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Motorsport Wheel Alignment: Measuring and Adjusting Castor

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Measuring and Adjusting Castor


00:00 - In this module, we're going to take a look at how we can measure and make adjustments to our caster in the home workshop or at the racetrack.
00:07 Now I know that a lot of enthusiasts will leave this particular step to a professional wheel alignment specialist, and this can makes sense since caster isn't an alignment aspect that we are going to make frequent changes to so it can essentially be set and forget at the beginning of our initial suspension setup and also we do require some specific tools in order to measure and adjust out caster.
00:32 In this case, what we're going to be using is a bubble camber/caster gauge.
00:37 These are available in both bubble style as well as digital, we've chosen the slightly cheaper bubble style here.
00:45 And this also does bring in another complexity in that the camber/caster bubble gauge does need to be accurately attached to our wheel.
00:53 Now if you've got a car with a hub that protrudes through the centre of the wheel and it's a steel surface then this can be quite simple, we just attach this magnetically to that surface.
01:03 Likewise if you've got a steel wheel, then as long as you've got a flat surface in the centre of the rim, we can also use this to locate the magnetic camber/caster bubble gauge.
01:13 In our case with aluminium wheels, this does get a little bit more complex as obviously that alloy wheel isn't magnetic.
01:20 To get around this, we've had our machinist machine up a simply press in adapter that just locates in the centre of the rim.
01:28 This gives us a nice flat mounting surface for our camber/caster bubble gauge and we know that due to the machining, this is going to be in line with our wheel.
01:36 The other tool that we're going to need is a set of turn plates.
01:40 Now in the perfect world, we would be using turn plates with degree markings on them and this is because, in order to measure our caster, we need to accurately turn the steering through 20 degrees in one direction, back through the centre and then 20 degrees the opposite direction.
01:55 So it's important to have those degree markings.
01:58 In our case we're using a relatively simple set of turn plates that are designed purely for caster measurement.
02:05 They don't have degree markings on them, but instead they have a set of lines at exactly 20 degrees.
02:11 Now alternatively you can get around this if you don't have turn plates because these can be a little bit pricey.
02:17 You can use a simple slip plate arrangement, which we've look at elsewhere in the course, and you can then use your string alignment and you can align this with the machined 20 degree markings on the outside edge of the camber/caster bubble gauge to give you a relatively accurate 20 degree measurement.
02:34 As usual, before taking any measurements, there are a few basic tasks that we need to tick off.
02:40 We want to make sure that our setup area is on flat and level ground.
02:43 We want to make sure that our steering wheel is pointed directly straight ahead, we want to make sure that our tyres are at the normal operating pressure, and we want to make sure that our suspension is settled.
02:55 Settling the suspension is important any time we're jacking the car up and down as even with slip plates there will be some stiction in the suspension system and it's a good idea just to bounce up and down on the car a little bit to make sure that everything is settled at the normal ride height.
03:12 And this can be checked by actually measuring the ride height before we take our measurements.
03:17 So with our basic setup out of the way, let's take a look at the process.
03:20 So we've got our camber/caster bubble gauge attached to our adapter in the centre of the wheel.
03:25 It's important to make sure that the adapter and the camber/caster bubble gauge are both concentric with the centre of the wheel.
03:31 Now that we've got that attached, the first task to do here is to level our gauge and we're just going to use the little bubble level at the end of the gauge to do this.
03:50 While not specifically part of the caster measurement, we can now read the camber off the gauge.
03:56 And in this case there are two bubble gauges, one for positive camber and one for negative camber.
04:01 We can see that we're running somewhere in the region of about 3.25 to 3.4 degrees of negative camber.
04:09 What we're going to do now is turn our steering.
04:12 And when we are starting here on the front right, we're going to initially turn the steering wheel to the right.
04:18 So we always want to begin by moving the steering wheel out from the car centreline.
04:23 We're going to use our slip plate here, our turn plate to measure 20 degrees.
04:27 And this particular plate is just marked with lines and all we're going to do is turn it until the inner plate that is turning with the wheel, aligns with one of the 20 degree markings.
04:38 So let's go ahead and do that now.
04:46 Alright we've turned our steering, we can confirm that the angle is 20 degrees, what we're now going to do is use the bubble level at the end of the gauge and we're again going to level the gauge.
05:05 Once our gauge is zeroed, the next task is to zero our caster gauge in the middle.
05:10 And we can see we've got a little thumb screw there.
05:12 What we're going to do is adjust that thumb screw until our bubble is central on the zero.
