BMW R 75/5

Cycle Magazine

Bavarian Motor Works has been manufacturing motorcycles for a long, long time, about 46 years. We recognize them as the big horizontal opposed twins that are smooth, quiet, and run forever. We're not sure if BMW originated the opposed twin idea in motorcycles, we do know they have been building this configuration since 1923.

The basic layout has tremendous advantages as opposed to one disadvantage. Motorcycledom's reason for popularity is its ability to go where automobiles can't. The width of the BMW, the distance from cylinder head to cylinder head, is the largest in the industry. You just can't squeeze it through those tight, narrow places as readily as with other motorcycles.

The reason for hanging them out there is cooling and a lower center of gravity. These two factors result in handling and reliability. In fact, the cast iron lined aluminum cylinders run so cool piston clearance is just a hair over .001 of an inch. That's unheard of for an air-cooled engine with a bore of 82mm. But BMW has refined and improved so much over the years that their reputation for reliability is unequaled.

Our test machine is the very latest, biggest, strongest, and best BMW offered to the public. Approaching its third year since the deletion of the Earles-type front fork, high mounted camshaft, and sidecar mounting lugs, BMW has changed its European image as a sidecar hauler to one of a high performance solo sports bike. The machine went through a complete redesign dropping nearly 50 pounds in the process while adding an electric starter and 150cc's to make the 600 a 750.

When they decided to build this latest generation BMW their only carry-over from the past was a 180 degree horizontal opposed engine, shaft drive, and a blue and white BMW emblem. The heart of the engine is a one-piece drop forged crankshaft. It uses plain bearings throughout, and the connecting rods have caps very much like the automobile's. A flywheel with a starter gear is attached, and power is transmitted through a single dry clutch plate whose lining is bonded instead of the previously riveted-type. Crankshaft throws are 180 degrees apart therefore the pistons travel up and down their cylinders together. This causes a lot of pressures and vacuums within the crankcase so the engine is fitted with a timed breather that vents into the right hand inlet upstream of the carburetor. The crankshaft runs in a near-vacuum atmosphere which is one of the reasons you don't see even the slightest of oil seepage from these engines.

The R75/5 stores its oil in a sump well below the whirling cam and crankshaft. An oil pump that looks like a miniature Wankel engine runs at half engine speed and circulates 200 gallons per hour at 6,000 rpm. Oil is fed to the overhead valve system through pressure to the rocker arm fulcrums and returns back to the sump down the pushrod tubes.

Earlier BMW designs had angled the valves at 90 degrees which requires a high dome piston in order to arrive at a reasonable compression ratio. The R75/5's included valve angle is 60 degrees and the piston is far less domed resulting in a stronger and lighter slug.

Camshaft location has been moved under the crankshaft for several reasons. It gets better lubrication down there, its weight is lower, and now there is room for an electric starter up top. The starter motor could come right out of an automobile, it's driven by a 12-volt, 15 amp battery and there is an electric safety device that will not allow the starter to engage whilst the engine is running.

A 12-volt three-phase alternator is driven off the front of the crankshaft and hides behind a massive alloy cover. Two 6-volt high tension coils live under the fuel tank and they are hooked up in series. This is supposed to draw less current than one 12-volt coil and still spew out gobs of blue electricity.

BMW's transmission is robust and automotive like. For the R75/5 both 2nd and 3rd gear have been increased in tooth width by nearly 100%. They use sliding dogs to engage the gears operated from a cam plate. The new models have gone from 6 dogs to 7 and cut a 10 degree relief on each dog for quicker and easier gear engagement. The throw of the shifter is short and it still makes the clunk when going into gear that BMW's are noted for. The mainshaft of the gearbox has fixed gears and rides on ball bearings at both ends. The output shaft also rides on ball bearings and the gears are bushed. In operation, transmission oil is thrown up into a catch trough located in the end plate of the gearbox. This oil fills the hollow output shaft and lubricates the bushings through drilled holes. Very clever these Germans.

A drive shaft has to have a universal joint and the BMW's is attached to the output shaft. This drive shaft is housed in the right hand swing arm tube and is mounted on a huge set of bearings and fits into the rear hub with splines. The swing arm pivots on a set of Timken tapered bearings that can be adjusted to center the swing arm in the frame. If you stand back and look at the bike in profile you will notice the engine is canted back towards the center of the rear hub. When you sit on the bike the rear suspension compresses allowing the swing arm to come into nearly the same plane as the engine. What you're seeing is a straight line from the output shaft to the pinion gear, and very little deflection of the universal joint. In a sense, the less movement a universal joint has to make, the more energy can be put to the rear wheel.

The rear wheel comes off quickly by removing a single axle. A large spacer drops out and the wheel can be pulled from its spline leaving the driving gear and braking mechanism in tact. Both wheels use full width hubs and straight pull spokes. Rims are aluminum with several notches in their bead. These notches are there to keep the tire from jumping off the rim in case your tire goes flat while in motion. Both wheels are electronically balanced and use S rated tires. An S tire is one rated at 90 mph travel for indefinite periods of time. Other ratings are H for 130 mph and V for 150 mph. This is a carry-over from sporty cars and a good idea. What other motorcycle manufacturers rate their tires?

Conventional expanding shoe in drum brakes are used on the BMWs. These have a special lining that works better the hotter, or more use, they get. We experienced absolutely no brake fade while twisting through the mountains packing double. When cold, or normal city driving, the binders have a hard feel, especially the rear brake which has a single leading shoe. The front is a double leading shoe-type that takes less pressure to make it work.

All control cables on a BMW are thoroughly waterproof utilizing molded rubber dust covers. In addition, the outer cable is nylon lined for a smoother pull. The two handlebar levers are also nylon bushed in their fulcrums and each have a spring washer to keep the lever from rattling.

BMW's exclusive throttle uses a set of gears, two cables, a cam, and a chain. The chain wraps around a progressive cam; as the throttle is opened the slides move up more slowly at first then increase their rate of travel. The R75/5 uses constant vacuum Bing caruburetors so it operates two butterflies and the venturi varies its size as the engine demands. (R50/5 and R60/5 BMWs use the conventional Bing slide-type carburetors and have a more progressive cam in the throttle linkage.) An adjustable drag or brake is attached to the throttle housing so you can set the throttle and not have it close on you by spring pressure.

Suspension and handling of the R75/5 is excellent. The forks have 81/2 inches of travel that is hydraulically damped both on the up and down strokes and at the top and bottom extremes. Rubber boots protect the fork stanchions and have breather holes incorporated in the bottom triple clamps. These fork boots no longer have the vent holes to take in and trap water as the older ones did.

Steering head bearings are Timken taper and the frame head itself is cross braced and gusseted to the double loop tubular frame. The two down tubes start at a much larger diameter where it is needed at the steering head weldment and taper to a smaller constant diameter for the rest of its journey around the engine cases. A large diameter single top tube is used, one that will not take too much area from the saddle of the gas tank. The rear section of the frame is simply bolted on in 4 places although it is a separate welded assembly. The seat mounts on this subframe and locks in with the same key that locks the forks. Under the seat is a water/dustproof tool and glove box that simply pulls out. There are also 2 wing nuts that mount the rear of the fuel tank. Remove these 2 wing nuts, disconnect the two fuel lines and the fuel tank comes off in less than a minute

Source Cycle Magazine