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Chassis Basics 2 - Chassis Materials

  Traditionally, the most common material for manufacturing vehicle chassis has been steel, in various forms. Over time, other materials have come into use, the majority of which have been covered here.

Steel

Steel Girder Bridge   Let's face it. Steel is easy to get. Machinery to manipulate steel is easy to get. People who know how to work with steel are easy to get. Steel is easy - and it's also cheap. This is the main reason why 99% of the cars you find are made from steel, although the fact that it's actually a very useful material plays no small part. Steel is by no means a "does the job" or "poor man's" option - the material has many attributes that render it perfect for vehicle chassis manufacturing. First, yes, the cost and availability (of both the material and what you need to process it) are a major advantage for commercial production, but the physical properties are also highly beneficial.

  As we've already covered, the main aim with a chassis is to build a stiff structure to ensure other components can work as they're designed to, and steel really scores in this respect, as it's a pretty stiff material. In addition, steel rates well in terms of both yield strength (how likely it is to bend permanently under load) and ultimate strength, particularly if it's carefully alloyed and processed. Steel also resists fatigue failure well (fatigue failure is where a material fails due to repeated loading and unloading, even though the loads involved may be far below the ultimate strength of the material). This last fact is extremely useful - even if the chassis flexes under load, such flexing need not lead to a critical failure.

  The fly in the ointment with steel is it's weight, or more accurately it's density (mass of material for a given volume). Steel is made from iron, and it's density isn't far off. Most of the time, this wasn't an issue, as the weight of a car didn't use to be of too much concern. As time has progressed, however, saving weight has become more of a priority - partly to aid fuel economy, and partly to allow for the addition of safety equipment without resulting in a vehicle that weighs as much as a small tank.

  Although steel does corrode when exposed to adverse environments, such corrosion is not too much of a concern: A good coating, properly prepared and applied, will offer excellent protection. Only when damage is sustained which reveals bare metal does this factor become an issue.

  Overall, the benefits steel has as a material for chassis building far outweigh the problems of using it, and it seems that this is likely to remain the case for the forseeable future where production vehicles are concerned.

  As a side benefit, no matter where you are in the world, you will always be able to find someone who is able to work on a steel structure. For a family hatchback, this is irrelevant. For a 4x4 being used as an expedition vehicle to the arse-end of nowhere, it's crucial.

Aluminium

Aluminium Cans   Aluminium is probably the material that springs to mind when you think about lighter alternatives to steel, and with good reason - the density of aluminium is in the region of 35% of that of steel. However, the first thing we should cover is the fact that when we talk about aluminium as a structural material, we are almost always talking about an alloy of aluminium - with an addition of magnesium, zinc etc depending upon the intended end use of the metal. The reason for this is that raw aluminium has too low a yield strength for structural use in a vehicle chassis.

  Once alloyed, however, aluminium's yield strength can be increased considerably, and is perfectly suitable for such applications. Alloying doesn't have so great an effect on the stiffness of the metal, though, and aluminium cannot compete like for like with steel, which is about three times as stiff. As such, in order to make use of the weight saving that can be achieved through the use of aluminium, a way of circumventing this stiffness problem must be found.

  If we think about a component of a chassis, such as a round tube, the loads acting on that tube can be assumed to act along it's centerline. The metal wall of the tube, being a certain distance away from this centerline, effectively has a leverage against these loads. The larger the diameter of the tube, the more leverage the material has to act against the load. This is a simple way of looking at the logic used to design a structure from aluminium - with the lower weight of aluminium compared to steel, you can have a large-diameter aluminium chassis tube that saves weight over a regular-diameter steel equivalent, while still maintaining the required stiffness. A similar effect can be obtained through simply thickening the wall of the tubes. If you were to build a structure from steel and another from aluminium, using exactly the same material thicknesses etc, the aluminium version would exhibit far reduced stiffness.

  It is critical, however, that an aluminium structure is stiff, as the material has a far lower fatigue tolerance than steel. It is this need to eliminate fatique that results in aluminium structures tending to be built stiffer than steel equivalents, not an inherent property of aluminium as a material. With all these differences, it is unsurprising that the weight difference between aluminium and steel designs with the same specification does not approach the 65% density difference, though weight is generally saved.

  Despite a much higher cost than steel, and additional problems in working with it, aluminium does have a secure place in chassis building. It should also be noted that aluminium alloys are also less likely to suffer from corrosion problems than steel, due to the material forming an outer oxide layer (surface corrosion, basically) that prevents further corroding.

Titanium

Titanium   Titanium has an association with space tech, and is regarded by many people as an "ultimate" material. It has a density roughly half that of steel, and also a little over half the stiffness value. It's a similar situation with regards to ultimate and yield strengths.

