A motorcycle frame is one of those parts that combines practicality and performance, mundanity and marvel. On one hand, it's a simple hunk of metal used to secure pieces like the seat, engine and the suspension. And on the other, it has to be an ideal structure: one that doesn't flex, weighs as little as possible and is accurately built with the proper geometry so things like tank slappers don't become the norm.
The basics are pretty straightforward (as basics tend to be). The frame has to hold the rider (and maybe a pillion), while mounting the engine, suspension, a steering mechanism and a stack of smaller systems like electronics, bodywork, luggage, a fuel tank and exhaust (just to name a few).
When talking about a conventionally-suspended motorcycle with a dual-sided swingarm and traditional front forks, the frame has to provide a steering head pivot position that's perfectly perpendicular to the swingarm pivot position. It also has to minimize any bending or flexing: If a bike is to track a true line in a bend (or even in a straight line), it's important to keep both wheels in line and with the direction of travel. Even a small amount of movement at the swingarm pivot causes problems, which are only magnified by the length of the swingarm and cause a larger movement at the wheel. The result is a weaving and wobbling effect because the movement in the frame allows the wheels to track their own paths rather than the one you've called for through the steering.
Modern frames are, by and large, very good and give fine handling. But how did we get here?
Early motorcycle frames can trace their roots back to bicycles. They were little more than a simple steel-tubed diamond with an engine that hung off it. As power outputs increased and speeds went up, primitive suspension systems began to appear. The stresses going into the frame increased and designers had to add more material to stiffen them, preventing flex and movement that cause instability.
Some builders got it right--most didn't--ending up with heavier frames that weren't much stiffer.
The Ingredients
Motorcycle frames have been made with numerous types of metals over the years. From basic steel to a carbon fiber hybrid, the frame has seen many forms.
Steel Tubes
Steel is an excellent frame material for many reasons: It's strong, simple to work with and pretty cheap. On the downside, it's heavy and prone to corrosion. Up until the early 1980s, steel tube frames were almost ubiquitous, and at their best offered pretty good performance.
Steel frames of the 1960s and `70s generally used a "cradle" layout where the engine was held by one or two tubes of steel that swept down from the steering head, under the engine and back to the swingarm pivot area. A steel "backbone" of thicker tube passed over the top of the engine and curved down to the swingarm pivot.
These long, curving shapes were very hard to make stiff, so many bikes of the 1970s had woeful handling. Kawasaki's two-stroke triples used hefty steering dampers to counteract the savage weaving provoked by the powerful engines and limp frames.
Building a stiff, light frame with steel tubes is all about triangulation. Triangles are stiff shapes, because it has to deform at the corners and in one straight line for the shape to change. By comparison, a rectangle only has to deform at the corners to collapse into a parallelogram.
Aftermarket firms like Harris, Bimota and Moto Martin built frames with smaller lengths of steel tube, using triangulation to add stiffness. These frames were expensive and not always practical, but returned much better handling characteristics than the standard Japanese frames at the time.
The ultimate expression of this triangulated steel tube design is, of course, Ducati. The firm's superbikes, from the 851 of 1987 right through to the latest 1098R have all used steel `trellis' type frames with small, triangulated sections.