BMW R 100CS

Who would call BMW "trend setting?" Even as motorcycles in general have become bigger, faster, more complex with more and more cylinders and valves and carburetors or even computer-controlled fuel injection, BMWs have come with the same basic engine layout since 1923—opposed Twin, two valves per cylinder, push-rods, two carburetors and a driveshaft to deliver power to the rear wheel.

The 980cc 1981 BMW R100CS is still an opposed Twin, still ohv with pushrods, still shaft-driven, retaining the original concept of the marque even though the design and execution have been refined and honed. It is expensive, selling for $6600, and it is very light, weighing 455 lb. with half a tank of gas.

Simplicity, weight and price are related. BMWs have long been light, some of the low weight coming from simple design and some from the use of lighter, but often more expensive, materials.

The latest trend in Japanese motorcycles is toward lightness. The Kawasaki KZ750 was big news when it was introduced last year, being much lighter than its 750cc competitors at 491 lb. with gas. The Kawasaki KZ1000J, light for a lOOOcc Four, weighs 519 lb., and the new Yamaha XV920 V-Twin weighs 519 lb.

BMWs were light before light became fashionable, before that particular trend began in Japanese machines.

Speaking of trends the R100CS is, in a way, one of a line of trend-setters. Elsewhere in this issue there is the Kawasaki 550GPz, which comes with low bars, semi-rearset pegs and a small fairing. Like the Yamaha Seca 550, Suzuki GS450S and Honda CBX, the GPz is the latest sign that the Big Four realize there are people who like sporting bikes.

BMW has known this for years. Back in 1973 there was a policy change and the German factory moved away from its rather stodgy image and got into sports. The result was the R90S. It had—as you've surely guessed—low bars, setback pegs and a bikini fairing.

The R100CS is a direct descendant. BMW has at the same time trimmed and broadened its model line. There's the middleweight R65, the explorer R80GS and four versions of the boxer Twin in lOOOcc form; the RT with full fairing and upright

riding position; the RS with a smaller, wind-tunnel-style fairing and lower bars, the standard RlOO, and the CS, with belly-button fairing.

The BMW's relatively low weight hasn't been used to turn the machine into a high-performance motorcycle. Indeed, the other lOOOcc bikes outdistance it in a test of acceleration. The BMW turns the quarter mile in 13.18 sec. with a terminal speed of 101 mph. The XV920 turns 13.06; the KZ1000J, 11.56; the KZ750, 12.26. Even the Seca 550 is quicker, turning 12.99.

The appeal of this BMW, like others of its make, is not rooted in raw horsepower or flashing speed. There is an image of BMWs being reliable and long-lasting. It is, in a sense, self-generating. The image attracts a certain type of rider, a man who values perceived quality above performance, a rider less likely to abuse his bike while hot-rodding around town. BMW buyers are likely to be more mature in age and attitude, and the way they ride contributes to long engine life. Which adds to the image, which attracts more of the same type of rider.

Then, too, BMWs come with relatively tall gearing. The R100CS engine, for example, turns over at a leisurely 3460 rpm at 60 mph in top gear, again contributing to long engine life.

Look at a BMW engine and the most obvious things are the two cylinders, one sticking straight out each side. The placement puts the cylinders and heads and exhaust pipes out into the middle of the airstream, which improves cooling. Each cylinder has a 40mm Bing CV carburetor, the intake manifolds angled inward to give the rider more shin room. Curved rubber tubes lead from the carbs to the airbox, which holds a flat, pleated-paper air filter. Air is drawn into the airbox through two forward-facing plastic snorkels.

The plain-bearing crankshaft runs parallel with the wheelbase, located front-to-back in the frame. The crank throw for the left cylinder is ahead of the crank throw for the right cylinder. Stroke is 70.6mm, while each piston is 94mm in diameter.

