Now that many bike makers have returned to constructing parallel twins for his or her decrease value, good efficiency, and market versatility, it’s time to evaluation the circumstances through which the primary actually profitable British twin got here to market. That engine was the 1937 Triumph 500cc “Pace Twin,” designed by Edward Turner.
The basic air-cooled British parallel twin was constructed on a spheroidal aluminum crankcase containing a bolted-together two-bearing crankshaft with central flywheel and 360-degree crankpin phasing. Connecting rods have been solid aluminum with plain pressure-oiled big-ends. On the left was an enclosed main chain drive linked to a separate four-speed gearbox. On the best was the timing case, with its distinctive polished alloy cowl, containing the gear drive for the camshafts and magneto, plus a twin plunger oil pump. Valve operation was by pushrods and rockers. This design was not solely a nice compromise between operate and worth, it proved dependable.
The accepted synopsis is that in 1936, Turner noticed that the Triumph bike firm, newly separated from its father or mother automaker and bought to wheeler-dealer Jack Sangster, desperately wanted a uniquely interesting new product that will stand out from Britain’s lengthy household of thudding singles. Turner was a person with a eager understanding of public style, even when egotistical and skeptical of engineering evaluation.
To “communicate” a recent language to the customer, this product needed to be a twin, not only a hotter single. To keep away from the price of a brand new chassis, it needed to match the place present singles match. That dominated out a V-twin. To make fashionable energy, it couldn’t have aspect valves; however as a result of the issues of overhead cam have been as but unsolved, it couldn’t be OHC. These standards, so the story goes, outlined the basic air-cooled OHV parallel twin.
After Turner made a gross sales success of Pace Twin, each producer needed to provide one; BSA, Norton, Royal Enfield, Ariel, AJS-Matchless. The British parallel twin got here to outline a whole epoch of motorcycling. Whereas tens of hundreds of Individuals realized to get pleasure from small-displacement Japanese step-throughs and lightweight motorbikes within the early Sixties, British twins continued to outline what the sporting motorcyclist wished.
How and why did Turner make the alternatives he did? Magic perception isn’t a proof.
Turner arrived in motorcycling by shopping for a small dealership, Chepstow Motors in SE London, which occurred to have a Velocette franchise simply as that firm was switching from two-stroke to overhead-cam four-stroke. Turner had beforehand hand-prototyped his personal bike design, additionally an OHC single, and had proven it to the press and provided a later construct on the market. Anybody who has constructed a fancy machine piece by piece is aware of this can be a sluggish, onerous sport, although one with a lot to show the participant. Velocette’s worth for its manufacturing 350 OHC single was far under what Turner’s design value to construct. This instructed him his designs have been ineffective with out a correct manufacturing facility to provide them.
He talked his approach into the precise bike trade. At Ariel, he evidently impressed wheeler-dealer Jack Sangster sufficient to be employed as a designer, working with the very skilled Valentine Web page, these days come from J.A.P., and draftsman Bert Hopwood, later to be an influential engineer in his personal proper. This was 1929, with the Nice Melancholy about to hit. Hardly an excellent time to launch one thing as advanced as what Turner had sketched: a twin-crankshaft four-stroke OHC sq. 4.
Pushed by scorching ambition and private certainty, Turner labored lengthy hours to push his sq. 4 and different initiatives into being. Unwilling to sharpen pencils for different males, Turner achieved and talked his approach into command of Sangster’s subsequent discount acquisition: the bike aspect of Triumph. The deal was made in mid-January 1936.
It was right here that Turner’s creativity would carry success. Shifting his considering away from progressive complexity to manufacturing practicality, in 1936 he drew a parallel twin that match into the chassis of a single, had a totally completely different sound, and propelled a motorbike way more easily than any single. It was proven to the press on the finish of July 1937 and entered manufacturing in 1938. Turner’s design launched little that was new, however broke away from a long time of single-cylinder sameness in methods consumers didn’t resist.
