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Suspension Tuning & Optimization: Sprung and Unsprung Mass

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Sprung and Unsprung Mass


00:00 - The concept of sprung and unsprung mass are important to discuss early in this course because we'll be referring to them heavily throughout.
00:08 Broadly speaking we can think of the sprung mass as being the chassis and everything somewhat rigidly attached to it.
00:14 This means the engine, fuel tank, driver, body panels etc.
00:19 Essentially everything that's supported by the suspension springs.
00:23 The unsprung mass is everything else.
00:25 The wheels, tyres, brakes and suspension.
00:29 To be a little more precise on that definition, some parts of the suspension are attached to both the sprung and unsprung masses.
00:36 Take the suspension arms for example.
00:39 They're actually partially sprung and unsprung because they're attached to both the fully sprung and fully unsprung masses.
00:46 But this is more academic than real world practical.
00:50 It's obvious that we want to minimise the overall mass of our vehicle in order to improve its performance.
00:56 But the key differences between sprung and unsprung mass are important to understand.
01:00 The sprung mass is more or less the dead weight we have to carry around with us as we move around the track.
01:06 Whereas the unsprung mass has a direct effect on how well our suspension can follow undulations in a road surface.
01:13 This distinction is going to be important to keep in mind as we move throughout the course.
01:17 A fundamental principle of physics is that F=MA, meaning that force is equal to mass multiplied by acceleration.
01:25 The higher the mass, the higher the forces required to accelerate it.
01:29 So it makes sense that reducing the mass of your car with all other factors being equal will lead to better performance.
01:36 Our suspension is tasked with keeping our tyres in contact with the ground to maximise grip and the higher the unsprung mass, the more difficult it becomes to minimise the variation of vertical load of the contact patch of the tyres.
01:50 This is because for a component moving at a given speed, the higher the mass of that component, the more kinetic energy it has.
01:56 As we'll see later, particularly when it comes to damping, the is something we want to minimise in order to control the movement of the unspung mass effectively.
02:06 In short, the more unsprung mass we have, the poorer our suspension will perform on undulating surfaces.
02:12 This will naturally lead to more compromises and follow on effects and the poorer the condition of the surface we're running on, the more detrimental higher unsprung weight will be to the performance of the vehicle.
02:24 There's no free lunch though and the big downside of lower unsprung mass is compliance in suspension components.
02:30 For a given suspension design and material choice, as we reduce the size of the components to reduce mass, we'll also increase the compliance or how much they flex.
02:41 We can optimise the design of the components or make use of exotic materials to lessen the trade off but both of these come with a pretty big manufacturing time and cost penalty.
02:51 Another important thing to consider with unsprung mass is its contribution towards lateral load transfer.
02:58 The larger our unsprung mass and the centre of gravity height of that mass, the more load transfer will occur.
03:03 We'll leave it at that for now but this is something we'll spend a lot more time discussing later in the course.
03:10 In summary, sprung mass can be thought of as the chassis and everything rigidly attached to it.
03:16 Unsprung mass is made up of the wheels, tyres, brakes and some of the suspension.
03:20 We want to minimise unsprung mass to help maximise the grip but it's important to understand that reducing unsprung weight is also a tradeoff thanks to the added cost and increased compliance.

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