Triumph Twenty One
Triumph Twenty One Background & History
The Triumph Twenty One was the first of the Triumph ‘unit construction’ twins. At 346cc, they called it a 350, which works out to 21 cubic inches, hence the name. All non-unit construction Triumph Twins fit into one engine family, from the 350 to the 650. Changes in displacement were accomplished through the installation of one of three different crankshafts and cylinder barrels, in the same set of crankcases. Pistons and connecting rods also had to be changed to get to the desired displacement numbers. They were all in one big engine family that shared architecture. But, when Triumph began it’s conversion to unit construction, starting with the 350cc 1957 Triumph Twenty One, what emerged were two engine families. Once the process was completed, the smaller engines, 350 and 500, would share crankcases and other major components, while the 650s took on a larger, stronger engine architecture similar to, but completely different from the 350/500 engine family. The 500 twins were the next to go unit construction, in 1959. It took until 1963 to make the change in the 650 line.
Prior to this, pre-unit or non-unit construction bikes had separate engines, primary chain cases and gearboxes that were all held together by a system of brackets. Unit construction combined these three components into one unitized casing, again hence the name. Unit-construction engines were more compact, lighter, stronger, leaked less oil, and, most important to Triumph, were easier to produce.
1958 Triumph 3TA Twenty One. This look was common in England due to the wet riding conditions there. Note the huge front fender, headlight nacelle and fully-enclosed rear end, known as the 'bathtub'. These and their 500 stablemates were called 'Bathtub Bikes'.
Triumph Twenty One Engine Design
The Triumph Twenty One model was launched in 1957 with the new unit-construction 350 twin engine. Other than being the first unit construction engine in Triumph’s inventory, the new engine was pretty much standard fare for the Edward Turner-designed Triumph vertical twin. Both pistons rise and fall together on a 360-degree crankshaft, but fire alternately. The crank is supported by just two main bearings. The two cams are gear-driven off the right side, the primary chain and clutch are on the left. Dry sump lubrication with an oil tank under the seat. Right foot-shifting and kick start (only). A single Amal Monobloc carburetor metered the fuel. The 1957 Triumph Twenty One was also the first Triumph to use an alternator instead of a dynamo, like the pre-unit bikes. In fact, the alphanumeric designation for the Triumph Twenty One is 3TA. ‘3’ for 350cc, ’T’ for Triumph, and ‘A’ for alternator.
The 349cc twin-cylinder engine was pretty typical for a Triumph with the exception of the distributor driven off the intake cam.
Triumph Twenty One 'Bathtub Bike'
The Triumph Twenty One and its 500cc stable mate the Speed Twin, both came with ‘bathtub’ bodywork, very popular in England in the late 1950s. Riding in England was a dirty affair, with rain, fog and mud. Rust was always a problem in that damp environment, so riders had to routinely wipe down their bikes after every ride. This full-coverage bodywork was thought to be a partial answer to that problem. However, in actuality, moisture still got in, but remained hidden inside the body until rust would form. It’s a very common problem with these Bathtub Triumphs. But all this practical bodywork didn’t appeal to US buyers at all and the bikes didn’t do well here. In fact, many US buyers took off the clunky pieces and replaced them with the slimmer bodywork of the other Triumphs. Those two factors are why you don’t see very many Triumph Twenty Ones in America.
Here in the US, Americans rode recreationally. But in Britain it was how they got to work every day. For those who couldn't afford a car, a motorcycle, or a bike with a sidecar, or maybe one of these enclosed bikes was better than nothing. Many of the classic British motorcycle marques made their own versions. Vincent was the first with the 1954 Black Prince, a fully enclosed blob based on the 1000cc V-twin Rapide. Then from 1959 through about 1965 several more attempts were made. Velocette had their Vogue. Ariel had the Leader. Norton's Navigator De Luxe was prettier than most. And then there were the Triumph Bathtub Bikes. No one made a bigger commit to the look than Triumph, both in boldness of design and it production volume. Many bathtub bikes were sold, in the form of Twenty Ones (350cc), Speed Twins (500cc) and Thunderbirds (650cc).
The rear 'bathtub' came apart in two halves and had to be removed often during maintenance.
In 1959 Triumph renamed the bike the Triumph 3TA, and it’s 500cc sister bike the 5TA. Both stayed in production through the 1966 model year. During that time, the fully-enclosed rear end slowly opened up until in it’s final year, 1966, the 3TA had dispensed with the rear enclosure altogether.
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