How Harley-Davidson’s Knife-and-Fork Conrod Shaped V-Twin Power and Sound
11 April 2021
Here's an engineering solution used on Harley-Davidson motorcycles from the very, very early years, when William S Harley was the lead designer. It's the "knife-and-fork", or "fork and blade" conrod.
While the design wasn't invented by H-D, it was selected as the best solution for several early problems:
- Single cylinder engines of the time didn't produce enough power
- A V-Twin or parallel twin could produce the power, but was wide and placed painfully hot engine cases too close to the legs of shorter riders
- The comfort of the rider would be compromised if the vibrations of the single-cylinder engine (vertical and fore-and-aft) were combined with the "rocking couple" side-to-side vibrations of a parallel or traditional V-Twin.
The solution? Put both conrods on ONE crankpin, and the knife-and-fork design allows this with a single-plane narrow engine. Fore-and-aft vibrations may increase, but there is zero rocking-couple effect.
This design has been used in almost every H-D V-Twin engine from the original 1909 V-Twin to the Milwaukee-Eight engines today. Other renowned engines using the knife-and-fork include the legendary Rolls Royce Merlin V12 aircraft engine.
The design is also responsible for the unique, soul-stirring Harley-Davidson sound. The two cylinders fire close together at the tops of their compression strokes (bang-bang close together), and the 4-stroke engine then goes through its power, exhaust, intake, and compression strokes before firing both shots again. The pulse has, therefore, an uneven sound. A beautiful uneven sound.