1951 TRIUMPH THUNDERBIRD


1951 Triumph Thunderbird, Triumph motorcycles, Triumph T110, Triumph Speed Twin

This lovely 1951 Triumph Thunderbird is a fine example of the look of most Triumph Motorcycles in the late 1940s & early 1950s. Still with a rigid frame until 1953, but with a sprung rear hub, the ride & handling were not spectacular. In fact it would be many years before Triumph Motorcycles gained a reputation as excellent-handling bikes. The bumps were soaked up by the sprung saddle & the natural flex of the frame. I rode recently next to a 1950 Triumph Speed Twin, which used the same rigid frame/sprung rear hug setup & noted that the frame flexed considerably on bumps. But it all seemed to work, somehow. This stunning Polychromatic Blue was a new paint color for the 1951 model year.




1951 TRIUMPH THUNDERBIRD - THE BASICS
The 1951 Triumph Thunderbird carried one model designation: "6T", and was known as the 6T Thunderbird, 6T or simply T-Bird. All 1951 Triumph Thunderbird 6T's were 650's & came equipped essentially the same, with the same rigid frame & sprung rear hub, and the same setup throughout. Unlike the later Triumph TR6 which came in a variety of flavors (ie: high pipes, low pipes, competition, etc.).

1951 TRIUMPH THUNDERBIRD - RIDING THE WAVE
The 1951 Triumph Thunderbird carried over largely unchanged from its 1950 model year introduction. There were the usual incremental improvements to engine & gearbox, mostly designed to improve reliability, ease vibration or simplify assembly. But Triumph Motorcycles & their ace designer Edward Turner, knew they had a hit on their hands & left things well enough alone with the 1951 Triumph Thunderbird. After all, don't fix it if it ain't broke.

1950 Triumph Thunderbird, Triumph motorcycles, Triumph T110

1951 TRIUMPH THUNDERBIRD - OVERVIEW
On the 1951 Triumph Thunderbird, the Amal carburetor was increased in size from 1 inch to 1-1/16 inch. The 7-inch SLS front brake was also improved. The same brazed-lug rigid frame soldiered on with the notorious sprung rear hub trying to pass as rear suspension. It was an atrocious compromise implemented (indeed invented) by none other than Edward Turner himself as a way to delay the expensive retooling needed to adopt a more contemporary swing arm rear suspension. It was crude & ineffective. It didn't absorb bumps very well & looseness would develop in the hub leading to uncontrolled wheel movements & erratic handling as the result.

1951 TRIUMPH THUNDERBIRD - COLOR SCHEMES
On a more positive note, the color scheme improved for the 1951 Triumph Thunderbird. Called Polychromatic Blue, it's a handsome metallic blue (pictured here). The Triumph T-Bird had its own logo, the so-called "Paper Dart" (unofficial nickname), pictured on the headlight nacelle above. This would later be cast into the primary cover.

1951 Triumph Thunderbird, Triumph motorcycles, Triumph Speed Twin

The 1951 Triumph Thunderbird was a handsome machine in its day. The headlight nacelle of the 1951 Triumph Thunderbird incorporated the headlight, gauges (clocks), and handlebar mounts all in one neat package. This was a British innovation, intended to help keep the bikes clean in typically inclement British weather & also make them easy to clean, once ridden. The full fenders on this 1951 Triumph Thunderbird are also in keeping with that school of thought. It wasn't until the American Triumph dealers made a fuss that Triumph Motorcycles began to get lean & mean, starting with the 1956 Triumph TR6.


