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Motorsport Wheel Alignment: Measuring & Adjusting Bump Steer

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Measuring & Adjusting Bump Steer


00:00 - As we've discussed in the body of the course, bump steer can make it quite difficult for us to accurately control and place the car on the racetrack and even standard production cars with unmodified suspension are likely to have some degree of bump steer.
00:14 Of course, this gets worse as we perform certain modifications to our suspension so it's something that is worth understanding and measuring.
00:22 This brings us to the problem of how we can accurately measure our bump steer and while there are professional bump steer gauges out there on the market, these are often quite expensive and for this reason we find a lot of enthusiasts never even bother checking their bump steer to find out what their car is doing.
00:39 We're going to show you in this module how we can quickly and cheaply measure our bump steer using parts that cost no more than about USD$50.
00:49 The parts we're going to be using here include an aluminium bar which we're going to cable tie to the rim.
00:55 This is going to act as a mounting point for our next product which is our laser pointer.
01:00 In this case I'm actually using a laser measuring device but anything that provides a laser point will work.
01:07 We're also going to need a mirror that's going to reflect our laser back onto some graph paper and I've also used a couple of angle brackets to support both the mirror as well as the graph paper that we're going to be making our measurements on.
01:21 The idea behind this technique is that we're going to attach our laser pointer to the aluminium bar on the wheel, we're then going to shine our laser pointer towards a mirror that's placed a specific distance from the wheel.
01:33 It's going to then reflect back onto our graph paper and by moving the suspension through its bump and rebound travel, and plotting the movement of our laser pointer on our graph paper, we're going to be able to see the amount of bump travel through the suspension travel.
01:48 You might be wondering at this stage why we need the mirror at all and why we can't simply place the graph paper at the front of the car and directly plot the movement of our laser pointer onto that graph paper.
01:59 The problem with doing this is that as we move the suspension through its travel from full bump to full rebound, we naturally find that the wheel moves outwards and inwards through its travel.
02:10 And this will gives us a false representation of our bump travel.
02:13 If, on the other hand we use our mirror to reflect the laser pointer and we attach our graph papers to the wheel itself, along with the laser pointer, we find that the laser pointer and the graph paper both move in the same arc and hence the only true movement that we're going to be plotting is that that's a result of our bump steer.
02:32 Before we can make any bump steer measurements, there are a few setup aspects that we need to go through.
02:37 The first of these is that so we can easily move the suspension through its full range of travel, we're going to need to remove the spring from our MacPherson strut assembly.
02:47 The other aspect of course is locating our laser pointer and our graph paper on the wheel and then mounting our mirror.
02:54 Now this does need to be done quite carefully.
02:56 What we want to do is make sure that the mirror is located five times our wheel radius away from our graph paper.
03:03 In this case our 18 inch rim measures 500 millimetres in diameter or 250 millimetres in radius.
03:11 If we multiply 250 by five, we've got a measurement of 1250 millimetres.
03:16 So this is the distance we need to locate the mirror from our graph paper.
03:20 Now you might be wondering why that's important.
03:22 The key here is that the laser will travel from the laser pointer out to our mirror and then it'll get reflected back to our graph paper.
03:31 The total distance there is 2500 millimetres or 10 times the radius of our wheel.
03:38 Now where that's going to become important is that once we've got our measurements plotted on our graph paper, these are actually going to be dramatically magnified over what our true bump steer is.
03:48 What we can then do, because we know that the laser pointer is going to travel a total of 10 times the wheel radius, if we divide the measurement that we've got on our graph paper by 10, this will give us our true bump steer value.
04:02 Once we've got everything set up, we want to adjust our suspension so that it's at the normal ride height and we can then make a mark on our graph paper where our laser pointer is located.
04:13 This is going to be our starting point.
04:15 From here, we want to move the suspension through its bump and rebound travel and plot the movement of our laser pointer onto our graph paper.
04:21 Since the bump steer tends to progressively get worse the further we move away from our natural ride height, we really want to be checking this close to our ride height.
04:31 In the case of our Toyota 86 development car, because it has a relatively limited range of suspension travel, I've chosen to check our bump steer within 30 millimetres of the natural ride height.
04:43 So what I mean by this is that we're going to move the suspension 30 millimetres into bump travel and then we're going to also check 30 millimetres into our droop travel.
04:52 This is going to give us a good idea of how much bump steer we have, over the normal range of suspension movement.
04:59 For our first test here, we've purposefully engineered a significant amount of bump steer into the front of our Toyota 86 and we can see that as we've moved into our bump travel position, we're measuring 17 millimetres of toe out.
05:12 Remembering here, that sounds drastic, but we do need to divide this by 10, so we've got 1.7 millimetres of toe out.
05:20 As we move into our rebound position, we're measuring 16 millimetres of toe in compared to the position at our normal ride height.
05:28 So again, dividing this by 10, this gives us 1.6 millimetres of toe in.
05:33 Now that you know what your bump steer's doing, you're obviously going to want to correct it and we've attached a file to this module that's going to give you some guidelines to correct your bump steer, depending on exactly what's happening to your toe as you move through bump and rebound travel.
05:49 In our case, what we're going to do is drop the tie rod, where it attaches to the hub, by 10.5 millimetres and we're going to go through this process again.
05:59 Since this is an iterative process, it's sensible to have a quick and easy way of making adjustments to our tie rod location so that we can plot and check the effect on our bump steer.
06:11 My preference here is to temporarily use a long bolt through our tie rod and our hub and then use a stack of washers in between.
06:19 This makes it really easy to make very fine adjustments.
06:22 With our adjustments made we can now go through the same process again, moving the suspension through its bump and rebound travel and plotting the position of our laser pointer on our graph paper.
06:32 We can straight away see that the changes we've made there have resulted in much less bump steer.
06:39 In this case, at 30 millimetres of bump travel we're now measuring three millimetres of toe out and with 30 millimetres of rebound travel, we're measuring four millimetres of toe in.
06:50 So remembering again, dividing these numbers by 10, that's 0.3 millimetres toe out, 0.4 millimetres toe in.
06:57 So much closer to our ideal of zero bump steer.
07:01 Of course we can continue this process and see how close to zero we can get it.
07:05 While in the perfect world we'd be wanting to achieve zero bump steer, on a production car, this usually isn't realistic and we're going to need to settle for having some degree of bump steer.
07:16 Generally if we can get bump steer to within half a millimetre or less, we're doing pretty well.