I tend to think that there's much room for improvement in these 'old' bikes. The geometry, weldments and fasteners are all teutonic goodness and such, but probably a little flexier than is optimal for the weight distribution. Many bikes, many models and manufacturers, suffer instability, shakes, wobbles, or outright 'tank slappers' under certain conditions; even highly tuned frame, wheel, fork setups on modern GP race bikes. But the R65 seems to have a bit more than its share... at least mine, and many other owners I've talked with.
Even after numerous tire options, replacing steering head bearings and swingarm bearings, preloading/torquing to spec, adding a rigid fork bridge across the fender mounts, experimenting with rear shock adjustments and front shock oil weights, and replacing fork springs..... hell even experimenting for while with about 30lbs of weight placed in various positions fore and aft to quell whatever harmonics were the devil. Half of those were done by the previous owner who well warned me about wobbling instability under certain conditions, and the other half were my own attempts. The good part perhaps is that it was primarily why he was willing to part with a showroom condition R65 with only 12K on it for 2000 dollars back in 1991.
Eventually, and a little controversially since not a few will insist these are worthless remedies, I installed a really nicely built 35 dollar Suzuki OEM NOS Ebay steering damper (tig welded to the frame and mounted with high precision fasteners/bushings for zero slop in the linkage), and filled the handlebar ends with steel rod up to the first bend in the bar. It's impossible that these resulted in merely curing an overactive imagination. They worked. I've been thus equipped for over five years, under extremely wide and varied conditions, and enjoyed a rock solid ride. 99 percent improvement.
It was pointed out in some research before doing this that all two wheeled vehicles will exhibit some degree of runaway harmonics at some point. It's mostly a matter of getting that point outside the envelope of expected performance/use. It could conceivably be the case that a twowheeled vehicle might wobble furiously at 5mph and be rock stable up to 200mph. But in 99.99 percent of motorcycles it's the case that somewhere in mid to max speed, under certain uneven pavements and sweeping corners, that the geometry's harmonics go awry.
A certain amount of flexibility is engineered into a whole system to benefit comfort and performace (most obviously at the shocks), but race mechanics who are tuning bikes to stay on their feet at up to 220 mph make different trade offs and concessions than for a bike that isn't expected to exceed 120mph. And different again for a vehicle that isn't going to exceed 45mph.
I'm not an engineer, but it's easy enough to see that the front wheel/forks/handlebars and the rest of the bike are the only two major structures of a motorcycle that are joined in such a way to allow anything more than even one mm of resonance to build (discounting the front/rear shocks and swingarm which have already been very well dampened against oscillations). That's the inherent weakness of a steerable two wheel vehicle.
Every part of the bike is going to resonate at a variety of frequencies (talking really long ones here) but will naturally quell anything beyond a couple of mm or so quite well on it's own if it's otherwise well maintained. But every bike will have a less than perfect tire roundness, less than perfectly adjusted swingarm pivot, or less than perfect engine rubber mounts, or other. To a degree, that's just 'real world'. But if a whole subassembly is attached to another, via a pivot that's wide open to mischief (the steering head), that's the flex point at which a few specific of those oscillations can complement each other and build. Let out of the bag like this, so to speak, they can enter the feedback zone and get the frame itself flexing beyond those initial bounds and all hell breaks loose. My R65 would still enter the tank slapping zone eventually, but like the race tuner, I've found a way to push that eventuality out past the 120mph point, seemingly for good.
Might just be beginner's luck. A very specific remedy that just 'happened' to work for me. But it's worth reporting on here if anyone else is concerned or curious about stabilizing their airhead. I don't mind any flame here. I've heard it. And I wouldn't suggest mounting your own steering stabilizer for anyone who was not highly confident in his mechanical skills. Bound or stuck steering at highway speeds would not be a happy outcome in any case.
But for me these fixes have added enormously to my pleasure in riding in many circumstances (exiting a highway into a cloverleaf at 60mph into the sweep on uneven pavement for instance) where once I lived in fear. It's just a different bike than the one I bought originally.
