I've got a 1985 K100RT. Good new tyre on the front, and a pretty good part-worn rear. Both at the right pressure.
The bike has just started following ridges in the road surface and feeling really squirrely on white lines. I've also twice had the feeling that it was tipping into the corner, almost like the front is breaking away.
Any ideas?
The wheels aren't loose - no play in the bearings. Forks good and clamped-up in the yokes.
It's odd - the bike used to follow ridges a bit with the old front tyre on (Battleaxe), but it became perfectly neutral when I renewed the tyre. And now this.
Perplexed.

mine does the same thing + dancing like crazy on fast corners... i have a new rear tire, pretty good front one, new rear chock and new oil in the front - what's going on?

You do not say what tyres you have fitted. K bikes are very fussy about tyres especially on the rear. When I bought my RT it had a Continental tyre on the rear which did everything that you describe. I replaced it with a Bridgestone BT45 which cured the problem. Even when well worn it was much better than the Continental. I now have an Avon Roadrider on it and so far it is every bit as good as the BT45. I am on my second Dunlop Arromax on the front, and find it OK. Another thing to check is the steering head bearings. The grease in them dries out making the steering stiff and causing strange handling especially at lower speeds.

Some of the Ks just take a dislike to a particular tyre. Mine likes a BT45 on the front and a Michilin Macadam on the rear.

The steering head bearings will cause you problems and if they are worn to notches they try to centre and so will push you all over the place.

Tyre pressure also has a bearing on what it is doing.

k freaks on the right thing ......could it possibly be the steering head bearings ? im purdy confident if nothing else has changed this is a place that i would be looking .....

I inflate to a minimum of 38 up to 40 cold psi in the front, solo, 40 - 42 cold psi in the rear, solo, 2 more plus psi if riding with a pillion or gear on Pirelli Sport Demons front and back. When I park the motorbike on the centre stand after a ride I swing the 'bars back and forth full lock a couple of times to 'spread' the now warmed-up grease in the bearings, and I check the top triple clamp lock nut for 'snugness' with a large spanner every so often. How's the condition of the rear shock? When mine began to follow grooves a bit too much I whacked a new shockie on the back and swapped to a pair of the then new Pirellis from Bridgestone S11, which were about the hardest and cheapest tyres one could fit to a motorbike of this weight and girth. These two improvements vastly changed the way the bike behaved on tar snakes and tarmac grooves.

Yep, I'm with TWB and KFreak on this one. If she feels queer in the rear around corners (wallowing), check the steering head bearings, then make sure the rear shock is ok. As for the tyres, I hated the Battlax BT45 front because od the centre groove, so I went for the Michelin Macadam ME50 with a central rib. It was excellent on the front when combined with a Metzeler Lazertec on the rear. Pressures at 38 psi front and 44 psi rear. Mine used to tip into the corners real nice, so much so that controlled peg scraping could be done with confidence. The K12 can't be punted as quickly 'round the Tassie bends as the 'ol K. Yes, correct, even with all the creature comforts, I miss the K.

Thanks guys. The rear shock is OK and the steering head feels fine. Had to do a fast run home last night so I "made progress" and forgot about the twitchiness. Which promptly faded away to a minor background noise. The rear tyre is a fairly squared off Battle Wing - a semi-trail job as the previous owner lived down a track. I'll try a full road tyre when its life is up.