Son #2 (David) noticed the other day that the front tyres on his 5 series BMW car had got very bald around the inner edges,with 85% of them almost unworn, so he whent off to a local alignment shop, not just tracking, but caster and camber as well, they pointed out to him that a ball joint was badly worn, and he had better fix it before setting up.So he returned, bought the joint and replaced it and then told me the story. 2 years ago,and about only 4000 miles he changed the same joint, he used a cheap pattern part, cost about 20 GBP, and replaced two tyres (200 GBP the pair) because he discovered that the genuine BMW joint was 135 GBP.This time he used the genuine article, because the cheap one had trashed 200 GBP's worth of tyres.
I should point out here that the matching joint on the other side of this 120,000 mile car is unchanged from new, genuine part (we know the cars history),so cheap bits are not always as good as they first seem.
CLEM