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The Harley Davidson V-Twin Engine

With special guest appearances by Vincent and Ducati

What gives the Harley Davidson its distinctive sound? Here, Dan explains how the famous 45 degree V-twin engine not only produces the most recognisable soundtrack in motorcycling but also successfully carried the company through thick and thin.

In music, syncopation goes against the ordinary and regular flow of rhythm with an off-beat and slightly unexpected flow that results in a tune which, depending on your taste, may or may not appeal. 

Similarly, in motorcycle engineering, syncopated engines will appeal to some folk and not others. I am firmly in the former group. I have dabbled in the multi-cylinder, smooth as silk and entirely predictable hum of the latest and greatest out of Japan but my love of motorcycling is inextricably linked to the disordinate clatter of twin cylinder engines. One such engine being the Harley Davidson v-twin. 

In 1994 Harley Davison (H-D) attempted to trademark the distinctive sound of their v-twin engine. They were unsuccessful and it seems redundant now but had they been able to trademark the Harley sound they would also have laid claim to the particular 45-degree engine layout that we will discuss further below. 

The implications of this are significant as the sound is linked to the very layout of the engine. A cynic might say the application was an attempt by H-D to prevent other manufacturers emulating their unique position in the motorcycle world.

History has proven other manufacturers did emulate the design and sound of the Harley v-twin but it took some time. My guess is, for a long time Japanese engineers just couldn’t get their head around designing an engine based on a pattern from the turn of the century. 

Everything about Japanese engineering is modern, smooth, innovative and reliable. Everything about the H-D v-twin is the antithesis of modern engineering. I’m going to draw a long bow here, but in sticking with their original design H-D was able to survive through famine and thrive in feast. 

Development was incremental, meaning research and development, tooling and manufacture was, for the most part, able to keep H-D relevant throughout the whole of the twentieth century. Through it all, the distinctive 45-degree v-twin has remained the heart of the beast. 

Some of my earliest and fondest memories of motorcycling include riding on the back of my Dad’s ’42 WLA Harley Davidson, slipping around on the rear mudguard and hanging onto the seat as it went up and down with Dad’s weight. 

I don’t know why there was no pillion seat, but, if I was lucky, Mum would let me grab a cushion from the lounge. I didn’t care, riding on the back of that bike set the scene for a life-long love of two wheels, cushion or no cushion. 

Some 12 or 13 years later, I became the proud owner of 1340 cc Harley Low Rider.

Out cruising on my 1980 H-D Lowrider

No sooner had I arrived home on my brand-new Harley and Dad jumped on the bike and was off for a ride on what was then a modern incarnation of the venerable v-twin. When he came back Dad said, “bloody things haven’t advanced from 1942.” And therein lies the rub. 

The most attractive thing about a Harley is the distinctive sound and feel of the engine. The off-beat cacophony has been with H-D since they began producing their 45-degree v-twin engine. It is the signature of the brand and for me it is the second-best thing about the Harley experience. 

The best thing is the way in which a Harley delivers the power. Coming out of a bend, or up a hill and winding the throttle on produces a meaty torque curve that was unlike any other bike I had owned, until now. 

I have discussed my Vincent motorcycle elsewhere on this site. Since acquiring the Vincent, I’ve been copping a bit of strife likening the power characteristics of my Vincent v-twin to those of the Harleys I’ve owned. The Vincent aristocracy don’t like that but as our discussion progresses the reasons for the similarities will become apparent.

Of course there are differences, some subtle, some not so subtle, but that unmistakable pull of the 45 and 50 degree v-twin is common to both my Vincent and all of the Harleys I have owned over the years. 

Let’s hear it again, just for the uninitiated. When you come out of a bend, or up a hill, preferably both, there’s no need to change down, just crack the throttle and enjoy the pull on the bars and bang, bang, bang coming from the exhaust. It’s awesome and it’s why I love my V-twins. So why are they so similar?

The off-beat ‘potato’ sound produced by the H-D v-twin is not entirely unique to that marque but it is the longest running, most enduring motorcycle engine design on the planet. It is rudimentary, bordering on agricultural. 

A design that dates back to 1909 when Messrs Harley and Davidson chose to plonk both con-rods on a single crank pin so that both cylinders could be set at 45 degrees, one behind the other. 

