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Practical Corner Weighting: Cross Weight Percentage

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Cross Weight Percentage

04.09

00:00 - As far as corner weighting goes, the cross weight percentage is usually considered to be the most important aspect.
00:06 As we've already discussed, the driver wants to be confident that the car will handle the same for both left hand and right hand corners and targeting a 50% cross weight will help ensure this.
00:17 Let's start by considering what happens as the car moves through the cornering sequence.
00:22 As you approach the turn in point to a corner, you'll be braking hard and this results in a weight transfer onto the front tyres.
00:29 When you then turn into the corner the weight is transferred across to the tyre on the outside of the corner.
00:35 As the weight transfers in this way, the opposite rear corner of the car will become light due to the weight transfer and the pitch of the car.
00:43 With this in mind, the cross weight percentage is critical as it's actually the two wheels diagonally opposite from each other that work in unison.
00:52 To demonstrate this, let's look at our wobbly table example.
00:56 Let's assume we have one corner of the car that's slightly lighter than the others.
01:01 We can show this as one of the legs in the table being shorter than the rest.
01:05 Now notice how the table rocks and the opposite corner is lifted.
01:10 The exact same thing happens with the car however given the compression of the suspension, we'll still end up with all four wheels touching the ground.
01:18 Even though the four wheels are touching the ground in our car thanks to the action of the damper and spring, the weight being applied to the ground at each corner will be affected.
01:28 The way you work out your cross weight percentage is similar to the previous module.
01:32 You want to calculate the combined weight of the front right tyre and the left rear tyre.
01:37 It's always these two wheels that we use for our cross weight calculation.
01:43 After you add these together, you again divide by the total weight of the car to get a percentage value.
01:49 Cross weight percentage can also be referred to as wedge, where a positive wedge would be a cross weight percentage above 50% and negative wedge would be the opposite.
01:59 Wedge is a term that's commonly used in circle track racing.
02:03 As I mentioned in the last module, most corner weight scales will automatically calculate the cross weight for you so you probably don't need to do this manually.
02:12 It's important however that you do note down your corner weights at each step of the process and you can do this with the setup sheet that we've attached to the bottom of this module.
02:22 When you want to make adjustments to your cross weight, there are two ways that you can do this.
02:26 The first option is to move weight around the car or add ballast as we've already discussed, and the second way is by adjusting the ride height on individual corners of the vehicle.
02:36 What you need to understand is that if we move weight from one corner, it will also have an effect on the other three corners of the car.
02:43 When adjusting your ride height or the ballast position, this can be an iterative and time consuming process in order to find the optimal settings.
02:51 Every car will respond differently to ride heigh and it could be more or less sensitive to changes than a different car.
02:58 Like with the left weight percentage, the target here for circuit racing applications is generally a 50% cross weight percentage and this split is more important than the left weight percentage because it will affect both the side to side weight transfer as well as the front to back weight transfer so overall this is where we want to focus most of our effort.
03:19 Again, like with the left and rear weight percentages, the cross weight percentage is not so clear cut with oval racing cars because of the nature of the tracks.
03:28 This is where it would be more common to hear somebody talking about a positive wedge where the car is only needed to turn in one direction and the cars can therefore run more positive or negative wedge to help compensate for the direction of the circuit and optimise the cornering.
03:44 When adjusting your cross weight percentage, you will certainly need some amount of patience as it is an iterative trial and error process.
03:51 You will quickly find out however how sensitive your particular car is to a change and how much adjusting one corner will affect the others.