FZR 600 `92 Front wobble

[A-JK]

Yesterday I installed new Racetech .95 springs, new seals and Motorex 15W fork oil with the oil level set to 100mm from the top. Now after litle trial and error with setting the correct preload and sag the front suspension feels just amazing after riding with leaking pogo-stick it used to be, BUT... As the front works othervise fine now I noticed a new issue with it:
At 120km/h the lover section of the front fork starts vibrating/shaking in a back/forward motion. The shaking is worst at 130-145km/h (almost 40mm motion) , however after exceeding 155km/h the shaking stops pretty much completely and dosn`t re-appear before 200km/h (my fighter won`t go faster... Yet ). I´ve ridden about 650km since yesterday and haven´t noticed any leaks or change in the problem.

Have you guys had any simalar problems in the past or have a good guess what could be the problem?

[thatkid]

I'm assuming you mean lower section and not lover section. Did you pull the fork legs from the triples to do the work? If so, did you reinstall everything that came out. Spacers in the wheel/axle, make sure the fork legs are even with each other, steering stem bearings are tightened properly, axle torqued properly...

[pefrey]

I don't know but that sounds pretty dangerous. There should be no flex between the upper and the lower, this would cause your forks to bind and not do their job. Since the onset was sudden, it is probably an error in assebmly. Trace your steps.

[M in KC]

You have something a foul. I have had my forks apart and together a half dozen times and never experienced anything as you are describing. I am also running the .95 Racetech springs and had been for several years before four weeks ago when I swallowed and added their gold valve emulators. So I know this setup isn't your issue. If you didn't experience any of these symptoms before your mod I wouldn't think that you have a tweaked fork leg. Dumb question I know, but did you reinstall your front fender and fork brace?

The great news is that once you get this sorted you are going to absolutely love your new springs.

[A-JK]

I can`t be sure if I allready had the problem before the rebuild as the front was in quite horrible condition, so it could have been there for a while but I just hadn`t noticed it.
I´m pretty sure I installed everything back to their places, or atleast I didn`t end up with any "extra"-parts...
The forks are set to same height within 1mm, so that shouldn`t be the case here, nor do I think I could have tightened the axle so much that it would have thin kind of affect ( think that would need huge over tightening).
The wobble was worse when the preload was excessive, also I think it helps when I brake so that the front dives little deeper into the lower part.
I´ve been thinking about few things that could cause the problem
1. Wheel bearings may be nearing their end.
2. Wheel could be too un-balanced. ( I know for sure they weren`t balanced when the tyre was last changed, but it was done 5000km ago.)
3. The nylon(?) bushings inside the forks could be worn out. ( In this case shouldn`t there be some wobble all the time?)
More advices would be appreciated

[M in KC]

Well, banging out the old wheel bearing and balancing the tire would be first up on my list if that is the case. If you didn't want to replace the bearing your local shop ought to be able to do it reasonably just before they balance the tire. I'm assuming your tires are in reasonable condition.? After these things Beyond that I'd start looking at your fork bushings. Not just the nylon ones. But one thing at a time.

Here's something else to consider though. Wobbles happen with all motorcycles when they reach a specific harmonic point based on the bike, its condition, the rider and riding position. Wobbles can be minimized or suppressed by riders who understand what mechanically is happening while they are riding by changing their speed and or riding position or both. In doing so they move the COG (center of gravity) and effectively alter the harmonic point of the oscillation (ie. the wobble). That being said normally wobbles can be experienced throughout the entire operating envelope or the service life of some parts normally suspension related but are most common at higher rates of speed. The reason I bring all this up is your issue(s) maybe elsewhere on your bike. Swing arm bushing, steering stem bearings, rear wheel bearings, chain alignment, rear shock, rear wheel bearings, etc. Now I don't mean to send you on a wild goose chase but, if after you have exhausted all the front suspension components as potential issues don't stop there.

[A-JK]

Once again the easiest solution was the best. Had the front wheel balanced today (60gr ) and the wobble was gone!
The front suspension is great now, racetech spring upgrade was well worth the money as now the front feels as good as new.
Thanks for all the advices guys!

[reelrazor]

Yeah, wobbles/vibration that goes and comes at speed thresholds is almost always a tire balance issue.

The fork bushings are often overlooked for replacement I bet there are forks out there that have had seals replaced numerous times and go back together with the same bushings everytime..