These were very much the experimental and emerging days of the motorcycle industry. Who knows what was going through the minds of the designers at the time, but to have the basic layout still with us 110 years later is something they surely could not have imagined in their wildest dreams. 

Elsewhere in the US, Henderson were producing inline four-cylinder engines that may have changed the face of motorcycling had Bill Henderson not been killed 11 years after the release of his first engine. 

Others manufactures were also experimenting with and producing inline fours, including ACE and Indian (who later joined forces with the Indian-ACE), but the Henderson is generally regarded as the finest of them. 

History has shown that the v-twin would be the victor in US engine design and application but things were a little different on the other side of the Pond. 

The British were also forging ahead with developing v-twin engines but they also remained committed to singles and, later, parallel twins. From early in the history of motorcycling the v-twin had made its presence felt in the UK. 

In 1907 Rem Fowler won the first Isle of Man TT event riding a Peugeot-powered, v-twin Norton. I know, that hurts. I’ve owned and loved Nortons from the days of my formative youth and I’ve only recently become aware of the part played by the French in Norton’s inaugural TT victory. 

From before the turn of the century, J.A. Prestwich (J.A.P) had been churning out single-cylinder proprietary engines for industry before they too went into producing a highly successful v-twin, one of the most famous applications being the Brough Superior. 

J.A.P had produced their own motorcycle up until 1908 but such was the success of their engine building enterprise they discontinued the motorcycle to concentrate solely on engines. 

To discuss the evolution of racing motorcycle engines would lead us far from the topic but H-D and, to a lesser degree, Indian, stuck with their v-twin engines in the face of an engineering revolution that was focused on single and twin cylinder engines. 

The singles and parallel twin engines would better lend themselves to tuning and development in the race for speed and horsepower but the curse of big singles and twins is vibration because the pistons change direction every 360 degree revolution. 

Imagine if one piston could continue to travel at full speed whilst its sibling is changing direction? Not only is the vibration reduced but power remains available in one cylinder whilst the other is reaching top dead centre. Hence, power remains more readily available through the engine cycle. 

Behind the clatter of the v-twin is a unique power-curve that, at least for your author, anchors the machine to the road through a series of pulses that urge the bike forwards. Of course, in an engine spinning at 5,000 rpm, you can’t actually feel every pulse, it just feels like you can. 

Coming out of a bend or going up a hill (preferably both), is where a big v-twin comes into its own. High revs are not needed, it’s all about pulling power that gives the American v-twin its soul. Of course, this is not unique to Harley or Indian motorcycles but they pioneered a unique method of harnessing the power of the internal combustion engine. 

The Harley V is set at 45 degrees with the con rods positioned one on top of the other described best by the analogy ‘fork over blade.’ The blade sits inside the fork and both push upon the same surface off the crank pin. 

Other v-twins, such as the Vincent and Ducati, have the con-rods sitting side-by-side but they too enjoy the unique power delivery available when pistons are fired upon at different intervals. 

This results in the cylinders being slightly offset to accommodate the position of the rods. All quite simple. Things get a little complicated when we start to listen to the firing patterns of the various engines, which is the syncopation and music each one brings us. 

The sounds from any engine comes from the point at which each cylinder is ignited. There is an explosion which is one of a series of explosions that are timed to produce power in usable quantities. 

What distinguishes various engines is the timing of these explosions and the position, or phasing, of the crankshaft as each one occurs. For the uninitiated, the crankshaft is the mass of spinning metal that captures the power produced by the afore-mentioned explosions and delivers it, through a series of mechanical processes to the road. 

Energy is captured in the combustion chamber and transferred from the piston, down the connecting, or con rod to the crankshaft. In a H-D v-twin that takes place at 315 and 405 degrees. The power of a twin-cylinder engine is delivered twice every two revolutions, or 720 degrees, of the crankshaft. In a single-cylinder it’s delivered only once in every 720 degrees of the crankshaft. 

By contrast, a traditional British parallel twin, where the cylinders are placed side by side, fires more evenly at 360 degrees in every two revolutions. This produces a flatter, less textured sound with the power being produced higher in the rev range. 

To complicate things, in recent times there has been a move in parallel twins to phase the firing order at 270 degrees. Traditionally the realm of Ducati, 270 degrees is the new black in parallel twins. 

