Battle of the British Twins: Triumph Bonneville vs. BSA Lightning

The bikesAlthough BSA and Triumph were competing brands, in 1951 both makers became part of one company when Triumph owner Jack Sangster sold his company to the BSA Group. BSA was by far the bigger company, involved in auto manufacturing, machine tools and numerous other engineering activities. But the Triumph division’s abrasive managing director Edward Turner managed to keep the BSA influence to a minimum until he retired in 1964. They were maintained as distinct brands, and until the late 1960s, each division’s machines were discrete designs sharing few components, built in different factories, and sold through separate dealer networks.

The BSA Group also 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, however, there really wasn’t much to differentiate them on the road, in spite of different philosophies. The Bonneville traces its roots to 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 x 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 Bonnie was still being built (although with a boost in capacity to 750cc) until 1984.

When BSA applied unitized construction to its Bert Hopwood-designed 650cc twin, the company took the opportunity of substantially revising the engine’s castings, producing the so-called “power egg” engine. Though based very much on Hopwood’s 1949 design, penned when he was chief designer for BSA, the engine dimensions were radically revised from an under-square 70mm x 84mm bore and stroke to an over-square 75mm x 74mm bore and stroke. Hopwood used a single camshaft mounted behind the crankshaft 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 bolted-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 Bonneville and Lightning had evolved into powerful, fast and reasonably reliable motorcycles, both capable of speeds in excess of 100mph (the magical “ton”) though the 120mph implied in the Bonneville’s “T120” model number was optimistic.

Visually, the bikes were also very different. BSA’s premier motorcycles almost always featured chrome plating on the gas tank and contrasting primary colors. Triumph, on the other hand, painted their tanks in a wide range of pastel shades and metallic finishes. Brakes, forks and many other components were different until 1968, when some standardization between the brands was introduced.

The verdict
For our comparo, we sampled a 1966 BSA A65L Lightning against a 1968 Triumph T120 Bonneville. Ahhh, but they’re not the same year, we hear you say. True enough, but the real truth is that between 1965 and 1970 mechanical changes were few and performance for each model stayed level. In the case of the BSA, there wasn’t much to differentiate a 1966 Lightning over a 1968, save a switch from Amal Monoblocs to Concentrics.

We sent our riders out on a variety of secondary roads, giving them ample miles for a true seat-of-the-pants assessment, and they came away from the comparo with renewed appreciation for both the Triumph and the BSA.

Over lunch, “Mike the Bike” and “Big Dave” offered their opinions after riding both bikes. Although a diehard BSA fan and owner, Woolley concedes that “the Triumph is a really sweet bike, and I think it would handle really well at high speed,” though he agrees the BSA probably accelerates faster. Woolley also liked the Bonneville’s “tossability.” “It’s very light, very agile,” he says. “It’s a very comfortable bike. You could ride it all day.”

Of the BSA, Woolley says, “I really like the motor. The BSA is a good ride-to-work bike, less agile than the Triumph, and the front brake wasn’t that good.” (Both bikes were fitted with the same twin-leading-shoe front brake from 1968 onward.)

Dickens is also a one-time BSA rider, having owned a Thunderbolt (the Lightning’s single-carb sibling) in the 1970s. “Riding the Lightning took me right back,” he says. “It felt like an old friend. But the Triumph felt more refined, more modern.”

Dickens also comments on the relative riding positions. “The pegs on the Triumph are much higher,” though he found both riding positions comfortable. And while the Bonneville “felt much more modern,” Dickens liked the low speed handling of the BSA. “It was as though you didn’t have to put your feet down when you stopped,” he says.

Woolley noted the Bonneville’s lightness, and this aspect is very evident in use. It’s the perfect bike for around-town riding with light, precise steering, good brakes, lively performance and a torquey engine. The Bonnie’s gears are spaced better than the BSA (which has top and third very close together). Relatively low overall gearing on the Bonneville, though, does limit performance, as the engine doesn’t really like to rev much past 5,000rpm (or maybe it’s my mechanical sympathy to the racket from the gear-driven valve train). Bert Hopwood learned from Edward Turner’s oversights to make the BSA engine much quieter mechanically.

The surprise was that in spite of their former and present allegiances, both riders preferred the Bonneville! I have examples of both bikes in my Big Shed: a 1966 Lightning (our feature BSA) and a 1970 Bonneville. I’d have used my own Bonnie for the story, but it’s not nearly 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, though, as Woolley noted, the older 8in 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. One other niggle: If I don’t ride my BSA regularly, oil drains back past the anti-drainback valve in the engine case, marking its territory by leaving a telltale puddle of oil on the garage floor.

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 definitely 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 have to make! MC

Press reports
“The Triumph big twin powerplant has come a long, long way since its initial appearance nearly thirty years ago. Because the original design was so basically efficient, company engineers were able to update the engine through the years to the point where it is as reliable and at least as powerful as anything in the high-performance motorcycle class.”
— Cycle, September 1966

“It hasn’t changed much, the Bonneville, in the last few years, but neither has Marlene Dietrich. And there’s Triumph’s secret: The Bonneville was right last year, and the year before that, and lord knows how many years before that. It doesn’t need to be changed.”
— Cycle, March 1969

“The Bonneville is more for the guy who would buy a Boss Mustang or a Z28. Performance-plus, flashiness in a moderate way, and respectable handling. Like the Sportster, the Bonneville is a stud bike, although the images aren’t quite the same.”
— Cycle World, May 1971
“Memory is at best a faulty instrument, with so many subjective factors coloring our view of the past, not the least of which is nostalgia. So we can’t really say how good the 1950 Triumph 650 twin was, but no matter how much we might wish it otherwise, there is simply no question that the 1972 Bonneville is a far superior machine. And of course that’s as it should be, considering the progress that has evolved in the industry.”
— Cycle World, January 1972

“The Lightning may not be the fastest motorcycle in the world, but its performance is certainly satisfactory, and it continues to be an eminently pleasing high-speed touring bike, which, when you get right down to it, is what it was designed to be.”
— Cycle, May 1969

“Although the [BSA] is fast, one of its most redeeming qualities must be the handling. Flinging any motorcycle around a roadracing course at high speed for long periods of time, under most circumstances,
is very tiring. Not so with
the Lightning.”
— Cycle Guide, March 1972

Essentials

Resources

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BSA and Triumph Parts

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