Over the years, racers have...
Over the years, racers have discovered that applying the effects of antisquat (A/S) improves traction, but not in the way we have assumed. Once we study how A/S works in our race cars, we can better understand how to properly apply the effect.
There are many dynamic effects related to chassis setup that have been discovered and applied over the years to improve the way our cars perform. Experienced crew chiefs know which effects cause which results. We need to know more about how these various effects work. Antisquat (A/S) is one of those mechanical tools that can add traction off the corners when used properly.
A/S is associated mostly with three-link rear suspension systems in which the upper third link is mounted at an angle to the ground with the frontend of the link attached lower than the rear Heim. There is also A/S effect from lift arm devices as well as, to a limited degree, the truck arm suspension systems. We will illustrate how the effect works using the three-link system.
Upon acceleration, the rear pinion gear tends to climb the ring gear in the differential, and this causes the rearend to rotate in a clockwise direction when viewed from the left side. There is a considerable force applied to the third link of the three-link system upon acceleration that pulls on the bar, trying to stretch it. This is the origin of the term pull bars, which is commonly used to refer to links that have torque absorbing springs or a rubber disk.
There are usually several...
There are usually several holes in the brackets for the upper and lower control arms so that the links can be mounted at different heights. The farther apart the upper and lower links are, the less mechanical leverage there is, and therefore the less antisquat effect. When moving the links, make sure you do not change the angles, and always move the front mounts in the same direction and distance.
The lower trailing arms have a small effect on A/S. They are usually mounted fairly level, up to two or three degrees, and are in compression during acceleration. Because the angle of these links is critical to rear steer characteristics and therefore not considered an adjustable item, we only speak of regulating the angle of the third link to adjust the amount of A/S in our cars.
It has been assumed over the years that by applying A/S to rear suspensions, more weight is applied to the rear tires by the mechanical effect produced through the third link. This is not true. The mechanical effect of A/S causes a portion of the load transfer associated with acceleration to be applied to the third link instead of the rear springs. This means we exchange some of the transferred load off the rear springs and onto the rearend where the link is attached. Since this is an equal tradeoff, there is no more load on the rear tires than before.
The statement that for every action (the A/S forces applying a vertical load to the rearend) there is an equal and opposite reaction (the removal of load from the rear springs) means that there cannot be additional load on the rear tires. This is understandable, because where would the added load come from? In the mechanical world, we cannot receive additional load without taking the same amount of load from somewhere else. So we receive the load from the action of the A/S, and it is taken from the rear springs, to ultimately find itself on the rear tires.
Let's look at it another way. If we had zero A/S effect, the car would transfer a certain load or weight onto the rear springs as we accelerated, the springs would compress to carry the added load, and the car would squat. If we applied 100 percent A/S (meaning the effect would prevent the car from squatting at all upon acceleration), then the springs would not have compressed and therefore would not carry any more load. The load that would have been carried by the springs is now applied to the rearend through the third link because it has to go somewhere.
There are several ways to adjust the magnitude of the force applied to the rearend through the third link. One is the angle of the third link, and another is the distance between the upper link and the lower links. The greater the distance between upper and lower mounts, the less mechanical leverage the system has, and therefore the less A/S effect.
The angle of the third (or...
The angle of the third (or upper) link helps determine the amount of antisquat. The greater the angle put into the bar, the more A/S effect. The addition of a pull bar does not change the A/S effect. It only cushions the application of the engine torque to the rear wheels.
Adding angle to the third link will increase the A/S effect, but this has its limits because the effect is reversed when we decelerate and brake into the corners. Instead of transferring load onto the rear tires, the reverse means that we reduce load on the rear tires when entering the turns, which can make the car very loose. Usually a 6-8 degree angle is sufficient to cause the desired effect without being excessive. If more A/S effect is needed, lower the third link and/or raise the lower trailing arms while maintaining the angles of all links.
If the third link is positioned laterally halfway between the centers of the rear tires, then the amount of load being applied to the rearend will be equally distributed between the rear tires upon acceleration. They will gain vertical load in equal proportion.
If the third link is moved laterally and closer to one side of the car, more of the vertical load will be applied to the closer tire. In this way, a greater load can be applied to one of the rear tires upon acceleration if that is a desired goal.
So, if the overall load on the rear tires by using A/S is no more than it would be without using it, how does the car gain traction by using A/S? Past experience indicates the car definitely can benefit from A/S to get better bite off the corners, so how does that work?
The amount of weight, or load, transfer that occurs during acceleration is determined by three things: 1. the magnitude of the accelerating force, which cannot necessarily be changed with the same motor/gear combination; 2. the length of the wheelbase-again not a variable; 3. the height of the center of gravity of the car. The last item is influenced by A/S, and load transfer is affected by it.
A higher center of gravity means more weight is transferred during acceleration off the corners. If the car squats during acceleration, the rear becomes lower, and the overall center of gravity is then lower in the car. A lower center of gravity means less weight transfer and less vertical load that is applied to the rear tires upon acceleration. By using A/S to keep the rear of the car from squatting, the center of gravity remains higher, and therefore more weight will be transferred to the rear tires.
Another way to increase rear load is through the aero effect of keeping the rear of the car higher and, along with it, the rear spoiler. A higher spoiler usually means a more efficient flow of air over it and more rear aero downforce.
Antisquat helps the car gain more vertical load on the rear tires, not from its own mechanical effect, but by helping to keep the rear of the car higher. Moving the third link laterally will allow you to determine where the added load will be applied between the rear tires.
The following are methods of further loading the left-rear (LR) tire upon acceleration: Use rear spring split with a softer right-rear (RR) spring; use a shock on the LR that has a higher compression resistance than the RR shock; and, as stated above, move the third link to the left.
The ideal load distribution on the rear tires for maximum traction off the corners is to end up with an equal load on each tire during acceleration. If the RR tire has more vertical load due to the weight transfer from the centrifugal forces that move weight from the left side of the car to the right side, there is a definite need to cause a shift in weight from the RR tire to LR tire upon acceleration to equalize the load on the rear tires.
If the rear tires are made to be almost equally loaded by using the spring split method, then the LR tire may be overloaded by moving the third link to the left, mistakenly ending up with unequally loaded rear tires and less traction. Be careful how far you go to try to get more traction-you might be going backward at some point in the process. Antisquat is just one of the ways to fine-tune your setup after you have processed all of the basic elements of a balanced chassis.