Kurt Sperry recently wrote about some of the problems found when buffing and repolishing aluminium bike parts.
On any typical bike from the period covered by the List there will be a signicant number of parts made from "aluminium alloy" eg stem, bars, cranks, chainrings, hub shells, brake levers, stirrups/calipers etc. These accessories were manufactured using a wide variety of engineering techniques - hot forging, cold forging, casting, machining (Turning, milling), rolling, extruding, to name but some of the them.
The use to which the parts are put dictates generally the engineering process to be used in the manufacturing, which in turn tends to dictate which of the many aluminium alloys should be used. These alloys are not fully interchangeable for any process.
It's a book in its own right to describe the various alloys and their applications, but aluminium alloys generally contain alloying elements such as copper, zinc, silicon, magnesium etc etc in varying proportions depending on the proposed end use of the material so that an alloy for use in extruded sections such as rims, would be no use for forging cranks. Theses alloying elements will also dictate to some extent the quality of finish that can be produced.
In the late 70s/early 80s everyone over here was heavily into anodising their bike parts as a way to customising then. A favorite "vehicle" to be customised was the Vitus 979 frame, in its original form ie silver anodised. So you could dress this frame up with red hubs laced on to blue rims, silver cranks with both a red and a blue chainring. You've got the picture..
One such frame that I customised could have been called the Velo d'Or, in that most of the aluminium alloy parts were anodised gold or failing that deep bronze. I recall that all the bits went into the same anodising vat at the same time - uniformity of colour being the desired effect. into the vat went a Cinelli 1A stem, Cinelli Mod 65 bars, Stronglight 93 cranks and rings, Maillard Team 700 hubs ( bearing housings blocked off with polyester resin), SR seat pillar. Instead of uniformity of gold the result was a hotchpotch varying from bright gold on the rings and bars to really grotty, and I mean grotty poxy dull gold on the Cinellt stem. the anodisers reckon that the higher the magnesium content in the parent alloy the better the finished job.
A classic example of selecting the wrong aluminium alloy took place with both Giant and Look - in the manufacture of the lugs for their early carbon frames (OK so it's out-of-time for the List.. or is it?) The first non-TVT Look frames and the early Giant Cadex frames had their lugs sprayed to match the frame. Quite a lot of frames from both companies developed problems of debonding due.. certainly in the case of LOOK, from the casting alloy having too much zinc in it. The zinc tended to effloresce out of the alloy, within the joint, causing the bonding to yield.
Hope that is some help to you polishers and would be anodisers out there.Rolled and extruded materials tend to polish up better, cold forged products do quite well, cast articles tend to be difficult... but these are only generalisations.
Norris Lockley .. from a very raw day in Settle, UK