wheels, plating and cracks (sort of long)

["Reid, Warren" <Warren.Reid@xxxxxxxxxxxxxxxxxxx>]

In alfa-digest V7 #298, there were a couple of more enquires regarding
chrome plating of steel and non steel wheels. Again I feel obliged to make a
few comments here from personal and professional experience.

The first is that as a metallurgist, I personally would never even
contemplate using anything that has been after market plated in a situation
where cyclic or non cyclic stresses are concerned. Sure 95% of the time you
will get away with doing this as the component in question has been well and
truly overdesigned. Steel (as used in car wheels) is usually pretty
forgiving stuff being not much more than a good grade of mild steel which
allows good formability and weldability. From its fatigue resistance point
of view its not too bad either and remember a road wheel is almost the
perfect fatigue loading situation. When cornering, that wheel goes through
one complete load cycle (ie compression/tension) every time the wheel does
one revolution. One of the advantages with steel is that it has what is
called a threshold fatigue cycle stress. ie if the stress stays below a
certain level the wheel won't (shouldn't!) fail due to fatigue.

That leads me onto my second statement -and this is going to upset a few
people, the light alloys such as Aluminium and Magnesium are probably one of
the worst materials to make wheels out of, especially when they're cast.
Aluminium and its alloys (and I'm pretty sure this applies to Magnesium also
- - this is all been done from memory) do not have fatigue cycling thresholds.
That is if you fatigue cycle an Aluminium alloy it will eventually fail. It
might take an enormous number of cycles to do it depending upon the design
of the component (wheel) however it will eventually happen (Joe
Bender-Zanoni being in the aircraft industry is probably better qualified
than me to tackle this topic). 

Part of my University studies thesis was looking at Aluminium alloy wheels
and their metallurgical structure. Lets just say that you usually get what
you pay for. A wheel from a certain expensive German automobile manufacturer
was the perfect sand casting with all the Aluminium Silicates nicely
Strontium modified to form a nice globular eutectic structure. The cheap
backyarder wheel had a nice sharp angular eutectic structure, which is
perfect for keeping those cracks growing. I might point out that the cheaper
wheel was already 100% fatigue cracked through 3 of its 4 spokes and it had
only been on the car 1 year! (sentimental note - I still have all those
samples in the bottom of my filing cabinet after all this time!) In addition
I might also point out that I did this work 17 years ago when "mags" for
cars were pretty rare. I dare say mass production aluminium alloy wheel
casting (usually not sand cast any more) has probably come a long way since
then.

The upshot of all of this is that for all those people with old aluminium or
magnesium alloy wheels, especially those doing a lot of racing and with
sticky tyres (a lot of those wheels were never designed for those tyres), I
would be having a very close look at your wheels - you may be surprised with
what you find (or don't want to find)! Dipping bare light alloy wheels in
acidic and alkaline solutions (eg for plating/cleaning), especially when
those wheels are already quite porous (hard to get away from that with sand
casting) could get you into even more trouble with something called "Stress
Corrosion Cracking" but that's a story for another day.

Ok that should give you guys something to talk about now!

Warren Reid BSc (Hons) MApp.Sc (Physical Metallurgy)
Melbourne, Australia

86 Spider Veloce (Magnesium Alloy wheels - arrgh!)
85 Alfetta GCL (Magnesium Alloy wheels - arrgh!)
68 1750 GTV (Painted steel wheels forever!)
58 and 61 Bugeyes (steel and NEW Al alloy wheels)

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