The Joy of Six... (cylinders)

The bike in question will be BMW’s forthcoming LT tourer, which will use the engine and chassis shown off in last year’s mad-looking Concept 6 prototype wrapped up in a new touring package. But regardless of the new LT’s intentions or appearance, all eyes will be focussed on just one aspect; the new 1600cc straight-six engine unit.

Ask any engine designer, and they’ll quickly explain that, in theory at least, the in-line, or straight six is one of the purest, simplest, smoothest layouts it’s possible to find for a reciprocating internal combustion engine, short of a V12. Everything else – singles, twins, fours and even V8s and V6s – are a compromise, with basic problems of balance and smoothness.

The basic ‘rightness’ of the six layout is best illustrated by the capacity variations of the design. Straight sixes have been made in every imaginable size, from the tiny Honda 250cc racers of the sixties, with thimble-sized pistons and internals more Hornby than Honda, to an incredible 10,920,000cc (yes, nearly 11 million) from a Wärtsilä-Sulzer two-stroke diesel ship engine – a 49,200bhp, 1160 tonne monster.

BMW’s design chief, David Robb, developed the Concept 6 and the forthcoming production bike said: “It’s our job to come up with solutions and I’ve been very impressed by our engine’s performance.

“But there’s also the feeling of a straight six, the power delivery is quite different. It’s a very sovereign power, very pure.”

Six problems and solutions

Putting six cylinders in line might give near-perfect balance (see panel), but it does present other problems.

From the engine designers’ point of view, a straight-six has one of the longest crankshafts of any configuration, and while cranks are hugely strong, under the forces inside an engine they can – and do – flex. And the incredibly high revs a six can operate at thanks to its balance can make the problem worse. Designers don’t want to add too much strength to the crank, as that increases weight and restricts revs, but they must ensure it’s strong enough to resist the twisting and flexing forces it undergoes.

Modern materials and computer-aided design technology are a massive help in reaching that balance but even with a perfect straight-six engine, the bike designer still has a problem; how to fit it into a bike. Almost inevitably, six cylinders will take up more space than four and when they’re strung out in a straight line across the bike’s frame, as on the new BMW and all previous production straight six bikes, that means adding width to the whole bike.

The enormous-looking engines of a Kawasaki Z1300 or Honda CBX1000 might be a vital element to their styling but they’re not conducive to extreme bank angles. Once again, however, modern technology is coming to the rescue, allowing thinner cylinder walls and tighter bore spacing to bring the overall width down.

When it comes to creating a production bike, and all the penny-pinching that entails, the six-cylinder has another drawback – it’s got 50 percent more cylinders, pistons, conrods and valves compared to a four.

When it comes to making a profit, that’s a potential problem and one that must weigh heavy on the mind of any bike firm hoping to offer a straight six. Certainly, earlier machines, like the Z1300, bore this out with high purchase prices which inevitably kept sales down.

But once again, it’s not quite as clear cut as it seems. While a six has more pistons, it can save in other areas, particularly when compared with the latest generation of four-cylinder engines. Its perfect balance means there’s no need for a balancer-shaft, or any of the associated gears or chains to drive one. Since many modern fours are opting for just such shafts in an effort to cut vibes and drive revs ever higher, it means the overall component count for a straight six is now comparable with that of a cutting-edge four – something that definitely wasn’t the case 30 years ago.

BMW Concept 6/K1600LT

Looking at the Concept 6, David Robb, who penned the machine says it’s in three parts: “The bottom of the bike, the suspension and wheels, comes straight from our current production bike range. The mid section – the frame and engine – is pre-production; something you will see on a future model. The top part, the bodywork, is pure concept.”

Unlike the naked Concept 6, the production BMW six-cylinder will be a replacement for the K1200LT tourer. Robb said: “On the touring bike, the fairing will cover much of the engine, so we wanted to do a bike where it was visible from every angle – even if you look down on this from above, you can see that it’s a six-cylinder.”

So while the Concept 6 is purely a non-running concept, we know it’s main specifications will be shared by the new LT. That includes, of course, the 1600cc, six-cylinder engine, as well as the K1300-esque chassis and Duo-Lever front suspension. While the capacity and the number of cylinders, as well as the old LT’s truck-like dimensions, suggest the final machine will be a Goldwing rival – more car than bike – Robb’s assertion that the new six is narrower than the Kawasaki 1400GTR’s four, as well as BMW’s decision to preview the new engine in a café racer-style machine, hints that perhaps the new tourer will have a sporty edge that’s missing from the Wing. We’ll find out this autumn, when the production BMW six is revealed.