New for 1981 are all-aluminum cylinders. Instead of cast-in iron liners, the latest BMW cylinders have a nickle/sil-icone-carbide coating applied to the bores. BMW engineers say that compared to iron liners, the bore coating increases heat conductivity (and thus, cooling efficiency), reduces break-in time and oil consumption, and makes each cylinder 3 lb. lighter.

sump crankcase below the crankshaft, and is driven off the crank by a duplex timing chain. Valves are operated by pushrods via rocker arms, and valve lash is adjusted by conventional screw tappets. The crank-case itself has been made stronger for 1981 and has a larger oil sump.

Also new in this year's BMWs is the clutch assembly, the unit weighing 40 percent less than the 1980 flywheel and clutch. As before, the flywheel is mounted to the crankshaft with a single automotive -type dry plate carried on a splined transmission input shaft. At the opposite end of the input shaft is a helical gear, which feeds power to the transmission mainshaft. The transmission countershaft is connected directly to the driveshaft coupling, and power is delivered to the drive-shaft through two u-joints. The driveshaft runs inside the right swing arm tube and transfers power to the rear wheel through a crown and pinion gearset. The final drive the single-arm, single-shock dual purpose GS but used on dual-arm, dual-shock road models as well.

To meet EPA emissions requirements, air is drawn from the airbox and fed into the exhaust ports through a system of oneway valves and external pipes. Like the system on 1979-and-later Kawasakis, the BMW clean air injection system doesn't use a car-type air pump, but instead relies upon vacuum created in the port by exhaust gases rushing out of the cylinder. When the clean air meets the hot exhaust gases, unburned hydrocarbons ignite, reducing emissions.

For the first time on a BMW, an electronic ignition system is used, firing the plugs through new, lighter coils, one coil per cylinder. The exhaust system is changed as well, now using two cross-over tubes (instead of one) between the two separate cylinder exhaust/muffler assemblies.

There was a time when BMWs could be described as being vibration free, at least relative to other machines. But now, when rubber-mounted four-cylinder motorcycle engines are common, the BMW cannot be realistically called smooth throughout the rpm range. Those big, 94mm pistons move back and forth in opposition, that is, each piston starts downward (inward, in this case) from top dead center at the same time. Viewed from the rider's position, when the left piston is traveling to the right, the right piston is traveling to the left. That arrangement tends to cancel out much of the vibration that otherwise would be produced. However, the cylinders fire alternately, so at low rpm (especially below 3000 rpm) vibration from power pulses can be felt through the bars, and images in the mirrors blur. At higher rpm, torsional vibration exists, caused by power being delivered first to the forward throw of the crank, (forcing the front end of the crankshaft to the right), then the rear throw of the crank, (forcing the rear end of the crank to the left). The amount of movement is minute, but it is enough to set up a vibration.

The 1000CS has a classic sweet spot. Between 3000 and 4000 the engine is nice. At 3750 rpm (65 mph) it is perfectly smooth. The mirrors are like, well, like glass. The engine has power and all those little forces cancel each other out, so when it's kept within the rev range it likes, the CS is as smooth and happy as any engine on the market.

The engineers must have known this because the gearing puts this sweet spot right where it does the most good. In top, it's from 50 to 65 mph, while the rider quickly learns to use the gearbox in traffic and when cruising around town.

It is important to note that even below and above that rpm, the kind of vibration produced by the BMW is very different from the high-frequency buzz noticed on higher-revving multis. The BMW's vibration is well controlled and of low frequency, and it is not intrusive or annoying even on long trips. Just how different vibration can be is best illustrated by jumping off a Suzuki GS850 and climbing onto an R100CS. Both are absolutely, perfectly smooth compared to a Harley-Davidson Sportster or a Triumph Bonneville. But in actual fact, the Suzuki has an underlying buzz felt through the bars, and the BMW has an underlying succession of pulses, a muted sort of low-frequency vibration, reminders that very large pistons are moving under the force of alternating explosions of vaporized gasoline.

Running at higher speeds for long distances gives the impression of the engine working, and the faster it runs, the more vivid the perceived pulses through the bars.

