This is one I’ve encountered more than once before. You have a bad ground connection between the battery and the engine block or transmission case, wherever the ground strap terminates. At the same time, you’ve got a good ground between the battery negative post and the car’s chassis. When you turn the key to start Illustration by Vic Kulihin the car, the starter motor needs several hundred amps for a few seconds, which typically flow through the starter to the engine and back to the battery. In your case, that engine ground path has too much resistance, so the current finds an easier, lower-resistance way from the engine to the chassis, passing through the wheel bearings, axles and CV-joint internals. The high current, transmitted through small points of contact between the machined races and the rollers or balls, strikes tiny arcs, damaging the smooth surfaces. Within a few hundred kilometres, the joint or bearing fails.
I’ve seen older cars with mechanical throttle cables and conventional rear axles do the same thing, only in their case, they sourced the ground connection along the throttle cable, which turned as red as the wire inside a toaster oven when you twisted the key. I’d replace all the battery ground straps and clean all the areas where the straps attach with a wire brush and DeoxIT contact cleaner. Assemble with new hardware and some copper-bearing anti-seize compound and your problem should cease.
The normal path for the current running back to the battery – from the engine block to the battery negative
– is a ground strap. There may be a separate strap to the body, or the strap may make a stop at the chassis ground point on the way.
If the ground connection between the engine/transmission block and the chassis is poor, the high current needed to run the starter motor will source the path of lowest resistance – through the wheel bearings, axles and CV joints.