Parallel twin engines have been used in motorcycles for over 100 years and are still in production today, from the traditional air-cooled Royal Enfield 650 to the sophisticated water-cooled Honda NC750 with optional automatic transmission. Because the cylinders are arranged side by side, manufacturers can pack a big punch into a small space. It has often been said that Edward Turner’s Triumph Speed Twin was readily accepted, even by conservative buyers, because it looked so much like a twin-port single cylinder machine.
Historically, the parallel twin’s big rival has always been the vee twin. Many manufacturers evolved from making single cylinder machines to vee twins because they could just graft two sets of their existing single cylinder parts onto a new crankshaft and case. The resulting machines tended to be longer in the wheelbase and more expensive to manufacture, so were usually premium products selling in quite small numbers to wealthy enthusiasts. This is still reflected in prices today, with vintage vee twins often selling for two or three times the price of the best singles.
While the initial investment required to produce a parallel twin may have been higher than for a vee, there were many compensating factors. Most parallel twins could be housed in the same frame as their single cylinder counterparts; there are many examples such as Ariel, BSA and Norton where this practice carried on right up to the 1960s. Parallel twins with the pistons rising and falling together also have an even firing interval so could run happily with either single or twin carbs. The inlet plumbing for a single carb on a parallel twin could be so effective that sporting models with twin carbs were often hard pressed to beat the performance figures of their lower spec siblings.
The Achilles Heel of the parallel twin has always been vibration. Early twins of smaller capacity, in modest states of tune, ran sweetly enough but as buyers wanted more performance and engines grew in size they became increasingly harsh. The Sunbeam S7 and S8 used rubber mountings to cut vibration but few manufacturers followed their lead because without the engine structure to stiffen the frame, a heavier frame design was required to maintain rigidity.
When the Japanese manufacturers launched their parallel twins in the 1960s, they tackled the vibration problem by sacrificing the even firing interval so beloved by British bike makers. Many Japanese twins had 180 degrees between the crank throws, rather than the established 360 degrees. This meant one piston was rising as its neighbour was falling. It didn’t eliminate vibration but did reduce it from a destructive shaking to a high frequency tingle; just as well with redlines of 10,000 rpm or more on models like the Honda CB250.
Modern parallel twins, from BMW, Honda, Triumph and many others, use rotating balance shafts or balancer devices to cut vibration. Though smoother and more useable than the classic twins, they lack the raw character of their predecessors which is perhaps one reason for the enduring appeal of the classics.