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Suspension Tuning & Optimization: Why Load Transfer Is Important

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Why Load Transfer Is Important

11.50

00:00 - Understanding what lateral load transfer is and how we can manipulate it is one of the most important concepts to grasp in suspension tuning.
00:08 I hope that after making your way through this section of the course, you'll be in a much better position to do just that.
00:14 What we're going to cover in this section is in many ways the culmination of everything we've talked about in the course, up to this point.
00:21 This is where we start putting together all of the pieces so we can start to do something truly useful with them.
00:28 Let's start by defining what lateral load transfer is.
00:31 When a car is sitting statically on a level surface, say when we're doing our setup on a flat patch, the vertical load on each tyre is largely defined by how the mass is distributed around the car and any corner weighting adjustments we've made.
00:45 When the car is driving in a straight line at constant speed, if we ignore transient loads from bumps and any aerodynamic forces, the vertical load on each tyre will be very similar to what we've measured on the flat patch.
00:57 When we corner, because we have a lateral acceleration acting on the car, the outside tyres end up with a higher vertical load on them than the inside tyres.
01:06 Again, ignoring any aerodynamic downforce effects, if we sum the total vertical load from each corner of the car, it'll exactly match the sum that we had on the flat patch.
01:17 Or driving in a straight line.
01:19 The only difference is how the outside tyres are now taking a greater share.
01:23 The amount of load transfer is a simple function of the vehicle's mass, its centre of gravity height, the track width and the lateral acceleration we have.
01:34 Let's take a look at an example.
01:36 This car weighs 1200 kg and is cornering at 1G.
01:40 The centre of gravity height is 400 mm above the ground and the average track width is 1600 mm.
01:48 Using the numbers from our example, we can see that this results in 300 kg of total load transfer from the inside tyres to the outside tyres.
01:57 For a given lateral acceleration, nothing we do to the setup of the car will change the amount of total lateral load transfer.
02:05 This is a really important point I want you to keep in mind because a lot of people misunderstand it.
02:10 Thinking that we can change the amount of total load transfer by tweaking our setup.
02:15 For a given lateral acceleration, this is physically impossible and breaks the laws of physics.
02:21 What we can do with the setup of the car is vary the proportion of the load transfer that the front and the rear axles take.
02:29 Let's go back to our example and we'll assume that we have a 50/50 weight distribution between the front and rear axles.
02:36 Let's again put the car in a constant cornering state.
02:39 If both the front and rear axles have an even share of the lateral load transfer, each axle would transfer half of the total 300 kg of load transfer.
02:49 So the front axle will transfer 150 kg from the inside front tyre to the outside front tyre.
02:56 And the rear will transfer the same from the inside to the outside.
03:00 The sum of these is of course the same as the total load transfer of 300 kg.
03:05 The convention in lateral load transfer calculations is to express the percentage of the total lateral load transfer that occurs on the front axle.
03:14 Taking our example vehicle, we can calculate this by dividing the front load transfer by the front and rear load transfers added together and multiplying everything by 100.
03:26 Obviously this gives us a percentage of a lateral load transfer of 50% front.
03:31 Let's take a look at another example with the same car.
03:34 Where the front axle instead transfers 165 kg and the rear axle 135 kg.
03:40 Plotting these numbers into the same equation we can see this results in 55%.
03:46 The front axle is accounting for 55% of the total lateral load transfer.
03:51 In a simplified theoretical case of having a 50/50 static weight distribution, if we had the same tyres fitted to the front and the rear and we're cornering on a skid pad in steady state, then a 50% lateral load transfer distribution is ideal.
04:06 In this case we're theoretically maximising the amount of lateral grip o the car by giving each of the outside tyres an equal distribution of vertical load.
04:16 In practice, the actual ideal lateral load transfer distribution which is often shortened to LLTD won't generally be equal to the static weight distribution of the car.
04:27 There are plenty of reasons for this.
04:29 We often don't have the same tyres at the front and rear.
04:32 Non linear tyre behaviour, compliance in the chassis and other components, changing aerodynamic force distribution, and that the car is never actually in a pure lateral steady state condition.
04:44 In reality, how close the total lateral load transfer distribution is to our static weight distribution isn't that important.
04:51 What we should be focused on instead is how much this lateral load transfer distribution changes as we modify this setup.
04:59 It's this difference in lateral load transfer distribution between the two setups we're most interested in rather than the absolute lateral load transfer distribution number compared to our static weight distribution.
05:12 This lateral load transfer distribution is an incredibly useful concept to understand.
05:16 It has a huge bearing on the balance of our car, calculating and tracking it can help you make informed setup decisions rather than guessing.
05:26 But why does this actually change the balance? The primary reason is due to tyre vertical load sensitivity.
05:31 Tyres are a massive subject all of their own.
05:34 In my opinion, they are probably the most complex part of a car and they're not something we're going to look too deeply into in this fundamentals course.
