• 1959 Triumph T120 Bonneville

  • 1959 Triumph Bonneville Author: Robert Kimberly, licensed under CC BY-SA 2.0,
  • 1959 Triumph Bonneville at "Gilmore Motorcycle Barn" Author: F. D. Richards, licensed under CC BY-SA 2.0

This was the original ’stupidfast’ bike

The Bonneville model-family is arguably one of the most recognized in the world, and it is the embodiment of the quintessential British cruiser that had taken shape during the 1950s. Named after the motor-vehicle proving grounds at the Bonneville Salt Flats, the first example rolled in 1959 and launched a legend that persists to this day. This model represents the heyday of British dominance and was one of the bikes to beat on both street and strip.

  • 1959 Triumph T120 Bonneville
  • Year:
    1959
  • Engine:
    Parallel Twin
  • Displacement:
    649 cc
  • Top Speed:
    115 mph (Est.)

Triumph Bonneville History

1959 Triumph T120 Bonneville
- image 886511
1959 Triumph T120 Bonneville
- image 886517
1959 Triumph T120 Bonneville
- image 886516

The ’59 “Bonnie” 650 didn’t occur in a vacuum, but rather was the culmination of the progression of design by the now-defunct Triumph Engineering. Back then, the factory had the habit of stating the machine’s top speed in its name, so the ’59 Bonnie rolled with the T120 designator to reflect its somewhat ambitious, 120 mph top speed that actually was clocked at 115 mph, but hey, who’s counting right?

The mill was based on the T110 parallel-twin engine that was punched out and stroked to achieve better performance and a higher top speed. At this point in time, the Japanese manufacturers had yet to make the jump to high-performance products, and that left the Brits in good shape on the world stage.

Depending on who you ask, the look of the original T120 wasn’t quite as refined as it would come to be, but performance and handling were at the top of the food chain even though the model was bound to have some growing pains over the next few years. The very next year, the T120 saw a number of aesthetic improvements to go with its powerful engine, but the line took a hit due to the ill-fated decision to change the frame, an issue that would persist through the next two model-years.

1959 Triumph Bonneville Design

  • Cyclops headlight
  • Blackout forks
  • Tank knee pads
  • Chrome badging
1959 Triumph T120 Bonneville
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1959 Triumph T120 Bonneville
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1959 Triumph T120 Bonneville
- image 886528

The look of the ’59 Bonneville T120 didn’t necessarily break any new ground, as it was fairly typical of the industry at the time. A full-length front fender took care of business with short sides that would give way to the unpopular full-valence fender the very next year. Sure, the valence is better at handling the wet conditions that were so prevalent on the island nation, but it just didn’t look good, and aesthetics do matter. It’s probably worth mentioning that the current T120 uses the earlier fender style that leaves the front hoop well visible and relies on an external strut for stability.

Another unpopular feature was the old-school (even for back then) look of the cyclops headlight housing, a detail that is now considered part and parcel on a classic machine. Blackout forks and bellow gaiters support the front end, and out back, the dual shocks rock a blackout beercan shroud that dresses up the rear.

The fuel tank is another component that contributes to the modern look, and it came with the knee pads and chrome badging that defined those early machines. Fans of the current Bonnies will recognize the 30.3-inch high, two-up, bench-style seat as well, because everything is better when you share it with a friend right? A rather long rear fender completes the coverage, but unlike the above, it’s cut down significantly on the newer machines. All in all, a very cute ride that looks very British.

1959 Triumph Bonneville Chassis

  • Brazed steel tubing
  • Laced wheels
  • Drum brakes
1959 Triumph T120 Bonneville
- image 886518
1959 Triumph T120 Bonneville
- image 886526
1959 Triumph T120 Bonneville
- image 886531

The original frame on the Triumph Bonneville was a single-downtube, cradle-type frame that completely supported the separate “pre-unit” powerplant and transmission. It relied on the strength of brazed steel tubing for its structure, though it was a bit wimbly, something later versions would address. Unfortunately for the factory, the ’60 and ’61 T120s ran on the much-maligned “duplex” frame that was prone to breaking up under the strain of use.

A non-adjustable, telescopic-fork suspension components floated the front end with preload-adjustable shocks out back, and as was typical of the time, laced wheels rounded out the rolling chassis with 19-inch hoops front and rear. Unsurprisingly, drum-style brakes were the only anchors to be had, and they came in a 203 mm diameter up front opposite the 178 mm drum that took care of business out back.

Frame: Brazed lug, rigid
Front suspension: Telescopic fork
Rear suspension: Twin shocks. Girlings 100 lb/ft spring tension, adjustable pre-set
Front tire: 3.25-19
Rear tire: 3.50-19
Front brakes: 203 mm (8.0 inches) drum brake
Rear brakes: 178 mm (7.0 inches) drum brake
Tires: 3.25x19 front, 4.00x18 rear

1959 Triumph Bonneville Drivetrain

  • 649 cc parallel-twin engine
  • 46 hp @ 6,500 rpm
  • Kickstart
  • Four-speed gearbox
1959 Triumph T120 Bonneville
- image 886521
1959 Triumph T120 Bonneville
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1959 Triumph T120 Bonneville
- image 886525

The real selling point on the T120 Bonneville was the beating heart and the performance it brought to the table. A four-stroke plant, the parallel-twin ran with a 71 mm bore and 82 mm stroke that gave it a total displacement of 649 cc (39.6 cubic-inches). It was air-cooled, something the factory still uses on a few select Bonnie descendants, and used a Lucas magneto for the ignition. (Y’all remember Lucas right; the Prince of Darkness?)

