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# Motorsport Wheel Alignment: Roll Centre

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## Roll Centre

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 00:00 - Roll centre is a topic that most enthusiasts are blissfully unaware of, however even those who have a broad idea of the concept often struggle to get their head around it. 00:09 Fortunately it's actually not that tricky when we look at some basic drawings. 00:13 In simple terms, the roll centre is a function of the suspension's geometry and it's the point around which the sprung mass of the car will roll as we turn through a corner. 00:23 This is easiest to understand if we look at a drawing of the car from the front. 00:28 In a MacPherson strut suspension design, we need to start by plotting the instant centre which we can find by drawing a line through the lower control arm pivot points. 00:37 Next we can draw a line perpendicular to the strut top and the instant centre is where these lines intersect. 00:43 Once we've found the instant centres, we can plot a line from each instant centre through the centre of the tyre contact patch. 00:50 Where these lines intersect is the roll centre. 00:53 In a double wishbone suspension system, the roll centre can be found by drawing a line through the inner and outer pivot points of both the upper and lower wishbones. 01:02 Where these lines intersect is the instant centre and again we can plot lines from the instant centres to the centre of the tyre contact patch. 01:10 As with our MacPherson strut example, the place where these lines cross is the roll centre. 01:15 The roll centre really needs to be considered along with the centre of gravity, as they both affect the way the car will handle. 01:22 If we look at the centre of gravity, we can see that in our drawing it's located above the roll centre. 01:28 The distance between the roll centre and the centre of gravity is important because this is the roll moment. 01:34 And the larger this distance the larger the roll moment, which means that the car will roll more in a given corner. 01:40 The reason we need to understand this is because as we lower the car's ride height, the centre of gravity will understandably drop, but typically the roll centre will drop by a greater amount. 01:51 This means that in a lowered car, the lever arm between the centre of gravity and the roll centre will be greater and all things being equal, the car will tend to roll more in a corner. 02:01 This can easily wipe out any potential improvements you may expect as a result of the lower centre of gravity. 02:07 We've only looked at a simple drawing to describe the roll centre and it is important to understand the the roll centre isn't stationary but rather it moves around as the suspension compresses and extends and the car negotiates corners and bumps. 02:21 One of the design challenges that engineers go through is trying to maintain a stable roll centre that doesn't move around too much to provide predictable handling. 02:30 Of course there are roll centre correction products available for many cars to address these issues when the cars are lowered, providing a similar roll centre height with a lower car to stock and we'll be covering these in a later module.

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