Turner had given his unique sq. 4 complexity that required a number of redesigns. He gave this new twin a simplicity achieved by combining confirmed present parts in probably the most fundamental approach. It could reward him by promoting in nice numbers for over three a long time.
Why, in making an attempt to switch the sameness of British singles with a twin, would Turner give his new design a 360-degree firing order, with each pistons shifting up and down collectively? Didn’t that vibrate like a single?
Bert Hopwood, Web page’s draftsman at Ariel, remembered, “Whereas Web page was nonetheless with us (he left for Triumph in 1932) we had modified a square-four engine by discarding two pistons and had developed a parallel twin on this approach.”
That entailed assembling the 2 rear crank halves onto their central flywheel at a 360-degree crankpin spacing, reasonably than the unique 180, and provision of an acceptable camshaft. All current have been shocked at how clean this 250 twin was. When Web page arrived at Triumph in 1932, a part of his work was design of an engine that’s acknowledged as the primary British parallel twin, the 6/1, which he gave 360-degree crankpin spacing. Why? This gave an excellent firing order that allowed operation on a single carburetor. It additionally didn’t rock backward and forward as 180-degree parallel twins do.
Turner, arriving as basic supervisor at Triumph throughout 1936, was irritated by what he thought to be the pointless complexity of Web page’s 6/1 parallel twin, so he did away with it and commenced his personal design.
Turner Influenced by the Riley ‘9’ Auto Engine?
Within the spring 2002 situation of The Vintage Bike, English journalist David G. Types urged that Edward Turner adopted options of the engine in his Riley “9” auto (a mannequin initially launched in 1926) in his “Pace Twin.” The Riley 9 sought the lowered valve prepare weight of OHC by mounting its cams excessive however not overhead, one on all sides of the cylinder block. This allowed use of lighter quick pushrods. (On this identical interval, Vincent featured “excessive cams” on its new 500 single.) The Riley’s engine had deep hemispherical combustion chambers, every with two valves at an included angle of 90 levels. All of those did grow to be options of the Triumph Pace Twin, however the hemi chambers and 90-degree valve angle have been already current in Web page’s Triumph 6/1.
Solid Aluminum Connecting Rods
An uncommon function of most British twins, originating with Turner, was use of solid aluminum con-rods, some working immediately in opposition to the crankpins as their very own bearing materials (though replaceable insert bearings have been later adopted). Anybody who, like Turner, had frolicked at Brooklands Speedway, knew of W.C. Devereux and his firm, Excessive Responsibility Alloys (fashioned 1927). Brooklands introduced collectively individuals from automotive, aviation, and bike backgrounds, making the racetrack a sensible college of contemporary transportation. Frank Halford, who would later design Napier’s 3,500 hp H-24 Sabre World Struggle II plane engine, had been an affiliate and good friend of Harry Ricardo, racing his Triumph-Ricardo 500 within the Isle of Man Senior TT of 1922. Halford had initiated using solid aluminum pistons and con-rods in his 1,500cc AM Halford Particular racing automobile of 1925, after which put RR56 aluminum rods in his mid-to-late Nineteen Twenties Cirrus engine for mild plane. RR56 was one in every of a brand new household of aluminum alloys whose uncommon energy got here from precipitated intermetallic compounds of copper, iron, and nickel inside them.
Why would Turner select aluminum? He’s stated to have remarked that aluminum’s lighter weight allowed such rods to be mild whereas giving larger bending stiffness. However there could also be one other facet. The 2 pistons and rods of a 500cc twin don’t collectively weigh greater than the one piston and rod of a 500 single, and in reality could very nicely be lighter. The consequence may very well be lowered vibration. Figuring out this, and sure being aware of Halford’s use of aluminum rods in his initiatives, made such rods engaging to Turner. Subsequently, throughout his second redesign of the Ariel Sq. 4 in 1935, he selected rods solid in RR56 aluminum alloy. The next 12 months, drawing the Pace Twin, he once more specified such rods.
Why Put the Flywheel on the Heart of the Crank?
