Quote:
Originally Posted by Cafe.Racer
I'm sure the owner and everyone else is a bit peeved (that's a understatement - thank god you're okay, sounds a right nightmare), but I agree with previous comments - earlier impact damage weakened the part, IMHO (although on thing that puzzles me is how "bright" the crack is).
As for comments about it being a weaker, cheapo part - I think that unless any of you have a background in stress in materials, FMEA and design engineering, well - best to assume that BMW know a little bit more than the rest of us, eh? And that's before they start using fancy software to analyse the stress suspension components are under.
As for the part failing at the weakest point where there is no lip, clearly (for the reasons given above) it was meant to be like that. It appears that that section is roughly in the middle (halfway along the span) which is the point at which there is the most stress.
Why did BMW include a cheap lightweight part in their suspension design, and then include a design feature that would, apparently, promote failure at point of highest stress?
The answer is - on purpose. My guess is that the arm bends under impact damage, but the wheel remains attached. A heavier, beefier part would pass the stress loading caused by impact damage to it's sister part - cracked upright, or even bent chassis.
Anyway, see what Soper say (or more likely what BMW say to Soper). Bearing in mind their stance on cracked alloys, I would practice counter arguements...
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also wouldn't the arm bend upwards if the wheel was hit against something and surely it would have to be some impact ? I keep going outside and looking at it and its not just a flimsy bit of metal but the sag in it must be 4" out of line