The Frame Game

A motorcycle without a frame is up there with a house without a roof... you kind of need 'em. Over the years we've seen hundreds of differing shapes, styles and stiffnesses, made from a multitude of different materials. Which one's best? Well, that's a very good question. Here's why...

- SIR ALAN OF DOWDS

Most of the material design choices on a motorbike are obvious nowadays. Engine crankcases are made of aluminium, as are pistons, blocks, and heads only a madman would try to make a cast-iron head, as happened 60 or 70 years ago. Cranks are steel, as are gears and shafts. Valves are stainless steel on normal bikes or titanium on performance machines. Wheels and fork outers are almost all aluminium, fork inner tubes are steel, fairings are plastic, and rubber tyres are, err, rubber.

There’s one major component where there’s still a fundamental choice, though - the main frame. Almost every bike on the road has a frame made from either steel or aluminium, and the choice varies by the type of machine and cost. Flagship adventure machines like the KTM 1290 Super Adventure and Triumph Tiger 1200 plump for steel tubes, as do most of the cheapest 125 learner bikes out there. On the other hand, almost all current supersports machines have aluminium frames, and that's gradually becoming true of most standard machinery too.

So, why are there still two options here? The fact is that both steel and aluminium are excellent choices as frame materials but there are so many factors involved that neither has taken over completely in the way that cast aluminium has for engine cases and wheels or forged steel for cranks and conrods.