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Suspension Tuning & Optimization: Total Lateral Load Transfer Distribution & Tuning

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Total Lateral Load Transfer Distribution & Tuning


00:00 - So far in this section of the course, we've discussed why lateral load transfer is important, how each of the components of lateral load transfer work and how they each contribute towards the total.
00:12 In this module, we'll look to bring together all of the contributions as a single system and discuss how to bring it all together in order to tune the steady state balance of your chassis.
00:22 We've largely been discussing lateral load transfer in terms of individual axles.
00:27 In the real world, we know we have an axle at each end of the car.
00:31 The end goal here is to vary the proportion of lateral load transfer between the front and the rear axles in order to change the balance of the car.
00:40 As we talked about in the first module of this section, it's the vertical load sensitivity of the tyres that makes varying the lateral load transfer distribution a useful balance tool.
00:50 From the simplistic perspective of considering basic lateral load transfer theory only, the end of the car that we increase the lateral load transfer on will end up with less cornering capacity than it had before as a result.
01:04 If we want to decrease steady state understeer, we can either increase the rear or decrease the front lateral load transfer distribution.
01:11 If we instead want the steady state balance from oversteer towards understeer, we can either decrease the rear or increase the front lateral load transfer distribution.
01:21 An important takeaway here is that we can use the opposite end of the car to improve the relative performance of the front or rear respectively.
01:30 This can be just as effective as making an adjustment on the end of the car that we have the problem with.
01:36 So let's look at some of the ways we can tune the lateral load transfer distribution.
01:41 Even though the unsprung mass is absolutely part of the lateral load transfer, we've already established that this isn't a practical tuning tool.
01:49 Because changing it requires either a redesign or upgrading of components.
01:54 So this is more useful when building the car rather than tuning it in the pits.
01:58 Before moving on, it's also worth nothing that in most cases, the total contribution towards the lateral load transfer of the unsprung mass is relatively small.
02:08 We know we make use of our elastic components to tune the balance.
02:12 In most cases, this will be with springs and anti roll bars.
02:15 We can increase the lateral load transfer at one end up stiffening one or both of these components.
02:23 While they work together in concert, we've already established that they work in quite separate ways.
02:29 We can make use of the fact that the springs and anti roll bars are partially decoupled from each other.
02:35 For example, if we're wanting to decrease the front roll resistance, and therefore the lateral load transfer distribution on the front axle, we could do this with either a softer anti roll bar or a softer front spring.
02:47 Let's say that we don't want the front to dive any more than it currently does under braking because we're already wearing out the front splitter in the braking zones.
02:54 This might be a reason to soften the front anti roll bar and leave the springs as they are as the anti roll bar has no effect on how much the front dives under brakes when we're travelling in a straight line.
03:06 In most situations, changes to the elastic load transfer makes the biggest difference of the 3 contributions we've discussed.
03:13 For this reason, the elastic load transfer is often one of the most effective tools we have for tuning our balance.
03:20 We also know from the previous module that we can affect the lateral load transfer distribution by modifying our geometry to change the position of the roll centres.
03:29 Increasing the roll centre height at one axle will increase the geometric lateral load transfer on that axle, and lowering it will do the opposite.
03:37 This means that if we have mid corner oversteer, we could either lower the rear roll centre or raise the front.
03:43 The geometric load transfer contribution isn't usually a large percentage of the total.
03:48 In most practical situations, it'll be significantly less than the elastic load transfer.
03:53 While not insignificant, this can make it a slightly less powerful tuning tool relative to elastic changes in practical terms.
04:02 In practice, the most common balance tuning tool on track is the anti roll bar.
04:06 This doesn't necessarily mean it's the best tool we have available, it's more that it's usually the quickest to change.
04:13 For example, while springs can be just as effective as anti roll bars in making balance changes, this usually requires removing the dampers to make a spring change.
04:22 Unless you're at an open test day, there's often not the time during practice sessions to make this change, meaning you'll usually wait until between sessions to do this.
04:31 Making the anti roll bar a more likely candidate for change when you're actually in a practice or test session.
04:38 It's a similar story for geometric changes, if we're going to make a roll centre change, in most cars with independent suspension this will require a relatively major rearrangement of the pick up points.
04:49 It's certainly no unheard of to make roll centre changes during a session, it just tends to make it less likely.
04:55 An exception to this is with cars that are designed to make quick roll centre changes possible but this really needs to be designed into the car from the start.
05:04 In most cases, you'll want to check the wheel alignment after making a roll centre change because we've changed the pick up points which again takes up more time by needing to go back onto the setup patch.
05:15 An important point to make regarding the roll centres we've been discussing throughout this course, is that largely we've been considering them as static.
05:23 In reality they'll be moving both vertically and laterally as the suspension moves.
05:28 This is what is often termed roll centre migration.
05:32 The migration behaviour of the roll centre is a function of the fundamental suspension design.
05:37 As such, we aren't going to spend much time on it here on this course other than to mention it exists.
05:42 Ideally we want to keep the roll moment at each end of the car as constant as possible.
05:48 And to do this we need to keep the roll centre as well controlled as possible.
05:53 If the roll moment is changing as we corner, this will cause the relative contributions of the geometric and elastic load transfers to change which will almost certainly lead to unpredictable handling.
06:04 This is made even worse when the front and rear rollment is changing at different rates throughout a cornering sequence.
06:11 In this situation we can have the lateral load transfer distribution changing drastically throughout a corner leading to variable balance.
