Conrod Stresses

As a moving component, the connecting rod must be lightweight to reduce inertia forces while maintaining sufficient stiffness and strength. Structural rigidity is particularly important at the small-end and big-end bores to preserve bearing geometry and ensure reliable load transmission.

Force transfer from the piston pin to the crankshaft occurs through hydrodynamic lubrication in the bearings. The actual force acting on the rod depends on the lubricant pressure distribution, which is influenced by bore stiffness. Inertia forces are balanced by lubrication pressure between the crankshaft pin and the cap-side bearing. The connecting rod bolts maintain the force flow between the rod and bearing cap and are critical to joint integrity.

Under high inertia forces, the big-end bore deforms into a vertically oval shape, and the bolts bend outward. If bolt preload is insufficient, the joint may open on the inner side, causing loss of bearing crush and increased fatigue risk.

Under maximum gas pressure, the rod shank presses on the crankshaft pin, and the big-end bore becomes transversely oval. In this condition, the bolts bend inward. These alternating deformation modes generate significant cyclic bending stresses in the bore region.

The most highly stressed areas in straight-split rods are the bolt threads and the fillet transitions between the shank and both the big-end and small-end bores. In angle-split rods, the upper portion of the blind hole thread lies directly in the force path, creating a local stress peak. Proper control of stiffness, bolt preload, and fillet geometry is essential to ensure fatigue durability.