and maybe I'm done? Most neutral handling I've experienced with this bike ever!

[81gs7501166]

Ok, I think she's fixed! Bike had always turned left easy, kinda like power steering. Now it turns equally well, left or right. Sometimes the road crown, screws things up, but I'm now tweaking the chain adjusters 1/8 turn at a time.
FYI, I'd recommend the flor light tube method to all. It's dead nuts straight, compared to the string method. As the rear wheel moves the tubes have to point straight ahead.
I did add a couple of steps to the read I found on web. I lay the tubes on 2 x 4 studs, this gets them up off the bulge in the bottom of tire, I also cut a scrap of wood the same thickness as the tire. This is used as a spacer between tubes at front of front tire. Small bungee straps hold it together at 3 points, 1 behind rear tire, 1 just in front of rear tire, and 1 in front of front tire around tubes and tire size spacer. I lay a wooden dowel under spacer to act as bearing so theirs no drag on left to right movement as rear wheel moves.
Get bike as level as possible, I have no center stand, so i shim kick stand with wood and place a floor Jack under footpeg on opposite side.
And PS- I did tighten my swingarm bolt bout 1/2 a turn.