Vintage Hall: Bonnie vs BSA

The Bonnie vs BSA? When it comes to making a choice between two classic cousins, Robert Smith is happy he doesn’t have to make one. However, he does document the differences.

Battle of the Twins

In the mid 1960s, two of the fastest bikes on the road were British: the Triumph Bonneville and the BSA Lightning. Though both were 650cc parallel Twins, they differed considerably in design and execution. But which was the better machine?
Although BSA and Triumph were maintained as distinct brands, both makers became one company in 1951 when Triumph owner Jack Sangster sold his company to the BSA Group. The company worked hard to maintain each brand’s distinctiveness. In terms of market positioning, BSAs were solid and reliable, while Triumph bikes were sold on performance. In practice, there was less to choose between them on the road, in spite of different design philosophies.

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The Bonneville traces its roots to Edward Turner’s 1937 500cc Speed Twin. The long stroke parallel Twin engine used two camshafts operating overhead valves through external pushrod tubes. A stretch to 649cc (71mm by 82mm) in 1949 for the Thunderbird was followed by a performance version (Tiger 110) and finally the twin-carb Bonneville in 1959. In 1962, the Bonneville engine and transmission were combined in one set of cases, the “unit construction” engine, in which form the Bonneville was still being built (albeit with a further capacity stretch to 750cc) until 1984.
When BSA applied unitized construction to its Bert Hopwood-designed 650 Twin, the company took the opportunity of substantially revising the motor’s castings, producing the so-called “power egg” engine. Though based very much on Hopwood’s 1949 design, the engine dimensions were radically revised to an over-square 75mm by 74mm bore and stroke. Hopwood used just one camshaft mounted behind the crank and operating the overhead valves through tunnels cast in the cylinder block.
Other major differences included Triumph’s use of a lug-and-braze frame with a bolt-on rear subframe, while BSA had introduced an all-welded steel tube duplex frame as early as 1954. The BSA frame was lighter and more rigid, though the Triumph engine was considered easier to tune. This led to many enterprising owners fitting Triumph engines in BSA frames, creating the TriBSA.
By the mid-1960s, both the Triumph Bonneville and BSA Lightning had evolved into powerful, fast and reasonably reliable motorcycles, both capable of speeds in excess of 100 mph—the magical “ton”—though the 120 mph implied in the Bonneville’s “T120” model number was pretty optimistic.
One UK magazine tested a new Triumph Bonneville in 1964, finding it good for 112 mph, yet quiet and comfortable to ride in traffic. In particular the writers singled out the handling and brakes for praise. The same magazine tested a Lightning five years later (the specification had remained largely unchanged) and reached just 104 mph. While they were impressed by the power and acceleration, they found the severe vibration to be tiring and caused a number of components, including the horn bracket, to fail.bonnievsbsa.1
The testers had hit on the Achilles heel of the BSA twin. Its short stroke meant that in order to produce its rated 48 hp—two more than Triumph claimed for the Bonneville—the engine had to rev higher. While 7,500 rpm was theoretically possible, neither man nor machine could tolerate much more than 5,500. Triumph’s torquier, slower revving, long-stroke engine was simply more practical for everyday use.
Visually, the bikes were also very different. BSA’s premier motorcycles almost always featured chrome plating on the gas tank and contrasting primary colours. Triumph, on the other hand, painted their tanks in a wide range of pastel shades and metallic finishes.
Even brakes, forks and other components were different until 1968, when some standardization between the brands was introduced.

I HAVE EXAMPLES OF BOTH BIKES in my Big Shed: a 1966 Lightning (the bike featured in this story) and a 1970 Bonneville. I’d have used my own Bonnie for the story, but it’s nothing like as pretty as Gil Yarrow’s 1968 UK-market example shown here.
Both are pretty easy bikes to live with. The BSA has the snappier performance and feels “tighter” than the Bonneville, and the older eight-inch SLS front brake is pretty weak. The BSA feels chunkier than the Bonnie and heavier to move around, but on the road it feels more solidly planted. The engine produces its power higher up the rev range, and that’s where its limitations really kick in.
To ride the BSA in a spirited fashion and use its performance means constantly replacing headlight bulbs, repairing split fenders and welding brackets back on. The buzzing spreads to the rider, too, and that makes the BSA really tiring to ride fast. Dynamic balancing the crankshaft might help, but that requires a full teardown. A couple of other niggles: If I don’t ride my BSA regularly, oil drains back past the non-return valve into the sump, leaving a telltale puddle on the garage floor. It’s just marking its territory, of course …
The lighter feel of the Bonneville is very evident in use. It’s the perfect bike for around town riding with light, precise steering, good brakes, lively performance and torquey motor. Gears are better spaced than the BSA (which has top and third very close together). Relatively low overall gearing on the Bonneville though does limit performance, as the motor doesn’t really like to rev much past 5,000 rpm. Or maybe it’s my mechanical sympathy to the racket from the gear-driven valve train. Bert Hopwood learned from Turner’s design to make the BSA engine much quieter mechanically.
Side-by-side, the BSA looks more purposeful, but the Bonnie has winsome good looks, especially the scalloped gas tank, instrument cluster and headlight. Triumph styling guru Jack Wickes knew how to make a bike look good. Both bikes are easy starters: if they don’t go within two kicks, I know I’ve forgotten to turn the gas or the ignition on.
So, if I could only keep one, which would it be? Putting everything together, and given its iconic status and performance record, it would have to be the Bonneville. Fortunately though, it’s not yet a choice I’ve had to make yet.

– Robert Smith (issue #271)