No matter what design of chassis is used for a vehicle, or what material is chosen to build it, as some point joints are going to have to be made.

Seam Welding

  Welding is where two metal components are joined by melting them into each other where they meet. The heat to do this can be generated through either burning gas or electrical resistance; Gas welding uses acetylene, and pure oxygen is added to produce a flame hot enough to melt metal (air is only 21% oxygen). For metals with a melting point below the 1500°C (roughly) of steel, other gases may be a viable option for welding. An additional rod of fillet metal, with a coating of flux, is dipped into the weld as it is made to create a stronger joint, and the flux helps prevent getting oxygen in the molten metal, which would weaken the structure.

  For electrical welding, the fillet material can be used as a conductor, with a current passing through the metal to generate the heat. In arc welding, the welding rod is literally just that - a rod, which has to be replaced as it is consumed. As with gas welding, the rod can also contain flux to help prevent oxygen getting into the weld, and this leaves a slag on the surface of the weld, which has to be removed afterwards (generally knocked off with a hammer). In MIG (Metal Inert Gas) welding, the rod is replaced with a spool of wire which is fed through a flexible tube to a hand-held "gun" whenever the "trigger" is pulled. This also opens a valve to allow inert (non-reactive) gas to flow, which covers the area of the weld to prevent oxygen getting in. TIG (Tungsten Inert Gas) is similar, with the exception of the reel of wire. Instead, a tungsten tip is used, which is not consumed during welding. As with gas welding, an additional rod can be used to add extra material. MIG/TIG welding can easily be robotised, which is a major advantage for production line work.

  Seam welding involves producing a long line of weld to join components - generally the entire surface where they meet. This gives an extremely strong join, though there is the risk of the amount of heat involved creating distortion problems, especially with thin material. It also takes time to run that amount of weld, and time, as they say, is money.

Spot Welding

  Spot welding is a form of electrical welding used to fasten two sheets together. Two stubby rods clamp the sheets, and pass the electric current through that point to join them. Although not as strong as seam welding, due to less weld area and load forces not being equally distributed across the joint, it's strong enough for most purposes, and is fast and easy to robotise. This makes it very suitable for production lines, and is the technique used for producing monocoque bodyshells in car factories worldwide. It is also effective at reducing the chances of distortion due to heating the surrounding panel compared to traditional welding.

  A spot weld can also be created by making a small hole in one panel, and then welding through to the other panel. This is an accepted way of welding in replacement panels so the joints look the same as the original, but without the need for a spot welder, and is often referred to as "plug" welding - because the act of welding fills up the hole with a plug of weld.

  Spot welding is only applicable where sheet materials are being used - it cannot be employed to join two tubes together, for instance. Also, because a spot weld does not seal a joint, the design of chassis panels must take this into account: Early Minis let in water because the seam-welded joints on prototypes had stopped anyone spotting where an overlapping panel could trap rainwater, which then got past the spot welds on production models. The design of the panels was later changed to overlap the other way.

Stitch Welding

  Stitch welding is very similar to seam welding, only short welds are spaced along the join rather than a continuous run being used. Other than that, it's pretty much the same. Using stitch welding can help avoid distortion, as less heat is put into the material.

  This technique is not used often in chassis manufacture: In most instances, you might as well just go all the way and seam weld. However, if a chassis is going to be galvanised to prevent corrosion, stitch welding helps: Galvanising is a chemical process that dips components in a very hot bath to coat them. Seam welds can sometimes trap air between parts, which expands with the heat and cracks the weld. Stitch welding leaves a gap that can allow the gases to escape.

Bolts & Threaded Fasteners

  Bolts, nuts and screws have the advantage that they can be removed non-destructively and re-used, so that an assembly may be dismantled and rebuilt with no new material or parts (unless the fittings stretch in use etc, but we'll ignore that one). This also means that separate components can be loosened and realigned as necessary, or replaced if damaged. Also, dissimilar materials may be joined together (subject to them having a reaction to each other, of course).

  The downsides are that the fasteners can come loose due to vibration etc, and that the join is not as stiff as a welded alternative. Also, in a similar manner to spot welding, the load at joints has to pass through the fasteners, and may not be evenly distributed.

  Generally, threaded fasteners are mostly used to attach components to a chassis rather than to build the chassis itself. There are exceptions to this, though, where components need to be varied for different models, for example, or where a part of the chassis needs to be removeable (e.g. a bracing strut that blocks access for engine removal).

Rivets

  A rivet secures joints in the same way as using a bolt, only the clamping force is generated by crushing the ends of a short rod that runs through the hole, rather than tightening the fastener down. Traditional rivets, as originally used for heavy-duty construction like bridges and ships, were literally forced into shape after being heated up to soften them. There is absolutely no problem with this technique - it's still used - but it is quite labour intensive.

  What is more commonly used in automotive applications is a "pop" rivet. These use a hollow tube with a flange on it, and a pin running from the end without the flange and protruding an inch or so out the other end. The hollow tube is pushed through the hole in the components to be joined, then the pin is pulled. This crushes the tube back against the flange, sandwiching the joint in between, after which the pin can be snapped off. This is obviously a much faster method, and is also easy to robotise if needed.

  Other than the speed, rivets have no real advantage over threaded fasteners (Though they are more resistant to loosening under vibration). They cannot be removed without destroying them (usually by drilling them out), and cannot be retightened if they loosen. When designing with rivets in mind, it is preferable to ensure that they are loaded in shear (across the rivet) rather than in tension (along the length of the rivet). The most common use for rivetted joints is in additional brackets, panels etc rather than major structural joining. Many vehicles (from kit cars to Thrust SSC) use rivets to hold their outer panelling in place, and aircraft have done so for decades - The picture above left is of a section of fuselage from a B52 bomber.

  Generally, riveting is not favoured in vehicle production, and most areas where a rivet may have been used in the past are now spot welded.

Bonded Joints

  Yes, we do mean gluing cars together. Another assembly technique time-served in the aerospace industry, most of the adhesives used are of the epoxy variety - some using an accelerant (hardener) to create a chemical reaction to cure the glue, others needing to be heated in an oven to attain full strength. Most are very similar to household brands like Araldite (left).

  A bonded joint is a very good way to join materials together, as it allows the load at the join to be spread across the entire mating surface evenly. As with seam welding, this prevents load "hot spots" that you can get with spot welding or mechanical fasteners, and so the materials used do not have to be designed to cope with this - allowing you to use thinner, lighter panels. Using an adhesive also avoids the issue of heat that can be a problem with welding, as even adhesives designed to be set by heat rarely require the kind of temperatures that would affect the metal they are joining.

  As far as strength goes, most commercial adhesives are very strong, and when a high quality, heat curing industrial adhesive is used, it's entirely possible that the component parts of the structure itself would fail before the adhesive.

  Joining materials this way does require good surface preparation to ensure maximum strength and durability, but has the advantage of allowing dissimilar materials to be joined. This does not affect recyclability, as adhesives are often chosen that will break down again if enough heat is applied, so that the various materials can be separated out for further processing.

  Bonded joints are used in the assembly of almost all composite-structure vehicles like F1 cars and supercars, as well as in some aluminium-framed vehicles such as the Lotus Elise. Sometimes additional fasteners, such as rivets, will be used as well to both provide backup, and so that the joints can be held in place long enough to fully cure without the assembly having to stay mounted on a jig.

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