Con-Rod Confidential

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Ever taken a look inside a big-twin American motorcycle engine or a large aircraft V-12? If so, you may have marveled at the unique way their connecting rods join two pistons to a single crankpin. As one of the connecting rods approaches its big end, it splits into a fork, resulting in two separate rings around the crankpin, like two tines of a dinner fork. The corresponding rod from the second cylinder terminates in a single big end, narrow enough to fit between the two “tines” of the first rod.

Why go through all this trouble? After all, the vast majority of Detroit’s V-8 automotive engines also join two con-rods to a single crankpin, but they get along just fine bolting those rods up side by side; all the designers do is offset one cylinder bank by the width of one rod. Vincent’s much-admired and much-sought-after big twins did the same, even with their roller big-end bearings. The well-respected S&S X-Wedge engine, a popular Harley replacement, also uses side-by-side con-rods, and so does Indian’s Thunder Stroke V-twin.

Beginnings of Fork-and-Blade Connecting Rod Design

Here’s what I think. Back in motorcycling’s early days, Glenn Curtiss went racing with the single-cylinder engines that he and chum Charlie Kirkham built. Eventually Curtiss wanted more power than a single could make, so he added a second cylinder. To minimize changes to the crankcase, he simply hinged the new cylinder’s con-rod to a lug sticking out of the existing con-rod’s big-end. It wasn’t ideal, but this kept the two pistons and rods in the same plane, plus it allowed him to just add a second cylinder-mounting pad to the existing crankcase patterns. Soon he was again winning races and his short but dynamic career of all work, no sleep, and ceaseless creation (he died aged 52). While this master-and-link rod system produced a secondary vibration due to the two piston’s different motions, it was small enough that Curtiss used this design in his V-12 aircraft engines.

S&S Cycle Heavy Duty Harley-Davidson fork-and-knife connecting rods. (S&S Cycle/)

I suspect that as other early motorcycle makers produced their own V-twins, they found the idea of having both cylinders and rods in the same plane simplifying. If they did this by using fork-and-blade construction, they also made the motions of the two pistons identical.

Allison and Rolls-Royce

In the aircraft world, there were additional reasons, rooted in history as well. Both Allison and Rolls-Royce produced V-12 engines which used knife-and-fork rods. Early on, Allison had been modifying some V-12 Liberty engines left over from World War I, especially fitting better con-rod bearings. My view is that designers at Curtiss decided to employ fork-and-blade rods because that would allow both rods to have the support of the single full-width bearing.

Rolls-Royce built its Merlin V-12 with the same con-rod construction. In fact, the Rolls and Allison designs are so close that US hydroplane racers would often build their Merlin powerplants using the stronger Allison rods.

Better Bearings, Better Designs

In the decades since, the underlying physics and the manufacturing practice supporting plain journal bearings have improved to the point that big-end bearings no wider than a normal con-rod big-end can easily carry the freight. This is how turbo Hayabusa rod bearings, originally sized for 200 hp, are able to survive at a modified 650 hp or higher.

And those modern engineers are happy to have the straight load path from wrist pin to bearing that side-by-side rods offer, rather than having to pass that load through the springy curved arch shape of a fork rod. At Harley, its Big Twin’s fork-and-blade tradition lives on, enabled by a very moderate peak of 5,000 rpm.

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