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Suspension Tuning & Optimization: Roll Center and Axis

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Roll Center and Axis

08.32

00:00 - Roll centre is probably one of the most commonly used terms that you'll hear flying around the pits when people are discussing setup.
00:07 Unfortunately, its meaning is widely misunderstood so in this module, we're going to look to demystify it before making use of the concept in the upcoming lateral load transfer section.
00:18 Simply put, the roll centre is the instantaneous point that the sprung mass rotates about when the chassis rolls.
00:25 We visualise the roil centre by looking from the front or rear view of the car.
00:31 We define a separate roll centre for the front and rear axles respectively and just to be really clear, the roll centre isn't a physical point we measure, it's a theoretical point we calculate.
00:42 To find the roll centre, we need to make use of the instant centre that we introduced in the previous module.
00:49 Looking from the front of this car that uses a double wishbone suspension, we draw a line from the centre of the contact patch for each tyre through their respective instant centres.
00:59 The point where each of these lines crosses each other is the position of the roll centre.
01:04 As we'll see later in the lateral load transfer section of this course, the position of the roll centre has a huge effect on how and when vertical load is distributed around the car as we corner which is one of the primary things that determines our chassis balance.
01:20 At this point, we should note that we'll be discussing the kinematic definition of roll centres in this course as opposed to force based roll centres.
01:28 The basic principles behind both of these concepts are similar but in order to use the more correct definition of a force based roll centre, we need a full understanding of the force and moment response of the tyres.
01:41 A moment being essentially another way of saying torque.
01:45 A force acting at a certain distance.
01:47 This takes force based roll centres outside of the realm of usefulness for those of us not operating at a professional level due to the lack of tyre data and the modelling complexity involved.
01:59 The good news is that by using kinematic roll centres, we an still get an extremely useful understanding of how our suspension geometry can affect the handling of our car.
02:08 Now that we've seen how to construct the roll centre, let's examine how it behaves as the suspension moves.
02:15 The important thing to understand here is that the roll centre isn't a fixed point in space.
02:20 In front view, it moves both vertically and horizontally as we move around the track.
02:25 Let's take a typical double wishbone suspension and move it through a few different positions to understand how the roll centre movement works.
02:34 Before we do that, keep in mind that while looking at examples like this is very useful, in the real world, as we move around the track, we constantly move through different combinations of heave, pitch and roll, all occurring simultaneously.
02:49 There's no such thing as pure movement.
02:52 Starting with the car at static ride height position, let's move the car through pure heave.
02:57 Both below and above the static ride height.
03:01 We can see that the roll centre moves vertically only and remains on the centreline of the chassis.
03:07 As long as the suspension geometry is symmetrical and the chassis is only moving in pure heave, this will always be true.
03:14 The same is true of pure pitch where the front and rear suspensions will be moving vertically only.
03:21 When we now put the chassis in roll, we can see that the position of the roll centre is moving both laterally and vertically.
03:28 And it moves from one side to the other as we roll the car in each way.
03:32 In most cases, and within reasonable limits, it's the vertical component of the roll centre movement that we care about the most.
03:39 Let's now look at the roll centre movement behaviour for some different suspension configurations to get an idea of how changing our geometry affects the roll centre.
03:47 Starting with our original double wishbone suspension, let's overlay a modified version of this by moving the position of the inner wishbone pickup points.
03:58 We can see that a relatively small change in pickup points can significantly affect the roll centre behaviour.
04:05 Let's take a look at a similar analysis but this time with a typical MacPherson strut layout.
04:09 Again, we can see clearly that with two slightly different configuration positions of the inner lower control arm pivot, we can have a lot of control over the roll cenre position and movement.
04:22 As we'll discuss in more detail later in the course, it's the position of the roll centre with respect to the centre of gravity of the sprung mass that has an effect on how much the chassis rolls.
04:33 For this reason, one of the things we're usually trying to keep as constant as much as possible is the vertical distance between the roll centre and the chassis' centre of gravity.
04:43 Here's an example of two different suspensions where the roll centre behaviour is quite different.
04:48 In both cases, the chassis is moving in pure heave by 50 mm.
04:53 We can see that in the blue configuration, the vertical distance between the centre of gravity and the roll centre is relatively constant.
05:00 In the red configuration, this vertical distance is changing considerably.
05:05 In this case, the blue configuration would be our preference because the roll centre location with respect to the centre of gravity is more stable.
05:13 Take note of the difference between these two configurations and the difference in roll centre behaviour here.
05:19 This will become important later in the course.
05:21 One suspension modification that's become more popular in recent years is the roll centre correction kit.
05:29 These are available for many popular makes and models and their purpose is to correct the geometry of lowered cars.
05:35 When people lower a car for performance reasons, they're usually trying to lower the centre of gravity of the chassis.
05:41 While it's true that lowering the car is generally speaking at least, a positive thing for performance, there are negatives too.
05:48 This is because we're now operating the suspension outside of its originally intended ride height window.
05:55 Let's visualise this by looking at another double wishbone suspension, starting at the original ride height intended by the manufacturer.
06:03 If we now lower the ride height, we can see how much both the vertical distance between the centre of gravity and the roll centre has changed.
06:11 Keep in mind that the only change we've made to the suspension here is the ride height.
06:16 Everything else geometry wise remains the same.
06:19 What's important here is not necessarily that the roll centre position has changed but that the starting point or datum at our ride height is different.
06:27 The problem with this is that we'll be operating the car at our new ride height at a roll centre position that wasn't intended by the designers.
06:36 The most common way to correct the roll centre in the aftermarket world is by changing the position of one of the outer suspension arm pivot points.
06:44 This is usually done by fitting a taller offset ball joint to the outboard lower control arm where it attaches to the upright.
06:51 If we then go back and overlay our newly corrected suspension geometry with the roll centre correction kit fitted, we can see we return the vertical distance between the centre of gravity and the roll centre to approximately what we would have had at the original ride height.
07:05 We'll dive further into how this affects the handling of the car in the lateral load transfer section.
07:11 Before moving on, let's tie it all together with what's called the roll axis.
07:15 As we mentioned at the start of this module, we define a separate roll centre for the front and rear suspensions respectively.
07:23 Looking at the suspension assembly together as a whole car in 3D, we can also draw a line through the front and rear roll centres and this line represents the roll axis of the car.
07:33 While the front and rear suspensions have an instantaneous roll centre, the roll axis is a theoretical line which the entire sprung mass rotates about as the chassis rolls.
07:44 As the roll centre positions are changing as the car moves on track, we can see how the roll axis moves in 3D.
07:51 You'd be right to assume that the roll axis can have some quite complex movements as we combine pitch, heave and roll of the chassis.
08:00 To summarise this module, we can construct a roll centre for both the front and rear suspensions respectively.
08:07 This is the theoretical point that each end of the car rotates about as the chassis rolls.
08:11 By joining these front and rear roll centres together, we can construct a roll axis which defines how the chassis will roll in 3 dimensions.
08:20 We've also learned how roll centre correction kits can be very useful in returning our geometry back to the factory set angles when we lower our car.