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Suspension Tuning & Optimization: Suspended Mass Geometric Load Transfer

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Suspended Mass Geometric Load Transfer

04.05

00:00 - The final component of lateral load transfer to cover is the geometric load transfer.
00:05 Which is sometimes also referred to as the direct or kinematic load transfer component.
00:10 This is the lateral load transfer that's reacted directly through the suspension arms, links and chassis and therefore bypasses the elastic elements entirely.
00:20 We change the geometric load transfer by changing the roll centre height.
00:24 If we position the roll centre at the same point as the sprung mass centre of gravity, all of the sprung mass lateral load transfer is geometric.
00:33 The implication with this, like we touched on in the previous module, is that if we place the roll centre at this position, none of the sprung mass lateral forces are reacted by the elastic elements.
00:45 At the other extreme, if we were to position the roll centre at ground level, none of the load transfer would be reacted geometrically.
00:52 It would be entirely elastic.
00:55 All the roll forces acting on the chassis would pass through the springs and anti roll bars.
01:00 At this point, it should be clear that we can use the roll centre to change how the lateral load transfer is reacted by both the elastic and the geometric components and how much each one takes.
01:13 So if we can reduce the amount of sprung mass body roll by raising the roll centre, why wouldn't we just have high roll centres? The answer comes from the term jacking force.
01:24 This is an often misunderstood term but it's simply the vertical force component of the direct load transfer that causes changes in the sprung mass height.
01:32 If we look at the front view of a suspension, with a realistic front roll centre position, we can draw the force line that runs along the instant centre.
01:40 This is the line of action of tyre force that's reacted via our instantaneous swing arm.
01:47 We can break down this force line into vertical and horizontal components.
01:51 In this case, we have a relatively small vertical component.
01:55 If we now raise the roll centre, this force line has a much larger vertical component.
02:00 This vertical component represents the jacking force and this raises the sprung mass during cornering.
02:09 In short, we're getting nothing for free, there's always a trade off.
02:11 Yes by raising the roll centre, we've reduced some of the chassis roll by reducing the amount of force the elastic components have to deal with.
02:20 However we've also introduced a large jacking force as a result.
02:24 Jacking is the primary reason we don't tend to see high roll centres in motorsport.
02:29 As we learned earlier in the course, the roll centre position is determined by our instant centres and these are constructed from our geometry.
02:37 So it follows that to change our static roll centre position, we must change our geometry.
02:42 And in doing so, we'll change the amount of geometric load transfer.
02:46 All this information should help to explain why the correction of roll centres when a car is lowered becomes so important.
02:52 As we discussed in the roll centre module, in most road car suspensions when we lower the car, we also migrate the roll centre.
02:59 If it drops more than the centre of gravity does, this will increase the roll moment and therefore the amount of roll.
03:06 With what we've covered over the last two modules, we now understand that moving the roll centre further away from the centre of gravity than the designer originally intended and therefore increasing the roll moment means we'll end up with more body roll as a result.
03:21 As the springs and anti roll bars now have to react to more force despite having lowered the sprung mass centre of gravity.
03:29 By using a roll centre correction kit, we can restore the position and behaviour of the roll centre while keeping a new lowered ride height.
03:36 In summary, geometric load transfer works by transferring the load directly through the suspension arms, links and chassis.
03:44 This means bypassing the elastic elements.
03:47 While we can reduce the amount of chassis roll due to the deflection of the elastic components, this results in higher jacking forces.