June 30 2011 by Jason Fogelson
When a designer conceives a bike, the frame has to be one of the first elements to consider. The frame determines a bike's structure, and every decision about frame design will determine function down the line. So, if you look at your own bike's frame, you can reverse-engineer your bike, and figure out how and why it is built the way it is. Once you understand that, you'll be able to evaluate any possible repairs and/or modifications that you might consider in the future, and you'll know whether or not they are compatible with your bike's very essence: its frame.
Motorcycles evolved from bicycles in the early days, and looking at an early motorcycle, it's easy to see the origins. Early builders took the tubular steel frame of a bicycle and mounted the engine, the gas tank and oil tank onto the bike frame, rigging a drive belt from the engine to the rear wheel. This early conception as a structure to hang the motorcycle parts on has continued on some bikes to this day, with some significant modifications.
As engine size grew, the bike's frame needed to grow wider to accommodate the wider engine. Instead of a two-dimensional frame of tubular steel, the frame became more three-dimensional, cradling the engine. As suspensions became more sophisticated, rear swingarms were added to many frames in order to provide room for some shock absorption. (Some bikes called "hard tails" still connect the rear wheel directly to the frame without a swingarm, but they are usually custom choppers or specialty bikes.)
Some bike designers even found ways to eliminate parts of the frame by making the engine a stressed member, or a part of the structure of the bike. The engine in essence becomes the frame for its section of the bike.
Other designers decided to integrate functions within the frame. So there are bikes, like some Buell motorcycles, where the frame not only provides structure for the bike, it is also the gas tank. Other bikes use the frame as an oil reservoir. Still others use the frame to conceal and protect wiring and other cables. Frame design is limited only by the imagination and engineering skill of bike designers.
So, what can you learn from your bike's frame? First, figure out what kind of frame your bike has. If you ride a Harley-Davidson, it's pretty sure that you're riding a bike with a traditional tubular steel frame. Next, discover any special functions that have been integrated into your frame. For instance, if you discover that your frame is also your oil reservoir or gas tank, you'll want to be very careful when mounting new accessories that you don't penetrate or crimp the frame, for obvious reasons. If your frame is concealing wiring or cabling, the same warning applies, and you'll also want to make sure that the frame is sealed at all points to prevent internal corrosion that might interfere with wires or cables.
Inspecting your frame can also give you clues about how you should be loading your bike. Just because you can fit that big set of panniers on your dual sport doesn't mean that the frame was designed to carry their weight. Pay careful attention to your frame, and remember that the further a load gets off-axis from the bike's direction of travel the more it can negatively affect handling, and there's even the possibility of bending a frame by loading it incorrectly or overloading it. A bent frame is really bad news -- and it's really hard to fix.
You may never have to mess with your bike's frame, but I'm convinced that spending a bit of time to understand it will make your overall motorcycling experience better and safer.