A suspension bushing separated from the disk on the rear wheel suspension of a racing vehicle while under operation on an express highway, causing the wheel to detach from the car. Visual inspection showed fresh turning grooves at four built-up fillet welds on the torus of the outside of the disk. The welding heat and rapid cooling caused the base material of the disk – already with a martensitic structure – to harden throughout the torus, both of which caused cracking in the inner and outer fillets at the transition from bushing to disk. Visual inspection of the other rear wheel showed similar stress cracks in the hardened base material of the transition region as well as the same four welds. Rough finishing and the sharp-edged formation of the cross section transition may have also contributed to the failure. The results of the investigation suggested that machine shops neither execute nor permit repair-welds on highly stressed machine parts and especially vehicle components.