Alloy & Alloys
Many articles in OBA give magnificent technical reviews of bike restorations and reviews on old and new models – great stuff well presented. It’s all good but I need to throw in some comments which nag me and it’s about the commonly used terms “Alloy” and
In OBA and real life Alloy or Ally is used predominately referring only to aluminium components such as cylinder heads, barrels & crankcases. Well that’s fair enough as far as it goes but it’s not the accurate whole story.
There are many Alloys used throughout the campaign of a bikes design and construction. To elaborate but not criticize, so let me extricate a few randomly selected examples;
In the early days of bike racing around 1940, Velocette and Norton were competing fiercely and both used “Alloy” heads – but (ref. Velocette Race Shop Memoirs by Peter Turnill) Velo were using aluminium heads and Norton bronze heads. They were both running “Alloy” materials because an alloy is any mixture of different pure metals in various proportions to create a new metal which has superior performance characteristics to the original singular parent metal. The Velo heads were Aluminium/Silicon alloy castings, heat treated for light weight and heat extraction with sufficient metal thickness mass for strength, cheap to manufacture and can be high production die cast.
The Norton heads were Copper/Tin/Zinc bronze alloy (probably admiralty bronze 88/10/2), a commonly used bronze much stronger and heavier than aluminium but possessing superior, faster heat dissipation properties and also considerably more expensive. Norton were not the only manufacturer using bronze alloy for cylinder heads.
In OBA#94 Alan Cathcart’s beautifully presented “Sunbeam one-year wonder” quotes an unknown steel material “Chromidium” used for the ribbed steel front brake hub. Well Alan that was also an alloy, being a Carbon steel Chrome alloy which does not appear in the ASTM American Society of Testing Materials because it was a private brand name for a special steel. Checking over my data on special steels available in the ‘30s and ‘40s this may have been a version of BS31 incorporated small additions, around 1%, of Carbon and Chromium for high strength, wear resistance, heat stability and importantly resistance to heat distortion generated from the brake. Having said all that, it is in my view unusual to specify a steel for a brake drum. Even in today’s modern allknowing times bikes have come out factory-fitted with stainless steel (an alloy) disc brakes which had to be changed (after market). It’s all about coefficient of friction characteristics where vermicular graphite alloy cast iron is most suitable. This material was not developed until the late 1950s.
OBA#27 Pg.71 states that in 1985 Ducati used a revolutionary square section Chrome-Moly frame. This again gets into the exotic stuff (typical Ducati). This is an alloy high strength thin wall carbon steel Chromium /Molybdenum alloy steel tube, well covered in ASTM Standards of alloy steels.
A lot of alloy steels are used in a motor cycle. Steel, not being a pure metal is manufactured as an alloy of Carbon and Iron as a base steel which is subsequently re-manufactured into hundreds of different grades of steel alloys by composition adjustments of (sometimes very small) Carbon, Manganese, Chromium, Nickel, Molybdenum to name just a few. Nev Murray (Foundry metallurgist)
“Alloys aren’t alloys, Sol!”