Ducati engines, sometimes referred to as L-twins, have the cylinders positioned at 90 degrees with firing phased at 270 and 450 degrees in the 720 cycle. Examples of this phasing in parallel twins include Yamaha’s 850 twin which pioneered 270 degree firing back in the nineties, emulating the Ducati sound in a Japanese twin.

Vee Two Ritorno engine
Vee Two Ritorno engine, also known as the L-Twin. Photograph by Vee Two

More recently, Norton, Triumph and Royal Enfield have all gone 270 degrees. So, what does that mean and how is it achieved? 

By now readers should be seeing a pattern emerging. What may not be so apparent is how this phasing is arrived at in the v-twin, although it is quite simple really. Consider the Vincent, it is a 50-degree v-twin. The cylinders are arranged 50 degrees apart. 

When the engine spins over the first cylinder will fire at 320 degrees, which is 360 less 50. The second cylinder fires at 410 degrees, which is 360 plus 50. In a 90-degree engine 360 minus 90 gives us 270 and 450 is 360 plus 90. 

So popular is the 270 degree layout, the figure has kind of weaved itself into the motorcycle lexicon and is now used to describe the modern range of parallel twins I’ve mentioned above. 

At the risk of going down a Newtonian physics’ rabbit-hole, the 270 arrangement has resulted in changing reciprocated forces from highly irritating up and down vibrating 360 degree engines to a more gentle the fore and aft where the forces are shared across the motorcycle in a more balanced way. 

Just to recap, in a 360 engine both pistons rise and fall together. They both stop at the top of their arc before then go back down. In a 270 engine only one piston stops at any one time. In an attempt to further smooth things out, there is a balance factor built into crankshafts but, again, this should perhaps be left to a physicist to explain. 

For now, consider the balance factor acts as a counter-weight to the reciprocating forces unleashed through the pistons changing direction every 360 degrees. So, now that’s clear, let’s return to the Harley engine. 

Fans of Harley Davidson would be familiar with the changes the engine has undergone over the past century but those less enamoured by the brand probably find the naming rituals a tad quaint. 

Purists assert there has been six major developments in the H-D engine, most of which is readily identified by the names attached to the top end. In H-D circles, a knucklehead is not necessarily someone of reduced intellect, a flathead is not a fish and you might dig shovelheads. 

See the pattern emerging here? I would hasten to add the latest incarnation is perhaps the most radical one yet and has arguably changed the soul of the brand and has thus far avoided a nick-name. We will come back to the current series after a short stroll down memory lane.

Way back in 1909, H-D come up with their first effort at a v-twin. The engine was promising but there problems sorting valve actuation which evidently took some time but, by 1911 H-D had a robust v-twin production engine which was referred to as ‘inlet over exhaust’ or simply ‘IOE.’ 

This was a reference to the valve layout that saw the carburettor mounted between both cylinders high on the left, entering the cylinder at a point higher than the exhaust valves. The original engine had a capacity of 820 cc which was soon enlarged to 1000 cc. 

Although, to be accurate and remaining with American naming standards, they were referred to variously as 50 and 61 cubic inch. 

In 1930 H-D introduced its flathead design. The term ‘flathead’ would later become a common description of all side-valve engines, however at the time of introduction, Harley’s new side-valve engine was state of the art. It was more powerful than the earlier IOE engine and was more easily maintained. 

At the time of its release, the flathead was 74 ci (1200 cc) and would grow to 80 ci (1340 cc) before being put out to pasture in 1948. 

Running alongside the big H-D v-twins were their smaller, 45 ci (750 cc) range of engines which was essentially the company workhorse. The most well-known of the flathead 45 ci engines is the WLA/C models. ‘A’ denotes army and ‘C’ is for civilian models. 

The 45 was also was used extensively in three-wheeler ‘servi-car’ utility vehicles that had an extraordinarily long run from 1935 to 1973. 

I was intimately familiar with this engine as it was fitted to Dad’s ’42 which was originally a WLA. A lot of army surplus machines found their way to Australia at the close of WWII and Dad’s bike had been used for cattle-mustering on a station in the North-West of Western Australia. 

Dad bought the bike from the pastoralist for $100. This was a fair amount of money in the seventies, particularly for a machine that was in a such a poor state, but the cult film ‘Easyriders’ was upon us and people were chasing Harleys to build choppers. 

Even more popular with the chopper builders was the next two engines released by H-D, the Knucklehead and Panhead. 

