With the demise of two-strokes due to increasingly strict emissions norms, almost all motorcycle engines these days are four-stroke units. However, they still come in multiple flavours, including the ever-popular single-cylinder mill that's still going strong even after a century, and the more advanced twins, triples, fours and even six-cylinder engines, all of which are currently in production. Here, we take a look at two of the more popular motorcycle engine formats – parallel twins and V-twins, examine their key differences, and take a look at the advantages and disadvantages they offer to the rider.
The majority of motorcycles produced today, especially in India and other Asian markets (except Japan), are powered by air-cooled single-cylinder four-stroke engines of 100-250 cc capacity. However, in markets like Europe, North America and Japan, where motorcycling is also a leisure activity, and where motorcycles are not used only for commuting, larger displacement engines are common, and two of the more popular formats for such engines are twin-cylinder units in either the V-twin or, to a lesser extent, the parallel twin layout.
Starting in the 1930s, motorcycle manufacturers like Triumph started using parallel twin engines to power their motorcycles and other British manufacturers like Norton, AJS, BSA and Ariel also produced parallel twin-powered machines. From the 1960s onwards, Japanese manufacturers like Honda, Yamaha, Kawasaki and Suzuki followed suit, with the parallel twin format becoming quite popular in the period from the 1960s to the 1980s. With parallel-twins, the cylinders are placed side by side above the crankshaft, with a firing order of 360°, 180° or 270°.
V-twin engines, where the two cylinders are arranged in a 'V' formation, have also been used on motorcycles for the last 100 years. Currently, manufacturers like Harley-Davidson, Indian and Victory use the transversely-mounted (cylinders in line with the chassis) V-twin engine format for their cruisers and touring bikes, while Ducati uses V-twins for its high-performance superbikes and dual-purpose machines. In fact, Ducati V-twins are some of the most advanced, high-tech V-twin motorcycle engines ever produced and are now not too far behind higher-revving inline-four and V4 engines in terms of specific output.
Manufacturers such as Moto Guzzi also use V-twin engines on their bikes, albeit theirs are horizontally opposed V-twins, mounted longitudinally (across the chassis), while BMW uses horizontally opposed 'Boxer' twins, where the two cylinder lie flat at a 180° angle from each other.
With V-twin engines, the angle of the 'V' formed by the cylinders affects engine behaviour, with a 90° angle (as used by Ducati and Moto Guzzi) providing the best primary and secondary balance. Narrower angles of 40-60°, as used by Harley, Honda and Aprilia, result in a narrower, more compact engine but sometimes at the cost of higher vibration levels. With such engines, manufacturers sometimes use offset crankpins, which can reduce engine vibration.
Coming back to parallel twin engines, these are no longer as popular with motorcycle manufacturers as they once used to be two or three decades back, but nevertheless are still being used by both European and Japanese manufacturers in a limited way. With two cylinders arranged side by side and with their pistons connected to a common crankshaft, parallel twins can be more compact than V-twins, cheaper to produce and sometimes, easier to service and maintain. Indeed, even car manufacturers have made use of the parallel-twin format in recent years, with the Tata Nano engine and Fiat's 'TwinAir' engine (used on the Fiat 500 and Fiat Panda) being two such examples.
Unlike V-twins, parallel twins do not use a common crank pin for connecting both con-rods and can be engineered with different crankshaft angles, with 360°, 270° or 180° firing orders. However, parallel twins with a 180° or 360° firing order usually have higher degrees of vibration, while engines with a 270° crank can feel more like a 90° V-twin. The Yamaha TRX850, launched in the mid-1990s, was one of the first bikes to use a 270° firing order on its parallel twin engine and while other manufacturers have followed in Yamaha's footsteps since then, it must be noted that such engines often need a balance shaft to reduce vibration.
The 270° crank is effectively a 'middle path,' with a more regular firing order compared to a 180° crank, and reduced vibration compared to a 360° crank. As with a 90° V-twin, the two pistons in a 270° parallel twin are never stationary at the same time, thereby reducing the net momentum exchange between the crank and pistons during a full rotation.
In the case of air-cooled engines, parallel twins are usually better positioned than V-twins for efficient cooling, with both cylinders placed up front in the centre of the airflow. With V-twins, cooling the rear cylinder (the one facing away from the direction of travel) can sometimes be a challenge and the rear cylinder runs hotter than the front, which can affect fuelling. This, however, is less of a concern with liquid-cooled V-twin engines that do not rely purely on air flow to cool the engine.
While parallel twin and V-twin engines have different traits in terms of power delivery as well as 'feel' and NVH levels, which is important for the rider, the engine format is even more critical for motorcycle designers and engineers. Parallel twins are usually more compact and provide more flexibility in terms of placement within the motorcycle's chassis, which makes life easier for chassis designers, who have a greater degree of freedom to move the engine forward or backwards in the chassis, thereby also controlling the bike's centre of gravity and, consequently, on-road behaviour. On the other hand, V-twin engines (except the ones with horizontally opposed cylinders, which Moto Guzzi uses) are typically longer and thinner, which lets manufacturers produce narrower motorcycles that some riders prefer.
In terms of engine vibration too, new rubber mounting solutions have mitigated a lot of the issues that were earlier associated with narrow-angle V-twins, while most modern parallel twins now come equipped with a counter balancer, which again reduces the engine's tendency to vibrate. Of course, 90° V-twins (the kind that Ducati uses) have perfect primary balance (because with a 90° firing order, the movement of one piston is always exactly and completely counter-balancing the other) and do not suffer from excessive vibration.
Of the two, no one engine format is necessarily better or worse than the other – parallel twins and V-twins both have their respective advantages and disadvantages, as described above. Credit has to be given to Ducati for pushing technology to unimaginable levels with the V-twin format and their engines are now some of the world's highest performance units, surpassing the power and performance of many inline-fours and V4s.
On the other hand, while the parallel twin has declined in popularity, manufacturers like BMW, Yamaha, Kawasaki and Triumph still appreciate its relative simplicity, lower costs of production and ease of maintenance, and continue to produce many parallel twin-powered motorcycle that motorcyclists worldwide love to own and ride.
With renewed interest in forced-induction engines in recent times, which use superchargers and turbochargers to boost performance from relatively smaller engine capacities, it remains to be seen whether the parallel twin format will see a big-time resurgence in the world of motorcycles.
Text: Sameer Kumar