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Motorsport Wheel Alignment: Adjusting Ride Height Using Coil Over Suspension

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Adjusting Ride Height Using Coil Over Suspension

12.07

00:00 - With modern coil over suspension systems it's incredibly easy to make fast and accurate adjustments to our car's ride height.
00:07 However when making these adjustments, there's a lot controversy around the correct way of doing this.
00:13 In this module we're going to address this and find out a little bit more about how the coil over suspension actually works.
00:19 Here we've got one of the MCA struts out of our Toyota 86 development car and this is pretty typical of a lot of aftermarket coil over suspension.
00:28 We can see that we have a threaded collar which is our lower spring perch.
00:32 And below this we've got another threaded collar and this locks up the lower portion or lower mount on our damper body.
00:39 This lower mount in this case for a MacPherson strut will mount to our hub.
00:44 Now a lot of people will make ride height adjustments by leaving the spring platform where it is and making adjustments to the lower mount.
00:53 Now this is actually the wrong way of doing this and as we'll see a little further through this module, what that's going to do is potentially adversely affect our amount of bump and rebound travel, making it hard for the coil over to do its job properly.
01:07 What we really want to do when making ride height adjustments is to raise or lower our spring perch in order to raise or lower the car.
01:15 Now let's find out why that's the case.
01:18 First of all, I know a lot of people think that raising the spring perch is going to have the effect of increasing the preload on the spring and there's all sorts of misunderstanding around how that's going to affect the spring rate or how it's going to affect the dynamics of the car.
01:33 The general consensus is that if we raise the spring platform up, we're going to end up preloading our spring and that's going to end up making it harder for the spring to compress as the car goes over a bump.
01:45 Now that's simply not the case but to understand this better, we need to also understand a little bit about how the spring works.
01:52 Now the spring that is fitted to this particular strut is a seven kilogram per millimetre spring.
01:58 If you prefer to work in imperial units, that's approximately 390 pounds per inch.
02:03 Now in simple English, what this means is that if we took this spring off the strut and we placed it on the bench in front of me here, if we placed a weight of seven kilograms on top of the spring, it would compress by one millimetre.
02:14 If we placed a weight of 14 kilograms on top of the spring, it would compress by two millimetres, 21 kilograms, three millimetres and so on and so forth.
02:24 What this means is that regardless of how much weight or in other words how much preload we have on the spring, adding another seven kilograms to the spring, will still have the same effect of compressing it a further millimetre.
02:37 It does matter if it's preloaded by 50 millimetres, or it's completely free to move up and down in the strut, we still have a spring with exactly the same spring rate so that's the first myth we need to dispell there, preloading the spring has nothing to do with changing the spring rate of the spring, it's not going to affect the dynamic operation of the car's suspension.
02:58 I do need to add one caveat here though in that we are dealing here with a linear rate spring.
03:06 There are some springs that use a progressive or rising rate but those are rare for our purposes and are outside the scope of what we're talking about inside of this module.
03:16 Hopefully at this point you now understand that preload isn't the enemy and it's not going to affect the way our car performs.
03:22 However in some instances, in order to achieve our desired ride height, we may actually find that at full droop or at full extension of our strut, we end up with our spring floating or non captive between the upper and lower spring platforms.
03:37 Now this isn't an ideal situation and in some instances we may find that if the suspension ends up at full droop and then recompresses, if the spring doesn't seat properly, this could cause some safety issues.
03:50 That's not really a problem though because it's easy to address.
03:53 In this case MCA have fitted a small low rate keeper spring which we can see here between the bottom of our main spring and our lower spring platform.
04:02 Now that keeper spring has a very soft spring rate.
04:05 In this case I can compress that easily just with some force from my hands.
04:09 And the idea of that keeper spring is just to make sure that that spring remain captive and will seat correctly if the suspension does end up at full extension.
04:19 So now we've dealt with the correct way to adjust our ride height, we should also talk about what the adjustable bottom mount is actually for.
04:26 And this is for setting our bump and rebound travel ratio.
04:31 Or in other words, how much bump and rebound travel we will have from our natural ride height.
04:36 Now making sure that we've got the right ratio of bump travel to rebound travel ensures that the suspension is going to be able to do its job properly, giving us maximum grip and the best possible perfromance.
04:48 While the correct setup is going to depend on your particular car as well as the type of motorsport you're involved with, a very general guide or rule of thumb is that we should maintain around about two thirds of our total travel in our suspension for bump travel and the other third for rebound.
05:05 Now another aspect with our lower mount is that it effects the amount of bump travel before we actually end up contacting our bump stop.
05:14 Now this is important because if this is incorrectly adjusted, we can get into a situation where the wheel or some of our other suspension components can end up contacting the chassis as we move into bump travel before we end up contacting the bump stop.
05:30 So this needs to be set so that we contact that bump stop, before we have any chance of contact between our wheel or suspension components and the chassis.
05:38 This is important for safety in the vehicle.
05:41 So now that we've got a basic understanding of the coil over suspension and how it works and how we should be making our adjustments, let's head out into the workshop and we'll have a look at those changes on our project car.