Pushrod-actuated over-head valves controlled the flow through the combustion chamber in the aluminum heads, and not one but ’two’ Amal monobloc carburetors managed the induction. This last was a bid for increased performance, and it held the Bonnie in good stead for several years to come.

As for output, the mill was rated for 46 horsepower at 6,500 rpm, and that power flowed through a wet clutch, four-speed gearbox, and chain-type final drive to deliver a top speed of 115 mph. The early engines were kickstart only.

Engine: Air-cooled, OHV 360° parallel-twin
Bore x Stroke: 71 mm x 82 mm
Displacement: 649 cc (39.6 cu in
Compression ratio: 8.5:1
Power: 46 hp (34 kW) @ 6,500 rpm
Torque: 37.8 lb-ft (51 Nm) @ 5,500 rpm
Cooling system: Air
Fuel system: Amal monobloc carburetor
Ignition: Lucas K2F magneto
Final Drive: Chain
Clutch: Multi-plate, wet
Transmission: 4-speed

1959 Triumph Bonneville Competitors

1955 - 1958 Yamaha YA-1
- image 886484
Yamaha YA-1, Yamaha Communication Plaza
Author: PekePon, licensed under CC BY-SA3.0
1959 Triumph T120 Bonneville
- image 886510
1959 Triumph Bonneville
Author: Robert Kimberly, licensed under CC BY-SA 2.0,

In 1959, we were still a few years out from Honda’s fabulously famous CB450, and Japanese motorcycle performance was still in its infancy. Sure, there was the Yamaha YA-1 that was a few years old, and the Rikuo models that were for all intents and purposes actually Harley-Davidson cruisers, the latter of which topped out at 60 mph which is exactly half of what Triumph was bringing to the table with its T120.

The American market likewise was doing little to give Triumph any real competition, and the closest it could come to the Bonnie’s sportiness was its XL (Sportster) that had evolved just a few years prior from the flathead-powered experimental K-Model.

Read our full review of the Yamaha YA-1.

He Said

“Anyone familiar with my work will already know and appreciate the affinity I have for classic British machines, and this is a perfect example of the kind of bikes that shaped my taste. I think that folks forget sometimes that there was a point in history when British engineering dominated the motorcycle industry, but it’s easy to see why that was in the legendary grand-daddy of all Bonnevilles.”

She Said

My wife and fellow motorcycle writer, Allyn Hinton, says, “I have to appreciate these old Triumphs. The T120 Bonneville was the leading sportsbike of the time. With a top speed well over the 100 mph mark, it was the bike to beat. The T120 engine was derived from the engine that was in the T110, and the T110, with a heavily modified engine, had just a few years prior set a world land speed record of 214 mph. That was amazing for its day.

1959 Triumph Bonneville Specifications

Engine & Drivetrain:
Engine: Air-cooled, OHV 360° parallel-twin
Bore x Stroke: 71 mm x 82 mm
Displacement: 649 cc (39.6 cu in
Compression ratio: 8.5:1
Power: 46 hp (34 kW) @ 6,500 rpm
Torque: 37.8 lb-ft (51 Nm) @ 5,500 rpm
Cooling system: Air
Fuel system: Carburetor. Amal monobloc carburetor
Ignition: Lucas K2F magneto
Final Drive: Chain
Clutch: Multi-plate, wet
Transmission: 4-speed
Chassis:
Frame: Brazed lug, rigid
Front suspension: Telescopic fork
Rear suspension: Twin shocks. Girlings 100 lb/ft spring tension, adjustable pre-set
Front tire: 3.25-19
Rear tire: 3.50-19
Front brakes: 203 mm (8.0 inches) drum brake
Rear brakes: 178 mm (7.0 inches) drum brake
Tires: 3.25x19 front, 4.00x18 rear
Dimensions & Capacities:
Dry weight: 402 lb (182 kg) (wet)
Power/weight ratio: 0.2514 hp/kg
Seat height: 30.5 in (770 mm)
Ground clearance: 5.0 inches (127 mm)
Wheelbase: 55 in (1,400 mm)
Fuel capacity: 4.0 gallons (15.14 liters)
Top speed: 115 mph (185 km/h)
Details:
Seat: Dual seat
Colors: Pearl Grey/Tangerine Orange

Further Reading

Triumph

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TJ Hinton
T.J got an early start from his father and other family members who owned and rode motorcycles, and by helping with various mechanical repairs throughout childhood. That planted a seed that grew into a well-rounded appreciation of all things mechanical, and eventually, into a formal education of same. Though primarily a Harley rider, he has an appreciation for all sorts of bikes and doesn't discriminate against any particular brand or region of origin. He currently holds an Associate's degree in applied mechanical science from his time at the M.M.I.  Read More
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