Instability at high speeds

[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.

[argonsagas]

I did not see this mentioned, so, do you have a fairing of windshield of any kind on the bike?

Depending on model and design of these items higher speeds can lift the front a bit.

My 1100G had something similar when the front lifted slightly as wind got under the Vetter fairing.

Turned out time and the extra weight had considerably weakened the springs so when the fairing lifted the springs were doing almost nothing and it actually was almost floating.

A new pair of Progressive springs and fresh fork oil improved everything.


Also....check front-rear wheel alignment. This should always be a part of the checks when you install bearings, especially swing-arm bearings..

[Big T]

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.

This points to an imbalance in the suspension, front vs back.
I suspect your shocks are too stiff and the forks are too soft

Better shocks and upgrading your fork springs should cure your issues