05:42 But vertical load sensitivity is something that we'll need to touch on to understand suspension tuning.
05:48 If we simplify down the lateral forces and look at them in terms of coefficient of friction, as we increase the vertical load, we decrease the coefficient of friction which is what we see here in this plot.
06:00 This is an important point to keep in mind and refer back to.
06:03 Remember when we're cornering, we're transferring vertical load from the inside to the outside tyres.
06:10 This means that the outside tyres will now have a lower coefficient of friction than the inside tyres.
06:16 The actual lateral force the outside tyres are producing is greater than what they were capable of before we transferred the load, simply because they now have more vertical load on them.
06:27 It's just that because we have a lower coefficient of friction of the outside tyres than we had before, doubling the vertical load, doesn't double the lateral grip.
06:35 Let's take another simple example to help illustrate this point.
06:38 We take a car with a 50/50 weight distribution again and assume the car is travelling in a straight line.
06:45 At this point each tyre has a 25% share of the total weight and for simplicity, let's say that we have 100 total units of vertical load so each tyre has 25 units of vertical load on it.
06:59 At this point, our car has 100 total units of vertical load on the tyres and 100 total units of lateral grip potential because right now, each tyre has a coefficient of friction of 1.
07:11 Now we enter a steady state turn at 1G and we have a lateral load transfer distribtion of 50%.
07:17 Let's say for this cornering state, we've transferred 5 units of load from the inside to the outside tyres of each axle.
07:24 So now we have 30 units of load on both outside tyres and 20 units of load on both inside tyres.
07:33 Again the total load of all tyres sums to But because when we increase the vertical load on the outside tyres, the coefficient of friction between the road and the tyre reduces, instead of gaining 5 units of grip on each outside tyre, we only gain 3.
07:50 The inside tyres have their vertical load reduced, means that their coefficient of friction has increased so even though they have lost 5 units of vertical load, they've only lost 4 units of lateral grip.
08:03 So in terms of lateral grip units, we now have 28 on each outside tyre and 21 on each inside tyre.
08:12 The thing to take note of here is that while we transferred 5 units of load, we only gained 3 on each outside tyre and lost 4 on each inside tyre.
08:22 This is because the coefficient of friction of the outside tyres has been reduced and the inside tyres has been inceased.
08:29 The sum of the lateral grip units is 98 so overall, we've lost two total units of grip.
08:37 Another way of putting this, what we've gained on the outside tyres by adding vertical load is less than what we've lost from the inside tyres.
08:46 The takeaway here is that while we've exaggerated the magnitudes involved, load transfer is always bad.
08:53 We always want to minimise the total load transfer that the car experiences because we'll always end up with a net loss.
09:00 The outside tyres gain less than the inside tyres lose.
09:05 The main tool we have for affecting the balance of the car is by changing the lateral load transfer distribution between the front and the rear axles.
09:13 To greatly over simplify things, increasing the lateral load transfer distribution of one end of the car will reduce its lateral grip potential.
09:23 This is because of the vertical load sensitivity of the tyres.
09:27 Again, in an over simplified way, and only looking from the perspective of lateral load transfer distribution, increasing the front or reducing the rear lateral load transfer distribution will tend to increase understeer.
09:40 As you'd probably guessed, the opposite is also true.
09:44 Reducing the front lateral load transfer distribution or increasing the rear will lead to more oversteer.
09:51 In the following modules in this section we'll be looking at the different mechanisms by which we can vary the lateral load transfer distribution.
09:59 Through our springs, antiroll bars and suspension geometry.
10:03 Just as importantly in the practical skills section, we'll give you the tools you need to be able to calculate the lateral load transfer distribution of your car for yourself.
10:12 Remember the only way we can affect the total lateral load transfer distribution is the terms in this simple equation.
10:20 For a given lateral acceleration, to minimise total load transfer, we need to reduce the mass, lower the centre of gravity, or increase the track width.
10:29 Nothing else you do with the suspension setup will implement the total lateral load transfer.
10:35 These are our fundamental goals.
10:37 While we should always strive to reduce the total lateral load transfer, factors such as build practicality, budget and rules and regulations generally come into play in optimising and choosing these.
10:49 One last point to clarify here, is that everything we discuss in this course related to lateral load transfer is for steady state cases.
10:56 We already understand that steady state is something we rarely have when we're running on track.
11:02 But we need to do it this way in order to keep things simple.
11:05 WIth that said, in my experience, these steady state concepts are still extremely useful to understand when tuning a car on track.
11:12 So in summary, the total lateral load transfer is governed by the lateral acceleration, the vehicle mass, the centre of gravity height and the track width.
11:22 Nothing we do to the suspension setup changes the amount of total lateral load transfer for a given lateral acceleration.
11:30 Lateral load transfer is always a bad thing for grip.
11:33 So we should do everything we can to minimise it within practical limits.
11:38 Changing the lateral load transfer distribution between the front and the rear axles is a critical part of tuning your vehicle's steady state balance.