So here we are, about to embark on the final part of our journey through the various chassis components. It is only fitting in that case to end with the chassis itself. It is, as many of you will already know, in essence the framework, the skeleton, and the backbone of virtually every car, van, pickup and truck. It is the part of the vehicle that everything else is connected to. There are many different types of chassis, the monocoque, the ladder frame, the tubular space frame, the backbone and many more. However, the two styles of chassis we shall be concentrating on primarily are the monocoque and the ladder frame. These two designs are probably found in more vehicles than any other.
Let us start by looking at the Ladder Chassis, see fig 1. You should expect to find this type of chassis on pretty much every vehicle made before 1960, and it is still being used today in many trucks and Sports Utility Vehicles (SUV’s). The name is derived from its simple ‘ladder’ like construction – several lateral cross members join two longitudinal side rails. The side rails are the largest and heaviest part of the frame. They are designed to support the body of the vehicle. This usually means that the frame is narrowest at the front to accommodate the engine, steering and front suspension. The narrow front end allows the vehicle to have a relatively short turning radius for the wheels, and consequently a smaller turning circle than if the wheels were mounted further apart. The frame then widens under the main body to provide additional support where the body is actually secured to the frame. Large trucks and trailers tend to have straight parallel frames giving the vehicle additional strength to help cope with the heavier loads. Whilst the sides of the frame carry most of the stress applied to the chassis, cross members are attached to the side rails in such a way as to help prevent ‘weaving’ due to lateral forces. The number of cross members used in a ladder frame will depend on the type of vehicle that the frame is to be used in. There is usually a cross member at the front of the frame to act as a support for the radiator and the front of the engine. A cross member at the rear of the vehicle can act as support for the fuel tank and rear cargo area. Other cross members may be added to provide mounting points for engine and transmission or for towing points on some trucks. On heavier duty vehicles there are often triangular shaped plates added to the joins between the side rails and the cross members. These are known as gusset plates.

Fig 1

Having spent a little time with the ladder chassis we are now moving on to the MONOCOQUE design. NO, not black and white roosters, those of you with a flair for languages will know this is French for single-shell. In a way this is exactly what we are dealing with, a one-piece structure that combines both frame and body. This unitised construction, as it is often referred to, is achieved by either welding all the components together or by forming or casting the entire structure as one piece. It should be pointed out that simply welding a body shell to a conventional chassis frame does not make that item a monocoque design. In a true monocoque design the frame and body structure is treated as a load-carrying member. It is this unit that reacts to all loads, be it stress, road or cargo. Welding preformed metal panels together forms the bodies for these integrated type designs. These panels are formed into various load-bearing shapes that are orientated in the bodies design so as to form a uniformly stressed structure. This results in some parts of the integrated structure resembling frame-like components whilst others look like body panels. This should not be that surprising as the structure must perform the functions of both these integrated elements. This type of design obviously has its pros and cons. The major disadvantage of this type of construction is the amount of noise transmitted to the passenger compartment. Because there are no rubber or urethane bushes the amount of noise transmitted to the passenger compartment is a lot higher than in the separate body and chassis frame design. However, modern sound proofing techniques do help to reduce this considerably. The advantages of this type of design do, however, out way this disadvantage. The main advantage is the substantial weight saving. This obviously leads to savings in other areas such as fuel economy. In trucks and vans it is possible to have a lower cargo floor area and vehicle height. This can be an advantage when loading and unloading cargo. There is also the all round advantage of a reduction in structural vibration.

Fig 2

Fig 3

Fig 4

Fig 5

. The shafts are bolted to the chassis, see fig 6. This as you may have guessed will be pretty much the same whether you have a separate chassis and body or a monocoque chassis. It should probably be pointed out here that not all front control arms are ‘A’ shaped. Take the 1960’s Mustangs for example, see fig 7.

Fig 6

Fig 7

Fig 8

Fig 9

So now you have a fair idea of what the chassis is, how it is constructed and what is mounted to it. But, and there is always a but, what can go wrong with the chassis, what should you look out for, is there anything that can be thought of as chassis maintenance. Well because there are no actual moving components in the chassis itself there is no need to concern yourself with any wear problems. The chassis can be classed as a non-wear item. There are two main enemies as far as your chassis is concerned, one is rust and the other is accident damage. If you are lucky enough to be able to garage your vehicle on a regular basis, then rust is some thing you need not worry about. However if your vehicle lives outside you should check your chassis regularly. There are of course many things you can do to help prevent the onset of rust, either wax oiling or under-sealing are probably the two most common courses of action. However if you are on a tight budget simply wire-brushing the excess rust from the frame and painting the affected area can help slow the rust until a more permanent fix can be made. We have seen cars where the rust has gone un-noticed, the result has been a car that gets lower and lower….and lower. This is due to all the suspension mounting points rusting away and allowing the frame to sink down past those points. This is an extreme case, but it can happen and it should go without saying that it is extremely dangerous. This kind of rust damage will affect the way a vehicle handles at times causing unpredictable glitches in steering or suspension response that will make the vehicle uncontrollable.
Accident damage, in a way, can be a lot worse than rust damage in as much as it is not always visible. Sometimes it takes only the slightest knock to bend a chassis frame enough to put it out of alignment. This type of misalignment usually manifests itself as a steering defect, with the vehicle tending to pull to one side. However, care should be taken not to diagnose chassis misalignment without first having checked the steering geometry, as we have discussed in a previous article. It is possible to remove minor bends from a chassis using a chassis jig and a system of clamps and hydraulic jacks and presses. Small cracks in the frame can be repaired by drilling the frame and then fishplating the affected area. This kind of repair should really only be carried by a professional because if care is not taken the frame can be weakened. Any welding on the frame should not be carried out using gas or arc welding unless actually specified by the manufacturer. The heat from these types of welding removes the temper from the metal and if allowed to cool to quick causes the metal to crystallise. This makes that area of the frame very weak and susceptible to further damage or cracking.

Fig 10.