This year's engine gains revs much more rapidly than last year's, due to the lighter flywheel and shifts better, too. The clunk that used to accompany each shift is gone, and because the engine can change speed quickly with the reduction in flywheel weight, the tendency to miss shifts or have difficulty shifting is gone. That's good, because in the case of a missed shift bringing rpm much over the 7200 rpm redline, it is common for those great big pistons to collide with the equally-big exhaust valves, requiring a top-end overhaul before its time. This BMW may not buzz like a Suzuki, but then it can't be revved to 10,500 rpm either.

While the R100CS accelerates and shifts better than previous BMWs, it still gets good gas mileage, 50 mpg on the Cycle World mileage test loop, and because it comes with a large gas tank, it has exceptional traveling range, about 265 mi. before reserve.

In keeping with BMWs penchant for simplicity, the R100CS has standard forks without air adjustability, without preload adjustability, without damping adjustability. The rear shocks have adjustable preload, period. Which means that the selection of damping and preload and spring rates must be a compromise suited to a wide range of uses and rider weights, and that compromise shows up when a 140-lb. rider encounters successive small bumps, like concrete highway expansion joints. Then the BMW's suspension is not as compliant as the suspension on a GS1100 set on the softest settings, and the jolts reach the rider and make the speedometer and tachometer bobble up and down on their rubber mounts. The bike does handle well at high speed on smoother surfaces.

In street corners, the footpegs on each side drag first, and it will take a brave soul to drag the cylinder heads. On the racetrack, it is possible to drag the heads on each side, with the stock Metzeler street tires.

Because it is light and because the center of gravity is relatively low, the BMW responds more quickly and easily to rider input in ess turns, making an accurate transition from right to left to right without hesitation. But the rider must be careful of two things if the bike is ridden at top speed. First, upon accelerating over a-brupt, drastic pavement changes or potholes at speed, the front end feels very light, as if the front wheel isn't staying on the ground, and the bike changes direction and the handlebars wiggle back and forth as the front wheel is deflected. To its credit, the bike will return to its original course without requiring any action by the rider other than hanging on.

Next, BMW engineers may have proceeded the Japanese craze for low weight and even the current trend of shaft drive, but they still haven't figured out how to eliminate driveshaft-induced torque reactions in the rear end of a motorcycle. The lighter flywheel seems to accentuate and draw attention to the unchanged drive-shaft reaction. Grab a handful of throttle and the rear shocks extend, the rear of the seat rising as the driveshaft pinion works against the ring gear in the final drive. Slam the throttle shut, and the rear end sags. Even when the rider is careful and the throttle is not worked rapidly to the extremes, the rise and fall of the rear end is pronounced and very real. Holding the gas on over a dip in the road will prevent the shocks from absorbing that dip, an advantage perhaps if the dip would normally cause the shocks to bottom, a disadvantage if the dip is at a corner exit and inaction of the shocks causes the rear tire to slide under power.

Enter a turn leaned over to the limits and close the throttle, and the ground clearance will be reduced. To be ridden at the limits the BMW demands adjustments in approach. Several other motorcycles with driveshafts, including the Suzuki GS650, the Yamaha Seca 750 and the Honda CX500, have less torque reaction at the rear wheel.

This year all the BMWs larger than the R65 get new forks, leading axle style with the usual long travel, but with better damping control than that of previous BMW forks. Instead of the rocking horse motion that used to come when a BMW's brakes were applied, the new forks behave much more predictably because of the improved damping. Ride quality of the forks is at least as good as the old models, perhaps better.

Evaluating the ride quality of a BMW means coming to grips with the seat. It's a reasonably sized and shaped seat, with very firm padding and a tendency to break-in or develop a new shape as it's sat on for long periods of time. Because the seat is much firmer than many of the other big bikes, it makes small bumps more noticeable and that makes the suspension seem to be harsher than it really is. In any case, the BMW doesn't have one of those seats that everyone immediately liked, but it is possible to survive on it for quite some distance.