06:19 This is never a good thing and will make it almost impossible to extract decent performance out of the car.
06:25 One important distinction in the way elastic and geometric load transfers behave is that elastic is more transient than geometric.
06:33 This is because geometric load transfer all happens essentially through rigid components.
06:38 The suspension arms, joints and chassis.
06:41 Because of this, there's essentially no lag in the load transfer.
06:44 For elastic load transfer, it takes a certain amount of time for the movement of each component to deflect and reach its final settling point.
06:52 Dampers also fall into this category although we won't be discussing transient load transfer a lot in this course as adding transient effects increases the complexity of the topic significantly.
07:04 The effectiveness of using lateral load transfer distribution to tune the balance will rest largely on the load sensitivity of the tyres we're using.
07:12 This is something that can vary considerably between brands and constructions.
07:16 A less load sensitive tyre will need a larger load transfer distribution split front to rear for a given balance change relative to a more load sensitive tyre.
07:26 Fully quantifying the load sensitivity of a tyre requires a lot of time and specialist equipment.
07:31 While it can be useful to understand this load sensitivity, it's not actually required to do any meaningful tuning.
07:38 It's just important to understand that some tyres have a higher load sensitivity than others.
07:44 Fitting a much larger or wider tyre to a car than was originally intended is another way vertical load sensitivity is changed.
07:52 If a tyre is more lightly loaded than what it was designed for, it'll tend to have less vertical load sensitivity.
07:59 Meaning the chassis will tend to be less responsive to balance tuning.
08:03 In this case, fitting a smaller or narrower tyre may actually increase the tunability of the car in some cases by increasing the vertical load sensitivity.
08:12 As you've no doubt already picked up, the concept of lateral load transfer distribution is a large one and in this course, we're only dealing with the basic elements of steady state load transfer.
08:23 Ultimately, even though we use a lot of simplifications here, these are fundamentally the same tools and concepts I make use of every day in my work as a race engineer.
08:33 In the following practical skills section, we're going to show you a simple spreadsheet based tool that with a little bit of upfront initial work from your side will allow you to calculate the lateral load transfer distribution of your car for yourself.
08:47 We'll be making the spreadsheet available for you to download too.
08:50 There are some situations where tuning the chassis balance with the concept of lateral load transfer alone may lead you in the wrong direction.
08:57 Let's take the situation where you're suffering from mid corner understeer.
09:01 Based on what we've discussed so far, you may make the decision to soften the front springs or anti roll bar which is a reasonable thing to try.
09:09 Let's say that the real underlying reason for the understeer is actually because of a lack of front camber.
09:15 The outside front tyre is rolling over into some amount of positive camber.
09:20 Now let's say we've softened the front anti roll bar to try to cure the understeer.
09:24 While we've reduced the load transfer, we've also increased the front roll angle.
09:30 In this case, softening the front axle may actually make the understeer worse by allowing the outside front tyre to get an even larger positive camber angle with the increased roll angle.
09:41 Counter intuitively, stiffening the front axle may actually increase our front grip by reducing the amount of positive camber we have on the outside front tyre.
09:51 To properly understand a situation like this, you should be making use of tyre wear and tyre pyrometer measurements on track which is something we cover in our motorsport wheel alignment fundamentals course.
10:02 Using the driver feedback in conjunction with reading this tyre data, can help you understand the problem.
10:08 To some extent, a situation like this is also very dependent on the type of tyres we're using.
10:13 It will depend on the vertical load sensitivity versus the camber sensitivity of the tyres.
10:19 The reason I bring up this example is not to say that lateral load transfer distribution isn't always useful, more that we always need to remember to look at the chassis performance and behaviour as a whole.
10:31 This being a suspension course, we haven't talked too much about aerodynamics so far however a brief comment on it when discussing balance is worthwhile.
10:39 How the downforce is distributed between the front and the rear axles will also affect the balance of the car.
10:46 A useful way to think about this is in terms of the centre of pressure.
10:49 Having the downforce concentrated on the front axle will lead to a more oversteer balance.
10:54 And shifting it to the rear will lead to a more understeer balance.
10:59 One way to determine whether you're dealing with a mechanical or aerodynamic balance issue is to look at the balance in slow versus fast corners.
11:07 in slow corners, say less than 50 mph or around 80 km/h, the aerodynamic forces will be largely indignificant in most cars.
11:17 By understanding and recording the balance in slow corners we can let this guide us on the mechanical balance.
11:24 If the balance in faster corners is different to that of slower corners, this could be an indication of an aerodynamic balance issue that needs addressing.
11:31 Clearly, the mechanical balance will also have an effect on the high speed balance.
11:36 But by looking at the different speed ranges we can start to decouple their relative effects.
11:42 In summary, we should always be thinking of the car as a whole in terms of a pair of axles.
11:47 We can make use of both elastic and geometric load transfers to tune the balance of the chassis.
11:54 Even though we've been considering lateral load transfer from a relatively simplistic steady state perspective and not considering the roll centre migration, this is still an extremely useful tool for tuning your car.
12:06 There is a wide spread belief that reducing the amount of chassis roll reduces the amount of load tranfer.
12:12 Hopefully at this point in the course you understand that idea is completely false.
12:17 You should now understand the total load transfer distribution is a function of lateral G force, centre of gravity height, mass and track width.
12:26 You can not change the total lateral load transfer without changing at least one of these parameters.
12:33 Equally, from looking at the different components of sprung mass load transfer, we understand that we can affect the roll angle and load transfer by either changing the roll centre height or the elastic stiffness.
12:44 Neither of these reduce the total lateral load transfer, they just affect the distribution between the front and rear axles.
12:53 And that by doing that we're exploiting the vertical load sensitivity of the tyres to our advantage.

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