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Chassis Basics 1 - Chassis & Bodyshells

  The chassis is the "skeleton" of the car - providing the structural strength, and the mounting points for other components. In this section, we will be looking at the various types of chassis design that have been used in cars.

Early Chassis Designs & Ladder-Frame Chassis

1920's Dodge Chassis   Many of the principles and techniques used to build cars evolved from the manufacture of horse-drawn vehicles, and early chassis design reflects this. The image on the left shows a 1920's Dodge minus it's body, and is a typical example of pre-war chassis design: Two long rails running the length of the vehicle, with the engine mounted between them and the axles suspended underneath. The body would then be mounted on top of the chassis.
  This kind of chassis is generally known as a ladder-frame design, as the members that run between the two rails resemble the rungs of a ladder when viewed from above. To start with, the rails would have been simple, straight lengths, but over time the design of these chassis incorporated bends to clear axles, and to bring the rails closer together at the front where the engine mounts and so on. Aside from such minor changes, the essense of a ladder-frame chassis is unaltered since it's inception.
  Building a chassis like this is technically undemanding: As long as you align everything properly, you can weld all the joints manually, and the materials used are cheap and easy to come by. However, it takes time to make all the joints, and the resulting structure suffers from a lack of diagonal bracing - it can be easily twisted along it's length. Also, the accepted technique for making a stronger, more rigid chassis involved adding extra members and using thicker material, which adds weight.

Series 2 Land Rover Chassis   In general, a ladder-frame chassis is a crude, heavy structure that does not really provide a good platform for building a vehicle on. But why, then, did the design last so long - the chassis on the right is from a '60s Land Rover, and is pretty much identical to the design used in a 2005-model Defender?

  To start with, the limitations of such a chassis design were irrelevant - the general crudity of other aspects of vehicles meant that it was hard to isolate what faults were purely down to the chassis, a factor compounded by a lack of effective analysis techniques. When your suspension, steering and drivetrain all have a large amount of play in them anyway, flex in the chassis is the least of your problems.
  Also, ladder-frame chassis do have some good points. First of all, when you are talking about loadings such as carrying heavy payloads and impacts, they are very very strong indeed - after all, what you are looking at are basically steel girders. This is actually a major factor, as one of the most common mistakes people make is to equate this kind of ultimate load capacity with strength, often at the expense of a stiffer structure (more about this later). It is this load-carrying capacity that results in ladder-frame chassis still being used for the majority of trucks and commercial vehicles.

Chassis Cab   The ladder-frame's trump card, though, is it's adaptability. Once you've built the chassis, you can shorten, lengthen, widen, or narrow the design quite easily. Also, with only slight restrictions due to the placement of major components, you can put pretty much any body on top that you want. As such, a saloon, convertible, pick-up and van can all be built on the same chassis - one of the secrets of successful ladder-frame designs such as that used by the Model T Ford and the Land Rover is to take this to the logical conclusion of having a "modular" design - all bodies have their mounting points in the same place, and so you use pretty much the exact same chassis design for every model, without even having to add extra brackets on. Most pre-war manufacturers would happily supply a chassis, with the mechanical components attached, onto which an independent company (a coachbuilder) would mount their own design of body. Again, this is why this design is so good for commercial vehicles; most ambulances, motorhomes, and other specialist applications are built independently on a "chassis-cab" unit supplied by the manufacturer (left).

  Overall, a ladder-frame chassis works well if you're not going to ask too much of it, and you're willing to put in the time required to align and weld the individual pieces together. For early vehicles, and for modern commercial vehicles and off-roaders, the brute strength and adaptability advantages outweigh the dynamic problems. For road and performance cars, though, as the problems with things like suspension and steering were engineered out, the limitations of ladder-frames became more and more of a hinderance, particularly with regard to building a stiff structure. And this leads us on to...

Space-Frame Chassis

Bracing on a Colin Chapman-Modified Chassis   As we mentioned, the problem with a ladder-frame chassis is that, although strong, it isn't very stiff. The suspension and steering systems on cars are designed on the basis that they are mounted to a solid object, so having a chassis that "squirms" under load prevents the suspension doing it's job as intended. Obviously, this is a major issue when you are designing a racing vehicle, and so it is here that we shall look. If you were to try and make a ladder-frame chassis stiffer, you're natural starting point would be to add bracing to prevent twist. The picture on the right shows bracing struts on a chassis built by Lotus founder Colin Chapman, based on the ladder-frame from an Austin 7, which stiffen up the structure significantly.

  Once you start to introduce the idea of bracing a chassis against twisting, the next logical progression is to consider the following fact: If the bracing is preventing the chassis twisting, do the main chassis members have to also resist twisting, or can you leave all those forces to the bracing components? Working on this principal, it should be possible to build a chassis where every single member has a bracing attachment, and has no twisting forces acting on it - just compression and tension. It is this principle that applies to spaceframes - removing bending forces acting on chassis members to allow you to make them smaller, thinner and lighter at the same time as building a stiffer overall structure.

