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Suspension Tuning & Optimization: Purpose and Role of a Suspension

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Purpose and Role of a Suspension

06.51

00:00 - Suspension is a deep and sometimes complicated topic to fully understand.
00:04 But its purpose is very simple, keep the tyres in contact with the road and isolate the chassis as the car traverses terrain.
00:12 What's important to understand is that while the broad purpose of the suspension of a road car versus a race car are similar, we place very different priorities on each.
00:21 In road cars, design is to prioritise passenger comfort, lower cost, ease of manufacture, stability, durability and even cabin space.
00:31 Of course, some effort is put into performance depending on the model but this still tends to be heavily compromised in favour of everything else that goes into a good road car suspension.
00:41 In motorsport, it's a completely different story.
00:44 The priority is purely performance with a focus on maximising both lateral and longitudinal grip of the tyres, quick response to driver inputs, high tunability and an ability to control the chassis to optimise the aerodynamics.
01:00 Generally speaking, all the things that we try to maximise in motorsport have a detrimental effect on the design priorities of road cars.
01:07 And conversely you'll often hear people complaining about the suspension of a given road car when it's being used for some type of motorsport.
01:16 It's important to remember that the designer of that car was likely prioritising very different things to what we might be looking for.
01:24 It's generally not a case of a suspension being bad, it was simply intended for a different purpose.
01:30 In truth, the design of a well optimised suspension intended for road use, is a vastly more complex task than that of designing something for motorsport use only.
01:40 One thing we need to make clear for this course is that we'll be ignoring some of the effects of tyres while discussing motorsport suspension.
01:47 The reality is that in addition to the tyres being critical in their own right, they're also a part of a vehicle's suspension.
01:55 For example, each tyre has a vertical stiffness that makes it operate as both a spring and a damper which is fitted in series with the rest of the suspension.
02:05 While tyres will come up at some points through this course, their contribution to the suspension will be largely ignored for the sake of simplicity and discussing suspension fundamentals.
02:15 Before we get into the course material, I first want to introduce some terms and definitions that you'll need to understand as we move through.
02:22 Let's start with the very basics, lateral is a term that we use to describe the side to side direction of the car.
02:29 It's often used when we're discussing cornering where the tyres will be predominantly exerting a lateral force on the suspension.
02:36 Longitudinal is how we describe the forward and aft direction of the car.
02:41 We'll often use this term when discussing acceleration and braking where the tyres are predominantly producing longitudinal forces.
02:48 You're also going to be hearing the terms heave, roll and pitch.
02:52 Heave is used to describe the vertical movement of the chassis.
02:55 Roll is where the chassis rotates around its longitudinal axis and pitch is where the chassis rotates about its lateral axis.
03:04 Next the terms compression and jounce refer to the suspension and wheel moving upwards towards the chassis.
03:10 While rebound and droop refer to the suspension and wheel moving downwards away from the chassis.
03:16 Ride height is used to describe the height of the chassis above the ground plane.
03:20 Any convenient point of the chassis can be used as a reference point to define the ride height but generally 4 points will be chosen.
03:28 Two at the front and two at the rear.
03:30 Using 4 points allows us to easily measure the height and inclination of the chassis.
03:36 We'll also be using the term static and dynamic throughout the course.
03:40 In this context, static will be used for measurements or parameters when the car is at rest.
03:45 Generally sitting on a flat and level surface.
03:49 Dynamic will be used for measurements or parameters when the car is moving around the track which will be constantly changing.
03:55 The term kinematics is used to describe the motion of different parts of the suspension.
04:00 How each part moves and through which arcs.
04:03 And that's defined by the geometry and the pick up points.
04:07 The key thing here is that kinematics has nothing to do with any forces, only the motion.
04:12 You'll also be hearing the term stiction which is used to describe the friction that occurs in a sliding component.
04:19 Most often in the context of components like dampers.
04:22 The end effect is a friction that must first be overcome before the component will move.
04:27 This often occurs in dampers that are used in MacPherson struts where side load on the strut makes a stiction force in the damper that must first be overcome before it'll move.
04:38 Next, independence is a term that's used to describe one side of the suspension being able to move without directly affecting the movement of the other.
04:46 Different suspension layouts allow different levels of independence and this is an important property to consider because the level of independence has a large bearing on the ability of a suspension to maximise the time spent by each tyre on the road surface when it's rough.
05:02 You'll be hearing the term compliance quite a bit as we move through the upcoming modules.
05:06 This can be thought of as the deflection of components that we don't usually want to deflect.
05:12 Compliance is an important parameter in suspension and is generally considered a bad thing because it means our components are flexing under load.
05:20 And that results in things being in different positions and angles than we intended.
05:24 While it won't be relevant for this course, it's worth noting that in high level motorsport, compliance is intentionally designed into components to achieve different handling characteristics.
05:34 When we use the term steady state in the context of suspension, we're talking about the car being settled and running at a consistent state.
05:42 An example of steady state is when we're in a constant speed corner as we would be on a skid pad.
05:48 All the driver inputs are constant, that means things like the throttle, brake and steering angle are all constant over time.
05:56 The opposite of steady state is transient which means things are changing over time.
06:02 An example of something transient would be the state of a car when we're transitioning from acceleration to braking or the steering input on corner entry.
06:11 In these cases, the driver inputs are changing over time.
06:15 So in summary, the purpose of a motorsport suspension is to maximise the grip from our tyres, give us quick response to driver inputs, allow high tunability and help us maximise our aerodynamics.
06:29 The goals of a road car suspension are very different from what we want in a motorsport setup.
06:34 And it's helpful to have a good understanding of the commonly used terms and definitions used in the motorsport world.