Load transfer is important in circuit driving, right?
Although its importance is widely recognized, both from a driving and mechanical perspective, I think relatively few people have actually calculated how the load on each individual wheel changes as a result of load transfer.
Compared with something like calculating damper stroke at 1G, calculating the amount of load transfer is quite complicated.
That’s because there are vastly more factors involved.
But load transfer is at the heart of performance driving. If you can understand what factors are involved and how they interact with one another, it can deepen your understanding of both driving and vehicle dynamics.
One way to approach this is to use a simplified model that leaves out some of the more realistic factors involved in calculating load transfer, such as the movement of the center of gravity and roll centers as the vehicle rolls, and focus on understanding the basic mechanisms.
Even the equations for a simplified model are fairly complicated, but I thought it might still be useful to get a feel for the trends in the calculated results without getting into the equations themselves. So I made a simple load-transfer calculation tool in Excel.
Simple Load Transfer Calculation Tool
The purpose of this tool is to help you understand how the various factors involved in load transfer interact with one another by limiting them to the basic elements.
It’s only a simplified model, so the values calculated by this tool are by no means exact.
Instead, it’s meant to help you understand trends, such as how changing the angle of the roll axis affects how much load each tire gains or loses, or what happens when the distance between the center of gravity and the roll axis becomes shorter.
Several values are already set as initial conditions.
Try raising or lowering the front and rear roll center heights by the same amount.
You should be able to see that as the roll axis gets closer to the center of gravity, the amount of load transfer becomes more strongly influenced by the front-to-rear weight distribution. Conversely, as the roll axis gets closer to the ground, the influence of the front-to-rear roll stiffness balance becomes greater.
For example, imagine the following situation.
A car has a 50:50 front-to-rear weight distribution, with very high front roll stiffness but very low rear roll stiffness.
As you progressively lower the roll axis of this car toward the ground, the handling balance shifts toward understeer.
In other words, the amount of load transfer at the front becomes greater, so more load is taken off the inside tire and more load is transferred to the outside tire.
At the rear, on the other hand, there is less load transfer than at the front. Therefore, less load is taken off the inside tire, and less load is added to the outside tire.
As a result, because of the load sensitivity of tires, the total grip available from the two rear tires decreases only slightly, while the total grip available from the two front tires decreases much more.
Now, if you raise the roll axis that you just lowered toward the ground, bringing it back toward the center of gravity, the handling balance that shifted toward understeer gradually returns toward its original state.
The way load transfer changes the front-to-rear grip balance reverses, and the understeer is reduced.
And when you raise the roll axis until the center of gravity lies on the roll axis, the car no longer rolls under lateral G in this simplified model, and the amount of load transfer becomes equal at the front and rear.
Real cars don’t behave exactly like this because of things like deflection in the body and bushings, movement of the center of gravity and roll axis as the car rolls, and changes in alignment.
However, by adding these factors to the basic trends, you can start to see the bigger picture of the forces acting on a car. That’s why it’s important to understand the fundamental factors first.
I’m not going to explain the actual equations here, but looking at the equations makes it clear which factors are proportional to one another and which factors work against each other, so understanding the equations is important, too.
Equations can look intimidating at first because they’re just a bunch of symbols, and they may seem difficult to get into. But an equation is simply a way of expressing how the different factors relate to one another using symbols. Once you understand that, they’re really not that difficult.
I hope more people can make sense of what the equations actually mean, too, so I’d like to explain them in an easy-to-understand way someday.
Someday. LOL