Taken from a 1950's engine design book. This photo shows the difference between the master rod's motion and the links rod's motion. As the master rod rotates on the crankpin the linkrod's relation to the crankpin changes causing the link rod to follow an elliptical path. Depending on the link rod's position on the master rod it can arrive at it's TDC a few degrees before or after it should in relation to the master rod's TDC. Put simply, a radial engine with an articulated rod setup like this will not have uniform motion.
HCI's design addresses this issue and arrives at a very smooth running radial.
Pictures showing development of master rod design. The left master rod was proven to be weaker than the one shown on the right. The "scallops" on the left rod added difficulty to manufacturing and the oil holes drilled in it were detrimental to strength.
The rod shown on the right with H sections turned 90 degrees was easier to manufacture and had reduced stress concentrations.
According to the text stresscoat analysis was largely used to determine the evolution of the Wright master rod.
