The BBC actually did a really interesting documentary about the birth of biker culture in the UK, which you can find here.

When 'teenage' culture began to flourish in the 1950s, the recent end of the Second World War meant that the market was flooded with knock-down army surplus motorbikes, which provided teenagers with a cheap means of independent travel. At the same time, the surrender of vast amounts of German materiel in 1945 had meant that equipment not kept by the allies had by now worked its way into the army surplus trade. This meant that Stahlhelms were now also a readily affordable means of some protection for bikers, and one whose symbolism also evoked a sense of teenage rebellion in its inherent edginess.