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Suspension Tuning & Optimization: Measuring Motion Ratio

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Measuring Motion Ratio


00:00 - Here in this example, we're going to be measuring the motion ratio between the wheel and the spring and damper assembly.
00:06 This is something we're going to be using for a number of calculations in later modules.
00:10 Now there's a couple of different ways you can do this and really a lot of it comes down to what's most convenient for you, what you can see and what you can get to.
00:18 The way I'm going to do this is by taking a measurement from the hub to this plate I've got sitting on the ground down here and I'm also going to be using some vernier calipers to measure how much the spring and damper assembly is going to be compressing.
00:31 So obviously in this particular car, because we've got a coilover type setup, the motion ratio between the wheel and the spring is going to be exactly the same as what we have between the wheel and the damper because the spring and the damper are going to be mounted concentrically which is going to be the case for the vast majority of situations.
00:48 The way we're going to be using motion ratio in this course is going to be using the average motion ratio over the entire travel.
00:55 So with any suspension that motion ratio is almost never going to be constant across the whole travel of the suspension so what I mean by that, so when we're starting in full droop, what's actually happening as the suspension compresses is the motion ratio between how much vertical travel the wheel has versus how much the spring and damper compress is going to change.
01:14 This particular situation the motion ratio is going to be relatively constant so making the assumption of a constant motion ratio will be totally fine..
01:21 In cars where you've got the motion ratio changing a lot over the travel, you may well need to use a non constant motion ratio.
01:28 But for the vast majority of cases, certainly for doing things like spring rate calculations like we'll be doing a little bit later, using a constant motion ratio will be totally fine.
01:37 Now the way we're going to do that is we're going to increment how much the wheel moves and how much the spring and damper moves.
01:44 We'll do that in a whole lot of different steps and then we'll take the average of the motion ratio for each one of those steps.
01:49 We're going to need to take this measurement once at the front axle and once at the rear.
01:53 As long as your suspension's symmetric you're not going to need to measure all 4 corners of the car.
02:00 Doing it once at the front and once at the rear is going to be fine.
02:02 In this particular situation, because the method we're going to use is so similar between the front and the rear, we're only going to look at the rear axle, it's going to be really obvious to you that you're not going to need to see me run through the front axle example as well.
02:15 But essentially you're just going to do this measurement once on the rear, once on the front and then you're going to have all the numbers you need to know.
02:21 So as far as the practicalities of how you're actually going to measure this, it's going to depend a little bit on the make and model and type of suspension you've got in your car.
02:28 So in this particular situation the way I'm going to do this is by measuring vertically from the centre of the hub here down to this plate.
02:35 And every time I increment the suspension up I'm going to take a measuring tape and measure from here down to this board and that's going to be my reference for how much the wheel is moving.
02:44 I'm going to use a set of vernier calipers on the spring and damper assembly and that's going to be my reference for how much the damper is moving.
02:51 Now equally you could have the wheel fitted on here and you could be taking a measurement from the top of the rim to somewhere on the guard or some fixed position on the car.
02:59 It doesn't matter too much how you're going to do that, you just need to use something that's going to be really repeatable.
03:05 Now in some cars the dampers are going to be less accessible than others.
03:09 In this particular car, the damper is a little bit buried in there but hopefully you guys will still be able to see pretty clearly what I'm doing as we go through the example.
03:17 So before I get started with the measurement, the first thing I need to do is remove the anti roll bar connection, the spring and the bump stop from this damper.
03:25 That's going to allow me to run the suspension through its full range of travel without having to overcome the spring force or anything like that.
03:33 So now I've got the spring and bump stop removed from the damper, and I've also disconnected the anti roll bar from the other side of the car, That's going to allow me to put the suspension through its full travel without any resistance.
03:44 So my method as I said before, I'm just going to be using a tape measure to measure the height of the hub as I jack this up and I'm going to be using a set of vernier calipers to measure the length of the damper.
03:56 Now I'm going to be doing this in approximately 20 mm steps of travel.
04:00 The reason I'm choosing 20 mm is it's somewhat arbitrary but I'm doing that just because if you try and go too fine on the measurements, your error in your measurements does tend to swamp your calculation of motion ratio as you go.
04:13 Obviously there is a limited accuracy to doing it this way, it's certainly going to plenty accurate enough for our purposes but you don't really need to be going too fine in those steps unless you've got a motion ratio that's going to be changing relatively rapidly which isn't the case here.
04:30 As I go with these measurements, I'm going to be punching them into a basic spreadsheet on my laptop here and as we go I'm just going to be filling out both sides of that column and then afterwards I'll come back and calculate the motion ratio at each step.