Maserati Birdcage Chassis   The picture on the left shows a Maserati "Birdcage" chassis, which is just about the perfect example of this design philosophy. Look at how thin the tubes are - because each one has only tension and compression loads, there's no need to worry about them bending. Remember how you used to make bridges out of glued-together spaghetti when you were a kid? Same principle - making sure each member is only loaded in the direction which it's strongest.
  Obviously, not every gap between tubes can be braced (you need enough room for the driver to sit in the car etc), so in reality the chassis members must retain some resistance to bending. However, you still have a structure that is much, much stiffer than a ladder-frame chassis at a far lower weight. The downside is, with all those tubes running everywhere, you aren't left with much space, which isn't good for carrying loads. Also, the design relies on even distribution of loading, so isn't suited to carrying a couple of tons on the back of a truck, for instance.

Gordon Keeble Chassis   A secondary problem is that a spaceframe is an utter nightmare to build. Even leaving aside the difficulty of ensuring that all required bracing has been designed in, the trouble is that at some point you're going to actually have to weld all the sections together accurately. Even for a relatively simple spaceframe, such as the Gordon Keeble chassis pictured right, this is a hugely labour-intensive exercise - far more so than any ladder-frame design.
  On the plus side, there's nothing that exotic about the materials and principles used, and if you're willing to dedicate the time to it, a very stiff, light structure can be assembled without any special techniques. It is for this reason that, although now overshadowed in terms of ultimate performance by other designs, the space-frame chassis remains a very popular way of building high-performance vehicles where speed of production, interior space, and ultimate stiffness & weight-reduction can be sacrificed in exchange for being able to use relatively basic materials and techniques: Most kit cars, as well as many racing vehicles, use space-frame construction.

Backbone Chassis

Lotus Elan Backbone Chassis   A space-frame chassis works by distributing the loadings on it across the whole structure. However, as mentioned, having to make room in the middle for the driver reduces the possibility of running bracing struts across, and so reduces the possible stiffness of the structure. If you instead hung the passenger area on the outside of the chassis, you would have no issues with being able to fit the bracing in place. Although this would necessitate moving the chassis rails closer together to fit between the seats, the loss of stiffness this would cause can be compensated for by the fact that you can now add almost unlimited bracing to this central area. Rather than using several tubes, the easiest way to accomplish this would be to use a panel across the surface of the frame - effectively making a single, giant square-section down the centre of the chassis. And if you're going to do that, you might as well just do away with all the tubing and make the central section into a square-section made from folded and welded sheet metal. On the chassis for the original Lotus Elan, pictured here, you can see that this has been taken to the logical conclusion, with most of the structure made from folded sheets.

Tatra Military Truck   A backbone chassis like this can actually be slightly stiffer than a spaceframe of the same weight. However, because the chassis has to fit within the confines of the centre of the body, it can only be made so stiff. As such, it tends to lend itself better to small or medium sized vehicles, as large, powerful cars do not necessarily have enough of a proportional increase in the room available for the central backbone to allow it to be scaled up to cope with the increased loads. This is not so much of an issue for extremely large vehicles, though, and Czechoslovakian manufacturer Tatra has successfully used a backbone chassis on large military trucks (right), although in this case the chassis structure is tubular rather than square. Although Colin Chapman is often credited with inventing the backbone design, it's actually Tatra who first developed it.

  A side note on backbone chassis is that, with the structural component of the car in the centre, the outer body panels can be made from very light material such as composites or aluminium. While this is good for weight-reduction, it also means that a backbone chassis car is not necessarily the place you want to be in the event of a side impact, as there isn't a lot there to protect you. Saying that, the DeLorean was designed using a backbone chassis (with the help of Lotus, as it happens), and was designed from the outset to be an extremely safe car, with good side-impact protection. The secret here is that the DeLorean was part backbone chassis, and part monocoque. In fact, this is true for most backbone-chassis vehicle designs. Monowhat? We'll talk about that later...

Floorpans & Tubs

VW Beetle Floorpan   Now that we've established the use of folded sheet metal for chassis components, it's time to look at designing a "chassis" purely from sheet metal. The hard way of doing this would be to take lots of individual pieces of metal, fold them into tubes and weld them together. The easy way is to press two large sheets of metal with lots of ribs, and then sandwich them together so that the ribs form tubes between the two sheets. You've now made a similar structure to a ladder-frame chassis, only without messing around lining up tubes. On the left is a VW Beetle floorpan - see how the pressed steel forms a backbone-like structure along the length of the car? The Citroen 2CV also used a floorpan like this. Doing this gives many of the advantages of a ladder-frame chassis (such as the adaptability for various body designs) without the time and labour penalties for manufacturing such a structure from carefully-aligned individual components.