Interesting, too, is the failure of BMW engineers to come to grips with sidestand and centerstand placement and design. True, the centerstand's position is such that if the rear wheel is removed, the bike rocks forward, perched on stand and front wheel. If the front wheel is removed, the bike rocks back, supported by stand and rear wheel. But it is also true that reaching and using the centerstand is difficult; that once the centerstand is lowered by a small extension on its left side, then the rider must shift his foot inward onto the main part of the stand to deploy it, and that during the foot shift the stand has an annoying tendency to spring upward into the retracted position; that the centerstand itself is too narrow to support the bike securely on uneven, rough, or sloped pavement; that reaching and deploying the sidestand is almost impossible to do without first getting off the bike and groping with the toe. There must be, and are, better ways to build stands.

Footpeg position, happily, has been improved. But the higher, more forward pegs are still so close to the passenger pegs that a passenger's feet hit the rider's feet if the passenger pegs are adjusted to the forward position. And BMW issued a recall earlier this year of right-hand footpegs, saying that the peg location was dangerous and prevented easy reach of the rear brake lever. Our test bike was delivered before the recall, and it was difficult to reach the rear brake quickly.

The rear brake is a mechanically-operated drum, the front a double-disc activated by hydraulic pressure with Brembo calipers. The front master cylinder is located on the right handlebar. The 10.125-in. discs are small compared to many other machines, but yield good braking power for the weight and speed of the bike. Brake feel was a matter of controversy among staff members, some thinking the front brake felt mushy, some thinking it was fine. The brakes on 1981 BMWs are better than those on previous models.

Horns. Happily, the horns on the BMW are state-of-the-art, making one wonder why all bikes don't have such wonderful, loud, effective horns. Same for the big quartz-halogen headlight. Instruments are well-lit and easy to read, including the fairing-mounted clock and voltmeter, and accurate.

BMW controls are different and interesting. After years of staying with the up down turn signal switch while everybody except Harley-Davidson used right-left, BMW has gone to right-left for the signals. At the same time the horn button is at the top of the cluster, a distance from the rider's thumb, and the signal switch is at the bottom, opposite to what the others have. No big deal; BMW buyers tend to keep their bikes for years and don't swap brands much, but for the first few days it is likely you'll intend to politely signal and will instead rudely honk, or vice versa.

More important in the long run is the sad fact that BMW no longer fits a standard throttle stop. There's no way to set the throttle for cruising and the return springs are much firmer than two CV carbs warrant.

The finish of everything—castings, paint, stampings ... everything—makes the typical mass-produced motorcycle look, upon close examination, cheap, unfinished. Examine the fins on a BMW cylinder or the finish of the cases, and you'll not find any defects hidden by thick paint. The welds are works of art, the pin stripes hand-painted instead of being stuck-on de-cals, the fenders light-but-pricy fiberglass instead of cheap and heavy stamped steel.

The tool kit is a real, usable set of tools, not pot-metal imitations of tools, that fit and can be used to do major work on the motorcycle. Included are tire irons capable of removing a tire, a patch kit capable of patching an inner tube, and a tire pump capable of inflating a tire.

Maintenance is simple and straightforward—one bolt secures each rocker cover, and valve adjustment is via screw tappets. There are, of course, only four valves total to adjust on the motorcycle.

The rear hub is cleverly designed so that the ring gear and brake assembly stay in place when the rear wheel is removed, the wheel hub including a spline that fits into the drive assembly. To remove the rear wheel, one pinch bolt must be loosened on the left end of the axle, and one axle nut (on the right side of the axle) removed. Then the axle easily slides clear and the rear wheel comes out while the bike is supported by the center stand and front wheel. It takes about two minutes.

It's possible to buy a BMW, ride off the showroom floor, with nothing except riding gear and the bike, travel out onto the highway, get a flat tire, repair the flat tire, and continue on your way, no truck needed.

That's something, right there. In fact, we can't name a single other brand of motorcycle that comes, standard, with the same degree of self-sufficiency and ability to be maintained and repaired by the owner.

We can't think of another brand of bike which is finished as nicely, either.

Which, we're sure, has a lot to do with why the BMW—1923 concept through 1981 execution—is alive and well in America today.

Source Cycle 1981