Anti-squat is a term describing how a car's rear suspension theoretically behaves during acceleration, and it is probably the most important thing that most modern drag racing rear suspensions are designed to exploit. Anti-squat is normally discussed in terms of percent, where 100 percent anti-squat means the vehicle's rear won't raise or lower due to acceleration effects. Less than 100 percent means the car will squat, potentially unloading the rear tires. More than 100 percent means the rear will rise, thus loading the rear tires.
For acceleration, loading the tires is good, and so more than 100 percent anti-squat is usually desirable for drag racing. As always, nothing comes free: high anti-squat will reduce rear braking ability since as the forces reverse during braking, the opposite effects occur. For road racing or street use, a better number is about 60 percent anti-squat; it's a good compromise between ultimate acceleration and ultimate braking.
Unfair's three-link setup is about 60 percent anti-squat, and the four-link is adjustable to allow changes in anti-squat and pre-load. The calculation itself is based on the lever length of the control arms divided by the lever length of the force generated by the tires, but it's a lot easier to envision if you make a line drawing of the car from a side view (see attached figure). As we've seen in our previous articles, the vehicle's center of gravity is very important when it comes to modeling its physical behavior, and figuring out anti-squat is no exception. Referring to the line drawing, first draw a horizontal line from the CG intersecting with a vertical line through the front tire's center point. From that intersection, draw a line through the rear tire's contact patch. This line is the 100 percent anti-squat line (sometimes called the "neutral line").
Now extend the virtual line for the linkage bars for the rear suspension forward until they intersect. That point is the instant center (IC) and from that IC we can calculate the anti-squat value. If the IC is below the 100 percent anti-squat line, we have less than 100 percent anti-squat. Above the line is over 100 percent anti-squat. The actual value is the quotient of the IC height divided by the 100 percent anti-squat height directly above or below the IC. If the IC were half as high, the anti-squat would be 50 percent. Twice as high would be 200 percent anti-squat.
With this understanding, we can see why drag racing four-links have so many adjustment holes. It's to change the IC to precisely dial in the right amount of anti-squat. Theoretically, it would be possible to do the same thing with the three-link since the IC calculations are the same. In practice, though, that doesn't work very well for a couple of reasons. First, the upper link runs directly over the driveshaft and is connected on top of the housing. Making that link be adjustable is difficult since space is very constrained. The second reason is that having two upper links gives the ability to preload the rear suspension. Many racers have found that pre-loading more weight on the right tire (by shortening the upper passenger side link) helps the car go straighter off the line, especially as horsepower goes up.
II Much Fabrication
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