- Antiroll bars are one of the go to tools when you're tuning chassis balance.
Typically if you're entering a corner and you're suffering from understeer,
you would soften the front antiroll bar or stiffen the rear.
Conversely if you were suffering from oversteer, you would tend to soften the rear
antiroll bar or stiffen the front.
It's pretty common knowledge that coil springs are rated in terms of stiffness.
The stiffness meaning how much force needs to be applied to compress a
certain amount.
An antiroll bar is just another type of spring.
When one wheel moves up, and the other one tries to go down, the antiroll bar is resisting
the movement between them.
In order to be able to predict the balance and handling of the car before we get to the
track, we can use some simulation.
In order to do simulation we need to be able to characterise the antiroll bar in the same
way that we would with the suspension springs.
Normally when you buy springs, they're pre rated, their rate, in either pounds per inch
or newtons per millimetre is given to you when you buy the spring.
Alternatively you can test them in a spring rating machine.
An antiroll bar is a little bit different when it comes to its stiffness.
When you buy an aftermarket antiroll bar, the stiffness isn't given to you by the
manufacturer.
Typically you only need the stiffness values of the antiroll bar if you're going to do some
simulation.
In order to do some simulation on a car we're going to be taking to the track,
I need to get some stiffness values from the antiroll bars fitted to the car.
I'm going to give you a walkthrough of how I'm going to rate these antiroll bars.
It doesn't require any expensive equipment to do.
Here I'm underneath the car, I'm looking at the rear suspension that has the antiroll
bar that I want to rate.
So this is the right rear wheel, this is the lower control arm.
The forward control arm, there's a upper control arm that's got two pin joints
at the top here.
And a toe control arm.
There is the damper which has the spring removed from it at the moment and back
here we have the antiroll bar.
This bar runs along the back of the car and connects to the lower control arm here.
This is totally symmetric across each side of the car.
You can see here this link that attaches to the lower control arm here has some
adjustment to allow you to equalise the link height side to side in the car to get
rid of any preload when you do your corner weight setup.
And we can see that on the end of the antiroll bar, on this one we have three
options to connect the link.
What's happening is the suspension moves up and down, if one side tries to go up and
the other side isn't going up, so let's say in this case, the car's turning a right hand
corner, so the outside suspension's going to compress, and the inside suspension, the
left side is going to go down.
That means the antiroll bar will get a twist or a torsion moment about it.
This side's going to want to do up and the other side's going to want to go down.
Looking at the back of the antiroll bar, that means this is the section here where
you're going to get the torsion happening with this side going up and the other side
going down.
The resistance that the antiroll bar provides is a function of its stiffness and its material
and how it's installed and also its shape.
The stiffer the antiroll bar is, the less differential deflection you'll get.
That means where one wheel goes up, the other wheel will tend to go up with
it as well if it's really stiff.
This is one of the tools you're typically tuning when you're tuning chassis balance.
Here we can see the point that the antiroll bar is secured in the chassis.
That is one of the points it is pivoting about.
So if that's the pivot, and this is the lever arm here, the further away the lower
control arm is connected from the pivot, the more leverage you have on the bar.
That means the bar has less mechanical advantage over the suspension.
What that really means in practical terms is the bar is providing less stiffness to the
suspension so it's less resistance.
That means that the further out you put this link, the softer the effect the antiroll
bar will be on the suspension.
If you were to put this link in the rear hole here which is closer to the pivot, that means
you'll get a higher stiffness rear antiroll bar and as you keep moving away it'll get
softer and softer.
What I want to know for the simulation is how much resistance this exact bar has
given me in the car.
So when I pull down on one side, how hard do I have to pull down to deflect it a certain
amount.
And I'll repeat that test for each one of these positions in each of the antiroll bars we've
got here in stock so we know what we've got before we get to the track.
One of the advantages of testing this in the car as opposed to doing it on a test bench
is that we're including all of the added up stiffness of everything that the antiroll bar
is connected to.
In an OEM car like this where you've got a factory brace down here to hold the
antiroll bar, as well as having the polymer bushes that are holding the antiroll bar,
this means you will get a significant amount of deflection, not just from the torsion of
the antiroll bar but also from a lack of installation stiffness.
In a proper purpose built motorsport application, all of these components
holding the antiroll bar would be much more solid.
If any of these components that are bracing the antiroll bar are moving, it makes the
antiroll bar a much less effective tuning tool.
Here I'm looking at the other side of the car, this is the left rear corner.
I've got some of the suspension components removed to give me access to the antiroll bar.
But this is how I'm going to measure the stiffness.
I've got a weight gauge here hooked up to a large ratchet strap that's on an anchor.
And I'm going to pull down on this end of the antiroll bar and measure how far it moves
and how much force I need to move it with in a few different increments.
Because the suspension on the other side of the car is on the damper and that's controlling
the droop travel, this side cannot move down any lower so when I pull down on the other side
of the car, this part of the antiroll bar will essentially be locked.
That means that the force I pull down with on this is only going into deflecting the
antiroll bar and nothing else significant on the car.
When I do these measurements I will take them in a number of steps per position and then
I'll move the shackle down to the next position and then the next position and record the
stiffness at each step.
I'll be repeating this method for both the front and the back of the car for each antiroll
bar that we have for this car and for each position in each antiroll bar.
It's quite a large test matrix but once we have that data, we can use that really
effectively in the simulation to get some good idea on setup before we actually turn
up to the track.
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