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1. Frame. If you've owned K-bikes, you know that nothing flexes
between the steering head and the swingarm pivot. Very stable. Not
so on BMW airheads, with their double-cradle steel tube frames, which
earned airheads (like the R90S) the nickname Gummikuh (rubber cow).
2. Brakes. The R90S has 70s-era brakes. Not so good by comparison
to late 80s-early 90s K standards. Many of the Ks also have ABS. Not
so the R90S.
3. Engine: K-bikes engines are liquid-cooled fuel injected, computer
controlled, and exhaust catalyzed. The R90S, like other airheads, is
carbureted and air-cooled. Stay out of traffic jams on the highways,
or pull over and shut it off.
4. Tires: The post-90 K-R bikes models have radial tires. I've
found the R90S, like the KRT/LT models, to feel somewhat tall with its
bias ply tires.
That said, several years back I bought an R100RS just to own one, and
to understand the experience was. Didn't hold a candle to a KRS in
terms of performance, but had a certain heritage-appeal.
