(This Retrospective article was published in the April 2007 issue of Rider.)
STORY BY CLEMENT SALVADORI, PHOTOGRAPHY BY RICK McNEILL
How much does a cylinder weigh on an original BMW K-bike, the one with the longitudinal engine?
If you look at the comparative weights between a K100 and a K75 a cylinder must be good for about 25 pounds…because all BMW visibly did when it made the 1985 three-cylinder version was lop off one cylinder. Invisibly, of course, a whole lot of stuff was done, including redesigned combustion chambers, 120-degree crankshaft, changes to the Bosch LE-Jetronic fuel injection, etc. The most drastic change was the triangulated tail end of the muffler, three sides for three cylinders, so that whoever overtook you would know you were on one of those slow triples.
The K75C, as this initial triple was designated, was the “standard” K75, though nobody ever explained why the letter C was used. It was an OK, if uninspiring motorcycle, appealing mostly to the BMW faithful—who had their faith sorely tested by the earlier introduction of the K100 in 1984. Nobody could argue with the need for the company to move beyond its boxer origins of 60 years past, but a longitudinal in-line four, flopped on its side no less, was a pretty strange concept. Worse, it became notorious for belching out smoke signals if left parked on the sidestand, as the cylinder heads were on the left side and oil could leak past the piston rings and into the combustion chambers.
The eight-valve, liquid-cooled, 987cc K100 engine was rated at 90 horsepower at 8,000 rpm, whereas the 740cc K75C had 75 horses at 8,500—which would translate to 100 horsepower in a liter engine, proving that those “invisible” changes did have a purpose. The K75 also had a respectable 40 lb-ft of torque at 3,000 rpm, maxing to 50 lb-ft at about 6,500 rpm. The K100 and K75 used the same five-speed transmissions, but the K100 had a 17-inch rear wheel, the K75, 18-inch; different rear-end gearing keeping them on a par. Curb weight for the triple, with five gallons of gas in the tank, was 500 pounds.
One mildly surprising aspect of this redesign was that one less cylinder meant a stronger jolt at the clutch, with three harder hits rather than four moderate ones, so the rubber-damped clutch as found on the K100 was tossed and a modified R80 clutch used—those twins gave the clutch a really good jolt. As a potentially sporty mount, test riders soon found that the K75C suffered from soft springing, with a conventional 41mm fork at the front, the single-shock Monolever at the back. While good for commuting comfort, the C pitched and weaved a bit when hard-pressed in the corners. Stopping was effected by twin discs on the front, a drum at the back.
A year later along came the K75T (for Touring), with no changes other than adding saddlebags and a windscreen. It kept the angled luggage rack that had appeared on the C, which nobody liked because any bungeed load would always be trying to fall off.
Like the C, this was a pleasant motorcycle, and if you could live without the extra 15 horsepower it was preferable in terms of smoothness and cost to a similarly equipped K100.
When the K75S (for Sport) finally appeared in 1987, it was like the heavens had opened and the light shone down on the benighted souls in BMW’s R&D division. The S was far from perfect, but it was probably the best two-valve K model to appear. It looked good, and handled well. A little down on power, perhaps, considering what a Japanese four-cylinder 750 was banging out (like a 90-horse GSX-R750), but in the late ’80s BMW was still trying to ignore the horsepower issue. Gentlemen, they felt, did not need excessive amounts of power, and whatever BMW deemed sufficient should be considered sufficient.
The engine was left pretty much the same, but the S also stood for Styling and Suspension. A frame-mounted half-fairing made a world of difference in looks and improved the aerodynamics considerably, adding a good 10 mph to the top speed. Which was still less than that of a R100RS twin, which caused many an S owner to learn the subtleties of drafting when riding with friends. A belly pan also helped with the sporty look, and probably with the speed, as the front of the engine was pretty barn-doorish.
The fork was still 41mm in diameter, but travel was cut down by 2 inches, from the 7.3 inches on the C and T to 5.3 on the S, with stiffer spring and damping rates. As a cost-cutter BMW figured the fork only needed a damping rod on one side…and it worked just fine. No adjustability was built in. The single rear shock, which came off the C/T with slight modifications to stiffen the action, did have spring preload adjustment. Perhaps the ride was not as plush, but the sporting rider did not really care too much about that; he or she wanted a good solid feel in the corners.
Enhancing the sportiness, the rear wheel became a 17-incher with the bigger K100 130/90 tire, rather than the 120/90 of the C/T; every little bit counts when you are hunting for traction. With the new wheel the S also picked up a disc on the rear wheel, so it had three Brembo calipers with dual pistons. All these changes stretched the wheelbase half an inch, from the C/T’s 59.7 inches to the S’s 60.3.
Riding the S was a charm, if you could get used to the very narrow bars, only 22 inches across, though they came with nice soft grips. The saddle was long and comfortable, but the seat height of 32 inches-plus kept the short of leg away—the bike in the photographs has a lower Corbin aftermarket seat. Even a 6-foot, 3-incher would find this very comfortable for long runs, 200 miles between fill-ups. And the stiffer suspension allowed for more spirited riding on the back roads, with less dive on the fork, less wobble under an accelerating rear wheel.
This was a nigh-on perfect sport-touring motorcycle, though it still had that ill-conceived small luggage rack with a 30-degree slope. Saddlebags were an option. Price was a little high at six grand, but you were getting a great ride as well as the blue-and-white roundel.
In 1995 the factory, for reasons never fully explained, decided to drop the triple while still spending wads of money on improving the four-cylinder version. Which, as we all should know, has metamorphosed into a transverse-mounted in-line four with oodles of horsepower—maybe BMW is planning to knock a cylinder off that one.