Turner’s Pace Twin has a two-bearing 360-degree crankshaft with its flywheel on the middle. Why not find the flywheel at one finish of the crank, as on Guzzi’s 120-degree twin or BMW’s flat twins? This was implicit in Turner’s imaginative and prescient of constructing the brand new engine match present chassis designed for a British single, an engine whose large-diameter crankshaft is its flywheel. He was simply placing the flywheel of his new twin the place room for it already existed: on the middle.
He had already carried out simply this when employed at Ariel in 1929 to construct his unique 498cc sq. 4. To make it slim, Turner gave this engine’s two crankshafts central flywheels, doubling as crank phasing gears by having helical enamel minimize into their outdoors diameters (this was later dropped). As on Scott two-stroke twins, the early Sq. 4’s curler crankpins have been “overhung”; they weren’t supported by outer crank discs and outer major bearings, narrowing the engine by the width of the 2 deleted crank discs and major bearings.
Turner’s Sq. 4 achieved each main steadiness, which means as a lot reciprocating mass was, symmetrically, all the time going up as was taking place; and the elimination of any side-to-side rocking couple by phasing diagonal pairs of pistons to rise and fall collectively. This identical self-balancing association has appeared in more moderen engines, equivalent to Suzuki’s Seventies RG500 two-stroke.
Solely Two Fundamental Bearings?
Pace Twin was given solely two major bearings, a ball on one aspect and a curler on the opposite. Why no middle help from a 3rd bearing? The apparent reply is that, on the Pace Twin’s max of 6,000 rpm, there was no want. And within the mid-late Sixties, when the Meriden Triumph group bought scientific about successful Daytona and even entered GP races with a twin, it was capable of rev the bike to almost 10,000 rpm. So why fear?
Turner Able to Error
In design, Turner may now and again have extra private conviction than understanding. Bert Hopwood, who labored with him for years, noticed that Turner dismissed analytical calculations as “schoolboy work” irrelevant to sensible design. That private shortcoming doomed his Sq. 4 to restricted energy in relation to its rising weight, as a result of its rear pair of cylinders have been “cooled” by air that had already been heated by passing by way of the fins of the entrance cylinders and heads.
Apologists for the sq. 4 have blamed that design’s persistent overheating on something however the apparent: You may’t cool second-row cylinders with scorching air. Profitable radial plane engines have been constructed with as many as 4 rows of cylinders, however all the time with cautious provision of one hundred pc recent chilly air to each cylinder. The sq. 4’s design may cool adequately within the early days of low energy, however when labored tougher, bother appeared. In the long run, Turner’s Ariel Sq. 4 was no Vincent-like road-burner and accepted the position of hefty and sedate sidecar hauler.
An Earlier Precedent for the Central Flywheel?
In 1947, when Bert Hopwood, by then a revered engineer in his personal proper, was designing Norton’s “export twin,” he too adopted the central flywheel, taking care to change particulars to keep away from patent infringement motion from Triumph. As a second line of protection, he discovered a design utilizing an identical bolted central flywheel that significantly predated each his personal and Turner’s. That was in Harry Ricardo’s 1922 design for the “Vauxhall TT,” an inline auto racing 4. Similar to Turner’s later Triumph Pace Twin, it joined its two crank halves to a central flywheel by way of flanges and bolts.
Val Web page, who left Ariel for Triumph in 1932, up to date that firm’s singles and designed a 650cc parallel twin, the 6/1, making 25 hp at 4,500 rpm. A lot of its options would seem on Turner’s Triumph Pace Twin 4 years later: deep hemi combustion chambers with two valves in every, set at a 90-degree included angle; OHV valve operation by pushrods and rockers from a gear-driven camshaft; 360-degree crankpin phasing; dry-sump oiling through a plunger oil pump, Val Web page’s typical alternative.
Which instance, if both, did Turner observe? Or was the two-valve deep hemi chamber with a 90-degree or larger included angle simply “within the air” amongst designers since being pioneered in Fiat’s early Nineteen Twenties GP automobile engines?