1951 Triumph Thunderbird SPECIFICATIONS:



MODEL DESIGNATIONS:
1951 Triumph Thunderbird 6T...................... Roadster

ENGINE:
Engine type....................................... OHV vertical twin
Horsepower at RPM............................ 34 BHP @ 6,500 rpm
Bore................................................ 71mm / 2.79"
Stroke............................................. 82mm / 3.23"
Displacement.................................... 649cc / 40 cu. in.
Compression Ratio
Home Market (UK).................................. 7.0:1
Export Market (US)................................ 8.5:1
Valve Clearance (cold):
Inlet................................................ 0.05mm / 0.002"
Exhaust............................................ 0.10mm / 0.004"
Valve Timing:
Inlet Valve Opens.............................. 34 degrees BTDC
Inlet Valve Closes.............................. 55 degrees BTDC
Exhaust Valve Opens.......................... 48 degrees BTDC
Exhaust Valve Closes.......................... 27 degrees BTDC

IGNITION:
Magneto Point Gap............................. 0.3mm / 0.120"
Spark Plug Gap.................................. 0.50mm / 0.020"
Timing (fully advanced):
Crankshaft position............................. 39 degrees BTDC
Piston position................................... 11mm / 7/16" BTDC

CARBURETORS:
Type................................................ Amal Monobloc
Throat Size....................................... 1-1/16"
Main Jet........................................... 240
Needle Jet........................................ .1065
Needle Position.................................. 2
Throttle Valve.................................... 376 / 3-1/2
Pilot Jet............................................ 25

TRANSMISSION:
Speeds.............................................. 4
Gear Ratios:
4th - Top........................................... 4.88
3rd - Third......................................... 5.81
2nd - Second...................................... 8.25
1st - Bottom...................................... 11.9
RPM @ 10MHP in Top Gear................... 638 RPM

CLUTCH:
Type................................................ Multi-plate, wet
Number of Plates:
Drive Plates...................................... 6
Driven Plates.................................... 5
Friction Material................................ Cork

SPROCKETS:
Engine............................................. 24 teeth
Clutch............................................. 43 teeth
Gearbox.......................................... 18 teeth
Rear Wheel...................................... 46 teeth

CHAIN:
Primary, pitch.................................. 1/2" X .335" X 5/16"
Primary, length................................. 70 links
Final Drive, pitch............................... 5/8" X .400" X 3/8"
Final Drive, length............................. 101 links

CAPACITIES:
Fuel (US versions)............................. 3 Imp. gal.
Fuel (UK & export versions)................. 4 Imp. gal.
Oil Tank........................................... 5 pints / 3 L
Gearbox........................................... 2/3 pt / 400cc
Primary Chaincase............................. 1/4 pt / 150cc
Front Forks....................................... 1/4 pt / 150cc

TIRES:
Front............................................... 3.25 X 19"
Rear................................................ 3.50 X 19"

SUSPENSION:
Front............................................... Telescopic Forks
Rear................................................ Sprung Hub

BRAKES:
Front............................................... 7" / 10.32cm SLS
Rear................................................ 8" / 17.78cm SLS

DIMENSIONS:
Seat Height...................................... 31" / 77.5cm
Wheelbase........................................ 55.25" / 140.3cm
Length.............................................. 86.25" / 219cm
Width............................................... 28.5" / 72cm
Ground Clearance.............................. 5" / 12.7cm
Dry Weight, unladen............................ 370 lbs / 168kg


TRIUMPH MOTORCYCLE BOOKS


TRIUMPH BONNEVILLE YEAR-BY-YEAR

TRIUMPH BONNEVILLE ESSENTIAL BUYER’S GUIDE

COMPLETE BOOK OF CLASSIC & MODERN TRIUMPHS

TRIUMPH MOTORCYCLES 1956-1983

FACTORY-ORIGINAL TRIUMPH TWINS

TRIUMPH MOTORCYCLES

TRIUMPH MOTORCYCLE RESTORATION, PRE-UNIT

TALES OF TRIUMPHS & THE MERIDEN FACTORY

TRIUMPH ILLUSTRATED SHOP MANUAL, 1935-1939

TRIUMPH ILLUSTRATED SHOP MANUAL, 1937-1951

TRIUMPH ILLUSTRATED SHOP MANUAL, 1945-1955

Triumph Thunderbird
Triumph Thunderbird, Pre-Unit
Triumph Thunderbird, Unit Construction

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