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Motorsport Fabrication Fundamentals: Crush Tubes

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Crush Tubes


00:00 - If you're performing more significant modifications to a vehicle such as perhaps an engine and a gearbox swap, you're going to need to give some consideration to how your new engine and gearbox mounts are mounted to the existing chassis rails.
00:14 The chassis rails will usually approximate a hollow rectangular shape which provides a good strength to weight relationship but this does make it a little bit harder to bolt components such as a new engine too.
00:26 This is because if we bolt through the chassis rail and then tighten up the bolt, we risk deforming and crushing the chassis rail.
00:33 To demonstrate this, let's take a square section of hollow steel with a hole drilled through the top and bottom to take an M10 cap screw.
00:41 If we install the cap screw, followed by a washer and a nut, and then use a ratchet to tighten it up, we'll find that it's relatively easy to begin deforming and crushing the steel once we start applying reasonable force to it.
00:52 Obviously this isn't an ideal situation as we damage the chassis rail as well as compromising its strength and structural rigidity.
01:00 It's also impossible to properly tighten the bolt since the more we tighten it, the more the steel is deformed and crushed.
01:07 The solution to this is to take a look at how OE manufacturers deal with this situation and we'll find what is referred to as a crush tube.
01:15 This is simply a hollow section of tube that's placed inside the chassis rail and the bolt then passes through the centre of the tube.
01:22 The tube acts as an internal support when the bolt is tightened and this prevents the chassis rail from being crushed.
01:29 There are a couple of options when it comes to installing a crush tube and the technique you use will depend on whether you can access the inside of the chassis rail in order to install the crush tube or not.
01:40 In our first example, we'll assume we do have access to the inside of the chassis rail and this makes things a little bit easier.
01:46 If we want to install a 10 mm bolt through the chassis rail, then we can start by drilling a suitable hole top and bottom through the rail using a drill press.
01:55 We now need a suitable section of hollow tube to act as the crush tube and in this case we have some tube with an outside diameter of 12.8 mm and an inside diameter of 10 mm which allows our bolt to just pass through snugly, making it perfect for the job.
02:11 What we want to do now is cut a length of the tube to match the inside dimension of the chassis rail and in our case this measures 44.5 mm.
02:20 It is important that we're accurate with this measurement and that we cut the tube end squarely so that the tube will fit neatly inside the chassis rail and provide good support when the bolt is tightened.
02:32 If the tube is too long, it's going to make it difficult or impossible to install, however it it's too short, it will still allow some deformation or crush of the chassis rail to occur as the bolt is tightened.
02:43 Likewise, if we've cut the end of the tube on an angle, we'll end up with only part of the tube contacting the inside of the chassis rail when the bolt's tightened.
02:51 Depending on the available access, you may be able to simply install the crush tube with your fingers while installing the bolt through the hole.
02:59 Alternatively you may find that holding the crushed tube with needle nose pliers offers a little more control and precision in placing the crush tube accurately.
03:07 Once we have the crush tube in place we can go ahead and fit the washer and nut and tighten the cap screw down as required.
03:14 With the crush tube installed properly there is now zero deformation of the chassis rail as we tighten the cap screw.
03:20 We can leave the crush tube just like this however of course the crush tube is free to move if the bolt is ever removed in the future so it can be advisable to tack weld the crush tube to the chassis rail internally to prevent it moving.
03:33 While the technique we've just demonstrated is effective and simple, more often than not you'll find it difficult to access the inside of the chassis rail in the location you're working and hence it's not always possible to install the crush tube like we've just demonstrated.
03:47 For our second demonstration we'll look at how we can deal with this situation by inserting the crush tube through the hole we drill in the chassis rail.
03:54 We're going to be using the same size bolt and tube that we've already used so we know the outside diameter of the tube is 12.8 mm.
04:01 What we can then do is use our drill press to drill a 13 mm hole through the chassis rail which will allow the tube to slip through neatly.
04:10 There's two options we can use with this technique though.
04:12 We can choose to drill both sides of the chassis rail to 13 mm, meaning that we'll need to weld the crush tube on both sides.
04:19 Or alternatively we can drill one side out to 13 mm and the other to our bolt diameter of 10 mm.
04:26 This means we only need to weld one side of the crush tube and this technique works quite well for vertical installations where we can drill the upper side to suit the crush tube and the tube will sit nicely on the inside of the chassis rail at the bottom.
04:39 Of course once the crush tube is welded in, it's never going to move.
04:43 For this demonstration, we're going to drill both sides of the chassis rail to 13 mm, using our drill press.
04:49 Once the initial hole is drilled, we'll also use a counter sink drill bit to produce a chamfer on the side of the hole which will help to provide a valley which we can then fill during the welding process.
04:59 We're using the same material for our crush tube that we used in our last example however this time we need the length of tube to match the outside dimension of our chassis rail which is 50 mm in this case.
05:11 Once we have the tube cut to length, we can use the linisher to put a slight chamfer on each end which will further help to create the valley which we can fill with our weld bead.
05:20 We can now test fit the crush tube into the chassis rail, ensuring that the tube is flush with each side of the chassis rail when it's fitted.
05:27 With the crush tube in place, we can also see the valley that's formed thanks to the chamfer that we placed on the end of the tube as well as the counter sink that we placed on the hole.
05:36 Before we weld the crush tube in place we want to remove the surface scale off the steel which we can do with an angle grinder fitted with a strip disc.
05:44 The last step before welding is to clean down the parts using acetone and a clean rag to remove any oil or surface contaminants.
05:52 In our example, we're using a TIG welder to weld around the circumference of the crush tube on each side of our box section.
05:58 The TIG process gives us a lot of control over the weld and provides a finished result that requires very little final finishing.
06:05 Speaking of finishing, we will need to lightly linish back the weld so that the surface finish is flat and smooth where the crush tube was welded and this can be accomplished using a sanding disc on our angle grinder.
06:17 Because we chamfered the edge of the hole and the tube, there's still a nice strong weld between the chassis rail and the crush tube even after this linishing is complete.
06:26 The last step is to run our counter sink drill bit on the inside of the crush tube.
06:29 This will provide a slight chamfer to the crush tube which helps locate the bolt into the hole as well as cleaning up any distortion of the tube that's resulted from the welding process.
06:39 We can now fit the cap screw and nut and tighten it with no risk of deformation to our chassis rail.

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