05:52 I know that the concepts we've just looked at regarding the base height adjustment are a little bit difficult to get your head around so before we actually make some ride height adjustments here, we're just going to go through a quick look at how this actually relates on the car.
06:06 So what I've done at this point is I've installed the front right strut assembly with no springs attached to it.
06:12 This is going to allow us to easily move the strut through its full range of travel.
06:16 Now these come preconfigured from MCA for the Toyota 86 chassis so they really should be a case of fit and forget, we shouldn't need to make any adjustments.
06:26 But we can see how these have been set up.
06:29 MCA give us a recommended ride height to start with and the ride height is measured from the centre of the wheel to the top of the wheel arch.
06:38 In this case, ride height should be 350 millimetres.
06:41 So we've got the car set at ride height now and what we're going to do is go through the full range of droop travel and then bump travel.
06:48 What we find is that if we allow the suspension to move into full droop, we end up with a measurement between the wheel centre and the wheel arch of 405 millimetres.
06:58 Taking into account our ride height of 350 millimetres, this means that we have available 55 millimetres of droop travel.
07:05 Now if we full compress the suspension, remembering again, we've removed the springs and I've also removed the bump stop, we find that the suspension bottoms at a measurement of 280 millimetres between the wheel centreline and the wheel arch.
07:19 This gives us a bump travel from ride height of around about 70 millimetres.
07:24 Those ratios there, 70 millimetres of bump, 55 millimetres of droop, they're in line with what we were talking about earlier.
07:31 The important thing to note here is that with MCA's base setup, we will not allow the tyre to contact the inside of the wheel arch at full bump, so this protects the car from any damage from chassis to tyre or chassis to suspension contact.
07:46 Now what we're going to do is effect a change in ride height of 20 millimetres.
07:51 We're going to lower the car by 20 millimetres but we're going to use the lower mount to do this, rather than the spring perch.
07:58 So let's go ahead and make that change and see how it affects our results.
08:06 With our 20 mil ride height change made at our lower mount, we can go through the same process of moving the suspension through its full range from full droop to full bump.
08:16 What we've started here is with our new lower ride height, remember that we've lowered the car 20 millimetres, so instead of 350 we're now looking for a ride height of 330 millimetres between the wheel centre and the guard.
08:28 Now we can move into full droop and we know that we still have exactly the same amount of droop travel, in this case 55 millimetres.
08:36 So we can see that at full droop we now have a measurement of about 385 millimetres from the wheel centre to the lip of the guard.
08:44 Now moving into full bump, this is where things get a little bit interesting.
08:47 You'll remember that last time we reached the extreme of the bump travel on the strut at 280 millimetres between the wheel centreline and the guard.
08:58 Now we can go a little bit further, we actually get to a point five millimetres further into bump travel, 275 millimetres from the wheel centre to the guard, where the tyre is actually now contacting on the inside of the wheel arch.
09:12 So this is the problem we strike when we're making adjustments to our lower mount to effect a ride height change.
09:19 What we can now see is that we're not utilising the full bump travel available from the strut, compromising the strut's ability to do its job properly.
09:28 More importantly we also have the issue where we will not be completely contacting the bump stop and we do risk that problem on a pronounced bump where the wheel may contact the body and this can be incredibly dangerous.
09:41 So with our demonstration out of the way, I'm hoping at this point you are convinced that using the lower spring perch is the correct way to make our ride height adjustments.
09:51 We'll now briefly cover the steps involved in actually going about this.
09:55 First of all with a set of suspension set up for our car like these MCAs, there's actually very little work to do.
10:02 The lower mount has already been set correctly for the particular chassis as we've discussed and our job is simply to install the suspension and check and make sure that the ride height matches what MCA recommend.
10:14 In this case 350 millimetres from the wheel centreline to the guard on all four corners of the car.
10:20 Now if this isn't the case, we're going to need to make an adjustment to our ride height at whichever corner needs work.
10:26 And the way we're going to do this is we're going to loosen the lower spring perch and we're going to either adjust it up or down as required using the supplied C spanners.
10:36 And it's always advisable to use the correct C spanner for your coil over suspension, I see a lot of people using chisels or punches to make adjustments and while this will get the job done, it's going to very quickly damage your expensive coil over suspension.
10:51 It's also worth discussing the magnitude of change that's going to be required.
10:56 So for example, with a MacPherson strut suspension design, we're looking at a motion ratio of 1:1.
11:02 What this means is that if we need to make a 10 millimetre change in ride height, we're going to change our spring perch height by 10 millimetres.
11:10 That's not always going to be the case though and particularly with double wishbone and multi link suspension systems, there will be a motion ratio.
11:17 So for example on a given suspension design, to achieve a ride height change of 10 millimetres, we may only need to change the spring perch height by six millimetres.
11:28 We'll quickly be able to get a feel for what magnitude of change is required in order to get our ride height where we want it to be.
11:36 Any time we are making a ride height change, it's important to allow the suspension to settle.
11:41 This can be achieved by taking the car off our jack stands or off our hoist, rolling it backwards and forwards a few metres and giving it a little bit of a bounce on the strut towers just to make sure that everything is settled in the normal ride height position.
11:55 Then we can remeasure.