Front end chatter/handlebar oscillation in corner | GTAMotorcycle.com

Front end chatter/handlebar oscillation in corner

Mike-the-Bike

Well-known member
I've just restored a 81 GPZ550 (the twin shock model) and am really enjoying riding it. I had one as my first ever bike and its bringing back so many good memories.

Having said that I have one issue. If I'm following a car or another vehicle into a corner and can't get on the gas immediately I get some chatter and handlebar oscillation in the front end. It's a little disconcerting.

If I can get on the gas immediately and accelerate through a corner there is no issue at all. its very well behaved.

The forks were just rebuilt with new seals and bushings and I installed new taper bearings in the head stock and wheel bearings when the bike was being rebuilt. I just rechecked the taper bearings

I checked the fork tubes when they were out with rollers and a dial gauge and they are straight, and I've also checked the front wheel with a dial guage.

I've been working my way through the Sports Rider suspension guide. They suggest too little rebound damping so either remove some of the preload on the fork or add some more rebound damping. I've tried both (shorter spacers and heavier oil but not at the same time) and there is not a lot of difference.

I've had the tire re-seated on the bead and the front wheel re-balanced.

The swing-arm bearings are new, as are the real wheel bearings and the rear shocks are a set of brand new Works Performance. The frame alignment was checked before I painted it.

I'm at a bit of a loss. Anyone have any ideas?

TIA
 
3 things to look at -
Have you changed the trail by installing the new shocks (longer or shorter than original) or raising/lowering the fork tubes on the crowns ?
Tire dimension and tread design can influence - what tires and are they significantly different in dimension to stock ?
Too much spring or damping in the rear end - try softening it up a bit and see how it changes what's happening (if at all) ?
Its a one-step-at-a-time process, go slowly and document your changes.
 
I've had tires that made bikes unrideable (head shake) for whatever reason. Swapping to different tires fixed the problem.
 
How are your front brakes? I would assume this is under braking making a right turn, but would be helpful if you could answer a few questions.

Btw, you might want to go to a big empty parking lot to do a few more controlled tests and understand when and when it doesn't happen.

Is it the same feeling when turning while braking and not braking?

Is it consistent at various turning speeds?

Constant throttle vs no throttle and clutch pulled in rolling through the corner vs deceleration?

If you make the same tests doing a tight left turn, do you get the same results as turning right?


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It goes without saying but make sure your tires are at the correct air pressure. Don't assume the shop set the pressure correctly, especially when re-seating the bead.
How old are the tires btw?


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Good day eh, I have to say you have done a lot of thoughtful work/upgrades. I luv the works performance shocks.

but what about the fork springs? and an emulator to boot? Maybe you did some upgrades to match those beautiful WP shocks?

if the front is still stock, the difference in springing would leave the rear just right, and the front soft and poorly damped. If the front spring rate is good, adding just emulators will transform the front damping. Have you set suspension sag?

nice bike

EDIT here is a link to racetechs fork spring rate calculator for the 81 gpz 550. You add your weight, choose race or street, and it will give you the recommended spring rate. This along with emulator valves damping will improve your bikes front suspension. The results will be very noticeable. This would be a great match for the WP rear shocks.
http://www.racetech.com/ProductSearch/2/Kawasaki/GPz550/1981
ebay may have springs and emulator valves for your specific bike. Sometimes you can find parts at about half the msrp
 
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I used to race one of those and have experienced this. I echo Boyoboy's comments about the Racetech emulators.

I found this it was caused by a little too much compression damping.

My settings were 0.8 Racetech Springs, 2.0 turns on the emulator with the 64Lbs spring, 15w maxima fork oil @140mm

I had initially installed the emulators at 3.5 turns but as soon as I removed the come of the compression damping it was a different bike.

They are very light anyway.
 
Thanks for all the comments

How is the back end? Is there any play in the swingarm or rear wheel bearings that could translate to front end handling anomalies?

- No play in the back-end at all. Both the bearings and the pivot shaft are brand new. The rear wheel bearings and sprocket carrier bearings also brand new
 
3 things to look at -
Have you changed the trail by installing the new shocks (longer or shorter than original) or raising/lowering the fork tubes on the crowns ?
Tire dimension and tread design can influence - what tires and are they significantly different in dimension to stock ?
Too much spring or damping in the rear end - try softening it up a bit and see how it changes what's happening (if at all) ?
Its a one-step-at-a-time process, go slowly and document your changes.

Forks are set flush with the top clamp which is the way the came from the factory.

The OEM shocks on that bike are 13.5 inches (343mm) the works performance are 350mm but they have an adjuster that provides +/-5mm and its set right at the bottom so that would make them 345mm. I hope 2MM wouldn't make that much of a difference.

Original OEM tires were 3.25. I'm running a 100/90 on the front which is supposed to be the equivalent metric size. I'm running Avon AM26's. I've run these on other bikes and liked them.

I'll try softening yp the back end and see whats happens. Spring and Compression at different times
 
How are your front brakes? I would assume this is under braking making a right turn, but would be helpful if you could answer a few questions.

Btw, you might want to go to a big empty parking lot to do a few more controlled tests and understand when and when it doesn't happen.

Is it the same feeling when turning while braking and not braking?

Is it consistent at various turning speeds?

Constant throttle vs no throttle and clutch pulled in rolling through the corner vs deceleration?

If you make the same tests doing a tight left turn, do you get the same results as turning right?


Sent from my iPhone using GTAMotorcycle.com

Calipers were rebuilt and the pads are brand new. The disks are also new (EBC) as the originals were warped and scored. the are directional and were fitted left and right.

Its worse when not braking, i.e. coasting into a corner behind a car. but is consistent as various speeds. Applying the throttle seems to make the problem go away.

Its exactly the same for left and right turns
 
It goes without saying but make sure your tires are at the correct air pressure. Don't assume the shop set the pressure correctly, especially when re-seating the bead.
How old are the tires btw?


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Yes, Checked the pressure. Tires are brand new. The rear is marked the 18th week of 2016, and the front is the 50th week of 2016
 
I used to race one of those and have experienced this. I echo Boyoboy's comments about the Racetech emulators.

I found this it was caused by a little too much compression damping.

My settings were 0.8 Racetech Springs, 2.0 turns on the emulator with the 64Lbs spring, 15w maxima fork oil @140mm

I had initially installed the emulators at 3.5 turns but as soon as I removed the come of the compression damping it was a different bike.

They are very light anyway.

Interesting you should mention this. I am running Racetech .8kg springs with emulators and 15w race-tech oil which is what they recommend. I do have about 3.5 turns on the emulator as their instructions are a little ambiguous. Let me fish them out and set them at 2.0
 
Thanks to all the replies.

Tried softening up the rear spring/damping and it didn't make a lot of difference.

Pulled out the emulators made sure they had the 64lbs springs, and wound them to 2.0 turns. 100% better. This was the problem. You sir, are a genius ;)
 

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