The mini bike frame that one purchases will depend entirely on the size, purpose, and structure of the bike being built. A motocross mini bike frame, for example, will feature a different shape and weight than a mini moto street-style frame. The mini-moto will have a different shape and weight than a frame made for a chopper bike, and all of these frames will differ from the mini bike frame used for a quad. The principles of frame design for a mini bike are quite similar to the principles of frame design used in making full-sized bikes; the mini bike frames are simply made on a much smaller scale.
A motocross mini bike frame will be designed to withstand constant and heavy impacts associated with the sport. The welds are likely to be fairly stout, though builders usually prefer to choose a frame that is fairly light as well, making jumping and quick turns possible. The design of the frame will be consistent with full size motocross bikes; the headtube angle is likely to be fairly lax to accommodate a long-travel fork, and the frame will come in two parts: the main triangle, and the rear swingarm. The rear swingarm allows suspension to be mounted in the rear of the bike, while still allowing the rear wheel to be affixed to the frame. The swingarm is attached to the main triangle at a pivot point, which is usually comprised of bushings and/or bearings.
A chopper mini bike frame will come in one piece in many cases, though some soft-tail versions may be available. The chopper mini bike frame features a very lax headtube angle to accommodate the extremely laid back riding position and the very long fork that will be affixed to the frame. The frame itself is likely to be fairly stout, especially if rear suspension is not being used; the frame will have to absorb road vibration and sudden hits.
A mini-moto frame will mimic the design of full sized street bikes. The head tube angle will not be as steep as the other types of mini bike frames, though the frame is almost guaranteed to come in two pieces: the main triangle and the rear swingarm. The fork affixed to a mini-moto will not sweep out at a large angle like motocross or chopper bikes; this aids in steering and aerodynamics. The frame will also be lightweight to make it appropriate for racing.