I emailed three different wheel repair organizations just to get some feedback from them on what they may have seen through the years. Soon as I get their info I'll type up the conversations and post them here.
Rolf, I know you did did research of your own concerning the BMW wheel cracks, and I did a little too and even through wheel cracking as we've been discussing is happening out there, for some reason, wheel cracking doesn't seem to be a widespread problem as the other type of wheel damage that repair shops fix.
One organization mention porosity as Alext mention, and often chrome plating alloy wheels can damage the surface area of the wheel and facilitate a slow process that allows cracks to develop where the spokes connect and that lug nut area.
Metro Wheels - wheel repair articles (Look about 3/4 down the page)
Quote:
Cracks can be Serious Business
Cracks are of particular importance, because they are the single most common type of damage that can leave a wheel beyond repair. When a wheel bends near the base of a spoke, haze marks in the finish can sometimes be seen and indicate cracks. Because repairing a wheel is not always best in a particular situation, it is important that the wheel is thoroughly examined.
- We value your safety above everything else and, for that reason, Metro Wheels considers a wheel to be irreparable if cracks appear: Near the base of a spoke
- Near the lug area
- On the inner lip of the wheel away from the spokes. (But if a crack is smaller in size, this type of damage is usually salvageable.)
Cracks are particularly common on chrome-plated wheels, where the chroming process tends to render the wheel more brittle than a standard-finish wheel. Sometimes, if a chrome wheel is bent severely, the chrome has to be removed because there can be cracks underneath the chrome. In the case of a cracked chrome wheel, Metro will grind the chrome out and re-weld it.
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AutoMotix - Alloy Rim/Wheel Repair
Quote:
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* Stress Cracks - Alloy wheels often suffer stress-cracks and air-pocket inclusions due to 'Hydrogen Embrittlement', which generally occur due to excessive heating. These cracks or inclusions can prove to be dangerous if it is not treated timely. This can be treated by filling the cracks with a suitable material. It is advised that these complicated issues should be handled by a qualified mechanic.
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Apart from the links that you provided, I found only a couple of references to wheels cracking on the inside rim edge, but there was no reason mentioned for it.
One question that keeps burning a hole in my brain is this: are the other premium auto manufacture marques' wheels suffering the same issues as BMW's. I'm sure they use a common manufacture for their wheels, but is there any difference in the metallic alloy ratios or is there a difference in the thickness of the rim itself?