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3D Modeling & CAD for Motorsport: Single Shear vs Double Shear

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Single Shear vs Double Shear


00:00 - Now that we've talked about shear force and discussed rod ends, let's look at the way they're supported when we use them in a design.
00:06 There are a couple of ways of doing this, single shear and double shear and it's always best to use double shear whenever possible.
00:14 So let's look at what these terms actually mean.
00:16 Single shear is where a rod end is bolted to a single bracket as you can see here.
00:22 The problem is that if enough force is applied to the rod end, the bolt will fail and shear in half.
00:28 Now if we added a second bracket on the other side of the rod end, we have double shear.
00:34 In this case, double the force can be applied before a shear failure occurs, even though we're using the same size and material for the bolt that attaches to the rod end.
00:43 Besides the added strength, double shear also means that there's no bending loads being applied to the bolt and the mounting bracket.
00:51 Looking again at our single shear installation, here we can see that any load being placed into the rod end will be transmitted through the centre of it.
00:59 Because this is offset from the bracket that the rod end is bolted to, it'll have the effect of trying to bend the mounting bracket.
01:07 If the material strength is questionable, we might see the bracket or the weld between the bracket and the rest of the chassis failing over time.
01:15 Even if a failure doesn't occur, single shear will always result in less stiffness and more compliance than the double shear installation, which of course we want to avoid, specifically for suspension systems.
01:27 Double shear is usually easy enough to incorporate when you're designing the way something like a suspension arm attaches to a chassis.
01:34 All you need to do is make sure to include a mounting tab on each side of the rod end.
01:39 Just keep in mind that we'll also need to make some allowance for the movement and angulation of the rod end.
01:45 If we literally clamp the rod end directly between two brackets, then we can severely limit the angular misalignment that would be possible before the rod end ended up contacting the bracket.
01:56 To get around this, we'll usually see that the gap between the two supporting brackets is significantly wider than the actual rod end width and the extra width is then accounted for with machined spacers on each side of the rod end and the bolt will then run through the centre of these spacers.
02:13 This also means that we don't necessarily need to use a bolt that has the same diameter as the inside of the rod end.
02:20 Instead we could use a 5/8th inch rod end which means that it has a 5/8th inch diameter hole through it but then we could use a half inch bolt to attach it.
02:30 Usually, these spacers would start with something like a 6 mm wall thickness which then tapers on an angle as they approach the rod end to allow for maximum angulation.
02:39 To make the job easier, these spacers can usually be purchased from rod end suppliers and we can then model this in our CAD to make sure they'll work with our design.
02:49 However any machinist will be able to easily machine something to suit whatever your design requires.
02:55 To be clear, double shear isn't always easy to incorporate in our designs, especially in the way the suspension upright attaches to the suspension arms and the steering tie rod.
03:05 Usually in a standard factory arrangement, the ball joints at these locations will be installed in single shear, purely because it's cheaper and easier to produce.
03:14 With that said, the engineers designing these parts completely understand the forces involved and have suitably sized the ball joints to make sure that they're safe and reliable.
03:24 Where things can get tricky is when we start to modify the suspension geometry.
03:29 Let's take a quick look at a couple of really common examples.
03:34 Say we want to correct the roll centre on a MacPherson strut front suspension which is something that often needs to happen once we start lowering the ride height of a car.
03:43 This is usually done by spacing down the outer ball joint from the attachment point on the strut and hopefully by now you should know that this is going to place a significant bending force into the ball joint and the spacer.
03:57 The more we lower the ball joint, the worse it gets.
04:00 This can be potentially dangerous and even if a failure doesn't occur, the bending load is almost certainly going to result in some amount of compliance in the spacer when arranged like this.
04:09 A similar situation occurs if we look at the way bump steer correction is addressed.
04:15 Usually this is tackled by spacing the steering tie rod up or down from the steering arm that attaches to the hub which puts that bolt that attaches the steering tie rod in bending.
04:26 And it also adds torsion to the steering arm which can cause compliance in that part too.
04:32 The reality is that it's difficult to properly fix the situation in a production upright but if we look at the way it's handled in a dedicated motorsport upright, things are very different.
04:42 Usually these are designed so that an enclosed spherical bearing in the suspension arm can be attached to the upright in double shear.
04:50 By adjusting the thickness of spacers placed above and below the spherical bearing, the same effect of adjusting roll centre can be achieved while still supporting the arm in double shear.
05:01 The most important thing to take away from this module is how important it is to understand the difference between single shear and double shear.
05:08 Wherever possible, we should always be designing with double shear in mind but when that's not possible, components should be sized appropriately to compensate.

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