Okay, I know this is about stuff that happened 50 years ago, but it’s an interesting comment on what makes a successful motorcycle.
Most of the major Italian makers built fast four-stroke 125cc Singles after Mondial showed the way by winning the World Championship in 1949, ’50 and ’51. Once there were 17-20-horsepower 125s, people naturally thought, “Man, what a 250 we could have by just putting two of those top ends on a common crankcase!”
Mondial built one in 1954-55, and it supposedly made a satisfying 35 hp. This wasn’t quite in the league with NSU’s Twin, which made 39 hp at 11,500 rpm, but it was getting there. The disappointing thing was the weight—more than 300 pounds. Mondial’s 250 Single was a much better racing machine.
MV was next, also using a pair of 125 top ends to make 36 hp at 12,000. This one was light enough to win some races from 1957-60, but the Honda Fours put it in retirement. And in 1958 or so, Ducati used a pair of its 1957 three-cam desmo 125 top ends with cam drive by gear to make 43 hp at 11,600. But the result was heavy and hard to manage—even for the great Mike Hailwood.
Here’s what Hailwood had to say about Twins:
“…I could not get on with any of them [the two exceptions he made were the Honda 125 and MZ 250]. They all had very narrow powerbands, which made them very hard work to ride for they felt rigid and inflexible and demanded constant gear-changing to get the best out of them. I persevered for a long time with the Ducatis because they had been built specially for me at very great expense as ‘one-offs’ and, in theory, should have left everything else in their capacity class standing.”
Being doubled-up 125s, these 250s naturally had narrow powerbands, and to stay in the power needed six-speed gearboxes that demanded constant shifting. A look at drawings of the Ducati Twin shows how weight can accumulate. While the bore spacing on a modern four-cylinder sportbike is only a bit greater than the bore itself, the 55mm-bore Ducati 250 Twin’s bore spacing was 175mm! The crankshaft had twice the mass of today’s 250 four-stroke MX engines. Length and width of that engine’s crankcase were 17½ inches and 13 inches. Why so big? I think it eased the problems of quickly coupling together a pair of proven 125 top ends, without involving a lot of time-consuming redesign for better packaging.
A bigger engine needs heftier chassis tubes, and the increasing weight needs bigger brakes, larger tires and more fuel. Soon, you’ve built a 250 that weighs more than a 500.