The history of the single

Ninety per cent of singles are learner bikes or big trailies. Does that make them irrelevant to modern motorcycling life? Are they just the poorest cousin of all bike engines, reserved for the crappiest bikes on the market? No, not on either count.

At the moment, there are some great single cylinder bikes around. The BMW F650GS might be the cheapest Beemer you can buy, but it’s a joy to ride, and the perfect bike to show off every advantage of the modern single cylinder. Honda’s NX650 Dominator and Yamaha’s XT600E are more serious, but just as enjoyable as the BMW on or off road. Then there’s stuff like the awesome Yamaha YZ426 and new Honda CR450F motocrossers, which have cleaned up in their relative classes. They’re not singles to save money – they’re singles to gain the perfect combination of weight-saving, rideability, traction and low-down pulling power.

History of the single:

It wasn’t always this way, though. The first petrol engines ever, more than 100 years ago, were singles. After all, when you’re still trying to get one cylinder working, you’re hardly going to add extras are you? But it wasn’t long before motorcycle manufacturers were adding cylinders in every configuration possible. Flat-twin, parallel-twin, V-twin, triple, in-line four, across-the-frame four… there was just no stopping them in the early 1900s.

But that didn’t mean that the single was forgotten, because it was still the mainstay of the industry. At a time when ideas were big but engine knowledge miniscule, the single was always going to be the safe option anyway less likely to throw a con-rod out of the engine and into the rider’s nads.

After World War One, the ideas got bigger still. Triumph hired celebrity engineer Sir Harry Riccardo, who designed them an all-new single with a four-valve head, and AJS followed with an advanced 350 single. Triumph watered down the Riccardo design, but AJS really went for it, and their 350 powered one of their Junior TT ‘Big Port’ racers to a 500cc Senior TT win – an amazing feat.

Norton turned their overhead-valve engine (with camshaft in the bottom end of the engine and pushrods operating the valves) into an overhead camshaft design (cam at the top, as in virtually all modern designs), It was said that these big Nortons revved so slowly that they fired just once every lamp-post, but they pulled over 100mph.

The single’s success story continued into the 1930s – one of the greatest ever singles was the BSA Gold Star, a 496cc sports bike that was created after the similar Empire Star model had topped 100mph at Brooklands, and so won a coveted Gold Star award. The resulting model was so popular that it lasted until 1963.

And then the Japanese arrived. Not with a fire-breathing four-cylinder but with a 49cc four-stroke single – the Honda Cub, better known as the C50/C70/C90 step-thru range. To think that these little things were first built in 1958, and are still sold new today is incredible, but that’s because they were so damned good. Over 26 million have been sold so far. If the Cub hadn’t been so good, then the Japanese might not have been able to develop the superbikes that followed. But it was, they did, and Brit bikes all but disappeared, taking the big capacity single with it.

It wasn’t until Yamaha introduced the XT500 in 1977, and then followed it up with more advanced big singles in the early 1980s, like the XT500 and the SRX600, that singles started to sell again. It’s these big singles that we’re most interested in, because they can be such an unexpected laugh to ride.

The good and the bad:

When big-capacity singles died out for a while it was mostly due to one defining characteristic of the engine – vibration. The whizzy-whirry four-cylinders had demonstrated that motorcycles didn’t have to shake and rattle, and anything that did was deemed archaic.

The poor old single had no chance. With just one piston thundering up and down inside the cylinder barrel, only firing on every other revolution (for a four-stroke at least), vibration is inevitable. If the engine is going to be a decent size, then the cylinder is going to be big and the piston that fills it will be heavy, however advanced the materials used to make it.

A 500cc four-cylinder would have four pistons, each one quarter the size of a 500cc single’s piston, and they’d be set to even out vibration, with two reaching the top of their cylinders as the other two reach the bottom. No such luxury for the single.

The only way to even out vibrations in a single has been to strongly counterweight the crankshaft: the crank is built up with heavy weights on the opposite side to the piston, to produce an equal and opposite force to the piston. Some engines have separate balance shafts as well, which work on the same principle. Trouble is, heavy crankshafts and balance shafts sap power and slow down the response of the engine.

Now, riders seem more tolerant of the vibration that a single dishes out, perhaps because four-cylinders have become so good that they’re almost bland in character (although they’re rarely bland in power delivery). More efficient designs have made modern singles run more smoothly anyway, and what vibration is left can optimistically be called ‘character’.

Their big advantage, though, is controllability. That’s why big four-stroke singles are so popular for trailies. Partly it’s because they’re actually not very powerful – there’s no point for such bikes – so the engines can be tuned for easy, gentle response, with low gearing that allows them to accelerate well all the same.

But it’s also because a single’s firing stroke happens so infrequently compared with a multi-cylinder engine’s closely-spaced firing strokes, that even when a single’s back wheel loses traction, there’s time before the next firing stroke to gain traction. That might sound unlikely, but it’s true, and it affects on-road cornering as well as clambering through the crud

Of course, every year we expect more power from new bikes, even singles. But, like V-twins, singles can lose all their low-down power if they’re tuned for ultimate top-end power – and it’s the low-down pull and tractability that’s essential in the sort of bikes that singles are fitted into.

What happens is that the one large-capacity cylinder of a big, powerful single needs to be fed with a large gobful of fuel and air mixture when it’s running at maximum engine revs. To supply that, it needs a big carburettor.

But at low engine speeds, the demand for fuel-air mixture isn’t as great, and the carburettor is so big that the air travels too slowly through the carburettor to suck the fuel into the airstream. The result is spluttering, uneven throttle response.

Yamaha sussed this out when they revived the single over 20 years ago, and introduced the Yamaha Dual Intake System (YDIS). This gave their singles two carburettors; at low engine speeds, only one carburettor was used, controlled by the throttle. But as the engine revved faster, the other carburettor was opened automatically by engine vacuum, to supply more fuel-air mixture. Job done. It’s a system that’s been used in different ways on loads of singles since then, with great effect.

The same job can be done by fuel injection tricks now, though carburettors are still cheaper. But while riders are buying big trailies (and they’re mad for them in Europe), the single will continue, and fuel injection will become more popular, if only to meet emissions controls. The single, the original petrol engine format, has years left in it yet.