BMW K75C SPECIAL FEATURES
BMW K75C SPECIAL FEATURES
BMW K75C :The "building-brick" principle of the BMW K75 as the next logical step in K100 technology goes something like this: The designers will tell you the bikes differ only in the disappearance of one cylinder and the rebending of the frame's front downtubes. But ask them about the K75's combustion chambers: "A little different," comes the answer. And the ignition curve? "A little different." Suspension front and rear? "A little different." Fuel delivery? "Also different, ein bisschen." Clearly the 75 is its own motorcycle—even a ride around the block tells that story.
BMW K75C : Out on the road, this weight redistribution makes a phenomenal difference. The rider's perception of overall trimness is such that unknowing riders jumping off a K100 and onto a K75 might think they'd also jumped out of 100 pounds or more. Indeed, weight loss and redistribution were claimed to be the key factors in BMW's decision to make the K75 a triple and not just a short-stroke K100. Although shortening the 100's stroke would have raised the redline and cost less than developing the triple, BMW estimates it would have saved only a pound of mass.
BMW K75C : In case you've forgotten how unique the K-bikes really are, let's review some engineering details. A longitudinal crankshaft transmits power to a second engine output shaft lying parallel to the crank. The last web of the crank, cut as a helical-toothed gear, meshes with the clutch drive and attached output shaft, while the nose of the crank drives two overhead cams via a roller chain and also controls ignition-pulse duties. The front of the output shaft drives the oil pump and the water pump. Spinning at crankshaft speed but in the opposite direction, the output shaft cancels any torque reaction. Bosch LE\Jetronic fuel injection feeds each cylinder at its intake port.
BMW K75C : Although the K75 closely resembles the four-cylinder K100 in general layout and concept, making the triple design work necessitated significant changes. First the crankshaft, set with its rod journals at 120-degree intervals, delivers one firing impulse for every 240 degrees of crank rotation. The crank's 70.0mm stroke pulls 67.0mm pistons through an undersquare cylinder configuration measuring the same as the K100's, but the pistons themselves are not the flat-crowned slugs of the 1000cc bike; rather they have slightly domed crowns with two valve-relief pockets to boost the 750's compression ratio up to 11:1 from the K100's 10.2:1.
BMW K75C : Give some thought to these compression numbers. The 750 CR king is Yamaha's FZ, running 11.2:1 figures; Suzuki's GSX-R has 10.6:1 pots. These bikes have complex four- and five-valve heads, along with sophisticated combustion-chamber design—both BMW's K-bikes use two-valve heads. K100 combustion chambers also have a prominent squish band, a design feature completely.eliminated on the 750. Yet our K75 never pinged during testing, even when run on unleaded super at 91 octane
BMW K75C : Furthermore, the K75's soft, long-travel suspension harries the rider engaged in hard braking. Staff opinion concerning the 750's front binders is divided: fast riders like the Brernbo's powerful grip; less racetrack-inspired individuals find its touch tough to modulate. The BMW has no shortage of braking power at hand and foot: the double discs will lock up the none-too-skinny front wheel on command, likewise the rear. Like those of the last K100 and R80 we rode, though, our K75's brake lever had a good deal of initial travel and a very narrow, medium-pressure arc of engagement. The combination of a hard-to-modulate, grippy front brake
BMWs, much of the package's appeal. Our K75C's bar was about perfect for all testers. The clutch is smooth, with a powerful, linear engagement, but, like the brakes, comes on strong in the last few degrees of throw; you'll not be speed-shifting on the Beemer. The K75 feels to have nowhere near the flywheel effect of the K100, and the K100 had nothing compared to the boxers. Still, the 750 takes off quickly from low engine speeds. The shifter is a long-travel device, but crisp, and gearbox ratios complement the engine's power curve well. Although the throttle is also a long-travel affair, fuel injection means it can use light return springs, a blessing after 150 miles of winding roads. The injection system itself was flawless on our 750, as we've come to expect.
specific features:
Make Model
|
BMW K 75C |
|
Year
|
1984 |
|
Engine
|
Four-stroke, horizontal three cylinder in-line, liquid cooled, DOHC, 2 valves per cylinder |
|
Capacity
|
740 |
| Bore x Stroke | 67 x 70 mm |
| Compression Ratio | 11.0:1 |
|
Induction
|
Electronic injection, Bosch LE-Jetronic |
|
Ignition /
Starting
|
Electronic ignition Bosch LE-Jetronic |
|
Max Power
|
75 hp 55 kW @ 8500 rpm |
|
Max Torque
|
68 Nm @ 6750 rpm |
|
Transmission /
Drive
|
5 Speed / shaft |
|
Front Suspension
|
Telescopic fork with hydraulic shock absorber |
|
Rear Suspension
|
Monolever - swinging arm |
|
Front Brakes
|
2x 285mm discs 2 piston calipers |
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