83 850 Bad Head Shake

[KickerATW]

New to the forum. Great site btw.

Long story short. Bought a 83 GS850 for $400 that wouldn't start. Carb clean plugs and new gas fixed that issue.

Problem is the PO swapped the stock forks over for a set of 550 forks to ditch the air assist and the stock forks were bent.

At 80mph plus the bike has a bad head shake. Checked axle trq tree trq and all bearings/fluids and its all fine.

What could be causing this head shake? I didn't know it was gonna happen and hit that speed around a sweeping curve on the turnpike and almost went down. Need to get this safety issue fixed quick.

Oh real quick. Does a 83 850 have a tube in the rear? I can't find.d the tubeless mark on the rim. And I know the front is tubed. Just not sure on the rear.

Thanks again.

[raygun]

I don't know a whole lot but I know on other bikes I've built, there needs to be a certain amount of preload on those bearings. Maybe the triple tree isn't tightened down enough? A little too loose perhaps.

[tkent02]

My first guess would be the 550 forks on the 850.

[Brendan W]

That would be my guess too. The springs are too soft. You could try seeing if the preload can be increased by turning the ramps in more if it has any or putting a spacer in. Check out what sag you are getting on the front end when you sit on it.
Both my rims are marked tubeless but one has the wrong size valve hole and a tube is fitted.

[KickerATW]

I will recheck the head bearings. Any idea on that spec or trq specs for the whole front end? I went off a Google search specs.

[Rob S.]

My first guess would be the 550 forks on the 850.

That would be my guess too. The springs are too soft. You could try seeing if the preload can be increased by turning the ramps in more if it has any or putting a spacer in. Check out what sag you are getting on the front end when you sit on it.

The 850 is a notoriously heavy bike.

If everything up front checks out, I have seen front shake caused by worn swing arm bushings.

[Datsa Noydb]

... the PO swapped the stock forks over for a set of 550 forks to ditch the air assist and the stock forks were bent.
...

The best way to "ditch the air assist" is to install proper springs so you don't need to add air. Good springs available from Progressive Suspension, EXCELLENT springs available from Sonic Springs (the owner is a member here, too).

Does a 83 850 have a tube in the rear? I can't find.d the tubeless mark on the rim. And I know the front is tubed. Just not sure on the rear.

In stock form (here in the USA), both wheels would have been tubeless by '82. Obviously one PO or another had other ideas about what needed to be there.

Maybe the triple tree isn't tightened down enough? A little too loose perhaps.

He mentioned that he had checked the torque, but those bearings are also subject to wear. Turn the bars, see if there is a 'notch' in the feel as they cross straight-ahead. If you feel a notch, replace the steering bearings.

My first guess would be the 550 forks on the 850.

I agree. Smaller tubes, lighter springs. No indication if either bike might have been an "L" with leading-axle forks.

You could try seeing if the preload can be increased by turning the ramps in more if it has any or putting a spacer in.

Adjustable preload on the forks was not a feature on US-spec bikes. That is why we were 'blessed' with the air assist.

... I have seen front shake caused by worn swing arm bushings.

I have, too, but the 850 (and other GS shafties) don't have 'bushings', they have tapered roller bearings for swingarm pivots. They need to be checked for proper adjustment, so the idea is good, but different parts are in question.

[rudeman]

Over torquing the steering head will sometimes cause a high speed wobble. Assuming that the bearings and races are in serviceable condition; back off the tension on the steering head and set it according to service manual specs.

Also, pretty certain that 550 stanchion tubes (36mm) will not fit in an 850 triple clamp (37mm).

[tkent02]

The 550 forks are only 35mm, barely adequate for the lightweight 550 which only goes about 100mph. The massive 850G on those spindly forks is idiocy.

[Big T]

You're overlooking the obvious here, even though every suggestion is good

1. Bent forks= crashed bike. Maybe the frame is bent?

Check your wheel alignment, vertical and offset

[hillsy]

No mention of the tyre - is it worn / cupped? That will cause headshake as well as the SHB's.

[Cyrano]

The only time I ever experienced a bad headshake (aka tankslapper) was at high speed on a borrowed GPz900.

Turned out to be low pressure in the front tyre. Easy fix and easy to dismiss.

[Rob S.]

The only time I ever experienced a bad headshake (aka tankslapper) was at high speed on a borrowed GPz900.

Then I guess you've never ridden the original Z1 at illegal speeds.

[KickerATW]

Thanks for all the great advice. Yea the guy said the bike was laid down. Not while riding though. He dropped it into a curb and put a dent in the upper tube right above the seal.

When i had bought it the front end was already swapped. 550 trees and all. And no original forks.

This 850 is an L according to the covers over the battery and electronics. Idk what the original forks are/were but these are leading axle 550 forks.

And i know they are 550 forks because they are identical to my 550s front end. Both in size/dia. and length.

So what would the leading axle change?

[tkent02]

If he didn't change the triples too it would give you less trail. And head shakes apparently. If the triples were changed to L triples too that doesn't really matter.