04:42 So here I've got my completed calculation table.
04:44 I've got all of the wheel positions shown in this column here that I measured.
04:48 And then all of the damper positions as well.
04:51 In this column I've got called wheel travel, that's just the subtraction between each one of these steps so you'll see that little formula pop up there which is just one minus the other so it's just taking the difference in wheel travel at each step.
05:04 Then I've got the exact same thing happening with the damper travel here, just looking at the difference between each damper step.
05:10 In this last column here I'm calculating the motion ratio.
05:12 The convention I always use is wheel over spring.
05:15 So that's the wheel travel, divided by spring and damper travel here.
05:20 So that's where we get this 1.28 from.
05:22 Now interestingly what we can see quite clearly as I've compressed that suspension as I've gone and moved down, that ratio has changed, wouldn't say a lot but it has changed somewhat so what we've got at the more compressed end of the spectrum where the suspension is more fully compressed, the wheel is moving more for a given amount of damper travel, we're starting at that ratio of roughly 1.3 and we're ending up at roughly 1.44.
05:48 Now as I said earlier, we were going to take the average so I've taken the average of those values there and that gives me a value of 1.35.
05:56 In the same way that we measure the motion ratio between the wheel and the damper and spring, we also want to do the same thing for the anti roll bar.
06:05 The setup for this is really similar to what we had when we measured the motion ratio between the wheel and the dampering spring.
06:12 The idea is we want the suspension to be fully free so we can run it through its entire travel and measure the displacements.
06:18 So what we've done there is we've got the spring and the bump stop removed as well as the anti roll bar disconnected from the other side of the car.
06:24 That just allows us to move the suspension through it full travel.
06:28 The steps are really similar, all we're going to be doing is moving the hub here up in about 20 mm increments and then I'm going to be taking that measurement from the anti roll bar to some fixed point on the chassis.
06:39 You'll see in the shot here I've got a fixed pen mark and that is where I'm going to be measuring to the anti roll bar.
06:47 It doesn't actually matter what position you use for that, it just really needs to be as vertically aligned as possible so you don't have the tape measure or the vernier callipers or whatever you're using on a big angle, you want it to be relatively vertical and it has to be some fixed point on the chassis.
07:02 So we're going to be going up in roughly 20 mm increments here and I'm going to be measuring the resultant deflection at the anti roll bar.
07:08 One thing to take into account here that because we do have 3 different position adjustments we can have on this anti roll bar, we're going to need to repeat this adjustment for each of those holes so I've got it in the middle hole at the moment.
07:19 I'm then going to move onto the softest hole, that would be the hole furthest away from the pivot point and then the hardest hole which is the one closest to the pivot point.
07:27 Because each one of those holes is going to have a different motion ratio with respect to the wheel.
07:33 I'll do this first measurement now and then I'll go straight into measuring all of the other steps as well.
07:37 So I'm just measuring here from this board to a position, a fixed point on the axle.
07:43 So that is 405 mm and I'll just type that in, that's our first measurement here on the spreadsheet.
07:53 You'll see that we've got, I've already set up two templates here, I've got one at the top here which is going to be the softest position and I've got the medium position which is the one I'm measuring now so I've just go wheel position in this column, anti roll bar position in this column, let's get my first measurement here on the anti roll bar.
08:13 So you do have to be careful because we haven't got a lot of movement on this anti roll bar, we want to be as accurate as possible with our measurements.
08:21 It can be a challenge to be repeatable so we just want to take our time with this.
08:25 I'm going to call that 247 mm.
08:35 And now we can move on and do the rest of them.
08:44 So there I've gone through the full process going from roughly full droop to pretty close to full compression here.
08:50 Now you can see in the table I've got the motion ratio calculation jumping around a little bit here, we've got a minimum of 1.8 and a maximum of about 2.1 but the average of all of those calculations comes out to about 1.93.
09:04 Now the sort of variability we're seeing in that motion ratio calculation is pretty typical when we're using the sorts of methods that we're using here which is just things like rulers and tape measures and vernier callipers.
09:16 The sorts of accuracy we can get with these methods is relatively limited but I would say we've still got something really useful at the end of it.
09:23 Now as I said at the start, we're going to need to repeat this process for each of the 3 holes we've got on this anti roll bar adjustment here and then we'll also go to the front axle and measure the motion ratio we've got there as well.
09:36 One thing to be aware of is that if you are fitted aftermarket anti roll bars, they won't always have the same geometry as your original anti roll bar, even if they don't have the adjustment so you need to go through and make sure you've actually got the same motion ratio if you've measured it previously with your factory anti roll bar.
09:51 Now with this information, in a later practical skills section where we're going to be measuring the anti roll bar stiffness, together with the distance and the stiffness information, we're going to be able to use that information in our lateral load transfer calculations later on in the course.

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