When I'm entering or exiting a freeway, I've noticed my '89 K100RS has a kind of waveform that seems to travel through my frame. For a while I thought it must be my suspension going out, but once I started paying closer attention, the wave/wiggle/bounce feels more 'side-to-side' rather than up-and-down. (Side-note: I'm not a speed-demon or major risk taker. Everything I'm describing is experienced at reasonable speeds and fairly pedestrian lean-angles.)

I've owned two shaft-driven R-series before, and am painfully familiar with the lateral torsion those bikes can generate when engine braking in a turn. What I'm trying to describe on the K has some similarities, but a much subtler and longer-lasting phenomenon... (R series produced a 1/4 second jolt, while the K wobble is more like a smooth 1.5 seconds.)

So far, this wiggle seems to be affecting the rider more than the bike. (Meaning that while its a bit disconcerting for me, the bike holds its line just fine.) Furthermore, under any other circumstances, the suspension feels solid/competent. The point is, I don't know how to interpret what I'm feeling. Is this part of the character of the K-series, or is it a symptom of something I need to address? 

Since I'm looking for useful frameworks from which to understand this thing, answers based on wild conjecture, and sober experience are equally welcome.

Bike shouldn't do that.....
I suggest you check some bearings for play, frontwheel, rearwheel, steering and swingarm.

and also tyres.

Tyre tread with a depth of less than two millimetres will 'track' badly. Check pressures - 38psi front / 42 psi rear seems to be the consensus around here.

And if the tyres are Golden Oldies BIN THEM

Can't see the rim, still have some miles on it

 more cracks in that than a Politicians answer. BIN TIME

Arlina you ride on canvas  anyway

Turning on and off a freeway in the US are both right hand turns I think. If you have a preference for turning right, that side of the tread can sometimes be more worn and it probably wouldn't happen on left turns. 

It only lasts 1.5 seconds, that sounds like when you cross a seam in the bitumen at a very acute angle, like going from the on ramp to the freeway.

Arlina wrote:Can't see the rim, still have some miles on it

That tire is like new. It still has the little nubs on the edges. Just keep turning it will be fine.

You may be feeling the tram tracking road surfaces get when used long term by trucks. You could be feeling the result of too many miles on a straight road and the turn in on your tyre now has an edge to it. Ambient cold at 36psi and put it through some bend swinging to see if you can round the tyre again. Then try to feel it again. Other than that check the final drive for wear by holding the wheel top and bottom and rocking it to see if any movement can be felt. Check swing arm for wear too. If all seems fine move forward and check your steering bearings for flat spots. But hey! onramps and offramps are worn out things at the best of times.

Those are all good suggestions. When you check your steering head bearings don't just check for looseness but also check for nochiness from a straight ahead position.

Some tires have a harder rubber compound in the center than they do on the outside edges. I've noticed sometimes when they get worn they tend to create a scalloping texture that gives me some wobble when I'm banked in a turn. That might be something to check and would indicate it's time for new tires, or give it to Arlina to ride it to the rim  . In the photo that harder center strip curves from side to side and in a bank the tire wobbles from high spot to high spot. Let us know what it turns out to be if (I mean when) you figure it out.

MikeyD wrote:Some tires have a harder rubber compound in the center than they do on the outside edges. I've noticed sometimes when they get worn they tend to create a scalloping texture that gives me some wobble when I'm banked in a turn. That might be something to check and would indicate it's time for new tires, or give it to Arlina to ride it to the rim  . In the photo that harder center strip curves from side to side and in a bank the tire wobbles from high spot to high spot. Let us know what it turns out to be if (I mean when) you figure it out.

I use Bridgestone BR45/46 and they are dual compound and prone to this scalloping and wander if run too soft. I find running them at 38psi front 42psi rear avoids this. This gets even more likely if the tyre is aged too much. Check the manufacture date too.

On the RS in particular it seems to be common, mine starts to get particularly nasty once there is less than around 4mm (!) left on the rear tyre. The front doesn't seem to matter.. My theory is that as the rear wears down it becomes more flexible, which allows this weird oscillating movement to start, and then the weight of the bike flexes the tyre even more and you soon find yourself wondering if you're actually riding some 1950s relic. I have replaced the steering head bearings and the shock and the crown wheel bearing and in the end the only thing that stops it is a new rear tyre. Or going slower..

Moving about in the seat helps as you can keep the bike stood more upright, and putting weight on the inside footpeg often helps calm it down too, and accelerating through the bend can be calmer still but you shouldn't have to put that much effort in to glide round a corner. Sometimes she's just a wobbly old Hector! I've got a Tourance Next to put on soon, I'm really hoping a stiffer radial will help stop it once and for all. By the way I run 40/42 PSI F/R.

I went through all the major culprits you fine folks suggested above. The only thing that jumped out was the rear tire which was definitely due for a replacement. I decided to use the opportunity to try my hand at replacing my own tire, which more or less went fine, save for my thumb which was properly crushed under one of the tire spoons that slipped while mounting the new rubber. (Seriously, it looks like someone else's digit now.)
In any case, it was during this process that I noticed the tire-balancing weights were missing. The adhesive contact patches were still there, but the weights themselves were gone...

Long story short, with the new tire in place and the wheel balanced again, the wobble is wholly gone. I'll never know how much of the wobble could be attributed to the old tire, and how much to the missing counter-balancing, but I'm okay with not looking this gift-horse in the mouth.

Hopefully this will serve as a reminder for others that Occam's Razor is a really, really useful thought exercise when attempting to diagnose/fix seemingly novel problems.