1938 Triumph Speed Twin

INTRODUCING THE 1938 TRIUMPH SPEED TWIN
It's hard to overstate the significance of the introduction of this bike. Until the moment of its launch, just about every British motorcycle manufacturer was producing mostly big air cooled pushrod singles, but as displacement and performance grew, so did engine vibration. They'd reached the single's practical limits at about 500cc and 30 horsepower or so, respectively. Any increase in either number led to wicked engine vibration. Many things were tried, but no one seemed able to tame the big twins shakes enough to coax more power out of them. Along comes Edward Turner, the genius engine designer behind the Ariel Square Four, now working at Triumph. He came at the problem from a whole new direction. He created a twin that was built like a single. Both pistons rose and fell together on a 360-degree crankshaft, which itself rode on only two main bearings, just like the big singles, which saved on tooling costs. The new 500cc twins were fast and smooth, much smoother than any single. The new engine begat a new motorcycle, the 1938 Triumph Speed Twin 5T. And it was an instant hit.

1938 TRIUMPH SPEED TWIN LAYOUT
Other than the radical new engine design, the 1938 Triumph Speed Twin followed typical British motorcycle convention of the day. And most of it was built from parts already being used on other Triumph Motorcycles. The rigid frame, springer front end, wheels and brakes were all from the parts bin of Triumph's big twins.

1938 TRIUMPH SPEED TWIN ENGINE DESIGN
The new 500 twin had cast a iron cylinder block and cast iron cylinder head with removable aluminum rocker boxes. Following the standard British pattern of the day, the cams were operated by a gear train on the right side of the engine (ie: the timing side), while a primary chain was driven off the left side of the crankshaft (ie: the primary side). The chain drives a multi-plate wet clutch, which turns the 4-speed constant mesh gearbox with right-foot shift. This was what is now called a 'pre-unit' or 'non-unit construction' arrangement with the engine, the primary chain case and the gearbox all separate components that were assembled in the frame with a system of brackets. All of this closely followed the British pattern of engine design at the time, and used as many off-the-shelf components as possible.


1938 Triumph Speed Twin SPECIFICATIONS

Model Designation

Engine Number prefix

Frame Number prefix

Production dates

Engine Type

Displacement

Bore & Stroke

Compression ratio

Engine output

Top Speed

Electrical system

Primary drive

Clutch

Gearbox

Ratios, overall:

1st, bottom

2nd

3rd

4th, top

Final drive

Frame construction

Suspension, front

Suspension, rear

Brake, front

Brake, rear

Wheel, front

Wheel, rear

Wheelbase

Seat height

Ground clearance

Fuel Capacity

Weight

Triumph 5T Speed Twin

8-5T

TF or TH

Nov 1937 - Oct 1938

Air-cooled OHV vertical twin

498cc / 30.4 ci

63mm X 80mm / 2.48" X 3.15"

7.0:1

28 bhp @ 6,000 rpm

85-90 mph

6-volt, Lucas magdyno, 40 watt

Chain, 5/16" X 5/8" X 78 links / 77 sidecar

Multi-plate, wet

4-speed constant mesh, right-foot shift


12.7:1 solo / 14.7:1 sidecar

8.65:1 solo / 10.0:1 sidecar

6.0:1 solo / 1.95:1 sidecar

5.0:1 solo / 5.8:1 sidecar

Chain, 1/8" X 5/8" x 92 links

Steel tube, brazed lug

Girder-type

Rigid

7-inch SLS drum

7-inch SLS drum

3.25" X 20.0" Dunlop ribbed

3.50" X 19.0" Dunlop universal

54.0"

29.5", adjustable

6.0"

3.25 Imp gal

355 lbs, dry


MORE SPEED TWIN PAGES:
.....1938
.....1946
.....1947
.....1948
.....1956
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Triumph TR7
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Triumph 5T Speed Twin
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Check out these TRIUMPH BOOKS

