Why ‘aerospace grade aluminium billet’ might not be all that cool
Why flash-sounding CNC’d billet aerospace grade alloy may not actually be what it seems…
By Mike Armitage
Aluminium has advantages over steel for motorcycle manufacturers, the main one being its lack of weight. Common mild steel is basically iron, alloyed with a small amount of carbon (0.05 to 0.3%, plus a dash of other elements to tweak its properties). So while it’s easy to work, cheap and strong, steel is also heavy. Aluminium is just a third of the weight. Its modulus of elasticity is just a third of steel’s too, meaning it has to be three times as thick to resist the same bending force. But with computer modelling, cunning load paths and clever profiles designers can save vital kilos by using aluminium.
Just as steel is an iron alloy, your bike parts are aluminium alloy. Other elements like zinc, manganese and silicon are added to increase strength. It may also have been heat treated to improve hardness, and the manufacturing process – casting, pressing – also affects properties. And then there are the serious aluminium alloys, engineered using other expensive elements to give the material very specific properties – like the very particular alloys that are used in military aircraft.
It’s therefore understandable to see an ‘aircraft-grade’ alloy bike part and assume it’s exotic. This is especially true if it’s also been CNC machined and is made from billet. Yet chances are it’s something rather ordinary.
‘It just refers to the size of the lump of metal...’
Aircraft use different aluminium for different parts: an airliner’s food tray bracket isn’t the same alloy as airframe parts. And fancy-sounding grade numbers only really refer to the type. ‘6061 aluminium’ sounds posh but just means an alloy with magnesium and silicon – and it’s used for everything from your bike’s wheel spacers to drink cans. ‘CNC’ stands for ‘Computer Numerically Controlled’, where the machine tool is run by a program. It’s the commonly used system and needs a less-skilled operator than manual machines. But the most misleading tag is billet. It’s become the byword for exotic material – but only refers to what size the lump of metal was that the part was machined from. Materials come in standard forms, usually rolled into either a billet (square section, between 40 x 40mm and 150 x 150mm), a bloom (square, minimum 150 x 150mm), or a slab (rectangular, minimum 250 x 40mm). And so ‘machined from billet’ simply means made from a solid chunk. It’s as relevant as describing a piece of bread as ‘sliced from loaf’, or pint of beer as ‘poured from cask’. It also means you can’t really have billet wheels… Yes, there are some bike parts skilfully made by hand from genuinely exotic material. But remember that ‘CNC’d from aircraft-grade billet’ could actually just mean an automated process chopped it out from a lump of Coke can material.