If you are going to build a motorcycle, the frame determines the basic look of the bike. Of course motorcycle frames affect not only the appearance of the bike but the handling and safety of the finished machine.
Frames are the basic skeleton to which other components are attached. They hold the motorcycle tanks and engine and provide support to the whole bike. Most frames are made of steel or aluminum but there are also other choices such as carbon-fibre.
Motorcycle frames generally fall into one of several categories.
Single Cradle: This is the simplest of the motorcycle frames, a design that harks back to some of the earliest motorcycles ever made. It has a single large tube for a spine and smaller tubes that cradle the engine. This is commonly used on off-road bikes. A variation is the split single cradle which is a single cradle that splits into two cradles at the back for the exhaust.
Double Cradle: An evolution of the single cradle, it uses two cradles on either side to hold the engine. These were commonly used on simple road bikes and some custom bikes, but have mostly been supplanted by perimeter frames.
Backbone: These are specialty frames that are used rarely. It has a solid metal backbone and the engine hangs suspended from this. It’s a simple and inexpensive frame with a distinctive look, but other designs are better in terms of strength and rigidity.
Perimeter: These frames originated on racing motorcycles. The steering head and swingarm are joined at a very short distance, reducing flexure and torsion substantially. They were originally made from steel but most are made from aluminum nowadays to save weight. Once the advantages of this frame were seen, they were adopted by most motorcycle manufacturers.
Monocoque: Another specialty frame that is even rarer than the backbone. They are very heavy and rigid, combining seat mounting, tank, and tail into a single sturdy piece of metal. They are used almost exclusively on specialized competition bikes and are not a good choice for street bikes.
Trellis: European manufacturers came up with this design although it is catching on in the US as well. It uses many of the same construction principles as the perimeter frame and matches it in rigidity and weight. Its distinctive look is formed by a “trellis” of short tubes that make up the body of the frame. It requires more effort to manufacture than the perimeter but the result is extremely strong.
One of the earliest decisions to make is which of these motorcycle frames is right for your bike. Many of your other decisions will depend on the type of frame you choose so consider the options and choose wisely.