Triumph 3T
The 350cc Triumph 3T was a handsome machine, based on the 500cc Triumph Speed Twin.
Triumph 3T Background & History
The Triumph 3T was the smallest displacement twin that Triumph ever built, and the smallest version of Edward Turner's world-beating vertical twin. Throughout the 1930s, British motorcycles were dominated by one engine design: the air-cooled OHV (Overhead Valve) single ranging in size from 250cc to 500cc. By the mid-1930s, they'd reached their practical limits at around 500cc of displacement and 23 horsepower or so. Anything more added to either number induced wicked engine vibration, which was hard on machine and rider. Many solutions were attempted but nothing seemed to solve the vibration problem. It didn't make sense to tune a big single to produce more power because the vibration would be so bad that the extra power couldn't be utilized. Some manufacturers increased displacement only to find that the larger bike had to be detuned to tame the vibes, making it little more powerful than the smaller bike it superceded.
Ariel Motorcycles hired a promising young motorcycle designer named Edward Turner in the early 1930s where he designed the first Ariel Square Four. Ariel owned Triumph so Turner turned his attention to Triumph's lineup. He was forced to address the vexing vibration issues and took an entirely new approach. Instead of trying to tame the big 500 singles, he split that 500cc into two cylinders rising and falling together on a 360-degree crank, but firing alternately, producing a smooth, even firing frequency. The net effect was a much smoother machine that was smaller, lighter, made more power and was much smoother than the singles. Turner called it the 'vertical twin', although technically it's a parallel twin (because some of them, such as the Norton Commando, don't have vertical cylinders), and the two terms are used interchangably. The new engine was introduced to the world in a new bike, the 500cc 1938 Triumph 5T Speed Twin, and it took the motorcycling world by storm. Suddenly, every other British motorcycle maker (those who could afford it anyway) were scrambling to develope their own vertical twins. Alas, World War 2 ended civilian production in Britiain in 1940 and didn't resume again until 1946. But when it did, the who race took off again, with Triumph in the lead.
Notice the cast-in rocker boxes. They don't bolt on like Triumph's. In this way the Triumph 3T is unique among Triumph twins.
By 1949, BSA, Ariel, Matchless/AJS, Norton and Royal Enfield all had their own 500cc vertical twins, just in time for Triumph to leave them all in the dust with another ground-breaking bike, the 650cc 1950 Triumph 6T Thunderbird. And again, the market rushed to catch up. Some built built 600s out of the 500s, others matched Triumph's 650, Royal Enfield even did them one better and launched a 700. Throughout the 1950s the bikes were getting bigger and faster by the year. The British motorcycle industry was in their heyday.
The Triumph 3T shows what happens when you go the other way. Everyone was trying to make their 500s bigger, the 3T dares to ask "what if we make a 500 smaller?". Well, for one thing, you can save a lot of money on development costs, especially if you're building it on the cheap, as the British usually did in those days. Think about what Honda and the other Japanese companies did in comparison. Honda launched it's earth-shattering 4-cylinder CB750 in 1969. It was a total hit and they made boatloads of them. Then they decided to make a 500cc 4-cylinder for the 1971 model year. Did they simply sleeve-down or destroke the 750 to make the smaller displacement out of the bigger engine? No, Honda engineers could never have lived with themselves, in fact they probably would have thought it was a stupid idea. Instead, the smaller engine would be of a new design, optimized for the 500's smaller physical size, torque loads, and so forth, and not just to the engine. The entire motorcycle was completely different than its CB750 forebear. The frame, wheels, brakes, forks and bodywork on the CB500 were different than those used on the 750. Other than both being Honda OHC 4-cylinders, they were completely different motorcycles. They shared no dimensions, the length, width, wheelbase, weight, and fuel capacity were all different. Then, when Honda wanted a 350cc 4-cylinder a few years later, they did it again. They didn't downsize the 500, they built a whole new bike, optimized for a 350. Triumph, and most of the other British motorcycle makers, would simply downsize the engine to achieve the smaller displacement and leave everything else the same, including the frame and running gear. Hence, a 350 would have all the weight of it's 500cc sibling but less power to pull it.
In the case of the Triumph 3T, they had developed their 350 twin for military duty during World War 2 as the 350cc 3TW from their 500 twin. When the war ended, they adapted it for civilian duty and turned it into their lowest-priced twin. The 3T was produced from 1946 through 1951. Like all Triumphs at the end of World War 2 and starting in 1946, the girder front forks had been abandoned in favor of modern telescopic front forks. The 3T had a rigid frame (no rear suspension) for it’s entire 6-year run, and of course it was pre-unit construction all the way.
The Triumph 3T had a 350cc version of the 500cc engine in the Triumph Speed Twin.
Triumph 3T Engine Design
The 350cc engine was based on Triumph’s vertical twin family which also included the 500 and 650 non-unit construction twins. The 3Ts engine was unique among Triumph’s other twins in two ways: 1.) Its rocker boxes were integrated into the cast iron cylinder head, rather than being removable like the other twins, and; 2.) The crankshaft was ‘built-up’, meaning it was pressed together from separate pieces, allowing for one-piece connecting rods. The 3T’s tiny 350 produced a decent amount of low-end ‘grunt’, but since the bike was built on exactly the same frame and running gear as the 500cc Triumph Speed Twin, it had to be geared down (numerically higher), however, they were still capable of a 75 mph top speed.
1948 Triumph 3T.
Triumph 3T meant to be Economical Commuter
The 3T was meant to be Triumph’s cheapest twin and it filled that roll well enough in Britain’s home market where cheap transportation of any kind was in short supply. But they would never had sold well in the all-important US market, so few were imported. Not enough power. By 1949, Triumph, BSA, Matchless/AJS, Norton and Ariel all had their own 500cc vertical twins and the following year Triumph blew the doors off of the competition again by bumping their 500 up to a 650, and a whole new arms race began. With Triumph in the lead, Triumph was always in the lead in those days.
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