BEEZUMPH TECH
Alan Cathcart explains how Bsa/triumph turned their three-cylinder production road bikes into howling, world-beating racers
How Doug Hele turned the BSA Rocket 3/Triumph Trident road bike into a race winer
In developing the Triumph T150 overhead-valve three-cylinder engine for its 1970 debut season in AMA road racing, Doug Hele and the Meriden factory race department drew on experience gained with the company’s 500cc twin. This had twice won the Daytona 200-miler (1966 and ’67), as well as giving a good account of itself in 500GPS. With the stock 67 x 70mm 741cc triple comprising an extra cylinder hung on the side of the Triumph twin, the potential for crossover engineering was obvious.
They maxed out the stock 741cc engine to the 765cc capacity permitted under AMA rules via the maximum allowed +0.040-inch overbore, which in theory allowed damaged cylinders to be bored out and reused. This entailed honing the austenitic cast-iron liners to accept specially made high-compression 11:1 cast pistons for dimensions of 68 x 70mm, mounted on stock polished and matched conrods. These were fitted to a lightened stock 120° crankshaft (with the standard triplex-chain primary drive retained) matched to a five-speed close-ratio Quaife gearbox and a lightened single-plate Borg & Beck diaphragm clutch.
TH6 cams from the 1968 500GP twin were used with larger-radius lifters, while the stock cylinder heads fitted with tougher valve seats were carefully ported, and the combustion chamber volumes measured and matched. Three 13/16in (30mm) smooth-bore Amal GP carburettors, with a single separate float chamber feeding all three carbs, were chosen for greater power at full throttle. Ignition was a Lucas energy transfer system, with an alternator feeding remote contact breakers without the need for a heavy
wet-cell battery, and with the three coils mounted in a separate carrier adjacent to the timing chest. The three-intoone exhaust employed a long megaphone measuring four inches wide at the exit, a system designed by Hele. The resultant engine delivered 81bhp at 8200rpm at the crankshaft. The Rob North chassis, manufactured in his shop in Bedworth, Warwickshire, featured a bronze-welded duplex frame in Accles & Pollock T45 chrome-moly steel tubing; it carried modified Triumph 15/16in (33.3mm) telescopic forks with welded-on brake caliper plates. The first-series 1970 North frames had a tighter 26º head angle (see Rob
‘THE RACE ENGINE DELIVERED 81bhp AT THE CRANKSHAFT’
North interview on page 34), which was later kicked out to 28º for the 1971 race bikes with 4.75in (121mm) of trail. Coupled with the taper-section tubular steel swingarm carrying twin Girling shocks, this resulted in a 57in (1450mm) wheelbase and a 50/50 disposition of the 396lb (180kg) half-dry weight (measured with oil but no fuel).
The 1970 bikes were fitted with a 250mm four-leadingshoe, cast magnesium Fontana front drum brake, with a single 10in (254mm) AP Lockheed cast iron disc at the rear, which transpired to be a Triumph Herald car component that had been machined down!
For 1971, the drum was replaced by twin Lockheed discs, while the steering head was lowered by two inches (51mm) for improved weight distribution. This also reduced frontal area for increased top speed, and widened the forks to provide room for disc brakes. The chassis was also lighter, scaling 35lb (16kg) in bare metal.
Besides the brake changes and the new ‘Lowboy’ frames for 1971, the oil cooler was also moved to the front of the new ‘Letterbox’ fairing, which was designed at the BSA Group’s Umberslade Hall technical research establishment. The power was increased to 84bhp at 8500rpm at the crankshaft, with a new TH13 inlet camshaft, squish pistons and suitably modified heads.
Carburettors for ’71 were 30mm MKI Concentrics for a smoother power delivery, while magnesium primary cases and wheel hubs, together with titanium fasteners, reduced weight. Titanium was not legal for axles, but with iron plugs fitted at each end, they passed AMA tech inspection, which consisted of poking a magnet at one end of an axle!