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Motorsport Wheel Alignment: Ride Height

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Ride Height

04.27

00:00 - One of the first modifications we're likely to make to a production road car when we're setting it up for competition is to lower the ride height.
00:07 The intention with lowering a car's ride height is typically to lower the centre of gravity which can help improve the ability of the car to corner faster.
00:15 A moderate reduction in ride height can achieve this aim, offering improved performance with few compromises.
00:21 Often however, we see cars built by enthusiasts that are lowered excessively in the name of aesthetics rather than performance and this can actually make the car perform much worse in some cases and the car could even become dangerous to drive.
00:36 Much like every concept we've discussed so far, ride height is a compromise and we need to understand how the ride height we choose will affect the operation of the suspension system.
00:46 One of the biggest concerns is maintaining sufficient suspension travel as lowering a car excessively can have the result of sitting the suspension on or very close to the bump stops.
00:57 The bump stops can be considered to be the suspension's last line of defence and they're there to prevent damage from the dampers bottoming or your tyres or suspension arms physically contacting the chassis under full compression.
01:10 The problem is that when we run onto the bump stops the spring rate on that corner of the car sharply climbs towards infinity and this can cause a sudden loss of traction and unpredictable handling.
01:21 That's not to say that we should never allow the suspension to contact the bump stops, however we do need to maintain sufficient suspension travel to allow the suspension to do its job first.
01:32 There are other fundamental issues with lowering a car excessively as well since the factory suspension geometry may now become less than desirable.
01:41 To illustrate this, let's take an example of a common MacPherson strut front suspension design.
01:47 It's quite common on a stock production car to find that the lower control arm is either parallel to the ground at the stock ride height.
01:53 Or potentially even angled downwards from the chassis mounting point towards the wheel.
01:59 This promotes negative camber gain as the suspension compresses which is exactly what we want.
02:05 On the other hand though, if we lower the car to the point where the angle between the lower control arm and the strut is greater than 90 degrees, now as the suspension moves into compression, we see the wheel move towards positive camber instead.
02:18 As a general rule, if the lower control arm is pointing upwards from the chassis towards the wheel, then the geometry is less than ideal and the ride height may be too low.
02:27 We also need to consider the effect of lowering the car on the relationship between the roll centre and the centre of gravity which we covered in the last module.
02:36 Essentially as the roll centre and the centre of gravity get further apart, the roll moment increases and the car will tend to roll in a corner more, which is something we don't want.
02:47 More body roll in a corner, coupled with a lower ride height that may compromise suspension travel and compression, is less than ideal.
02:54 Another consideration with excessively lowered ride height is bump steer.
02:58 As we've discussed in the bump steer module, most production cars will have some amount of bump steer due to various design compromises, however this bump steer is usually minimal at the stock ride height and gets progressively worse as we lower the ride height.
03:12 Naturally, as we'll see further into the course, there are aftermarket parts available that address some of these issues and in some purpose built racecars, significant design changes are made to the suspension geometry and mounting points, in order to allow a low ride height without compromising the suspension operation.
03:30 The next question that we need to consider is, how do we measure ride height? This is actually not as straightforward as it might sound as there are a wide variety of places we can make that measurement which may include the sill panels, cross members or chassis rails.
03:45 In tightly controlled racing classes which specify a minimum ride height, the rule book will tell you where the ride height must be measured so you know exactly what to do.
03:55 If there is no specification then it's really up to you.
03:58 And in reality it doesn't really matter where you're making your measurements from, as long as you use the same point each time.
04:05 Common sense would suggest that you use a solid or rigid mounting point on the car such as a sill or the chassis rail, that's unlikely to move from one setup session to another.