Honda CB500T
- Engine: Air cooled, DOHC 4-stroke parallel twin
- Bore and stroke: 70mm x 64.8mm
- Claimed power: 34hp @ 7,000rpm (period test)
- Transmission: Helical gear primary, multiplate clutch, 5-speed, chain final drive
- Fuel: 4.2gal, 41.5mpg
- Dry weight: 455lb curb
- Max speed: 96mph
Until 1938, the two most popular motorcycle engine formats were the single and V-twin. Not surprisingly, singles dominated the smaller capacity class up to around 500cc, and V-twins above that. Why V-twins? Adding another piston in-line with the first and sharing the crankshaft seemed like the simpler engineering solution.
Then came Edward Turner’s big idea: the 360-degree parallel twin, which was essentially two singles side-by-side on a common crankcase. Cylinder barrels and heads could be siamesed, thus reducing costs. Turner’s genius was in making the Speed Twin look like a sporty twin-port single, so as not to appear too radical, and to use a 360-degree firing sequence for a regular exhaust beat, so it “sounded right.”
And that was a critical decision. Throughout their life of 50 years, all production Triumph twins (and most other Brit twins) used the 360-crank. But there was a problem: Turner’s twin was just as vibratory as a single of the same engine size, because both pistons still stopped at top dead center and bottom dead center.
Turner understood that 500cc was just about the acceptable limit for a 360-degree parallel twin. Calls for more power meant more cubes; and parallel twins grew to as much as 850cc, requiring a way of dealing with the hammering vibration. Sometimes, this was achieved by reducing compression (like the Norton 750 Atlas) or isolating the engine (Norton’s Isolastic system).
In the Sixties, Honda experimented with 180-degree crank throws, but the pistons still stopped at TDC and BDC, so the vibration remained. Yamaha and Laverda tackled the problem head-on in the mid-1970s with a balance shaft driven from the crankshaft featuring offset weights to negate the vibes. The issue was later resolved by using a crankshaft with 270/90-degree firing intervals, as Triumph did in their 900 Scrambler. Ironically, a 270-degree parallel twin emulates the primary balance of a 90-degree L-twin — like a Ducati, for example …
When the CB450 was released in 1965, it was praised for the power from its advanced dual overhead camshaft parallel twin — with a 360-degree crank in some markets and a 180-crank in the U.S. But while it was fast, sure-footed and technically impressive, it was still a shaker. Within five years, the CB750 set new standards for smoothness, and twins were no longer the bikes to beat. So rather than drop the twin, Honda repurposed it.
Over the years, the CB450 had gained a fifth gear, a front disc brake and revamped styling; but it was no longer the standout performer it had been. Honda’s solution was to turn it into a commuter and touring mount. Those expecting the CB500T to be another relative rocket ship were to be disappointed. In spite of gaining another 54cc from a lengthened stroke, the 500 produced no more power and actually lost performance. It was 17 pounds heavier than the 450, had lower compression (8.5:1, down from 9:1), taller gearing and emission control changes that stifled breathing: Cycle magazine chose the word “subdued” to describe the CB500T.
But Cycle‘s biggest criticisms were aimed at the suspension and ground clearance. This became a major issue on mountain roads, where the foot pegs, side stand and center stand would all successively hit the deck even in modest turns, causing “skitterish” handling as the rear tire lost grip. Testers tried cranking up the rear preload to maximize clearance, but all that achieved was suspension that was “stiff and hammering.”
Overall, said Cycle, the CB500T was a victim of “convenience engineering” where functionality was sacrificed to aesthetics. “The CB500T broadcasts its fresh classic looks, its ‘newness’ everywhere. And like a 1975 Chevy Nova, it’s a lot more entertaining to look at than it is to ride.”
Contenders: A couple of smooth operators
1975-76 Yamaha XS500B
- Engine: Air-cooled, DOHC, 4-stroke parallel twin, 180-degree crank, eight valves with torsion bar springs
- Bore and stroke: 498cc (73mm x 59.6mm)
- Claimed power: 36.5hp @ 8,000rpm (period test)
- Transmission: Multiplate clutch, 5-speed, chain final drive
- Fuel: 3.4gal, 40mpg
- Weight: 459lb curb
- Max speed: 103mph
In many ways, Yamaha’s XS500B looked to the future (eight valves and a balance shaft), while the CB500T’s simpler 2-valve architecture was anchored in the past. Cycle World called them “practically identical.” And superficially, their performance might suggest that: but under their alloy engine cases, they were quite different. Double overhead cams, yes. But the Yamaha had four valves per cylinder (better breathing and reduced valvetrain inertia allowing higher revs). In terms of engine vibration, the XS was “infinitely superior” thanks to its Omni-phase balance shaft: “Lack of vibration is just what you need for extended periods in the saddle,” wrote CW.
What the XS did have were lots of chains and sprockets: driving the overhead camshafts from the right side of the engine; a separate gear and chain driving the two oil pumps (feed and return) and another driving the balancer; and another connecting the starter motor to the crank.
Curiously, while Cycle had been scathing of the CB500T’s cornering, Cycle World thought it a minor issue, giving the CB the nod over the XS in handling while noting the XS’s “top-heavy feel.” The seat on the XS also came in for criticism, though the brakes were superior to the CB. Cycle also criticized the XS’s “hair-trigger” Keihin carburetors and the “lost motion” in its drivetrain, making for “unlovely low speed manners.” Cycle World concluded, “Taken in total, the 500T is the better machine.”
1975-81 Ducati 500 GTL/Sport Desmo
- Engine: Air-cooled, SOHC parallel twin, two valves/cylinder, 180-degree crank. (Desmodromic valve operation: Sport Desmo)
- Bore and stroke: 497cc (78mm x 52mm)
- Claimed power: 50hp @ 8,000rpm (Sport Desmo)
- Transmission: Multiplate clutch, gear primary, 5-speed, chain final drive
- Fuel: 3.7gal, 40mpg approx.
- Weight: 407lb dry
- Max speed: 115mph
Fully 10 years in gestation, Ducati finally introduced two new 500cc parallel twins to the U.S. market in 1975. (This without Taglioni’s genius to guide the project — he was working on what would become the 500 Pantah). Both were undersquare, air-cooled 180-degree twins with chain-driven single overhead camshafts. The 500 Sport used desmodromic valve control, while the touring 500GTL used springs. The two could be distinguished by the single-downtube frame, wire-spoke wheels and distinctive Georgetto Giugiaro 860GT-like styling on the GTL, while the Sport used a dual downtube frame, FPS cast wheels and sports styling. Because the mandarins who now ran Ducati needed space at Borgo Panigale to make diesel engines, the 500s were assembled (and the Sport restyled in the likeness of the Darmah) at Leopaldo Tartarini’s Italjet factory across town.
Journalist Mick Walker rode a 500 Sport Desmo from Italy to the U.K. in two days in 1981 and noted that it “had a character quite unlike the usual parallel twins. [It] vibrated at low revs, but above 70mph, everything levelled out.” Handling was “excellent,” the clutch was “beautifully light,” and the electrics “unaffected” by the rain he encountered. “The engine is the model’s outstanding feature,” wrote Walker, except that “it needed a 6-speed gearbox,” thanks to the narrow powerband. Less impressive was the poor finish of the black painted exhaust and Lafranconi muffler. Walker concluded: “In spite of all that’s been said … actually they were not bad [his italics] motorcycles.”
