The 1950 Triumph
Thunderbird was not only the first Thunderbird, it was the first Triumph 650 &
it began a long legacy that lasted until the demise of Triumph Motorcycles in
1983. It is said that Edward Turner got the idea for the name from a motel he
stayed in while visiting the US on business.
He had another brilliant idea as well. As the quest for ever more power had
pushed the 500cc Triumph Speed Twin to its limits, Turner simply expanded those
limits. He bored it out to 71mm & stroked it out to 82mm & in doing so, created
a 649cc fire-breather. The 6T Thunderbird was born & was instantly one of the
fastest motorcycles on the market!
A huge advantage was that most of the major engine castings remained relatively
unchanged from the 500cc units already in production. And of course this meant
that it all fit into the same frame as well, saving even more development &
tooling costs. Just as he had done back in 1937 with the seminal Triumph Speed
Twin, Edward Turner had once again created a game-changing motorcycle out of
components already on the shelf.
FRAME & RUNNING GEAR These included the brazed-lug rigid frame, the rest of the
running gear & the infamous "sprung hub". This was another Edward
Turner invention designed to bring some rear suspension travel
(however slight) to the Triumph rigid frame without investing the
money needed to replace it with a proper swing arm rear suspension.
Essentially it placed springs radially between the rear axle & the
hub/spokes/wheel. In theory, the springs would absorb the bumps
while keeping everything in line. The problem should be obvious: any
play in the system results in uncontrolled wheel movement & erratic
handling. But, it remained in service until the swing arm came out
in 1954.
MORE STRENGTH To cope with the added power, the 1950 Triumph Thunderbird
got a new stronger 3-piece pressed-together crankshaft & the engine
cases were beefed up around the main bearings. The oil pump's
capacity was increased by 20%. The gearbox was strengthened with
larger pinions, a greater number of teeth (finer pitch) & improved
shift forks.
GEAERING
Primary gearing was raised by replacing the Speed Twin's 22-tooth
engine sprocket with a 24-tooth. Changing the engine sprocket like
this is actually the way the factory preferred to change gearing, as
the final-drive sprocket remained 18-teeth on all twins up to &
beyond unit construction in 1963.
OTHER CHANGES Another interesting feature was the crankshaft shock
absorber. Introduced with the 1950 Triumph Thunderbird, it remained
in service until the conversion to alternator electrics in 1953,
which was now mounted in its place on the left end of the crank,
inside the primary cover. This brilliant device decoupled the engine
from the primary, linking them through a spring-loaded, cammed
interface that allowed a controlled amount of 'give', smoothing out
harmful vibrations. It is said to have made everything last longer:
engines, primary chains, clutches, gearboxes & even final drive
chains! When the alternator replaced it, Meriden attempted to do the
same job with rubber shock absorbers inside the clutch hub, but it
never worked nearly as well.
ENGINE
For the most part, the new 1950 Triumph Thunderbird 650 engine was
the same as the 500cc Triumph Speed Twin it evolved from. The cast
iron cylinder block & head were similar, but the 650 head was more
generously finned & utilized a copper gasket instead of a 'spigot'
arrangement, common for the day. The valve train was identical. Home
market 650s had a 7:1 compression ratio to cope with the low octane
British Pool fuels available at the time. However, export versions
(read that US) had 8.5:1 which was enough to boost power to 34hp @
6500 rpm, enough to push the 370lb 1950 Triumph Thunderbird past
100mph!
THE T-BIRD LOOK The look of the 1950 Triumph Thunderbird was established
early on & followed normal convention for a machine of its type. It
had generous fenders, made to cope with wet British riding
conditions, & a headlight nacelle housing not only the headlight,
but the gauges, top yoke, fork tube tops & the center section of the
handlebars. All of this weather protection was also supposed to make
the bike easier to clean after riding in the rain.