Lotus Elise Chassis Tub   An improvement on the floorpan idea is to build a tub out of sheet metal or composites, so that the "walls" of the tub help add stiffness. The Lotus Elise chassis shown right is a very good example of a tub design, made from glued-together aluminium sheet. This gives the car a very, very stiff structure onto which the other components can be mounted - one of the reasons why the Elise is proclaimed by many to be the best-handling production car ever built.

Monocoque Chassis

Cutaway Lotus Seven   Ok, up until now, we've been looking at designs where a chassis takes all the structural loading, and the body panels just sit on the outside and look nice. The problem with this is that no matter how light you make the chassis, you're going to be adding weight when you put the body panels in place. There are two solutions to this problem. First, you could fit only the bare minimum of panels required to cover the chassis, made from the thinnest, lightest material easily available to you (say, sheet aluminium). If you did, then, you'd probably end up with something like the Lotus Seven pictured left, which is little more than a thinly-skinned chassis.
  Alternatively, you could make the body panels work for you. As we saw previously, you can use shaped sheet metal to make chassis sections as well as body panels. Logically, therefore, you could use the body panels themselves as chassis members, and eliminate unnecessary weight from having both. If you did this, what you would have is called a monocoque ("single shell"). The idea of having the panels of a structure that give it it's outer appearance also providing the structural strength is nothing new (most aircraft are constructed this way - often referred to as "stressed skin" construction), and is simply a progression of the use of pressed panels we discussed in the previous section. However, instead of only making "sandwich tubes" for the floor, you do the same for the side panels as well. If a floorpan is like a ladder-frame made from pressed sheet, then a monocoque is like a similarly-constructed spaceframe. Because you're able to press complicated shapes, rather than just using straight tubes, you're not as limited with where bracing members can be placed, so you can build a stiff structure that still has plenty of room inside, without bloody great tubes running everywhere.

Monocoque Bodyshell   With the ability to build light, stiff structures that can still be very "open plan" inside, coupled with the ease of construction in a production-line environment (especially with modern robotised factories), monocoque construction is now the technique of choice for mass-production vehicles. Although lacking the absolute strength of a heavy ladder-frame chassis, and bettered by other designs in terms of weight and stiffness, the pressed-steel-panel monocoque is currently the best compromise available once you have to take cost and production concerns into account, and is likely to remain so for several years to come.

  Although the thinner material used in monocoque designs compared to tubular-based structures is easier to buckle, if it wasn't for this factor, we wouldn't be able to have the safety-enhancing "crumple zones" built into modern cars. By allowing the energy of a crash to be distributed around several panels, crumpling them in the process, the length of time it takes the vehicle to come to a full stop is extended - thereby reducing the accelerative forces on the occupants. Although such systems can be incorporated into the ends of chassis rails, for example, it is nearly impossible to provide as high a level of protection with a traditional separate chassis arrangement. The payoff, of course, is that you completely trash the bodyshell in the impact - but this is regarded as a price worth paying to save lives. These crumple zones can be made to act in any direction, so allow you to design for side impacts too, and this highlights the major advantage of pressing sheets to shape compared to using tubular material - you can choose which directions you want a component to be stiff in, and which to make it easily collapsible.

Subframes

Mini Front Subframe - This is a Titanium Space-Frame Version Made by Force Racing   In most instances, a car will have all it's major components mounted directly to the chassis. However, in some cases, a subframe will be used. This is an individual "mini-chassis" that carries certain components (such as the engine and drivetrain, or the rear suspension) as an assembly, and is then attached to the main chassis or bodyshell. The best known use of subframes is in a mini (left), where a front subframe carries the engine, gearbox and front suspension, and a rear subframe carries the rear suspension, but many cars use a simple subframe as the mounting point for the rear suspension.
  In most cases, a subframe design is used in conjunction with a monocoque design - There's no reason why you couldn't use a subframe with a ladder-frame or space-frame chassis, though there would be no real advantage to doing so.

Ford Sierra Rear Suspension with Subframe   The advantage of a subframe is that it allows you to assemble components together before you assemble the whole vehicle, and gives you the adaptability benefits of a ladder-frame chassis - you can use one subframe assembly with several designs of bodyshell, and you can get many of the stiffness benefits of a space-frame or backbone chassis at critical points (such as the suspension mountings) without sacrificing the benefits of using a monocoque bodyshell to build the vehicle's main structure. This is particularly true where the first monocoque designs are concerned, as such early examples were not always as stiff as they could be, and being able to have a rigid "traditional" chassis for things like suspension mounting points gave a certain amount of belt-and-braces reassurance. In modern designs, however, the chief advantage is that you can have very tight, accurate tolerances on the sub assembly, with the accuracy of the alignment of the major structure less critical to things like correct suspension alignment.

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