Suzuki Stratosphere

Probably the single most interesting concept bike shown in the last decade, Suzuki’s Stratosphere briefly looked like it might reach production. Those hopes have now receded, with sales for expensive naked bikes dropping away sharply worldwide.

It’s sharply set aside from most concepts through the fact it’s actually a runner, rather than a clay-and-plastic mock-up. Official videos showing the bike in action prove it worked and was under serious production consideration. Even for a company with the might of Suzuki, making an all-new, working six-cylinder engine isn’t a task taken lightly.

Some suggest the engine’s origins lie in a secret MotoGP prototype, designed in the early days of the four-stroke class before the 800cc capacity limit and alterations to the minimum weight regulations made four cylinder designs the only viable choice. That certainly ties in with rumours from Japanese insiders that the motor’s actual capacity was somewhat less than the claimed 1100cc, and that its power was more than the 180bhp mentioned by Suzuki.

The Stratosphere is now exceedingly unlikely to reach production unless there’s a serious turn-around in the bike market in the wealthy, Western world – and even then, Suzuki’s less than amazing sales for the B-King, the last of its concept bikes to be turned into a showroom model following pressure from a star-struck public mean the firm isn’t likely to be too keen. However, it can be thanked for bringing the in-line six layout back into the public eye; BMW might already have been working on its own six but the publicity generated by the Stratosphere will have calmed nerves in Munich and planted the idea of showing BMW’s own design in a naked concept bike rather than waiting for the production-ready machine.

Benelli Sei

Virtually forgotten now, the Benelli Sei was briefly a legend in its own lifetime – the world’s first full-production six-cylinder bike, and still the smallest production six ever to grace our roads.

Initially launched as a 750 in 1972, it was the first European superbike to hit back at the Honda CB750, launched three years earlier as the world’s first four-cylinder superbike. After all, the logic went, if the CB750 is great with four cylinders, a six will be better still.

The engine wasn’t a million miles from the Honda’s either, being basically a Benelli 500 Quattro motor with two extra cylinders grafted on. And the Quattro’s engine was, put politely, a homage to Honda’s CB500-Four.

Regardless of its roots, as a six the Benelli was unique, and the three stacked exhaust trumpets on either side left onlookers in no doubt they were staring at something positively exotic. Its 76bhp was initially impressive; in 1973, Popular Science magazine said it had “a top speed of more than 125mph, almost twice as fast as you’ll ever want to travel on the highway.”

But despite a capacity boost to 900cc, the performance never grew – by the time it was killed in 1987 you could get a Japanese 600 that was much, much faster.

V6s – the missing link

Production bikes have taken many forms over the last century – singles, twins, triples, fours, sixes, even V8s and Wankel rotaries have all rolled out of factories and into showrooms – but one layout has been conspicuous by its absence; the V6.

It’s a strange omission, particularly given the popularity of the layout with car manufacturers, who appreciate the relatively high power and compact dimensions a V6 offers. The reason is that, despite sharing the same number of cylinders, a V6 is a totally different proposition to the perfectly-balanced straight-six which is set to make its biking comeback this year.

In fact, a V6 could hardly be more different; with three cylinders on each side, it’s a layout that’s inherently unbalanced. In a straight-four, a straight-six or a V8, the pistons form a mirror image; when the first is at top dead centre, so is the last. That means the movement of the pistons doesn’t try to rock the engine. In a triple or a V6, that rocking action does happen. As a result, achieving smooth running on a V6 tends to need the addition of balancer shafts and big crankshaft counter-weights. Add in the fact the layout needs twice the number of cylinder heads, camshafts and cam drive systems as an in-line engine, and it becomes increasingly unattractive.

The saving feature, however, is the fact that a V6 can be physically tiny. And that’s vital – V4’s share the same benefit, and look at their current popularity in bikes, both on track and on the road. When F1 cars were restricted to a 1500cc capacity limit in the 1980s, V6s were the default choice, combining high power, impressive strength – their crankshafts are half the length of a straight six, reducing stress even if vibrations are higher – and compact dimensions.

And despite the fact you’ve never been able to waltz into a dealer and actually buy a V6-engined bike, there have been several efforts to make just such a design work.

Honda CBX

Not the first production six, but probably still the most iconic, Honda’s CBX still defines what a six-cylinder bike should be.

Even when it was launched, in 1978, its makers were trotting out rhetoric about the six-cylinder engine being more compact than you might imagine. Indeed, a CBX was narrower overall than a CB750 four, despite the seemingly massive engine.

