- Now that we've talked about shear force and we've learned a little more about rod
ends, we can talk about the way they're supported when we install them.
There are two ways of doing this which are referred to as single shear and double
shear and it's sound engineering practice to ensure that we always use double shear
where possible.
Let's dive in a little deeper and learn what each of those terms mean.
Single shear looks like this, where the rod end is bolted to a single bracket.
The downside of this is that if enough force is applied to the rod end, the bolt
will fail and shear.
Essentially, it'll be cut in half.
On the other hand, if we added a second bracket on the other side of
the rod end, we now have double shear.
In this case we'd be able to apply double the load to the component before a shear
failure occurs, even though we're using the same size and material for the bolt
that attaches to the rod end.
As well as the increased strength that double shear offers, it also means there's no bending
load being applied to the bolt and the mounting bracket.
If we consider again our single shear installation, any load being placed into
the rod end here will do so through the centre of the rod end.
Since 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.
If the material strength is marginal, this can result in the bracket or weld between
the bracket and the rest of the chassis failing over time.
Even if a failure doesn't occur, single shear will result in less stiffness and
more compliance than a double shear installation.
Double shear is usually easy enough to incorporate when we're considering the
way something like a suspension arm attaches to a chassis.
We simply need to ensure that we include a mounting tab on each side of the rod end.
However we also need to make some allowance for the movement and anglulation
of the rod end.
If we literally clamp the rod end directly between two brackets then we'd severely
limit the angular misalignment that would be possible before the rod end ended up
contacting the bracket.
To get around this, we'll usually see that the gap between the two support 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.
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.
Instead, we may perhaps use a 5/8th inch rod end which means that it has a 5/8th inch
diameter hole through it but we can use a 1/2 inch bolt to attach it.
We can then account for the difference between the outside diameter of the bolt
and the inside diameter of the rod end with the machined spacers.
Usually these spacers would start out with perhaps a 6 mm wall thickness that then
taper on an angle as they approach the rod end to allow for maximum angulation.
These spacers can usually be purchased from rod end suppliers to make our job
easier but if you need something a little more specific then any machinist will be
able to easily machine something to suit your requirements.
Where double shear can become a little trickier to incorporate is in the way the
suspension upright attaches to the suspension arms and the steering
tie rod.
Usually in a production car the ball joints at these locations will be installed
in single shear for reasons of manufacturing ease and cost reduction.
Of course, the engineers designing these components understand the forces involved
and the ball joints are suitably sized to ensure safety and reliability.
Where we can get into trouble though is when we start modifying the suspension
geometry.
Take for example a situation where we want to correct the roll centre height on a
MacPherson strut front suspension which is a common handling modification
for a lowered car.
The specifics of the term roll centre doesn't really matter but what it means
is that the outer ball joint is spaced down from the attachment point on
the strut.
If you've been following along so far you'd hopefully have picked up that this is going
to place a significant bending force into the ball joint and the spacer, compared
to the factory installation and in fact the more we lower the ball joint, the worse
this situation gets.
This can be potentially dangerous and there have been plenty of instances of suspension
modified in this way failing in use.
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 it's arranged like this.
A similar situation occurs if we look at the way bump steer correction is handled.
This is usually achieved by spacing the steering tie rod up or down from the
steering arm that attaches to the hub.
Not only does this place the bolt that attaches the steering tie rod in bending
but it also adds torsion to the steering arm which can again introduce compliance
into that part too.
While it's difficult to really fix this situation in a production upright, if we look at how
this is handled with a dedicated motorsport upright, things are very different.
These are designed so that an enclosed spherical bearing in the suspension arm
can be attached to the upright in double shear.
By adjusting the thickness of spacers placed above and below the spherical bearing,
the same effect of adjusting the roll centre can be achieved while still supporting the arm
in double shear.
This particular upright is actually designed for a double wishbone suspension
design and we can see that it offers the same double shear arrangement to support
the top wishbone as well as the steering or toe control arm on the side.
So the key point to take away from this module is that whenever possible, we want
to ensure that our rod ends are supported in a double shear installation.
This may not always be possible when modifying production parts and in this
case it may be necessary to compromise the geometry we're trying to achieve,
use oversize parts or both in order to ensure safety and reliability.
That was just one module from our Motorsport Fabrication Fundamentals course.
If you're interested in learning how to fabricate components for your vehicle, then this course
is going to be perfect for you.
You'll start by learning about the various tools that you're going to need for motorsport fabrication
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You'll learn about techniques such as bending and notching tube, drilling and tapping holes
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You'll also learn about MIG and TIG welding which are both popular techniques for welding
in the world of motorsport fabrication.
While you don't need to be a mechanical engineer or have a degree to fabricate parts, you'll also
learn some fundamentals of design so that you're able to design and fabricate strong and
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All of this knowledge is then tied together with a range of worked examples so you can
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If you're a motorsport enthusiast and you want to expand your skillset so that you can start
fabricating parts for your vehicle, our Motorsport Fabrication Fundamentals course is perfect
for you.
If you'd like to find out more about this course, click the link in the description.