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 perform 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 on 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.
- Support the weight of the vehicle (except the unsprung mass) and everything inside it including passengers and cargo.
- Provide the frame for the suspension, engine and drivetrain to be mounted. It must be
strong to hold these components in place during various driving manoeuvres.
- Handles or resists any torsional stress (such as bending or twisting of the body) that the vehicle may be subjected to as it drives.
- It allows the vehicle to pull objects as these heavy objects must be directly or indirectly attached to the chassis.
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 vehicles. 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 aided design (CAD) makes unibody platform sharing much easier. For body on frame vehicles its
easy to build another body even from another bodystyle and place it on a ladder chassis as long as they are of similar dimensions.
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.