Unlike the earlier Benelli, the CBX was actually quite near the cutting edge in engine technology. There were 24 valves, driven by two camshafts, and while the original 103bhp wasn’t earth-shattering even in the late 70s, it was achieved in a very low state of tune.

It might have been four times the size, but that engine was a direct descendant of the original biking six, Honda’s 1966 RC166 250cc racer, and its performance lived up to the heritage – a 11.5s standing quarter made it the fastest production bike in the world.

Despite early promises of race kits to unleash its real performance, the CBX mutated into a tourer and was killed in 1982.

Primary and Secondary balance

A conventional in-line four-cylinder engine seems to meet all the basic requirements for good balance. For each pair of pistons on an up-stroke there’s a matching pair on a down-stroke, cancelling out the kinetic energy and resulting vibration.

While that four-cylinder arrangement has perfect ‘primary’ balance, it fails on ‘secondary balance’. With two pistons rising as the other two fall, twice in every revolution there’s a moment where all four come to a complete halt before accelerating away again in the other direction. The problem is the pistons at bottom dead centre accelerate upwards much more slowly than the downward acceleration of those at TDC (Top Dead Centre). It’s an unavoidable result of using a crankshaft and connecting rods.

During the BDC (Bottom Dead Centre) phase, much of the crankshaft’s rotating movement is used to swing the con rod from one side to the other, rather than moving the piston. In the upper part of the stroke, as the con rod goes up and over the crank, the “dwell” – time which the piston is stationary – is far less. The result is a secondary imbalance caused by the pistons in the top part of the stroke having greater kinetic energy than those in the bottom half. Theis happens twice in every rotation of the crank, causing a ‘secondary’ vibration – a buzz with a frequency of twice engine revs.

It can be solved by using two, counter-rotating balancer shafts. These look like camshafts, but rotate at twice engine speed, and are weighted to create a vibration that cancels out the engine’s secondary vibes. But they’re big and heavy.

On an in-line six, the problem is solved. Because the pistons (still in pairs) have their throws evenly spaced at 120-degrees rather than the 180-degrees of a four, there’s never a moment when all the pistons are stationary. While there may still be some slight imbalances between the upward and downward kinetic energy, these don’t have the same harmonic rhythm as in a four and they’re less than half as severe because you never have a moment where some pistons are at TDC and others at at BDC. The vibrations become so slight as to be undetectable.

Kawasaki Z1300

Just as with the Honda CBX, the Z1300 was originally seen as a sports bike but quickly morphed into a tourer when it became clear it was too big, heavy and expensive to attract superbike riders.

Unlike it’s short-lived Honda rival the Kawasaki six did manage a long life, remaining in production for a decade between 1979 and 1989. Its high-tech engine not only featured twin cams and 24 valves, but water-cooling – something that hadn’t appeared on four-stroke, in-line-engined bikes before (the two-stroke Suzuki GT750 of 1972, the Wankel-engined RE5 and Honda’s 1975 flat four GoldWing all beat it to liquid cooling, as did British Scott two strokes from 1908!). So it was a trend setter, albeit not in the way originally imagined. It was also an early-adopter of fuel injection, starting in 1983, as Kawasaki struggled to improve the big six’s fuel economy. A change that upped power from 120bhp to 130bhp.

Although the Z1300 was, in many ways, the template for modern sixes like the forthcoming BMW – water-cooled, injected, shaft-driven – it also explains why the layout has been long ignored. Sales were never impressive, and although production ended in 1989, as late as 1993 some dealers were still selling ‘new’ old-stock machines.

Honda Six

Straight-six production bikes might not have the most glowing histories, but in racing terms the one and only effort with the same configuration was a remarkable success.

Starting with the 250cc RC164 in 1964, and continuing with the RC165, RC166 and the 297cc RC174 – which competed and won in the 350 GP class despite being closer to a 250 – Honda’s tiny sixes were fundamental to Soichiro Honda’s intention to beat two-stroke opposition with a four-stroke engine (the same desire also led to the five-cylinder 125cc RC149 and later to the oval piston NR500 GP bike).

The fact Honda’s only production straight six didn’t arrive until more than a decade later, and was four times the size of the original racer, illustrates just how complex the 250-6 GP machines were.

George Beale, who built six perfect replicas of Honda’s RC174, appropriately over a six-year period, said: “The original idea was to get more breathing. Six cylinders meant more valves and the related performance was a lot better – you could get more fuel in and more gas out, and of course more revs, which means more power. Less vibration from a six also allows higher revs.

