The Bmw-Puch 250/350 Prototype
THE BMW-PUCH 250/350 PROTOTYPE
Honda's Influence Finally Reaches The Continent
A NEW SORT of BMW will be on the roads soon, if not in 1972, in 1973. It is the result of an agreement between the German firm of BMW and the Austrian firm of Puch.
The BMW-Puch prototype Twin is a four-stroke, highly reminiscent of the old Honda CB77 and CB72 designs. It is being tested in 250and 350-cc versions.
The all-alloy, wet sump engine splits its crankcase horizontally, and the erankpins are opposed at 180 degrees. A single overhead camshaft, driven by a cam chain between the cylinders, times the opening of the valves. Quite a la Japonais.
It seems curious that the. makers are bothering with a 250 version, as. for purposes of standardization, it is virtually identical with the 350. The engine is designed as a 350 with a bore/stroke ratio of 65mm/52mm. To achieve the 250ec displacement, the alloy cylinders of the 350 are narrowed down to 55mm with thicker cast iron sleeves. The only other difference between the 250 and 350 at the present time is carburetion: the smaller engine has two 24-mm Bings, while the larger engine employs Bings of 26-mm choke size. Both machines have a 9.5:1 compression ratio. The output of the 250 is quoted at 29 bhp at 9000 rpm, while the 350 produces about 35 blip, also at 9000 rpm. Peak torque on the 250 occurs at 7900 rpm: on the 350. it is at 7500 rpm.
The battery-coil ignition is by Bosch, with current supplied by a 12-volt three-phase alternator. An electric starter will be optional equipment.
Perhaps the most novel aspect of the machine is that it offers a six-speed gearbox, with shift lever on the left side. Shock damping is incorporated in the gearbox. The clutch is of the multiplate. wet bath variety.
Drive to the rear wheel is accomplished by an exposed chain. There has been some discussion among the makers about whether the production version would incorporate chain or shaft drive. As the crankshaft is disposed transversely in the BMW-Puch frame, shaft drive would seem highly inappropriate. More likely, the chain will remain, running in a fully enclosed oil-bath chain guard, and incorporating an automatic tensioner.
The tubular frame of the prototype is of the conventional double cradle variety, suspended with telescopic forks (8-in. travel) at the front and swinging arm at the rear. Wheelbase is 53.5 in. Present tire size for both models is 3.00-19 front and 3:50-18 rear. Drum brakes are featured, a 7-in. double leading shoe at the front, and 6-in. single cam at the rear.
Claimed “top speed" figures from Europe, with the rider in a sitting position rather than in a racing crouch, are 84 mph for the 250, and 90 mph for the 350.
When the new model arrives on these shores, it could greatly expand the BMW line and the impact of BMW in the U.S.
Traditionally, the BMW motorcycle output has been modest, as the firm concentrates mainly upon its automobiles. Apparently, BMW desires to increase its market penetration, and so has turned to Puch, Which has the production capacity to produce large numbers of motorcycle engines.
