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83 850 Bad Head Shake

  • Thread starter Thread starter KickerATW
  • Start date Start date
K

KickerATW

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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.
 
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.
 
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.
 
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.
 
tkent02 said:
My first guess would be the 550 forks on the 850.
Click to expand...

Brendan W said:
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.
Click to expand...

The 850 is a notoriously heavy bike.

If everything up front checks out, I have seen front shake caused by worn swing arm bushings.
 
KickerATW said:
... the PO swapped the stock forks over for a set of 550 forks to ditch the air assist and the stock forks were bent.
...
Click to expand...
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).


KickerATW said:
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.
Click to expand...
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.



raygun said:
Maybe the triple tree isn't tightened down enough? A little too loose perhaps.
Click to expand...
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.


tkent02 said:
My first guess would be the 550 forks on the 850.
Click to expand...
I agree. Smaller tubes, lighter springs. No indication if either bike might have been an "L" with leading-axle forks.


Brendan W said:
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.
Click to expand...
Adjustable preload on the forks was not a feature on US-spec bikes. That is why we were 'blessed' with the air assist.


Rob S. said:
... I have seen front shake caused by worn swing arm bushings.
Click to expand...
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.
 
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).
 
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.
 
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
 
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.
 
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?
 
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.
 
Yea the triples are 550 as well. I have researched and know the 850 forks were larger by a few mm. And with size come strength.

But under all conditions the front end is solid. Barely dibes on hard braking. I put the bike on center stand and ratchet strapped the rear down to lift the front tire. And there is no binding or play anywhere. He gave me reciepts for the front end and it was a complete swap. Plus the reciepts for the new SHBs, wheel bearings, fork seals, dust boots, and progressive springs.

Plus i am a honors graduate from a motorcycle tech school. I know what to check, how to check it and how to service all this stuff. As well as a PA motorcycle state inspector. So it all checks out wear and condition wise.

This head shake is baffling though.

Reason i ask is i googled and recieved results from the old gsr.com amd it stated some older heavy big 4s were known to have head shakes from the factory, just trying to find out why as my searches were inconclusive with an answer.

Good example in a Homda CBR954RR. They have a bad head shake from factory due to power on a light bike and the front end lifting and unloading slightly. And the only cure for that other than a damper is to power through it.

But since were on topic. Would a damper help this or should i just find a front end from a newer gsxr and swap out. And ditch the brat style and make it a modernized restomod cafe build. Then swap these newly eebuild 550 forks onto my 550.
 
Even if the 550 forks are using the 550 yokes (triples), are the 550 fork legs not a fair bit shorter than the GS850 ones anyway? That alone would reduce your trail a bit.
I've got GS1000 forks on mine, and those are 19mm shorter than the 850 ones. I wouldn't care to go any shorter - skinnier, no way.

None of the GS850s are prone to head shakes unless something's worn beyond sense or is way out of whack. Bad loading plays a part, too.
 
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