UPDATE: Problem solved -- it turned out to be the rear tire! An Avon Storm 3-D XM, it was most likely defective in some way.
---
Hey, everyone,
I have been working on a problem with my 2001 Concours for weeks now. Let me describe what is going on, and what steps I've taken so far to fix it. I'd like to know if there's anything else I can try before I just throw my hands up and give up here! This will be a bit of a read, but I'm trying to be thorough and offer as much information as I can.
SOME BACKGROUND
I've owned the bike for a little over 2 years so far. In that time, it's always been rock-steady on the freeway, up to and over 80 miles per hour. Recently, the bike sat outside under a CycleShell cover for about 2 1/2 to 3 months, as I didn't have time to ride. At that time, the tires on the bike were about 6-8 months old, and they were both Avon Storm 3D X-M radials, in the standard sizes for the Concours. The windshield on the bike is non-stock, and is a Gustaffson 11" shorty shield. I do not ride with a trunk or with the saddlebags, and have the side-covers installed and the antlers for the bags are currently removed. The fairing is completely stock, and in excellent shape and has no cracks or other issues that should abnormally affect aerodynamics.
THE ISSUE
I started riding it again at the end of April. While doing so, I noticed that at highway speeds, there seemed to be a bit of a wobble -- the bike wouldn't track straight, and instead felt like it was... wiggling? I think that's the best way to describe it. Below about 70, you wouldn't really feel it beyond sometimes kind of getting a sense of unease. Above 75-80 miles per hour, it begins to manifest itself as an oscillation of what feels like the entire bike at about 3-4 Hz (so about 3-4 wiggles per second). In other words, it's a slow wiggle or wobble, not a vibration.
The issue can also be felt at slightly lower speeds by performing very quick maneuvers, such as quickly switching lanes or performing an emergency swerve maneuver, such as you'd use to avoid debris in the road. At lower speeds, the bike will oscillate and eventually seem to settle down. At higher speeds, such as around 80 miles per hour, it becomes pretty much constant until you slow down again.
I just want to stress again, this doesn't seem to be a problem that gradually manifested itself -- when I put the bike away at the beginning of the year, I was regularly riding at about 80 miles per hour on the freeway, and it was solid as a rock, with no wavering.
WHAT I'VE DONE SO FAR
I have tried an awful lot of stuff to get rid of this. Here's the complete listing of the fixes I've had done. All the work was performed by a competent local shop that specializes in vintage Japanese motorcycles, and they've always done right by me.
New Steering Head Bearings
The first thing I tried was having the steering head bearing replaced. I've heard that high-speed stability can be an issue with a worn head bearing. The bike now has a brand-new steering head bearing, OEM Kawasaki, installed professionally.
New Front Springs
The suspension on the bike was worn and quite old, so I had the front springs replaced at the same time with a set of Progressive straight-rate springs (1.1 kg). This was performed at the same time as the steering head bearing replacement.
New Rear Shock
The rear suspension was also quite old and soggy, so I also had that replaced. I used a Progressive 465-series shock absorber from Murph's Kits, and again, the installation was handled by a professional mechanic at the same shop. This was performed at the same time as the two previous fixes.
Front Fairing Repair
While I don't think this will have anything to do with the actual issue, I'm mentioning it for completeness. My front fairing was cracked and inexpertly repaired by the previous owner right next to both turn signals. Since the fairing was coming off to do the front-end work, I had the cracks fixed and the fairing re-painted. I also installed some sick-ass Ninja decals because no one can tell me not to.
Front Fork Brace
As a final capper, I also ordered a fork brace from Murph's, and had that installed at the same time as the front suspension work was done.
After this point, I first got the bike back. I took it out on the freeway, and noticed that the problem had not been noticeably improved at all, but neither had it been made worse. Still, given all the work I'd just had done, it was discouraging. I spoke to my mechanics, who then said that there was some sign of unusual or uneven wear on the front tire. So...
New Front Tire
Contacting a local motorcycle dealer (as my regular shop is over an hour away from my house), I ordered a brand new Dunlop sport-touring tire and asked that it be mounted, balanced, and installed on the bike. Took a couple of days to get the tire because of Connie's weird sizes, but they did it without incident.
That was the second time I got it back. I once again took it out to the freeway, testing for both the wobble at high speed as well as the induced wobble from quick lane changes. It had not changed in either case, and was still occurring. So, once more, I called my favorite shop and talked it out with their lead mechanic. I said that I had read a few articles online, and that one of the other issues that apparently can cause similar symptoms is a missing or broken engine mount bolt. Apparently, it's not uncommon for these bolts on the ZG1000 to back out, shear, break, or simply vibrate loose over time, and because the engine is a stressed member of the frame, giving it lateral freedom of movement can result in some odd handling effects, including a sort of gyroscopic, side-to-side movement that sounds like it feels something akin to what I am experiencing.
Engine Bolt and Frame Inspection
I returned the bike to my favorite shop, and they spent two days going over it with a fine-toothed comb. The lead mechanic put a wrench on "every exposed bolt he could find", including the engine mounting bolts, frame connectors, swingarm mounting bolts, suspension mountings, and everything else he could reach, making sure that everything was up to the factory specifications. They discovered that the two front engine mounting bolts were each about half-a-turn loose, so those were re-tightened. In addition, they looked over as much of the frame as they could see, searching for any sign of stress fracture or cracking -- peeling paint, rust spots, actual fractures, etc. They found nothing of the sort. The only other issue they found was that the right-side footpeg was a little loose, which they also fixed (hey, you never know!). They test-rode the bike and said from what they could tell, it was improved.
At that point, I reclaimed the bike for the third time, and rode it home. While it seemed improved and more stable on the road, once we hit the freeway and I was able to get up to 80 again, I was disappointed to discover that the same issues began to appear. I wrapped it up to about an indicated 90, and the shaking only got worse -- it was visibly moving the handlebars, and my long-suffering girlfriend driving behind me was even able to notice it before I told her about it.
SO WHAT NOW?
So now, here I am, having spent way more than this bike is actually worth to diagnose an issue that remains MADDENINGLY out of reach. I still want to try to fix it, because at this point, in for a penny, in for a pound, but I'm not sure what else to do here. About the only thing I can think of that I have left to try is also replacing the rear tire. Short of that, I honestly don't know what to do here. I don't understand how this changed so suddenly -- this bike was always absolutely rock-steady in the fast lane for the two years prior to this. I don't see any missing parts or linkages, all the bearings and fasteners seem to be correct and fitted, there's nothing wrong with the frame...
I would greatly appreciate any words of wisdom that anyone else can offer for this. I don't want to have to get rid of this bike, I love it dearly, but I mostly ride freeways, and I just don't feel safe riding it with this kind of instability at the kind of speeds that I regularly hit on my commute!