Instability at high speeds

[wilsonsandy07]

Having some pretty squirrely/light shakiness at about 90 and above mph. Had one instance of tank slap that’s caused a deep dive into what’s going on. This has all been done after the tank slap happened. Single rider, 150lbs ish, 5’5

- brand new battlax bt46s / balance beads
- forks rebuilt/serviced
- front wheel bearings replaced
- air assist serviced and verified not leaking, set to 3 top and bottom
- rear wheel w/balance beads, centered on axle
-chain slack verified 20-30 mm
- tire pressure front 30 rear 35
- aftermarket rear shocks, 1 inch lower than stock no damping adjustement only preload set to 3 as well

plan on checking
- steering stem bearings/torque
- replacing rear wheel bearings/swingarm bearings


is there anything else I should take a look at? Maybe missing something?

 

[Rob S.]

Maybe you should have checked the swing arm bushings/bearings first. And not lowered the ride height. And I'd say you're about 10 lbs light in both tires, despite your light weight.
What model & year are we talking about?

 

[wilsonsandy07]

Crap I always forget to post year model 82 1100e

I only lowered the rear to accomodate for my height a bit, after having the bike for a bit I’m actually looking to put stock height suspension back on I just don’t want to put the stock ones I have as they are ancient.

Is it common to run 40+ psi?

 

[Rich82GS750TZ]

If you create a signature with your bike in it, you’ll never have to remember to include that info in your posts.

 

[Grimly]

Is it common to run 40+ psi?

No. Far more common is 32F and 36R.

 

[wilsonsandy07]

If you create a signature with your bike in it, you’ll never have to remember to include that info in your posts.

Very true need to do it lol

 

[wilsonsandy07]

No. Far more common is 32F and 36R.

I thought so, seen a lot of 32/36 but lots of different theories and preferences. Either way I don’t think I’m far off enough to have my instability caused by pressure

 

[bonanzadave]

Crap I always forget to post year model 82 1100e

Same,same,same. My solution is to just not go 100mph. I know.....IT SUX !!!

My list of replacements and checks is twice as long as yours.

My anomaly is more like a "weave" as if the bike is a hinge.

 

[alke46]

I used to have a similar wobble, but mine was as I was doing a right hand sweeper at speeds near 80 mph and up. Swing arm bearings had already been replaced, so the next thing was the steering head bearings. Now I only have memories of the front end wobble.
Good luck and let us know when you get it fixed and the wobble disappears.

 

[wilsonsandy07]

Same,same,same. My solution is to just not go 100mph. I know.....IT SUX !!!

My list of replacements and checks is twice as long as yours.

My anomaly is more like a "weave" as if the bike is a hinge.

I would say that’s a pretty accurate description of my issue as well, trying to really focus on what’s happening first when I feel it front or back. I’ll update after I replace swingarm and steering stem bearings hopefully that’ll do the trick!

 

[oldGSfan]

Not a fan of balance beads. Maybe others like them, maybe I didn't use the right amount, but they didn't work.

 

[Rob S.]

Same,same,same. My solution is to just not go 100mph. I know...IT SUX !!!

That's no solution. Are you trying to live forever?

My '73 Kaw Z1 had a high speed wobble. Killed the PO and tried to off me a few times. What I learned is braking, especially panicked braking, only makes it worse. So I would ride the footbrake, and gently apply throttle.

 

[rphillips]

Thanks for the input Tom. I've done lottts of reading and searching on the beads. Most everybody that said they didn't work "had not tried them", it was rare to find folks, that "had tried them" say they didn't work... I wish somebody could explain how they work for some & not for others. You'd think either they do or they don't. You'd think it would be very easy to prove scientifically yes they work or no they don't. It's been a debate for yrs but still nothing concrete that I can find.

 

[oldGSfan]

Thanks for the input Tom. I've done lottts of reading and searching on the beads. Most everybody that said they didn't work "had not tried them", it was rare to find folks, that "had tried them" say they didn't work... I wish somebody could explain how they work for some & not for others. You'd think either they do or they don't. You'd think it would be very easy to prove scientifically yes they work or no they don't. It's been a debate for yrs but still nothing concrete that I can find.