By the point Edward Turner arrived at Triumph in 1936, he’d misplaced his urge for food for complexity (these square-four redesigns!) and for pointless expense (the herringbone main gear drive in Val Web page’s Triumph 6/1!). As soon as at Triumph, he had a transparent thought of what he wished in a brand new twin.
As valve operation by overhead cam was in vogue, and since he had adopted OHC for the 498cc and 601cc variations of the Sq. 4, his first drawings for a brand new twin included it. He quickly reverted to pushrods and rockers. Pace Twin’s crankshaft “was constructed up from three elements, a central flywheel and two single [forgings], every of which fashioned a crank, bobweight, and mainshaft” (web page 21, No matter Occurred to the British Bike Trade, Hopwood).
Connecting rods have been of solid RR56 aluminum. The double-piston plunger oil pump that Web page had given all fashions at Ariel in 1929 was chosen to function Pace Twin’s dry-sump oiling system, pushed by an eccentric pin on the top of the consumption camshaft. The smaller plunger equipped oil stress to the best finish of the crank, thence by drillings to the 2 crankpins, whereas the bigger plunger scavenged the oil that drained to the underside of the two-piece crankcase, returning it to an exterior oil tank.
Selecting an Oil Pump
How was the plunger oil pump chosen? Within the Nineteen Twenties bike engines gave up total-loss oiling in favor of pumped recirculating techniques, however there have been alternate options. BSA selected double-gear oil pumps, and Turner put one into his early sq. fours. Val Web page adopted plunger pumps, presumably for his or her fame of being higher capable of raise oil after lengthy use. As a result of cavitation may develop at excessive pump velocity, it was typical to drive oil pumps from a camshaft, rotating at half crank velocity.
Though Turner had initially given his Sq. 4 a radical (for the time) unit three-speed gearbox (“unit” which means positioned inside the primary engine castings), his realized distaste for novelty and the unproven put a standard separate four-speed Triumph gearbox behind the Pace Twin’s engine.
Dry Sump or Moist?
Why dry sump reasonably than moist? Turner had given the unique Sq. 4 a moist sump, with curler connecting-rod big-ends lubricated by “dippers” and a gear-type oil pump. If a moist sump is designed with a flat backside, oil can transfer away from the oil pump pick-up throughout acceleration or braking, interrupting oil circulate to the pump. A dry sump was an answer to this drawback, as its oil flowed by gravity to the oil pump from a distant oil tank.
The Pace Twin’s valve tappets have been paired in spherical tappet guides, one earlier than and one behind the cylinder; the guides seemed identical to these adopted by Turner for the 1935 Ariel 4G 997cc Sq. 4. Cams have been supported by bronze bushings, not by the ball bearings of the Sq. 4.
Though Val Web page had designed his 6/1 parallel twin with uncovered valve springs, Turner noticed that full valve prepare enclosure was the longer term.
Cam Drive: Gear or Chain?
Bert Hopwood, in his later British twin designs for Norton and BSA, drove their cams as Harley-Davidson does as we speak on the Milwaukee-Eight: by quiet chains, reasonably than by gears whose clicking backlash will increase because the aluminum crankcase warms up and expands.
Turner drove his cams by gears. Hopwood famous in his e-book that “…the attribute ticking and clicking noises gave the impression to be accepted and forgiven by Triumph fans” (Hopwood, web page 30).
Imitation the Sincerest Type of Flattery
As a result of Turner’s Pace Twin was the primary British Twin to attain market success, its design impressed opponents to repeat—and enhance. An instance of that is the extra compact shallow combustion chamber with the narrowed valve included angle of 57 levels, given to Norton’s line of OHV twins by Hopwood in 1947.
Past scholarly argument and historic paperwork lies the truth that Turner had actual perception into what motorcyclists wished and would purchase. He could have been as unpleasant as some contemporaries bear in mind him, nevertheless it was his model of the parallel-twin idea that grew to become the enduring gross sales chief.
Who could have copied whom is unimportant. The underside line is that having educated himself by way of onerous expertise, Edward Turner selected for his Triumph Pace Twin the only options that added as much as engaging performance, produced at an inexpensive worth.