05:17 And it's important to mention here, when we are reading any of these bubble levels, what we're looking for is the mark closest to the centre of the bubble.
05:25 So let's go ahead and zero our caster gauge.
05:35 Alright, with our caster gauge zeroed, we're now going to turn our steering wheel back through a total of 40 degrees so we're pointing to the left of the centreline by 20 degrees, so let's do that now.
05:49 Alright we've got our steering turned back through to the left and we are on 20 degrees so we're going to go ahead and zero our caster gauge again using our level on the end.
06:02 Once we've got the unit correctly levelled, we can now read the caster and we don't want to adjust our thumb screw this time, we are just reading the caster off the level gauge and we can see that we're running currently five degrees of positive caster which is pretty much in line with the factory specifications for this car.
06:20 So at this point, we've looked at how to measure our caster using the turn plates.
06:23 However if you don't have access to turn plates, you can use your string wheel alignment equipment as I've already mentioned.
06:30 So let's get our string set up and we'll see how we can go about doing that.
06:34 So with our string set up, we're going to use our string as a reference for the centreline of the vehicle.
06:40 And when we're turning our steering wheel, we can use the machined angles on the end of the camber/caster gauge as a reference for 20 degrees.
06:48 What we're going to do is turn the steering wheel until those angled, machined edges align perfectly with our string.
06:55 When this happens, we know that we are at 20 degrees.
06:58 So at this point we've seen how we can use our camber/caster gauge in order to take a caster measurement.
07:04 Now of course we need to know what changes we should make if we want to adjust our caster.
07:09 So in this case, let's look at how we can increase our positive caster, given that our reading there of five degrees is a little bit less than what we'd want.
07:17 In general, there's two ways we can go about doing this.
07:20 Given that this car uses a MacPherson strut suspension design in the front of the car, we can either move the top of the strut rearward in the chassis or we can move the hub at the bottom, forward in the chassis.
07:32 In order to move the strut top rearward in the chassis, we could use a camber adjustable strut top and rotate this so that it's operating in line with the car centreline.
07:43 Now in this case we're not going to do that, we have got camber adjustable strut tops but we want to retain their functionality for camber alone.
07:49 So instead what we're going to do is move the lower ball joint forward in the chassis and we can do this with our adjustable bottom arms.
07:57 Let's get the car up on the lift and we'll have a look at where we can make those adjustments.
08:02 We've got two points of adjustability on our aftermarket lower control arms here.
08:06 We've got a rod end located here and while extending this will to a degree have the effect of moving the lower ball joint forward in the chassis, it's also going to have a bigger effect on our track.
08:19 Essentially it's going to be pushing the wheel further out from the car centreline.
08:22 On the other hand, the rod end that we have here at the front of our lower control arm, is going to predominantly affect our caster with very little impact on our track width.
08:34 What we're going to find here is that by shortening that rod end this is going to have the effect of pulling the lower ball joint forward in the chassis, increasing our positive caster, so this is exactly what we're going to do.
08:44 Unfortunately, because there's no left and right hand threaded adjusters on this particular lower control arm, in order to make adjustments, we're physically going to need to drop the rod end off the chassis mounting point, make out adjustments and then reattach it.
08:59 So we're going to do exactly that now and we're going to start by making adjustment of three turns.
09:29 We've made our adjustment there of three turns and we've just locked up our jam nut to make sure everything's nice and tight.
09:35 It's always a good idea when you are making any adjustments like this, just to make sure that when you do tighten those jam nuts that the rod end is approximately in the centre of its travel.
09:44 In other words we don't want it offset hard in one direction which may limit its range of movement.
09:50 Let's get the wheel back on the car and we'll get it back down on the ground and we'll go through the same process and see the effect of that change.
09:56 So with our change made there to our lower control arm, we've just gone through and we've checked our caster again.
10:02 We've moved from five degrees to 6.5 degrees which is a little closer to our target of seven degrees.
10:08 Now making adjustments to our caster is an iterative process.
10:12 However, what we can do is use some basic math and in this case we know that we've made three full turns of adjustment to our rod end and we've achieved a change in caster of 1.5 degrees.
10:24 So on this basis, we can roughly assume that we're going to get about a half a degree caster increase for every full turn of adjustment to our rod end.
10:33 And this can help guide us with our next set of changes.
10:37 Once we've got our caster correctly set on the right hand side of the car, of course we need to go through and repeat this process on the left.
10:44 But we want to be very certain that our caster is equal left to right.
10:48 Following this process, we would also want to make sure that all of our jam nuts or anything that we have touched on our suspension is correctly tightened.