1960 Triumph T120 Bonneville

Some enthusiasts regard the debut of unit construction with the 1963 models as the first major change in the Bonneville's history, but the fact is the Bonnie was subject to a number of running upgrades and improvements soon after its 1959 introduction, with the 1960 model year being no exception.
To backtrack a little, the debut Bonneville was a development of the Thunderbird and Tiger T110, adding the Tiger's optional twin carburettors, along with a strengthened crankshaft, strengthened valve springs and other modifications as standard. As most Trumpy enthusiasts will know, the name came from Triumph's record-setting success on the famous salt flats at Bonneville, Utah. The American-inspired name was appropriate for another reason in that the spur for development of the T120 came largely from the US market. Americans liked the handling, performance and style of the big Triumph twins, but wanted more power. Fitting a twin carburettor cylinder head, along with other modifications, achieved this. However, there were some shortcomings in the first iteration of the Bonneville. For starters, twin carburettors meant potentially twice as many problems, with the added issue of getting both carbs balanced so they delivered the optimum amount of power, especially at the higher end of the rev range. Increased engine output also stressed the clutch, requiring a different grade of friction material to ensure the clutch was up to the task. The additional power showed up deficiencies in the handling and suspension, too. While it still used pre-unit (ie. separate engine and gearbox) construction, the 1960 Bonneville featured improvements over the 1959 model, both visually and mechanically.
The first Bonneville was identified by deeply valanced front and rear guards, a result of the 'streamlining' craze which Triumph and other manufacturers bought into during the late 1950s. The 1960 model's front guard was changed to a slimmer alloy unit, while the rear was a similarly pared-back steel item. The headlight nacelle which housed the instruments on the debut model was replaced with a chrome shell (making the headlight a standalone unit also made it easier to remove for racing) and separately-mounted gauges, including an optional revcounter (tachometer) that became standard in 1965. Oddly, the headlight switch was moved to a position near the offside front edge of the seat for 1960. The petrol tank, although it looked the same, featured different mounting points and a broader, rubber-backed chrome strap holding it in place. The seat was longer and better padded, especially at the rear, making it more comfortable for pillion passengers, and finally, the unpopular 1959 colour scheme (tangerine and duck egg blue would be the best way to describe it) was replaced with a more attractive pearl grey over mid-blue. This last change seems minor, but the debut year colour was so unpopular in the USA that 1960 Bonnevilles were redesignated as 'TR7A' and 'TR7B' for the US market, primarily to distance them from the negative feedback the Bonneville name attracted due to the 1959 colour scheme.
More important (and less obvious) than the 1960 Bonneville's aesthetic changes were its mechanical updates. The single top and seat tubes from the 1959 frame remained, but the switch to twin front down tubes and a steeper steering head noticeably reduced engine vibrations and front end lightness at high speed. The trade-off was slightly heavier steering. The 1959's slim front forks were replaced with sturdier units identified by distinctive ribbed gaiters. In an effort to reduce complaints of spring friction, more damping oil was added and the units themselves were redesigned to provide what Triumph claimed was two-way damping. The rear swingarm was carried over from the 1959 models. Unbraced, it could lead to a loss of rear end stability at very high speed. This and the insufficiently small drum brakes counted among the Bonneville's few mechanical weak points.
Engine specs were largely unaltered for 1960, but gearing was revised - largely as the result of US market feedback - to a 22 tooth front sprocket (down from 24) and a 43 tooth rear sprocket (down from 46). The Bonneville's signature twin carbs were fed by a central reservoir, which required a fair amount of skill - or practise - to prime properly. Even in optimum tune, this setup made the bike run rough at low speeds. The addition of 1 1/16" Amal Monobloc carbs late in the 1960 model year removed both the reservoir and the problems that came with it. Electrics were largely carryover from 1959, with the exception being the addition of an alternator to replace the dynamo. The notoriously unreliable Lucas electrics remained, and were an ongoing issue, not just with Bonnevilles, but many other British-built motorcycles.
The Bonnie's affordability when new, at least compared to some other big twins, partly explains its success, but it wasn't a winner on price alone. It was fast, handled better than many of its contemporaries, looked good and was generally reliable. Oil leaks and weak 6 volt electrics aside (12 volt electrics didn't arrive until 1966), the 650 twin was a durable unit, provided it was well maintained.
Modern riders are unlikely (and unwise) to take this bike to the "Ton Up" limits of its heyday, but the pre-unit Bonnie still has more than enough grunt for swift country road touring and weekend blasts. Unit (post-1963) Bonnevilles are more widely available, cheaper to buy and, some may argue, the "best" of the original Bonnies, but each era has its own fans, with many appreciating what a pre-1963 Bonneville has to offer. If you accept the compromises that are required to ride a 50+ year old bike, and have a reasonable level of mechanical competence to keep it running, then a 1960 pre-unit Bonnie is a very desirable classic.

Specifications - 1960 Triumph T120 Bonneville

Engine: 649cc 4 stroke OHV parallel twin
Bore/Stroke: 71 x 82mm
Compression: 8.5:1
Power/Torque: 46bhp@6500rpm (approx) / N/A
Fuel system: 2 x Amal Monobloc carbs
Cooling system: Air
Transmission/Drive: 4 speed manual/chain drive
Starting:Kick
Electrics:6 volt
Front Suspension: Triumph telescopic forks
Rear Suspension: Swingarm / Girling twin shock
Front Brake: 8" sls drum
Rear Brake: 7" drum
Front Tyre: 3.25 x 19"
Rear Tyre: 4.00 x 18"
Weight: 403lb (183kg)
Seat Height: 31" (787mm)
Wheelbase: 56.5" (1435mm)
Ground Clearance: 5" (127mm)
Fuel Capacity: 3 gallons (13.6 lt)
Top Speed:115 mph (183 kph) approx.

TRIVIA
In 1956, Johnny Allen set a new motorcycle speed record of 345.08 kph on a twin carb Triumph 650-powered streamliner at the Bonneville salt flats. Although the record wasn't officially recognised, the publicity was great for Triumph and the event was credited with giving the T120 Bonneville its name.

While the exploits of Allen and other US racers showed the potential of twin carb-equipped versions of Triumph's 650 parallel twin, UK-based racers had been successfully using modified twin-carb heads in scrambles competition for some time before the Bonneville's introduction.

On revcounter-equipped 1960 Bonnevilles, the take-off for the tacho ran out of the offside timing case, requiring a special change in the bend of the adjacent exhaust pipe to accommodate it.