Confusingly in the bike business, alloy means specifically aluminium alloy. An alloy (technically) is a metal with other things added to improve some of it’s properties. Lots of mass-production bikes are now being made out of aluminium alloy. The manufacturers say they do it because it is light – it is, but not really any lighter than a good steel. One reason it is becoming very popular is that aluminium tubes are thicker than steel, so are easier for computer-controlled robots to weld. It doesn’t rust, although road salt can make it go a bit funny. It is also nice and shiny! Aluminium can feel rather harsh to ride, though – it is very stiff, but the newer good alloys are addressing this problem. You should be aware that aluminium has a limited life – aluminium frames usually have a guarantee of five years maximum as they fatigue.