“I grew to know Mike Hailwood quite well towards the end of his career, and when I asked him he said there was no question the Honda six was the best bike he ever rode.”

To create his perfect replicas, Beale dismantled one of only two original RC174s in existence, and was stunned by the engineering in the bike. “When we took the engine out,” he said, “It was so light that I thought there were no internals in it. It turned out they were all there, and not only that but they were brand new. It seems that at the end of 1967 it was rebuilt with all-new parts for the ’68 season. Then Honda announced it wouldn’t be racing any more.”

MV Agusta 500/6

Most people credit Honda’s RC166 as the first six-cylinder GP machine, but that’s to forget MV’s effort ten years earlier, the 500/6.

It showed up at Monza for the 1957 Grand Prix of Nations, but didn’t race until making its only competitive appearance at the same event a year later, ridden by championship runner-up John Hartle. It didn’t make the chequered flag. Sadly by then all MV’s serious Italian rivals – particularly Gilera, which had taken the ’57 title with its own four-cylinder – had pulled out of the World Championship, removing the impetus to develop the six-cylinder.

Against a field made up mainly of privately-run Manx Nortons, the older MV Agusta four was more than fast enough to stroll to the title largely unopposed for the next few years, making the six redundant and consigning it to gathering dust at the back of MV’s race department’s garage.

Laverda V6

Ask any expert about V6 bikes, and this is the one they’ll mention. Despite racing only once, at the 1978 Bol D’Or – where it didn’t finish – Laverda’s V6 is still one of the world’s most legendary bikes.

At the time, Laverda had been churning out triples for a decade, giving it an insight into many of the balance problems associated with a V6; three cylinder motors share the same basic imbalance, end-to-end, just without the complexity of a second bank of cylinders thrown into the mix. And it thought a 1000cc V6 would give it the power it needed to win in endurance racing.

The bike’s result at the Bol didn’t live up to that, although a top speed of 175mph showed promise and Laverda hoped to press on with the project until the series organisers, fearing spiralling expenses, imposed a four-cylinder limit in the regulations for 1979.

Honda FXX

Honda FXX: never revealed

It’s unlikely we’ll ever know quite how close Honda came to making a V6 superbike, but the firm developed just such a device in the late 1980s and prototypes are almost certainly still under dust sheets somewhere in the vaults of Honda’s R&D department.

Believed to have gone under the codename FXX, the V6 was just one element of a vast Honda thrust into superbike racing, largely driven by the then-new World Superbike Championship. This effort saw fruit not only in the V4-engined RC30 which went on to reap success but also lead to a machine called the CBR750RR (which gained an extra 150cc before reaching production, becoming the FireBlade) and to the near-mythical oval-pistoned NR750 V4, which was in reality closer to a V8 in its design.

Designed by Tomoo Shiozaki, who was also a key player in the design of the NR750 and more recently designed the original RC211V MotoGP bike and the revolutionary dual-clutch gearbox on the VFR1200, the FXX was intended to explore the possibilities of a V6 as a halfway-house between the ridiculously exotic NR750 and the more prosaic RC30, and according to insiders the prototype performed just as expected; beating the RC30 on all counts.

The cheaper-to-make RC30 was clearly more than good enough, and offered the bonus of giving a marketing boost to Honda’s mass-produced V4 VFR750. Fearing that a pricey V6 machine may prove unpopular with buyers, and that it could be easily banned from racing just as the Laverda V6 was a decade earlier, Honda’s decision to follow the V4 route was sensible. But we wonder what the superbike market might be like today had the FXX project been taken forward.

A clue to its potential did finally appear in the form of the RC211V. Faced with the need to create a purpose-made four-stroke prototype GP engine, Honda dug out the old V6 designs – those with long memories might remember the rumours that the firm’s 2002 contender would be a six rather than the V5 that emerged.

As it turned out, the 10kg weight limit reduction for having five rather than six cylinders was enough to persuade Honda to create the now-legendary V5 that achieved so much success in the MotoGP era of 990cc engine capacity.

Blata V6

The potential performance benefits offered by a V6 led to speculation that the birth of the four-stroke MotoGP class in 2002 would see a new breed of hyper-tech six-cylinder machines.

The familiarity of four-cylinders proved too tempting for most manufacturers, but there was at least one project that attempted to create a V6 MotoGP machine – from the unlikely source of Czech mini-moto makers Blata.

