You’ll often read about how some manufacturers opt for one crankshaft design over another. Several crankpin layouts are used in multi-cylinder engines, but to simplify things, we’ll talk about crankshafts in parallel-twin engines. There are currently three crankshaft configurations available in parallel-twin engines: a 360-degree crankshaft, a 180-degree crankshaft, and one of the later developments, the 270-degree crankshaft, currently employed in various models by Honda, Triumph and Yamaha. The degree value in each layout refers to the crankpin configuration.
Each crankpin arrangement has certain requirements to deal with vibration, and each will give the engine certain torque characteristics. The firing order of each arrangement will also give the engine a distinctive sound, and a rider with a trained ear can pick out what type of crankshaft hides inside the crankcase by listening to the exhaust sound alone, without having to look at the spec sheet.
All right, first the basics: A four-stroke engine’s crankshaft must make two full turns, or 720 degrees, to complete all four strokes, regardless of the number of cylinders (“Mojo Garage” July 2013 describes how a four-stroke engine works). Ignition happens once every 720 degrees on a single cylinder, and four times on a four cylinder. This is why the more cylinders for a given engine displacement, the more power the engine will produce. However, the timing in which those power strokes arrive within those 720 degrees will also affect how the powerband feels.
Back to twins. Familiar bikes that use a 360-degree crankshaft include BMW’s F800 twins and older Honda CB400s and 450s, among others. In this layout, the crankpins for both cylinders are on the same plane, so both pistons rise and fall at the same time. Without additional help, this engine will have considerable primary vibration, as both pistons rise and fall at the same time. Unchecked, the vibration would be very similar to an unbalanced, single-cylinder engine, with an up-and-down shaking, called primary imbalance, that can break peripheral components like fenders, engine mounts and gas tanks. Primary imbalance, simply put, is the up-and-down vibration you feel as the piston rises and falls (the actual vibration is more complex, but that’s basically what it is). To negate this vibration, a counterbalancer is used, which is a bob weight placed on a shaft timed 180 degrees from the crankpins, and which spins in the opposite direction of crankshaft rotation. With this configuration, the ignition fires once every 360 degrees. These equally spaced power strokes produce a flat, yet pleasant, droning exhaust note identical to a boxer twin, which also has power pulses spaced 360 degrees apart.
Many parallel-twins of the 1970s, and Kawasaki’s current 650 cc twins, use a 180-degree crankshaft. This crankshaft configuration has opposing crankpins – one piston reaches the top of the cylinder (top-dead-centre or TDC) as the other reaches the bottom (bottom-dead-centre or BDC). This layout has good primary balance, but it will exhibit rocking-couple vibration. Primary imbalance is negated in a 180-degree layout, because the pistons rise and fall alternately. Rocking couple is a kind of sideways vibration, and it, too, is quelled through the use of a counterbalancer.
With this configuration, the first cylinder fires, followed by the second cylinder 180 degrees later. The crankshaft must then rotate an additional 540 degrees before the first cylinder fires again (180 + 540 = 720). This produces a lumpy exhaust note due to the unevenly spaced power pulses, which sound offbeat and not very sexy. This crankshaft layout enhances torque, though, as the two closely spaced power pulses (180 degrees apart) give a significant initial push on the crankshaft within its 720 degrees of rotation.
Finally, there’s the 270-degree layout, which you’ll find in bikes like the Triumph Scrambler, the Yamaha Super Ténéré and FZ-07, and the Honda NC700 and 750 twins. In this configuration, one crankpin trails the other by 270 degrees, so the second cylinder fires 270 degrees after the first, and then the crankshaft must turn an additional 450 degrees before the first cylinder fires again (270 + 450 = 720). The power pulses are spaced exactly as they are on a 90-degree V-twin, such as on a Ducati or Suzuki SV engine. As such, it produces a very appealing staccato exhaust note, and more torque than a 360-degree layout. The primary reason for using this engine layout is its sound, as a 180-degree layout can achieve similar torque characteristics, but will lack the rich exhaust note.
A 270-degree engine exhibits a mixture of vibration characteristics of the two configurations mentioned above, so it, too, needs a counterbalancer.
Although the engine configuration discussed here is the parallel-twin, the same basic rules apply to other engine configurations. For instance, in the 1980s, Honda produced a visually appealing, narrow V-twin angle of 52 degrees, but by using offset crankpins, mimicked the sound and primary balance of a 90-degree V-twin.
Yamaha applies offset crankpin technology on the R1, using what the company calls a crossplane crankshaft that features uneven intervals between the crankpins of its inline-four. This produces a firing order that enhances torque, and gives it a unique, almost V-twin-like sound.
Next time you see a parallel-twin, listen to its exhaust note and try to figure out which crankshaft configuration it uses; chances are you’ll be able to tell the difference.
Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.
