- Wed Nov 22, 2017 12:28 pm
#72423
The 270 degree crank for the new twin 650 seems a wise choice and the configuration of the Enfield engine crank and balance shaft seems directly derived from the successful Yamaha twin. The 270° crank is a fairly recent development of the straight-twin engine, which imitates the sound and feel of a 90° V-twin, but requires a balance shaft to reduce vibration. Effectively, the 270° crank is a compromise which allows a more regular firing pattern than a 180° crank and less vibration than a 360° crank. As with a 90° V-twin, the pistons in a 270° inline twin engine are never both stationary at the same time, thereby reducing the net momentum exchange between the crank and pistons during a full rotation. The oscillating momentum manifests itself as an oscillating crank rotation speed, which, when paired with a driven-wheel rotating at the more steady road speed, will introduce an oscillating torque in the drivetrain and at the tyre contact patch.
Examples of 270° parallel-twin motorcycles in production in 2014 include the Donnington Norton Commando, Triumph Thunderbird, Honda CTX700, and Triumph Scrambler.
A feature of the 270° parallel twin is that it provides most of the benefits of the popular V-twin layout, along with further advantages: it is simpler and cheaper both to produce and to maintain than a V-twin; it needs only a single cylinder block and head; induction, exhaust and cooling are simpler; the engine is lighter and has better potential for an optimum CG position; battery location is easier; rear suspension design is simpler; and it allows a shorter wheelbase for sharper handling.
Yamaha devoted much time and effort to the “quality†of the torque produced by the 270 degree engine. While analysing the power and torque produced by the combustion inside the chamber, a theory emerged: maybe the unevenness in the torque curve is created by the forces acting on the two moving pistons when converting the power created by combustion into drive force.
This led the team to altering the angle of the two connecting rods where they couple to the crank, and the prototype engine they produced smoothed the torque curve. While doing prototype test rides in the Algerian desert, the engine’s uneven firing interval (combustion) resulted in a machine that was easier to ride. The torque characteristics were very similar to those of a 90° V-twin, but the new engine was shorter in length, allowing for more chassis design freedom. This in-line 2-cylinder engine configuration represented an innovation.
Norton Motorsports' earlier prototypes had 360-degree crankshafts, like the original Commando. "We were aiming to retain the classic Commando sound," says R&D; chief Paul Gaudio, "but even with a counter-balancer, vibration was too intense. That's why we switched to a 270-degree crank."
With grateful acknowledgments to Wikipedia and other on-line sources (I didn’t make this stuff up myself!)
