An accident occurred during operation of a custom-built beet planter in which the two support wheel kingpins fractured, causing it to collapse and injure the workers on-board. The operator was testing the planter to determine if they wished to purchase it from the current owner. An investigation into the failure was performed that included a site investigation, visual evaluations of the tractor and planter components, optical and scanning electron microscopy, metallographic analysis, hardness testing, and fracture mechanics calculations. The evaluations determined that pre-existing cracks were present on the kingpins on both the forward and aft surfaces. Strength calculations based upon the crack sizes and locations determined that the king pins were sufficiently strong for normal operations and that failure was primarily due to operation related overloading and not by poor maintenance or gross defects in the design.