Summary:A motorcycle’s frame is the core structure to which all of its other components are attached. It car...
A motorcycle’s frame is the core structure to which all of its other components are attached. It carries the engine, provides a location for the steering and rear suspension, supports the rider and often any luggage or passenger and holds the fuel tank. In addition, the frame protects the more vulnerable parts of a bike in the event of a crash. Modern frames are usually made from welded aluminium, steel or metal alloys, although carbon fibre is occasionally used in high-end or custom bikes.
Most bikes have front and rear suspension to help smooth out the ride by absorbing some of the impact forces created by road surfaces. A major part of a motorcycle’s frame is the headstock, which is the part that the front forks mount to. A separate part called the swingarm is attached to the other side of the frame, and it is this that the rear suspension is attached to.
There are various different types of motorcycle frames, and the type chosen depends on the specific requirements for a particular model.
For example, an ultra-lightweight race bike requires a much lighter and stiffer frame than a commuter or sport model, and so requires the use of expensive exotic materials such as carbon fibre.
The most basic and economical of all motorcycle frames is the backbone, which is essentially a long strip of steel that resembles a spine and keeps the different parts of the bike together. It’s one of the oldest frame designs around, and it was used on a lot of old bikes that had low output engines that wouldn’t require too much stiffness.
More recent designs have come up with ways of making bikes lighter and stiffer, but at a cost. One of the most popular is a design known as a twin spar frame, which is based on research from motorcycle racing that showed significant advantages could be gained by joining the steering head and the swingarm pivot in as short a distance as possible. To make this possible, the two beams (known as ‘twin spars’ after their design) that join the headstock with the swingarm pivot are bent in a way that allows them to carry the load of the engine without affecting the frame’s rigidity.
This type of frame is a good choice for lightweight sports models, and it’s also used on some mid-range production bikes. Modern high-end models however, often have a perimeter frame which is a more sophisticated version of this design. This is based on further research from motorcycle racing, and it shows that you can improve a frame’s rigidity significantly by making the section which joins the steering head with the swingarm pivot even shorter.
Many riders choose to make their own motorcycle frames as it is a satisfying project and not terribly difficult, provided that you have a TIG welder and the right tools. A lot of planning is required, and a little bit of experience helps, but the rewards are great when you roll down the road on a motorcycle that you yourself have fabricated with your own hands.