In 1936 Harley went to an overhead valve v-twin. It looked like it had two big fists coming out of the valve covers; the Knucklehead was born. Arguably the most collectible of the all the Harley v-twins, the Knucklehead came out first in 61 cubic inch and grew to 74 about halfway through the 11 years the engine was in production. 

This was the engine that carried Harley through the war years. Like elsewhere in the allied countries, motor companies were dedicating their resources to the war effort. The demand for large, road-going motorcycles during the war would have naturally been low. But returning servicemen would change that.

The cliché records hoards of idle young men returning to the US seeking adventure and danger, drawn to a booze-fighting life on two-wheels to which there may be an element of truth. 

Or, it may simply be they were seeking the freedom of the open road, a pleasure that had been denied whilst they were away defending that very freedom. 

H-D wasn’t quite ready for the post-war period. In the US raw materials such as steel and aluminium had been quarantined for the war effort which restricted manufacturing for the populace, however the Brits weren’t so afflicted. 

Whilst H-D couldn’t keep up with the demand of their big v-twin, the Brits were in full-swing, exporting the majority of motorcycles it was producing and they found a ready market in the US where the smaller, faster and more available machines were a big hit. 

This would have been welcome by the British motorcycle manufacturing industry but no doubt caused some alarm in H-D as they transitioned to a post-war economic expansion. The transition included a new engine that would help H-D maintain, or perhaps regain, its crown as King of the Road. 

The Panhead engine was introduced in 1948 with modern upgrades including aluminium heads and hydraulic valve lifters. It was indeed a handsome engine and one that would increasingly find its way into tall, raked chopper frames which exposed the top end of the engine. 

On this engine, the valve covers were said to look like overturned pans – enter the Panhead. 

Continuing along with the unofficial naming conventions, the Shovelhead arrived in 1966. The Shovel quickly built a reputation as outdated, troublesome and overpriced, although they did build a loyal following of adherent fans and owners, including me. 

In defence of H-D, it wasn’t that they were particularly bad, although there is no shortage of people who will argue they are, H-D refused to engage in the race to refinement. Within three years of the release of the Shovelhead engine, and its 1000 cc Sportster sibling, the superbike era would be upon the world. 

The Triumph Trident, Honda CB750 and Kawasaki Z900 were game changers but H-D steadfastly remained true to its historical roots. 

By comparison to the superbikes of the seventies, H-D was an antiquity, an outlier that connected the company with its heritage, indeed, they would even celebrate such heritage some 20 years later by releasing a model of the same name. 

By the late sixties H-D was in financial trouble. American Machine Foundry (AMF) stepped in, purchasing H-D and applying their text-book manufacturing techniques to the fiercely proud, all-American company.

The result was a more mechanised production that saw many workers laid off and the company beset by strikes and low morale. H-D’s reputation for an inferior product continued to gain momentum. For the purists, these are considered dark days but there was a fix at hand. 

In 1981 a conglomerate of investors, including Willie G. Davidson, the Grandson of H-D founder William Davidson, purchased the company from AMF. The AMF logo was torn off like a dirty band-aid, which perhaps it was. Had AMF not purchased the company they most probably would not have survived the seventies. 

Prior to the purchase, Willie G had been busy creating H-D’s first factory custom motorcycles. This became Willi G’s forte and launched H-D to a world or pseudo-bikers who had neither the time nor the skills to produce their own custom motorcycle. 

This became the strength of the company. In the face of an onslaught of refined, smooth-running machines coming out of factories in the UK and Europe, H-D stuck with the syncopation of clatter. 

The long, low design, chunky primary drive and twin pipes exiting the right side of the machine are synonymous with H-D, as is the sound. Any other engine just wouldn’t work, the Japanese proved that with some truly horrible attempts at factory custom choppers (for want of a better term). 

Harley would eventually bend to the will of refinement and produce a more balanced, oil-cooled, 8 valve engine, but, through it all, they have remained true to fork-over-blade, common crank pin design of the forefathers. 

The distinctive 315, 405 firing pattern remains with us today. 

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Mick Arthur

Monday 17th of February 2020

Thanks for the article Dan, I loved it. I've had a Z650, a Triumph Bonneville, a Norton 850 Interstate and 2 Shovelheads. Unfortunately age and circumstance prevent me from riding these days.