Which Bonnie is the Best?

Conventional wisdom says the 1968-1970 Triumph 650 Bonnevilles were the best, the product of a decade of continuous development. The crack-prone pre-unit duplex frame was history, and the 1963-on unit-construction engine was more compact and needed less maintenance. Balance factor and flywheel changes moderated vibration, camshafts were nitrided to reduce wear, and the 1968-on twin leading shoe brake was a potent stopper. The engine was mostly oil-tight, thanks to improved breathing and revised pushrod tube seals. Already becoming a classic, the 1970 Bonneville wasn’t broke and didn’t need fixin’.

But they fixed it anyway. BSA Group’s in-house think tank, a safe distance from Triumph’s Meriden factory, dreamed up a new welded chassis that carried oil in its frame tubes, dispensing with the oil tank. Though built around BSA’s unit construction 650, it was also supposed to fit the Triumph 650.

At Meriden, everything that could go wrong, did. The new frame would not accept the Bonneville engine with the rocker boxes in situ, slowing production, while the seat height leapt to an ankle-stretching 34 inches. Frothing inside the frame “tank” meant reducing oil capacity, with the potential for overheating. Frames fractured around the center stand mounts, creating oil leaks. Because of these and other issues (a new computerized ordering system for one) BSA/Triumph missed the lucrative Spring sales season in 1971 — one of the reasons they ran out of cash in 1972. Fairly or not, the unloved oil-in-frame Bonnie got the blame.

But was this dissing deserved? Aside from its faults (and its eccentric styling) the OIF 650 of 1971-1972 brought in much-needed upgrades, like the welded frame, 5-speed transmission and turn signals. Most of its flaws were corrected during 1972, and paved the way for the disc-braked, 750cc Bonneville T140V of 1973.

Good, better, best

The success of the “new” 1963 unit-construction Bonneville — like its progenitor, the 1938 Speed Twin — owed much to Edward Turner’s flair for combining classic looks with contemporary technology.

Inside the unitized powertrain was a 71mm x 82mm dry sump parallel twin with a one-piece crankshaft running on ball bearings (drive-side roller from 1966) and plain bearing big ends. A duplex chain drove the wet clutch and 4-speed transmission. Two gear-driven cams operated overhead valves by rockers with screw adjustment. Battery/coil ignition replaced the pre-unit magneto.

A single downtube lug-and-braze steel tube frame housed the engine, with a bolt-on rear subframe (that’s why Triumphs were popular with hardtail builders!). Triumph fitted their own telescopic fork, while Girling spring/damper units controlled the swingarm. Brakes were floating single leading shoe, 8-inch front and 7-inch rear.

U.S. dealer feedback prompted changes for 1966. In came smaller gas tanks and wider handlebars; electrics went 12 volt; front brake lining area was increased; exhaust cam lubrication was improved; the rear drive sprocket was detachable from the brake drum; and the tach gained a right-angle drive to prevent cable snagging. 1968 introduced the potent TLS front brake, and for 1970 the crankcase breathed through the primary to atmosphere, replacing the camshaft-timed breather.

A Cycle World test in 1966 recorded a top speed of 109mph and a 14.2 second standing quarter at 88mph, while Cycle magazine’s 1970 model recorded 112 and 13.9 at 97mph, thanks to engine upgrades. Cycle also noted extra stability at high speeds from an increase in fork rake. Though steering felt heavier, it was still “head and shoulders above previous models” in handling and “roadability,” they wrote. CW noted that the increase in brake lining area made the front stopper “fade free.” Wrote Cycle, “Spend some time with this prince among two-wheelers … and you’ll come to an understanding of why it has been around so long.” They lauded its “great mechanical presence,” “power” and “silky smoothness.”

“The Triumph Bonneville for 1966 must still be considered the standard by which all medium-large displacement high performance touring bikes are judged,” wrote Cycle World. “None offers quite the same combined package of performance, and reliability, and handling.” MC

Other Bonnevilles to Consider

1959-1962 Triumph 650 T120R Bonneville

• Claimed power: 46hp @ 6,500rpm
• Top speed: 115mph
• Engine: 649cc (71 x 82mm) 2-valve air-cooled OHV parallel twin; 4-speed, chain final drive
  Weight: 404lb (wet)
• Fuel capacity/MPG: 4.8gal/52mpg
• Price then/now: $1,100 (est.)/$7,000-$28,000

The 1959 Bonneville was essentially the 1958 Tiger 110 650cc with a new splayed port cylinder head, dual remote-float Amal Monoblocs and a new one-piece crankshaft. It wore the Tiger’s dowdy headlight nacelle and valanced fenders. U.S. dealers asked for a sportier look.

That came in 1960 with a new duplex frame, slender blades replacing the frumpy fenders and the nacelle nixed in favor of paired tachometer and speedometer. Unfortunately, the new frame was prone to breaking below the steering head, and a triangulating brace was added during the year. A 60-watt Lucas RM15 alternator and selenium rectifier replaced the 6-volt DC generator, though a Lucas K2F magneto still supplied sparks.

British magazine The Motor Cycle tested the 1960 Bonnie, finding it “very fast” with “tremendous acceleration.” Handling was “first class” though heavier than usual for a Triumph, and the then-new floating-shoe brakes worked “really well” while the engine remained oil tight.

1971-1972 Triumph 650 Bonneville

• Claimed power: 47-50hp @ 6,500rpm
• Top speed: 112mph (period test)
• Engine: 649cc (71mm x 82mm) 2-valve air-cooled OHV parallel twin; 4-speed (5-speed optional), chain final drive
  Weight: 399lb (curb, 1/2 tank gas)
• Fuel capacity/MPG: 3.5gal/53mpg
• Price then/now: $1,479 (1971)/$3,000-$9,000

When the 1971 Bonnie finally arrived in the U.S., the cycle press fawned: “One of Triumph’s best,” said Cycle World — though they noted that the seat was too high, and the Lucas turn signal switches were awkward and looked “as they’d be easy to break off.” But U.S. dealers were aghast: “The one thing Triumph didn’t have to change was the appearance,” noted dealer/racer Bob Leppan at the time. “That’s why people bought Triumphs!” In time, riders got to appreciate steadier handling that came with the lighter, more rigid frame, improved fork action, and taper-roller steering head bearings. Cycle Guide thrashed a 1972 Bonnie round Orange County Raceway averaging over 70mph (including stops) before the clutch gave up at 318 miles — and the engine had burned two quarts of oil! Both CG and CW found the brakes worked well, though many owners would differ. The retro-styled 1973 T140 finally got it right: “Triumph should have built the 750 Bonnie five years ago,” said Cycle World in 1973.

The Verdict

So which Bonnie really is best? For historical significance and cachet, it’s the 1959. The duplex pre-units win on graceful, austere styling and street cred. Early unit-construction Bonnies carry the McQueen seal of approval. And for sharp handling, it’s the 1971-1972. If only Triumph had built a 5-speed, disc-braked 650 … but they didn’t. The 1968-1970 Bonnies were capable, reliable and refined over a decade without losing their classic good looks. So they still get the nod.