Friday, February 11, 2011


After legendary Triumph boss Edward Turner retired from the motorcycle factory in 1963, he holed up in a BSA subsidiary, CarBodies Ltd of Coventry, and simply couldn't keep his hand out of the old business.  Having entered the hallowed pantheon of Motorcycle Greats with his popular, stylish, and sometimes avant-garde machines from the 1920s onwards, he is best remembered as the man who made a parallel-twin engine look like a twin-exhaust-port single cylinder machine (the 500cc Speed Twin of 1938), which fit snugly into the existing 'Tiger 90' (a beauty as well) single-cylinder chassis.  This new combination had magic in name, looks, and performance, and set the tone for the British motorcycle industry for the nearly 50 years, for better or worse (worse at the end!).
Turner with his 650cc Triumph engine, ca.1960
Edward Turner visited Japan in 1960, and saw firsthand the technical superiority of the motorcycles being produced there, even if, at that time, bikes built there were small capacity (250cc and under), or clones of larger foreign machines (eg, the Kawasaki 'W1' copy of the BSA A10, and the Rikuo H-D clone). Unable to rouse his Board of Directors to make the necessary investment and produce a modern design in England, Turner chose to retire, unhappy with the direction of the British industry as a whole.  Still, he had always done interesting work as a freelancer, having come up with an advanced overhead-camshaft single-cylinder bike in 1925, and penned the foundation for what became the overhead-camshaft Ariel 'Square Four' in 1928 - which got him a job at Ariel under Valentine Page, and his radical design developed into metal by 1930.
The radical 1930 Ariel 'Square Four' ohc 500cc engine
From the sidelines in 1967, Turner sketched out a direct challenge to the Honda CB450 'Black Bomber', whose performance nearly equaled his beloved but aging line of 650cc Triumph twins... the Honda rubbing salt in the wound with an electric starter and leak-free, reliable running.   Turner poached a few Triumph employees to build up a running prototype of his double-overhead-camshaft, twin-cylinder 350cc bike with a short-stroke, 180degree crankshaft - exactly the spec of the Honda, but with 100cc less capacity.  Turner was confident his decades of experience squeezing power from his twins would yield excellent performance from this smaller engine, and so it proved to be.  The little bike hit 112mph in tests, faster than the Honda.  The styling was swiped from the current Ducati Monza, which wasn't the first or last time the English took a leaf from the book of Italian bodywork.
Wesley Wall of the NMM staff tests the prototype.
While an advanced machine on paper, with a mechanical disc brake, those cams up top, and excellent performance, the reality was, Turner had designed a hand grenade.  The Triumph brass (chairman Eric Turner - no relation), instructed chief engineer Bert Hopwood to ready the experimental machine for production.  Hopwood, performing an autopsy on the little machine after it broke its crankshaft on test, considered the design "fundamentally unsafe",  and set about, with Doug Hele, designing a wholly new motorcycle, with enough of the 'ghost' of Turner's idea clearly visible to satisfy the Board.
Chain-driven camshaft drive...
Hopwood's version of the DOHC twin, called the 'Bandit', had a stronger crankshaft, a chain primary drive instead of expensive gears, a 5-speed gearbox, electric starter, and a frame based on Percy Tait's 500cc grand prix racer, designed by Ken Sprayson of Reynolds Tube.  The Bandit was a real winner, with the same performance as Turner's machine, but promised reliability, excellent handling, and truly modern specification.  BSA shifted its mighty girth and tooled up for production in 1971, but less than 30 machines were built before the plug was pulled on the whole enterprise, as the British motorcycle industry began a period of free fall.
Pull the pin, lad, and it'll shortly explode...
Turner's prototype has been restored to running condition by John Woodward, on staff at the National Motorcycle Museum in Birmingham.

Many thanks to Mick Duckworth for forwarding these photos and information about the prototype!


Anonymous said...

I am a new reader, learning of your site from David Blasco of royalenfields. Great site!

I noticed on the fotos of the 1938 speed twin and the 'Tiger 90' both equipped with girder front ends. My question to you is "Whatever happened to the girder?"

Harley currently produces a springer, and in the mid 90s I had a mountainbike (ProFlex)with a girder style front fork, haven't noticed any in production today and I suppose, some decades.

Why the demise of the design? When did they cease? What was the last production bike with it. Did anyone ever make chopper (extended) style girder forks?

Thanks for a great clean historical site on motorcycles.


The Vintagent said...

Hi Matt,
'girder' forks still exist, they just look different now. BMW's Paralever suspension uses the same basic principle, and the Britten race bike too. Telescopic forks have their problems, like diving while braking, which girders don't have. The travel of girders is shorter though...

occhiolungo said...

Hi Matt. Girders were used in production bikes through the 1930s and in a few bikes after WWII. The last production bike to use conventional old-styled girders was probably the BSA M20 / M21.

Like Paul said, they typically don't offer as much travel as telescoping forks. They also cost quite a bit to manufacturer, with a lot of parts that need to be brazed together, plus a lot of machining operations to make all the various links and bushings. Teles were pretty simple and cheap to build. Teles also have simple methods to have damping, which girders never did well.

There were a lot of home made extended girders on choppers in the 1970s, some were 20" longer or even more. The strength/weight ratio of a girder is better than a tube for a long span.

Some flat track dirt racers still prefer to run girders, as they give good strength during side loading as the bike is pitched into a corner.


Anonymous said...

I rode an Ariel 600cc square 4 some 50yrs ago,girder forks/rigid frame,super torque but the rear cylinders tended to get overhot.
Turner's twins motors all used the vertically split"clamshell" crankcase design,too bad the Brit's didn't see the wisdom of Horex' horizontally split 1950s design as Yamaha did.Hardly rocket science,all the automobiles of the 1930s had used it.
The XS650 and all the Japanese inline 4's are more basically robust due to that old innovation.

istvan said...

Indian had a very good one in the late '40s.Good travel and no 'stiction',and also good damping due to an auto type shock.They need careful setup but handle well. S.

istvan said...

Paul,I'm new to your site,got a link somwere,great blog!Now girder front ends,leading link or trailing link?Ithink the English style works best,but the Harley guys think otherwise.In my opinion there are too many bushes and pins to keep up with,but they do work. Travel is not a big one ,if you hit a pothole it hurts no mater what front end you have.

Cbm said...

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please may i take to put in my blog