Ran it up to 115mph, and ...

[Shuriken]

I did this, put progressive springs in.. Mostly because I dropped my front end an inch and the front fender was rubbing when I braked. It changed my speed wobble a little bit in terms of what speed range it occurred. I did the "drop test" where you lift the front wheel and allow it to drop when you turn it. I ended up tightening the the steering bearing nut and it solved the speed wobble up to 120 (there's only so fast you can go down my street at 10 pm.) Then I had it in the shop getting it rejetted due to an exhaust change and they said my bearing and race were worn out. Now my slow speed wobble is back. I will try tightening the head nut again since it's new. Mine is a 2007.

Yuck!

Looking at things in a positive light..... all of these things I've looked at, mostly one at a time, has let me observe and feel what (if any) difference each change makes on the system. Additionally, it has caused me to find and resolve various (including safety) issues. So this has been great. Now I know the steering bearings are good. The swingarm bearings are good. The wheel bearings are good. The brakes are rebuilt and safe. The rotors are true. The tires are new. Rear shocks are new and working. I am still working on the front end because I've touched about everything and haven't solved the issue to my satisfaction yet. But now I don't have to speculate on parts of the system that are already corrected.

Most alarming so far was how much lateral slop I found in the swingarm. I mean, it must have been 2-4mm(!) and now, with new bearings and torqued to spec, there is 0 slop. That's just one of the bigger issues I found. But that did not remove the wobble!

So... my theory I'm working on at the moment is that the issue is 100% with the front end and probably specifically with the forks. I've read other people's descriptions of the "wobble" and the best way to think of it is when you are at 65 or faster and you jiggle the handlebars a little (small inputs!), the bars won't quickly stabilize; instead, it takes 6-8 or more oscillations back and forth to stabilize or "settle down." On other modern bikes, the front end settles way sooner, maybe 2-4 oscillations, or none at all. So on the max, when the front end feels this wiggly, you'll notice at 75 or better, changing lanes, bumps in the road, wind buffeting, etc., will cause the bike to start weaving or wiggling gently, which is a warning to slow down, unload the front end, and go resolve the issues.

The Shinko 230 tires feel noticeably more responsive than the ME880s that were on it so the effect of going to the 230s was actually an increase in the onset of the wobble. The front end became more responsive and I believe the 230s just aggravated the underlying other issues. This will be a disappointment to anyone who thinks just a tire change will fix a wobble, when the issue is elsewhere.

What I did notice is with no spring preload and 5w oil, with the bike on the center stand and a little weight on the front wheel, wiggling the handlebars seems to really reveal the front end slop. In a way, this feels exactly like it does at speed. So this test served to help me theorize that the front end is entirely where the issue is at the moment, and specifically in the forks. So the next step was to add preload back and try to reset sag and this was noticeably better up to highway speed, especially regarding how quickly the oscillations would settle out. Unfortunately, I didn't have time to try a higher speed test yet but may tonight.

I can definitely say that the front forks absolutely should be properly configured for the specific rider, AND that the steering head bearings must be in good shape and at proper torque. So far, these 2 items seem to be contributing, at least in my case, more to the wobble than anything else, and since the bearings are good, now, that leaves the forks....

Oil level must be the same on both sides. Spring rates must be the same on both sides. Both forks must move the same on both sides, and neither should have more stiction than the other. The correct amount of preload and static sag must be set for the rider. The oil weight must be set for the rider and for the forks to move freely.

Many of us just want the front end to stop diving during braking, so we go to heavier oil, progressive springs, etc. I think this can be fine IF there are no other issues, such as especially the steering head bearings. But if you take more play out of the forks, it may transfer more stress and movement to other areas of the bike, such as the steering head bearings, frame, etc.

The spring and the damping have to work together to keep lateral stability. In the 85 model, the only option for tuning the damping is to experiment with oil! Yamaha changed the quantity of oil holes as well as the oil hole diameters in various ways across the years as a way to fine tune the forks. Also, I don't think this 40 year-old design is super sensitive, either. I feel like you really have to get into the bumps and get the forks moving a LOT to notice the damping. In contrast, I notice damping action with the economy Progressive 412 shocks with as little as a half-inch of spring compression. I imagine more expensive suspension is even more sensitive.

So the consequence of screwing up the fork settings on (at least) the 85 forks is that they are configured way too stiff (causes improper sag/no sag/too tall in relation to back suspension, harsh ride, springs are preloaded too much so damping is ineffective, etc.) or they are configured way too soft (too much sag, bike leans forward in relation to back suspension, no spring preload causes too much reliance on damping/oil and spring rates for lateral stability, etc.). Setting proper sag on front and rear should place the forks and shocks a little into their travel, which will preload the springs just enough and also activate the oil and damping.

In other words, damping and spring rates have to work together on the old forks. Damping, by itself, sucks on the 85 forks, in my opinion, and OEM springs by themselves, without damping, suck on the 85 forks, in my opinion. I think the trick, after making sure every other part of the system is in good, properly-configured order, at least on the 85, is to get a little bit of spring action and a little bit of damping action set properly with static sag, ride test, and go from that point. And there may not be additional improvements after that without moving away from OEM altogether and drilling out the damper rods and then installing valves or cartridge emulators or other modern tech.

If one OEM spring in one side is a little more tired than another, then this could definitely contribute to instability and oscillation. If one fork has more or less oil than the other side, or viscosity isn't the same in both sides, or if one damper rod is halfway obstructed, or if one fork has less air pressure than the other side or even bushing wear isn't the same, I could see how all could contribute to the problem.

So I am really refocusing only on the front end again and a logical choice is to ditch the springs I have now and go with new springs and start over with a known spring rate, lighter oil weight (5 vs 10), proper oil level, and finally, a preload/sag of at least 1" or more. Also, it is worth a double-check of the damper rods to make sure they are both identical and not mixed from different years, so I'll do that tonight as well.

 

[Fire-medic]

You mentioned the air being the same in both sides, you have a '92 so there should be one schrader valve servicing both sides. Unless you have some material plugging the pneumatic line on one side or the other, that is.

I forget what you said you are for size/weight, but I think 5 wt is too-little for most riders, unless you're something like 120 lbs ready-to-ride. Race-Tech, YSS, and RICOR all have wave-washer stacks to remove damping from the damper rods. I have a RICOR set and upgraded to the 1993+ 43 mm forks & brakes in my '92.

 

[Shuriken]

So the 5w-20 synthetic and 5.5" spacers puts the sag in a good place and the forks are doing the job now. This made a noticeable improvement and by simple testing means, seems to have raised the wobble onset speed by 10-15mph alone.

Too windy to reliably test but it feels much better at freeway speed.

I notice with throttle roll ons and throttle chops that the forks move probably too much of course but they are active.

What I think is happening now on higher speed runs is that the forks are lengthening/extending to the top of their travel. On the freeway, I believe this mostly removes the sag and thus most if not all of the damping. The springs are mostly then handling the bumps and that's what I believe I'm feeling: higher speeds extend the fork and riding sag!

I think if I can tune up the forks properly, I may be finally in business.

Your idea of 7.5w oil may be about right as well. At least I'm trending in the right direction!