  Understandably, this means that the methods used to build with titanium are similar to those for building with aluminium: Tubing should be larger in diameter than for steel, to compensate for the lower inherent stiffness, though this does not need to be as pronounced as with aluminium. Again, when we talk about using titanium to build structures, we are referring to alloys rather than raw metal, though straight titanium is not as weak as straight aluminium.

  A major advantage of titanium is it's resistance to corrosion, and also to fatigue failure. It costs, though: Titanium is not a cheap material by any stretch of the imagination, and is impractical to use for any normal road vehicle.

Magnesium

Magnesium   Magnesium is the lightest metal that's likely to be used in a vehicle chassis, with a density about quarter that of steel. This weight advantage helps to compensate for the fact that it's strength and rigidity is below even aluminium, and with careful design can be used to build a light, stiff structure.

  Magnesium can react quite easily, and will ignite under extreme circumstances. Although in most cases the sections of material used in vehicles are too thick to be at risk from this, it does mean that special care needs to be taken during manufacture - particularly with filings from machining operations etc.

  Currently, the use of magnesium in vehicles is generally restricted to cast shapes for mounting brackets, braces and so on, though several manufacturers are working on using magnesium sheet and extruded sections where possible. The fact that magnesium is a very common element, and that it is easy to recycle, are attractive properties for manufacturers, particularly with legislation moving towards things like guaranteed end-of-life recyclability for new cars. One area where it has been used for decades, however, is in the construction of high-strength, low weight wheels. Most "alloy wheels" you see on vehicles are made from aluminium alloys, but competition-spec equipment (e.g. the original "Minilight" wheel, and the Compomotive-brand wheels used by World Rally Cars) is often made from a magnesium alloy. Aluminium alloys are much cheaper, which is why that's what most road vehicles use if they're not fitted with steel wheels. A mag-alloy wheel comes in at four to five times the cost of an aluminium-based equivalent.

Fibreglass

Glassfibre Reinforced Plastic Being Laid Up   Raw plastics do not have anywhere near enough stiffness to be used for structural components in cars. If strands of glass are added to the mixture, though, their properties improve remarkably. This gives you a Glass Fibre Reinforced Plastic (GFRP or GRP), most commonly referred to as fibreglass.

  Like a plastic, fibreglass can be moulded to practically any shape. Although nowhere near as stiff or strong as steel, the ability to create practically any shape allows you to compensate for this. A bodyshell or tub may be created in a single piece, with no seams that could be weak points, and made with variable thickness - Extremely thick near high-load areas such as suspension mountings, and very thin in unstressed panels, all in the same unit. It is this infinite variability across a structure that allows a properly designed fibreglass construction to be both stiff and light.

  It takes time to lay up (make) a fibreglass structure, though, and this is not always practical - though the material can be squeezed into shape by a mould well, and sections can be joined without the join representing a change in material structure. About the only concerns with using fibreglass are the possibility of the material being attacked by chemicals (e.g. certain types of paint cannot be applied directly to fibreglass), and issues with creep. At higher temperatures, it is possible for fibreglass to soften and flow slightly, which can cause damage in the event of a fire, or if hot engine-bay components such as the exhaust manifold run too close to panels.

  Traditionally, fibreglass has been used for specialist applications like sports cars most of all, and is often used in conjunction with a separate chassis or subframes rather than alone. Even if a bodyshell is made to be a stand-alone fibreglass structure, metal inserts are still usually used to spread the load at mounting points etc.

Carbon Fibre

Carbon Fibre   Carbon fibre is very similar to fibreglass, only with carbon strands rather than glass strands as the reinforcing medium - it's correct description would be Carbon Fibre Reinforced Plastic (CFRP/CRP), though almost everyone refers to it as simply "carbon fibre".

  It is a lot like fibreglass, only despite a density that is almost exactly the same, it can have the strength of an aluminium alloy and the stiffness of steel. The key to this is that, unlike fibreglass, where the strands are pretty much random, carbon fibre uses a woven matt of fibres - this is what gives it it's distinctive appearance. Getting the full strength and stiffness advantages requires maintaining the correct alignment of this weave, and so carbon fibre structures cannot be compression moulded, they have to be laid up in layers. This requires time and skill, and is probably the biggest factor in the high cost of using carbon fibre.

  If you are on a fairly impressive budget, and need the maximum possible stiffness and strength combined with minimum weight, carbon fibre is possibly the best option around, which is why the tub chassis used in F1 cars are all carbon fibre. For road vehicles, though, the cost is just too frightening, and the use of carbon fibre tends to be restricted to large, reasonably flat panels (such as roof panels and bonnets), where the best "bang for buck" weight savings can be found.

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