What is the difference between unibody and body over frame?

Answer 1

Monocoque vs ladder chassis or unibody vs body on frame respectively. These are the two main types of vehicle chassis. The chassis is the frame or skeleton of the vehicle and here are the two most popular types.

Monocoque or Unibody are the terms used to indicate when the body of the vehicle is designed and built to support the weight of the vehicle and any other function of a chassis. This is achieved by strengthening the floor of the vehicle, the pillars and using bracing along various parts of the body.

Ladder Chassis or Body over Frame are the terms used when the body of a vehicle is mounted to a separate frame or chassis. This frame is similar to a ladder in design as two long pieces of steel (approximately the length of the vehicle) are held parallel to each other by shorter pieces running across.

Either chassis should perform the following functions at least at a workable level.

Unibody, body on frame or any type of chassis has to strike a balance between weight/materials used, intended function, vehicle design and cost but the type of chassis used will have certain benefits and ultimately determine the function and character of the vehicle.

Performance - The monocoque is a lighter design which is a plus for fuel efficiency, it has more torsional stiffness and is by far the better chassis for performance oriented vehicle. The heavy nature of the ladder chassis makes it tough and it is much better than the monocoque for carrying heavy loads and towing heavier objects.

Design - A unibody bodyshell is difficult to design, build and modify (platform sharing) when compared to the body on frame but computer assisted design (CAD) has greatly reduced this advantage. Another body can be placed on a ladder chassis but changes have to be made to major parts of the vehicle.

Materials - Unitary bodyshells can be made from a variety of materials steel and steel alloys, aluminium and aluminium alloys and even carbon fiber or combinations of these materials whereas ladder chassis are usually built from steel.

Other Benefits - It is much easier to incorporate crumple zones into a vehicle with a monocoque bodyshell but a vehicle with a ladder chassis is much easier to repair after an accident.

Monocoque chassis are used in most vehicle types and is more suited for everyday, luxury and sport oriented vehicles but the ladder chassis is better suited for commercial and heavy duty work. Debating on monocoque vs ladder chassis will come down to the intended function of the vehicle.