Big Twin Development

Big Twin Development

In spawning one of the greatest designs of all time - the Vincent big twins - a flash of sheer inspiration achieved more than any amount of premeditation. Sitting at his board in Stevenage one day in 1936 was Australian Phil Irving, as brainy an engineer as ever graced the motor-cycle industry; on the board were a drawing and a tracing - both of the timing-side crankcase half of the firm’s 500cc high-camshaft single. Idly, Irving turned the tracing over on top of the drawing and lined up the centres of the crankshaft pinion and the idler wheel in the timing gear.

His brilliant intuition recognised a sudden posibility. “Here, look at this”, he called to Phil Vincent, boss of the outfit. For there, clear as daylight, was a ready-made layout for the timing-side crankcase half of a 1000cc 47 degree vee-twin. It was an odd cylinder angle, to be sure, but it meant that the extra crankcase holes for the extra cylinder could be produced simply by inverting the original jig, just as Irving had inverted the tracing. For the idler gear of the five-hundred happened to be offset at 23.5 degrees from the cylinder axis.

At that time, the high-camshaft single had been in production for only a year because Vincent - after buying his way into the industry by acquiring the defunct HRD concern - had initially been content to fit various propietary makes of engine, while concentrating his own design activities on rear springing and dual brakes. But a bellyful of JAP engine troubles in the 1934 Senior TT had convinced him he must design his own engine, too.

Made in standard (Meteor), sports (Comet) and racing (TT Replica) guises, that engine was bang up to date, to say the least, and in many ways was advanced. The crankcase mouth was carried halfway up the five-stud barrel. The short, tapered pushrods were splayed at 62 degrees to lie parallel to the valves, which they opened through straight, transverse rockers. And each valve had two short guides, the rocker end being forked to bear on a valve-stem collar between them. Valve closure was by exposed double-hairpin springs at the top.

Soon, that drawing-board crankcase layout was translated into metal and two Meteor top halves were clamped on - the rear one offset to the right to assist cooling and obviate the need for a forked connecting rod. Fortunately, a suitable frame was handy - built with the top tube three inches longer than standard, to house a JAP engine for Eric Fernihough.

In production, the frame was lengthened a further half inch (stretching the wheelbase to 58.5in). Even so, the springs for the front exhaust and rear inlet valves left precious little room for the enamel on the down tubes! And the only way to house the front carburettor was to specify a horizontal mixing chamber.

Affectionately dubbed the “plumber’s nightmare” because of the maze of external pipes, the Series A Rapide (as the first twin was catalogued) set unprecedented standards in effortless high performance. Eloquent of its sheer versatility was an achievement by Jim Kentish, an early private customer.

After commuting one Saturday morning to the Kew Theatre, where he was stage manager, he took time off during the matinee to nip over to Brooklands. There he won a gold medal for lapping at over 100mph, before trundling quietly back to the theatre in time for the evening pereformance. More seriously, George Brown (then an experimental tester at the factory) entered a Rapide for the Motor Cycle Clubman’s Day at Brooklands, clocked nearly 113mph through the kilometre trap and lapped at just under 106mph.

However, the Burman four-speed transmission had never been designed for massive torque of the Rapide. First weakness brought to light by uninhibited use of the twistgrip was clutch slip. When that was cured, and the torque got through to the gearbox, the layshaft bearings failed. Modify those, and the casing split....

No sooner had the Second World War run its course than Vincent and Irving came up with an answer that had connoiseurs and young bloods alike falling over themselves to place their orders. A complete redesign, the Series B Rapide did full justice to the complementary talents of the two Phils - Vincent imaginative and fanciful, Irving sound and practical.

As before, the theme was a Jekyll and Hyde sort of machine - a comfortable, mild-mannered roadburner capable, with a little tuning, of world-record performances. Just about all that remained of the pre-war specification were the 84x90mm bore and stroke, the pivoted-triangle rear springing and the duplex brakes.

To suit the available magnetos, the cylinder angle was increased to 50 degrees. The wheelbase was shortened by dispensing with a conventional frame and using the integral engine and gearbox as a structual member, suspended from a six-pint, box-section oil tank that doubled up as a top tube.

Both carburetors had vertical mixing chambers. Driven by a blade-tensioned triplex chain, the clutch had an ingenious self-servo action, in which engine torque forced a pair of pivoted shoes into contact with the drum. And the whole machine bristled with riders’ points, such as the minutely adjustable riding position, instant chain setting without tools, and so on.

Inevitably, sporting (Black Shadow) and racing (Black Lightning) models followed, their cylinders and heads anodised black, crankcase and covers stove-enamelled. On the Series C versions, the friction-damped Brampton front fork was superseded by the Girdraulic pattern, with forged light-alloy blades, two pairs of long, soft springs and a hydraulic damper. A similar unit took over from the friction bands in the rear springing.

Magnificent roadsters though the post-war twins were, their renown spread quickest through their sporting successes. George Brown was the most persistent protagonist - terrorising the short circuits, hill climbs and sprint strips for years, with many a British and world record thrown in.

Gunga Din, the first of his three models, was evolved from a factory hack. Then, when George Brown left the factory in 1951 to set up his own business, he went from strength to strength on Nero (unsupercharged) and Super Nero (supercharged). On these, he was free to make modifications forbidden on Gunga Din by factory policy. They included propietary suspension, a lighter clutch, faster oil circulation and caged-roller big ends. Designed for long life at normal revs, the standard crowded-roller big ends were inclined to jib at sustained high speeds and could sieze, locking the engine.

While George Brown was hogging the limelight in Britain, others were doing so elsewhere. Rollie Free set the ball rolling on the Bonneville Salt Flats in 1948, when he pushed the American record beyond 150mph, lying prone on a Black Lightning in his bathing trunks. About the same time, Rene Milhoux chipped in with solo and sidecar records on a Belgian autoroute. But no achievement was more praiseworthy than that of Russell Wright and Robbie Burns on a damp, bumpy, 22ft-wide road near Christchurch, New Zealand, in July 1955. With their unblown Lightning encased in a home-made shell and pulling a 2.5-to-1 top gear, they pushed the world solo and sidecar records up to 185.15mph (Wright) and 163.06mph (Burns) respectively, winning the Motor Cycle trophy and £1000.

Final version of the Vincent twin was the 1955 Series D, with extensive weather-shielding, Armstrong suspension struts and coil ignition. But worldwide sales were taking repeated knocks from rising insurance, national bans, import quotas and market closures. Alas, the Vincent went down for the count.