COMPLETE BOOK OF CLASSIC & MODERN TRIUMPHS


TRIUMPH MOTORCYCLES IN AMERICA


TRIUMPH MOTORCYCLE RESTORATION


TRIUMPH MOTORCYCLE RESTORATION, PRE-UNIT


TRIUMPH BONNEVILLE & TR6 RESTORATION GUIDE: 1956-1983


TALES OF TRIUMPH MOTORCYCLES & THE MERIDEN FACTORY


TRIUMPH 650 & 750 TWINS OWNERS WORKSHOP MANUAL, 1963-1983


TRIUMPH BONNEVILLE: ESSENTIAL BUYER’S GUIDE


TRIUMPH BONNEVILLE: PORTRAIT OF A LEGEND


ILLUSTRATED TRIUMPH BUYER’S GUIDE, by Roy Bacon


TRIUMPH BONNEVILLE, 2001-2015


TRIUMPH: PRODUCTION TESTERS TALES FROM MERIDEN


TRIUMPH MOTORCYCLE RESTORATION GUIDE


TRIUMPH OWNERS HANDBOOK, UNIT-CONSTRUCTION 350 & 500, Kindle


TRIUMPH ILLUSTRATED WORKSHOP MANUAL 1945-1955, by Clymer


TRIUMPH TWINS & TRIPLES


FACTORY-ORIGINAL TRIUMPH TWINS, 1938-1962


TRIUMPH BONNEVILLE & TR6, by Timothy Remus


TRIUMPH MOTORCYCLES: FROM SPEED TWIN TO BONNEVILLE


TRIUMPH MOTORCYCLES: THEIR RENAISSANCE & THE HINKLEY FACTORY


TRIUMPH PRE-UNIT TWINS OWNERS WORKSHOP MANUAL


TRIUMPH THRUXTON BONNEVILLE 1959-1969


TRIUMPH MOTORCYCLES ILLUSTRATED WORKSHOP MANUAL 1937-1951, by Clymer


TURNER’S TRIUMPHS: EDWARD TURNER & HIS TRIUMPHS


TRIUMPH TWIN RESTORATION, by Roy Bacon


TRIUMPH TIGER CUB & TERRIER 1952-1968, Haynes Repair Manual


TRIUMPH 675 DAYTONA & SPEED TRIPLE REPAIR MANUAL 2006-2010, Haynes


CLASSIC MOTORCYCLES: TRIUMPH


TRIUMPH BONNEVILLE YEAR BY YEAR


TRIUMPH TRIPLES


TRIUMPH RACING MOTORCYCLES IN AMERICA


TRIUMPH MOTORCYCLES


TRIUMPH, by Roy Bacon


TRIUMPH 750/900 TRIPLES & 1200 FOURS 1991-1999, Haynes Repair Manual


TRIUMPH TROPHY BIBLE


TRIUMPH DAYTONA, SPEED TRIPLE SPRINT & TIGER: 885/955 1997-2005, Haynes Repair Manual


BSA & TRIUMPH TRIPLES GOLD PORTFOLIO 1968-1976


TRIUMPH TRIDENT OWNERS WORKSHOP MANUAL, by Haynes


TRIUMPH: A CENTURY OF PASSION & POWER


TRIUMPH BONNEVILLE, by Osprey


TRIUMPH TIGER 100 & 110, 1939-1961, by Roy Bacon


TRIUMPH TIGER 100 & DAYTONA


TRIUMPH BONNEVILLE, by Crowood


TRIUMPH by Don Morley


TRIUMPH TRIPLES & FOURS 1991 & 2004


TRIUMPH 350 & 500 1957-1974 ESSENTIAL BUYERS GUIDE


BONNIE: THE DEVELOPMENT OF THE TRIUMPH BONNEVILLE


TRIUMPH SINGLES: EARLY DAYS TO 1974, by Roy Bacon


TRIUMPH: RETURN OF THE LEGEND


TRIUMPH FUEL INJECTED TRIPLES 1997-2000


BOOK OF TRIUMPH TWINS 1968-1969, by Floyd Clymer


THE TRIUMPH STORY: RACING & PRODUCTION MODELS 1902-PRESENT


TRIUMPH 350 & 500 TWINS 1958-1973, Haynes Repair Manual


TRIUMPH BONNEVILLE 2001-=2008 ROAD TEST PORTFOLIO


HINKLEY TRIPLES & FOURS


TRIUMPH PRE-UNIT TWINS


HINKLEY TRIUMPHS: THE FIRST GENERATION


TRIUMPH BONNEVILLE BIBLE: ALL MODELS 1959-1983


TRIUMPH TWENTY-ONE TO DAYTONA: C CLASS 350 & 500 TWINS


ORIGINAL TRIUMPH BONNEVILLE


TRIUMPH 650 & 750 TWINS


TRIUMPH MOTORCYCLES & CUSTOM BIKES


TRIUMPH MOTORCYCLES


TRIUMPH, THE LEGENDS SERIES


TRIUMPH THUNDERBIRD SUPER PROFILE


TRIUMPH SPEED TWIN & THUNDERBIRD BIBLE


TRIUMPH BONNEVILLE T120/T140


TRIUMPH MOTORCYCLES FROM 1950-1983


TRIUMPH & BSA TRIPLES: THE COMPLETE STORY


TRIUMPH THUNDERBIRD, TROPHY & TIGER ESSENTIAL BUYERS GUIDE


TRIUMPH TIGER CUB BIBLE


ROAD RACING HISTORY OF THE TRIUMPH 500 UNIT TWIN


EDWARD TURNER: THE MAN BEHIND THE MOTORCYCLES


SAVE THE BONNEVILLE: THE INSIDE STORY OF THE MERIDEN WORKERS CO-OP


TRIUMPH DAYTONA 1991-2006 ROAD TEST PORTFOLIO


TRIUMPH DAYTONA 2003-2009 ROAD TEST PORTFOLIO


TRIUMPH TIGER 100 / DAYTONA: DEVELOPMENT HISTORY


TRIUMPH, by Daniel Gilpin


BOOK OF TRIUMPH MOTORCYCLES ILLUSTRATED SHOP MANUAL 1935-1939


TRIUMPH: THE RACING STORY


TRIUMPH BONNEVILLE T120 1959-1974, by Roy Bacon


TRIUMPH BONNEVILLE GOLD PORTFOLIO 1959-1983


TRIUMPH TRIDENT SUPER PROFILE


ILLUSTRATED TRIUMPH BUYERS GUIDE, by Roy Bacon






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