Today's motorcycles are technological marvels, far removed from the simplicity of earlier bikes. As many modern motorcycles can reach speeds near 200mph, motorcycle chassis design has had to evolve to cope with these increases in performance.
What is a Motorcycle Frame
The frame is the backbone of a motorcycle where all components are mounted and connected. The frame provides mounting points for the engine and fuel tank, as well as a pivot point for the steering head, where the front fork is mounted. A separate pivot point is designated for the swing arm, a forklike frame that holds the rear wheel and suspension components. The rider saddle, or seat, is often mounted to the frame for most cruiser-type motorcycles. Sport bikes, however, use a separate structure called a subframe that is bolted onto the frame to provide seating for the rider. Ideally, a frame should be lightweight yet strong enough to support the motorcycle's components. Additionally, the frame must provide structural rigidity while maintaining flexibility when needed.
Cradle-type frames, used since the early days of the motorcycle, appear in variations. Single cradle frames are characterised by a single down tube that extends below the steering head and under the motor to reconnect at the spine to form a cradle for the motor. A full (or duplex) cradle frame uses a pair of down tubes to form two cradles, providing additional support for the motor. Modern full cradle frames are used mostly by cruiser and chopper-style motorcycles. Although pressed steel frames have been used in the past -- a method used to great success by Honda's first bikes -- modern cradle frames are generally made from steel or aluminium tubes welded together.
Twin-Spar (Perimeter) Frames
Used mostly by sport bikes, twin-spar frames maintain their strength against the tremendous power output of these high-performance machines. Twin-spar frames consist of a set of beams that enclose the motor and run from the steering head to the swing arm pivot, reducing flex under acceleration. The motor is mounted to the frame as a central stressed member, acting as the main load-bearing component of the frame. Cast aluminium frames are commonly used by manufacturers, although titanium, magnesium and carbon-fibre is occasionally used for race bikes.
Trellis frames use short sections of aluminium or steel tubing that are welded together similar to a twin-spar frame. The trellis wraps around the motor and connects the steering head to the swing arm pivot as directly as possible. Trellis frames are found mostly on European motorcycles, with Ducati's frames often referred to as artful masterpieces. Lightweight and very rigid, the trellis frame offers greater strength than a twin-spar frame, but are more complex and difficult to build.
Acting as a single unit, a monocoque frame gains strength by placing its load on the external surface of the frame, offering certain advantages offset by greater weight. Kawasaki's Concourse 14 is one of a few production street motorcycles that uses a monocoque frame because of higher manufacturing costs. These frames are mostly reserved for specialised race bikes.