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Motorsport Wheel Alignment: Roll Centre & Bump Steer Correction

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Roll Centre & Bump Steer Correction

04.57

00:00 - As we've discussed already in this course, lowering a car can end up producing adverse effects, particularly in regard to the roll centre height and the introduction of bump steer.
00:10 Without addressing these problems, it's very easy to end up lowering your car with the aim of improving handling only to find that you've created the exact opposite effect.
00:19 Fortunately there are products available for many cars that will address and correct these problems.
00:25 Correcting the roll centre can be achieved by dropping the outer ball joint on the lower control arm in relation to the hub.
00:32 It's easiest to understand this if we look at a drawing of what's actually happening.
00:36 You will remember that the roll centre can be found by first of all plotting the instant centre and then plotting a line through the instant centre to the centre of the tyre contact patch.
00:46 As we lower the car we can see the effect on the roll centre, which has now dropped significantly.
00:52 By lowering the outer ball joint in relation to the hub, we're changing the angle of the lower control arm, which in turn moves the instant centre.
01:00 We can see that the effect of this is that it raises the roll centre height and reduces the roll moment.
01:06 This roll centre correction is achieved either with a spacer plate that locates between the bottom of the strut and the lower ball joint, or a specially designed extended ball joint that replaces the stock component.
01:18 There is a limit to how much correction can be applied using this technique though, since there's a huge amount of force placed through this ball joint and obviously a failure here could be catastrophic.
01:29 If you're considering a roll centre correction kit, it's strongly advised that you deal with a reputable manufacturer and only use the product as they intend.
01:38 Since correcting the roll centre height on its own is going to make any bump steer significantly worse, the roll centre adjuster is normally sold with a bump steer adjuster as part of this kit.
01:49 These parts are designed to work together and when installed by following the manufacturer's recommendations, you'll end up with roll centre correction and minimal bump steer.
01:58 These bump steer correctors work on the same principle as the roll centre corrector, offsetting the outer tie rod end in relation to the steering arm on the hub.
02:07 Everything that we've just looked at so far is relevant for a MacPherson strut front suspension system however there are many different suspension designs out there so let's discuss your other options.
02:19 If you're dealing with a double wishbone or multi-link suspension system, then things can be a little bit trickier.
02:24 It's still possible to move the outer ball joint for the lower wishbone down in relation to the hub.
02:30 However this will also affect the relationship between the upper and lower wishbones which will affect the rest of your suspension geometry.
02:37 In the rear of a car equipped with a multi-link suspension system, a tidy way to affect the roll centre height is to actually raise the complete rear factory subframe up inside the chassis.
02:49 This has the affect of raising the inboard mounting points for the suspension arms and can leave you with stock geometry in a stock roll centre at a lower than factory ride height.
02:59 Raising the subframe like this is possible because OE manufacturers isolate the rear subframe from the rest of the chassis using rubber bushes.
03:07 Usually there will be part of this bush sandwiched between the subframe in the chassis and by removing these rubber bushes, and using a specially designed alloy bush instead, we can raise the subframe by the thickness of the original bush.
03:20 Admittedly this may only be 10 to 20 millimetres but everything helps.
03:25 As an added bonus, you'll also be removing the compliance in the rear subframe and improving the precision and feel albeit at the expense of more noise being transferred into the chassis.
03:36 Another common option is to use an aftermarket hub assembly that corrects the roll centre and bump steer by relocating the mounting points on the hub.
03:44 These are available for a range of popular cars and in particular these are a common upgrade for drift cars where there are a range of manufacturers offering options.
03:53 The last option is to start moving mounting points for the suspension linkages on the chassis or the subframe, however this requires in depth modelling of the suspension geometry and is beyond the scope of this course.
04:06 With a solid or live rear axle, things are a little trickier and the method of adjusting the roll centre will depend on the lateral location of the axle.
04:14 Probably one of the more common methods of adjusting the roll centre is to use a Watt's linkage mounted the chassis and this Watt's linkage will provide the lateral location of the axle.
04:24 In this case, the roll centre is located at the centre of the Watt's linkage.
04:29 By raising or lowering the Watt's linkage on the chassis, the roll centre height can then be changed.
04:34 Now that might sound like a lot of work however in purpose built racecars the Watt's linkage can be located on a screw thread with an adjuster located inside the boot or trunk area.