Cashing in on the then-booming popularity of his eponymous firm’s cheap mini bikes, boss Pavel Blata reckoned MotoGP would be the ideal launch pad to move into “real” motorcycle production, and hooked up with WCM, which was about to be kicked out of the Championship for using a production Yamaha R1-based engine. Explaining the use of a V6, Pavel Blata said: “As an engine layout it offers a good deal in terms of power production and balance in a compact envelope. The layout allows us to design what will be a good racing package.”

After a flurry of press releases, including specifications claiming over 220bhp, several drawings and finally one photograph showing a half-built machine, the project disappeared.

Benelli

A recent effort to breathe new life into a V6 came from Benelli in 2007 when, flush with cash from its new Chinese owners, they penned radical projects including a turbo triple and a range-topping V6.

Unlike the Blata, Benelli’s V6 project was kept tightly under wraps in the knowledge that it might never happen, no images of the bike exist, few details emerged. Just like Laverda in the ’70s, Benelli was well-versed in making a smooth-running triple, so the leap to a V6 design wasn’t too daunting. Leaked information suggested it would be a 1000cc superbike, but the only engine to appear from the project was actually more than twice that capacity – a V6 made by mating two of Benelli’s 1130cc triples on a common crankshaft. Shown by Benelli’s watercraft offshoot, HSR-Benelli – which made a concept jetski with a 2260cc motor, claimed to make a frightening 342bhp, and seemed set for production in 2009. The monstrous machine was cancelled in May last year, with HSR-Benelli blaming the recession for a lack of demand from customers.

With that, hopes for a revival of the Sei legacy also appeared to vanish.

KTM V6: light of day denied

Another in the ‘what might have been’ drawer is Austrian manufacturer KTM’s little-known and top secret project to make a V6-engined bike in 2004.

The idea was first mooted by KTM’s boss, Stefan Pierer, who confirmed it was a project under consideration. Asked why he thought a six would be better than a four or twin-cylinder motor, his reply summed up the true appeal of the idea: “Who wouldn’t want a bike with six exhaust pipes sticking out the back?”

Insiders later confirmed the idea was more than merely Pierer’s whim – a prototype engine was up and running in the firm’s R&D department, it seemed like KTM were going to venture into six cylinder engines before they’d even produced a four.

Although details of the engine were never released, and KTM wouldn’t be drawn on what type of bike it could have been fitted to, the reason for its failure to appear wasn’t down to any problems with the motor itself but the difficulties involved in building a chassis around it. Whether that was down to the engine’s shape – like V4s, V6s are notoriously difficult to package thanks to having exhaust pipes exiting from both banks and relatively little space for an airbox in the centre of the V – or down to the sheer power of the motor, is unknown. However, the KTM V4 MotoGP motor used – spectacularly unsuccessfully – by Team Roberts in 2005, gives a none too glowing hint at what a V6 rushed into development on slender resources might have been like. We won’t know now.

The other six: Honda’s Goldwing

Of course, if you want the six-cylinder experience and can’t wait for BMW, there’s an option out there: Honda’s GoldWing is the longest-serving six in biking, and has stood alone as the only production six for the last two decades.

But it’s not to be confused with the in-line six and V6 machines we’ve been talking about; its flat six ‘boxer’ engine is yet another kettle of conrods.

In typical Honda style, the flat-six is a slightly left-field, but eminently intelligent solution, aimed at achieving the smoothness of a straight six and the compact dimensions of a V6.

Just like an inline six, a six-cylinder boxer has the benefit of perfect primary and secondary balance; the movement of the pistons inside each bank mirrors those in the other, effectively cancelling out the vibrations. Allied to a short crankshaft, like a V6’s, the boxer design has loads of potential for high revs, smooth running and very impressive power, and it keeps the centre of gravity lower than any V or in-line arrangement – there’s a reason why Porsche has steadfastly stuck to its guns for so long with the same layout in its seminal supercar,
the 911.

It does have one, elemental problem. It’s shape. With cylinders pointing out of each side, the same thing that gives it the low centre of gravity also limits cornering clearance on a bike. Fine on a GoldWing – or the Valkyrie and Rune cruisers it spawned – but don’t expect to see any boxer-six-powered sports bikes.

The appeal of the ‘multi’ endures. In the days when single cylinder machines were the norm, a twin was considered exotic and anything endowed with more cylinders was truly a thing of great wonder. Now that the four-cylinder motorcycle engine is the ‘norm’ the six has another chance to make it. Whether it will prove to be an idea whose time has come, or the white elephant of before, only time will tell.