ALT-3 Engine Configurations on Motorcycles
The defining moment for motorcycle design occurred in 1901, with the "new" Werner. It was a heavier, stronger bicycle with a single-cylinder air-cooled engine, with the new electric ignition, fitted in the place more conventionally occupied by the pedal crank assembly, and driving the rear wheel by belt. Hardly radical by today's standards, it was a great improvement on the 1899 Werner, which had its engine mounted above the (driven) front wheel. As was usual at the time, its ignition was by "hot tube" - a platinum tube, closed at the outer end, was screwed into the cylinder head and kept incandescent by a Bunsen burner. It must have been uncomfortable to ride in Summer, and falling off would probably result in its destruction by fire. It was probably no more unstable in practice than the notorious Pennington, which had the cylinder behind the rear wheel which it drove directly, the crank forming part of the wheel. It had a crude form of direct fuel injection, no cooling system and an electric ignition system best described as "optimistic".
Many of today's engine configurations were already known before the lights went out in August 1914. FN produced a four-cylinder bike from 1906, with the engine in line with the frame. Douglas produced a transverse flat-twin (boxer), and Scott produced a water-cooled two-stroke parallel twin. Most other bikes were singles or V-twins.
Between the World Wars, the parallel twin challenged the dominance of single-cylinder sports motorcycles, in the form of the Triumph Tiger 100 while Ariel made a brave attempt at capturing the large tourer market with the Square Four, best described as a collection of solutions to problems of which only the designer was aware while creating a few more of its own. Scott made a 3-cylinder water-cooled two-stroke prototype. In Italy, Guzzi made horizontal singles.On the other side of the Atlantic, V-twins and along-the-frame fours were available, and Henderson produced a handful of inline sixes whose wheelbase must have been measured in yards. Fortunately, most American highways are straight, or nearly straight.
As Solomon famously wrote, "There is nothing new under the sun." Why so many configurations? Which ones have survived?
Two-stroke engines, sadly, are mostly of the past, with only KTM and Aprilia showing any serious interest in their further development. So, I'll concentrate on four-strokes.
Some of the odder and less practical types have disappeared. No one now incorporates a radial engine in the front wheel. No one now makes an inline six, either across or along the frame. The square four is gone, which is a pity in a way, given the performance of the Suzuki RG500.
Single-cylinder engines are compact, simple and light. They are available in surprisingly large sizes these days. They are good for economy, off-road use and commuting. However, if you do a lot of medium or long trips, especially if you ride briskly, you'll probably come to hate them.They don't like high revs, and tend to lose their economy advantage if pushed hard. Because of their large, widely spaced power impulses they are hard on chains and rear tyres, which is a good reason to have belt drive.
Parallel twins are less common than a generation ago. They are almost as light and compact as singles, and smoother and kinder to transmission and tyre. They are generally economical and easy to handle. V-twins are more popular, both for sport and touring, as they are usually smoother (most have a 90-degree angle between the cylinders, which gives perfect primary and secondary balance) and have an abundance of medium-speed torque. Guzzi mount their engine with the crankshaft along instead of across the frame, which gives a shorter wheelbase and better cooling, and warm knees in Winter. It also necessitates shaft drive instead of chain, adding weight and absorbing a bit more power than a chain, although this does not detract from sporting appeal. Guzzis are an acquired taste.
Flat twin engines have traditionally been associated with BMW (although they are also made in Russia, Ukraine and China). The principal disadvantaged of this inherently balanced layout is that cornering clearance is limited by the valve covers grounding. In practice, if you ride a BMW that hard on a public road, you're on the wrong bike. BMWs are for gentlemen who like to travel fast without appearing to.
Three-cylinder engines are associated mostly with Triumph, although older readers will remember Laverda, and maybe BMW's K75. Smoother than a parallel twin, lighter and more compact than a four, and a lovely sound. Groups of three cylinders sing with a euphony unavailable to twins or fours.
Inline four-cylinder engines are abundant. From 400 and 600cc sportsbike engines revving past 15000 to big easy-going all-purpose road bikes, they are mostly smooth, with some secondary vibration intruding at times, durable and reliable. They are also unavoidably bulky, and large-capacity fours tend to be heavy. They are not usually chosen with fuel economy as a major consideration.
Flat fours and V-fours are made mostly by Honda, and mostly as tourers. They are very smooth, and on a touring bike their bulk doesn't matter much. Honda also make a flat-six. I'm tempted to describe it as decadent.
My own preference is for twins, between 500cc and 800cc. There are so many engine configurations available now that you're sure to find something that suits you.
