Open cradle frame

A cradle frame wraps tubes around and under the engine like a cradle. An open cradle frame leaves out the lower tubes: the front downtube and the area around the swingarm pivot hold the engine, which then works as a semi-stressed or stressed member and carries part of the load.

Leaving out the lower rails saves weight, improves ground clearance and makes engine access easier. Many single-cylinder road bikes, older Japanese twins and some enduro bikes use variations of this layout; related designs include the diamond frame and the backbone frame.

Because the engine is part of the structure, its mounting bolts and their torque are important. Loose engine mounts can cause vibration, odd handling and cracks.