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Suspension Tuning & Optimization: Instant Center

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Instant Center

07.13

00:00 - As we discussed in the previous module, the components of our suspension are all moving through different arcs as the suspension compresses and extends.
00:09 The component of an instant centre describes the point around which a given part of the suspension is rotating.
00:16 The instant centre of rotation is not a real point that can be physically measured in space.
00:22 It's a construction that we can make based on the orientation of our suspension components.
00:27 Usually we're most interested in the instant centre for each wheel.
00:33 An important property to understand about instant centres is that generally speaking, the position of the instant centre of each wheel is always moving.
00:41 That means that as the wheel moves through space, as the suspension moves, the wheel is rotating around an instant centre but the instant centre is also moving.
00:51 When looking in 2D, we can define the instant centre in front and side view of the car.
00:56 Which we see here.
00:58 When we're considering cornering, we're most interested in looking at the front view instant centre while under acceleration and braking we're most interested in the side view instant centre.
01:09 Let's now look at the construction of the instant centre for the most common suspension types we'll come into contact with.
01:16 Looking from the front of a MacPherson strut suspension, we draw a line through the centre of both pivot points for the lower control arm.
01:23 We then draw another line at 90° from the top pivot point of the strut relative to the axis of the strut.
01:31 The point of intersection between these two lines is the instant centre.
01:35 This is the point that the wheel will instantaneously rotate about as the suspension moves.
01:42 As we move the suspension through its travel, we can see the position of the instant centre changes.
01:48 Now looking at a double wishbone we draw lines through the pivot points of both upper and lower control arms.
01:55 The point of intersection between these again gives us our instant centre.
02:00 We can also construct the instant centre in side view.
02:03 Here we've done this for the same double wishbone suspension.
02:06 We again draw lines through the pivot points and find the intersection of them.
02:10 These examples are useful to teach the basic construction of instant centre but with that said, all the suspensions we've looked at so far are heavily simplified and to accurately find the instant centre of most real suspensions or any other kinematic property for that matter, we need to make use of either our own further calculations or a software tool.
02:32 Don't worry too much about those details now, we'll go through some examples later in the module.
02:37 One thing to keep in mind is the construction of the instant centre using the simple methods we've shown so far isn't always possible.
02:45 Depending on the type of suspension we're working with, multi link is one good example, for this type of suspension, it'll require some simulation with software designed to deal with the suspension design in order to locate the instant centre.
02:59 With the instant centre defined, we can now construct the virtual swing arm.
03:04 This is done by drawing a line between the instant centre and the middle of the tyre contact patch.
03:10 This represents a hypothetical single suspension arm that we could use to replace all the actual suspension and represents the motion at this particular point in the suspension travel.
03:21 We can construct the virtual swing arm from both the front or side.
03:25 Which are referred to as the front view virtual swing and the side view virtual swing arm respectively.
03:31 We can look at the vertical and lateral components of the virtual swing arm to tell us something about how the track width and camber variation will change as the suspension moves through its travel.
03:42 The larger the vertical component of the virtual swing arm, the more track width and camber change we'll get for a given amount of suspension travel.
03:51 As we see here in these examples of 3 different suspensions, the heavily inclined swing arms are leading to a large track and camber change.
04:00 Likewise, the smaller the lateral component of the virtual swing arm length, the more track width and camber change we'll get for a given amount of suspension travel which is what we see here in these 3 examples.
04:12 These are both important properties to understand.
04:15 To a degree we normally want to minimise the amount of track width variation because too much can upset the tyres.
04:21 Effectively we start to drag them across the surface of the road as the chassis heaves which can lead to unwanted stress on the tyre.
04:29 Particularly within the tread.
04:31 These track changes also introduce a slip angle into the tyre which will lead to an unsettling effect on the chassis.
04:38 Camber change is important to understand because it affects our dynamic camber as the chassis heaves, pitches and rolls.
04:45 If we wanted to maximise pure longitudinal tyre grip which is braking and acceleration we'd want to keep the tyres completely vertical as the suspension heaves.
04:54 To do this, we want to make the virtual swing arm length as long as possible.
04:59 To minimise the amount of camber change in heave.
05:02 In fact, it's possible to make the virtual swing arm length essentially infinitely long if we use equal length parallel wishbones.
05:11 In this case, as the suspension heaves, the tyres remain straight up and down.
05:15 Now if we take the same suspension but instead put the chassis in roll as if we're taking a corner, we can see that the outside tyre has leaned over, giving us positive camber.
05:26 This is not what we want.
05:28 On sealed surfaces anyway, all modern motorsport tyres make more lateral grip with some amount of negative camber so we need to avoid positive camber when cornering.
05:40 If we instead, make use of a more conventional style suspension, the uneven length and non parallel wishbone system, by repositioning the pivot points of our suspension, we can design what's referred to as camber recovery.
05:53 That is, as the chassis rolls, the outside tyre recovers the camber it would otherwise lose from the chassis itself rolling.
06:00 The downside of this style of suspension is that we've now hurt our camber angle when the chassis is in pure heave.
06:07 As we can see here, as the chassis moves vertically we're changing the camber which will hurt our longitudinal performance.
06:14 Ultimately the design of the suspension system becomes about finding a suitable compromise.
06:19 Like most things in life, there is no perfect solution.
06:23 So in summary, for each suspension type, we can construct a theoretical point about which the wheel rotates in space called the instant centre.
06:32 This point can be found from both the front and the side view, though in most real world practical suspensions, the accurate calculation of this point is too complicated to construct with simple 2D methods and we're much better off using some software to help.
06:48 Once we've constructed the instant centre, we can define both the side and front view virtual swing arms which can tell us a lot about how the track width and camber variation will change as the suspension moves through its travel.