They were OK up to high speed, then on the dyno at above 100 the rear started jumping around like crazy and had to be cinched down super tight. I could see the dyno getting very jumpy on several of the runs. The shop owner said those are terrible for very high speed. This is not a scientific analysis by any means, and like I say, maybe I didn't use the correct amount (but I followed the directions). In everyday legal speed riding they were OK.

That could be why my bike only put down 182 HP - another guy who knows big HP bikes said 'there was more on the table'.

 

[bwringer]

I used to have a similar wobble, but mine was as I was doing a right hand sweeper at speeds near 80 mph and up. Swing arm bearings had already been replaced, so the next thing was the steering head bearings. Now I only have memories of the front end wobble.
Good luck and let us know when you get it fixed and the wobble disappears.

I suspect this is a good chunk of the answer.


Also, air pressure in the front suspension is a terrible, imprecise, leak-prone way to manage spring rate. Install a proper set of straight-rate front fork springs from Sonic springs or Racetech and set sag correctly. Lowering only in the rear can also cause instability.

The original 40+ year old shocks are junk. There are multiple threads here on choosing shocks, but Hagon or Progressive brand with proper spring rates make decent shocks for not too terribly much dough.

Until then, keep it under 90 I guess? Speed costs.

 

[Rob S.]

In everyday legal speed riding they were OK.

Limiting a turbo 11EZ to "everyday legal speed" ought to be against the law.

 

[KiwiAlfa156]

All good info above.

If it was me, and I'm assuming it's a bike that's new to you, I'd start by confirming the frame is straight using string-lines. If it's tweaked, nothing else you do will compensate for this. And repair will be necessary. This one below has the steering head is twisted anti-clockwise (seated on the bike).

Then I'd check that the fork tubes are straight by taking them off the bike, clamping them in a vice by the caliper lugs, setting up a static pointer and rotating the stanchion tubes, if bent this will be obvious.

If the frame and forks are straight, I'd make sure the have the correct oil at the right level

Then I'd check the tapered roller steering head bearings replace if wear is evident, regrease if they look good. The stem nut needs to be torqued down 29.0 - 36.0 lb-ft, seated by rotating the stem once or twice and the nut backed off a 1/4 to 1/2 a turn. There should be some resistance to rotation, but it should be smooth; it shouldn't rotate under its own weight. This is important as the preload on the steering stem bearings provides some steering damping and so impacts stability and resistance to the resonance that sets up dynamic instability of wobble/tank-slappers.

The other critical chassis part is the swing arm pivot. The cross-bracing around the swing arm pivot means that shimming the swingarm pivot is a better way of removing end play on the swing arm, as the tension on the pivot can't deform the frame to close this up to any great extent. If you look at a parts diagram, there are shims either side under the needle bearing dust caps. If you can squeeze another one in, then do it. Even if you need to tap the swing arm in with a mallet. Definitely replace the bearings and sleeves as these never get greased and are likely to be shot.

​​​​​​Correct preload at the pivots is critical to make the GS chassis handle.

Assuming the shocks are good, lowering the rear end should improve stability as it increases trail on the front wheel.

Tire pressure should be what feels good to you. Less pressure is a bigger contact patch, more grip, more heat, more wear and more tread squirm/slightly heavier steering more feed back. Higher pressure gives a small contact patch, less grip, less heat, less squirm/lighter steering and less feedback. I run about 29/32 it works for me.

 

[wilsonsandy07]

quick update i did put the stock suspension back on and although they are old and feel terrible for bump dampening as expected with 42 year old shocks, i did feel a bit more stable at higher speeds.

 

[wilsonsandy07]

another update, ride to work today was about 40 degrees out, and i didnt even realize i was past my usual rpms, so the cold helped my instability a little. usually i dont push past 6500 (not sure of exact speed since speedo stops at 85) but today i was pushing 7500. maybe ill revisit my tire pressure and recheck my air pressure in the forks just to be sure.

 

[rphillips]

Dang stupid 85 mph speedos... Ball park figure, most big GS's run appx 15 mph. per 1000 rpm., so appx #'s, at 7500 you're near 112 mph... Check your bike, hold steady at 3000 in 5th gear and check your speed, if you're doing 52 mph. divide that by 3 and you're doing 17.3 mph per each 1000 rpm.. Keep us updated, I've had a couple of big GS's that with age got a little insecure. I changed everything I could think of, but never got them back to how they were when new.