THINGS YOU NEED TO KNOW…
Originally evolving from bicycles, motorcycle frames simply used bigger and stronger tubes as engine weight and power increased. For ease of manufacturing, complex elements such as headstocks, swingarm pivots and other frame elements were made from weighty steel castings, braised onto the frame tubes. As welding and production technology evolved, manufacturers were able to replace the heavy cast sections with lighter components, typically fabricated from steel pressings and more sophisticated cast and machined pieces. Steel quality also improved over time, producing stronger materials enabling the use of thinner, lighter tubing without compromising stiffness.
Because of the relatively high cost of aluminium, it is often regarded as being rather exotic, but it actually makes up around 8% of the earth’s crust. Unfortunately, it doesn’t turn up in handy lumps but in various mineral compounds and it is the refining process, requiring huge quantities of electrical energy, which ramps up the cost.
By the 80s, the need for stiffer frames, the higher value of premium motorcycles and improved material technology paved the way for aluminium frame construction.
In general terms, aluminium is approximately one third of the weight of steel by volume and about one third as strong. As a tube’s diameter increases it gains stiffness, so aluminium can be used in bigger, stiffer sections than steel without any weight penalty - hence the proliferation of beam frames. The relative ease with which aluminium can be formed, or machined, into complex shapes also trumps steel.
Carbon fibre composites get their strength from long, precisely aligned fibres glued together by a polymer that is cured at high temperatures and pressures.
Its fantastic strength to weight ratio does give it potential to be the perfect frame material however high costs mean that carbon fibre is unlikely to become mainstream any time soon.
Given the particular benefits of each material, it makes sense to optimise their use. Bimota were early adopters, using aluminium for the swingarm pivot and bolting it to a tubular steel frame. Bimota designer Massimo Tamburini carried this idea forward to the entire generation of modern MV Agustas.
Steel framed bikes regularly feature aluminium sub-frames and carbon fibre bodywork is a sure fire way to reduce weight on race bikes.
Ultimately, no frame at all would be the ideal. Mounting of the swingarm pivot through the gearbox casting is not uncommon and with the Panigale’s monocoque frame incorporating the headstock into what is effectively the airbox, you could argue that Ducati achieved the ideal. Not many engine configurations lend themselves to these minimalist designs and there will always need to be somewhere to sit!