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Suspension Tuning & Optimization: Measuring Anti-Roll Bar Stiffness

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Measuring Anti-Roll Bar Stiffness

07.24

00:00 - If we want to know the stiffness of our aftermarket springs, this is usually as simple as reading the part number printed on the side of the coil.
00:07 If you're getting really accurate, it's not uncommon to rate each coil individually.
00:12 But in general, reading the number off the side is more than accurate enough for most cases.
00:17 However when it comes to anti roll bars it's not as common for manufacturers to specify the stiffness of each of their bars.
00:24 When we're looking at upgrading anti roll bars, it's much more common to see these described as being a certain percentage stiffer than factory.
00:32 While that tells us it'll help reduce the amount of body roll and increase the amount of lateral load transfer on that axle, it doesn't help when it comes to any calculations.
00:42 In a later module where we'll be looking at an example of calculating lateral load transfer distribution, we need real stiffness numbers to deal with.
00:50 In this module we're going to do a practical example of measuring the stiffness of a rear aftermarket anti roll bar on a Toyota GT86.
00:58 However the techniques and concepts will be generally applicable to all other applications.
01:04 There are a range of methods we can use to rate anti roll bars but in this example we've tried to strike a balance between accuracy and something that will be both practical and manageable for most people.
01:14 In order to characterise the stiffness of an anti roll bar, we're interested in knowing how much force we need at the point of connection between the bar and link for a given amount of deflection.
01:26 So we'll apply a force at the connection point and measure the resultant deflection.
01:31 As discussed earlier, these measurements can be done in a few different ways.
01:35 Often they'll be bench tested on a jig independently from the car.
01:40 The advantage of this is it allows the stiffness properties of the bar to be understood extremely accurately, independent of any other component.
01:48 Unfortunately though, this probably isn't something that's going to be practical if you're not working in a well resourced race team.
01:55 Another approach is to test the anti roll bar in place on the car.
01:58 This can be a lot easier to achieve as you don't need to manufacture as many components to carry out the test.
02:05 The other big advantage is that we're including the compliance of the whole any roll bar system as it's installed on the car.
02:12 In road cars, the anti roll bar is usually mounted to the chassis with polymer bushes, the reaction forces at the chassis from the bar being loaded will normally lead to significant deflection in these bushings.
02:24 The mountings to the bushings in the chassis can also be quite flexible.
02:27 Often these are made from relatively thin sheet metal for ease of manufacture.
02:32 All of this can add up to a significant difference in stiffness compared to considering the stiffness of the anti roll bar only.
02:38 This compliance becomes even more significant when we fit anti roll bars stiffer than the original manufacturer intended.
02:45 To carry out the stiffness test, we're going to apply a torque to the anti roll bar to simulate how it will be twisted when working as part of the suspension.
02:54 We'll do this by disconnecting one side of the anti roll bar and leaving the other attached to the rest of the suspension.
03:00 There are a number of ways the bar can be twisted and the exact method you use will depend on your particular application.
03:07 We'll be doing this by pulling down on the disconnected end of the bar and measuring the force required using a tension scale.
03:14 Because the suspension is already at full droop, the other end of the anti roll bar which still has its link attached to the lower control arm is restrained from moving.
03:23 All we need to do is pull down on the bar while measuring the resulting vertical deflection and the force in a series of steps.
03:31 We'll be measuring the vertical deflection relative to a fixed point.
03:35 The first step is to make any fixtures or tools to connect the scale and measure the deflection.
03:41 In this case, we've made a simple part that attaches to the anti roll bar at one end and the weight scale to the other.
03:47 At the bottom of the weight scale, we're using a ratcheting tie down strap that'll allow us to pull down on the scale in a series of steps.
03:54 The tie down is secured to something heavy at the other end.
03:58 Alternatively you could attach the other end to a fixed anchor point if you have one.
04:02 You do need to take care that when you're carrying out this test, you're doing it in a safe way.
04:07 In our case, we have to be careful not to pull down with such force that the car could become unstable on the hoist. You could equally test the anti roll bar by pulling it up or even leaving it connected to the lower control arm and pulling the hub up towards the strut tower.
04:22 The method you choose will be based on the equipment and resources you have available.
04:27 At this point, I do need to remind you that if you're not confident in carrying out a test like this safely with the equipment that you have, you should not attempt to.
04:35 In this case, this anti roll bar has 3 different positions we can attach the drop link to.
04:40 We'll carry out the test on the middle hole and then repeat it for the other two holes.
04:45 For each test, the link needs to be attached to the accompanying hole at the fixed end of the anti roll bar.
04:52 So when we test the middle or medium position the fixed end will also be in the middle position.
04:57 You can see here I've taped a small ruler to my test fixture.
05:00 This will be my reference for the amount of vertical deflection.
05:04 I've done this so it's simpler to get a relatively accurate measurement.
05:07 it's definitely going to be worth going to the trouble of making the measurement easy to take and read if you can.
05:13 First, I apply a small amount of force with the ratchet.
05:17 Just to take up any iniital play in the system.
05:20 Then zero the force scale, now we're ready to start loading the bar.
05:24 We'll do this in a series of steps, the amount you increment the force will vary depending on your application.
05:31 In this case, I'm aiming to get at least 6 or 7 data points for each adjustment.
05:36 After recording the initial deflection measurement with the weight scale zeroed, we can start loading up the bar.
05:42 At each step, we record the applied force and the resulting deflection.
05:47 With the data gathered for each of the 3 adjustments on the rear bar, we can then go ahead and repeat the same process on the front anti roll bar, noting that in this case it only has two adjustment options rather than 3.
05:59 Looking at the data for the rear anti roll bar, we can see that if we make a plot of force on the Y axis versus displacement on the X axis, that behaviour for each setting is relatively linear.
06:11 We can see that each data point is following a relatively straight line.
06:15 This won't always be the case for every car but does slightly simplify things.
06:20 We can now calculate the rate of each setting.
06:23 There are a few different ways we can do this.
06:25 One is shown in this table of measurements where we convert the force to newtons then divide the force by the total displacement at each step.
06:32 This gives us units of newtons per millimetre.
06:35 The other way is to fit a linear trend line through the data which is simple in any spreadsheet software.
06:41 In this case, Microsoft Excel.
06:43 After adding a trend line to each series, we can now choose to show the slope value.
06:47 This also has units of newtons per millimetre.
06:50 One thing we can see with this data is that while each of the adjustments produces a relatively linear rate, the stiffer setting looks to have slightly larger differences in stiffness from the medium setting.
07:01 Compared to the difference between the soft and medium settings.
07:05 This is clear when looking at the slopes between the trend lines.