Well, I have a friend who is a metallurgist that runs a prepared Alfa Spider
in time trials. I asked him about "mags" and problems. Here's his reply:
_____________________________________________________________________
Subject: Re: Mag Wheels
Yep, the name 'mag' wheel is really a misnomer nowadays. However, my spider
came stock with true magnesium alloy wheels! (Campagnolo). The advantage,
of course, is the light weight -- about 10-15% lighter than aluminum, for
the same size/configuration wheel.
I don't know the specific alloys offhand, but I can check our reference
material in the lab, and get back to you.
You have a good understanding of the problems with older magnesium wheels.
General corrosion is probably the most insidious problem -- magnesium and
its alloys are VERY ACTIVE (meaning they corrode easily) in even very mild
environents. Today's very effective coating systems can reduce corrosion
much more significantly that in the 60's and 70's. Therefore, if you can
clean off the corrded areas, removing all the white powdery oxide, and
recoat the wheels, you can stop the attack that has already occurred, and
slow (to a crawl) any new corrosion.
Next is 'stress' or fatigue. Mag alloys have lower fatigue resistance than
aluminum, and steel. Also, fatigue is cumulative, meaning that whatever
fatigue loading the wheel had sustained in the past has caused microscopic
damage that remains in the wheel when you reintroduce the wheel (that has
presumably been sitting in someone's basement) to service. There is no good
way to "zero time" the metal. However, a thorough visual inspection, with a
10X magnifying glass, in conjunction with flourescent penetrant inspection
(we do these things at my place of employment) can identify if any
macroscopic cracking has occurred.
Hope that helps. Let me know if you need anyt other info.
Jim Hayes Boston, MA USA
www: http://www.fotec.com/jim.htm
veni, vedi, veloce
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