S197 Suspension: Adjusting Instant Center & Anti-Squat
Whether you have a basic street car, or an all-out race car, BMR has the parts to add the adjustability needed to correct any geometry issues.—Brett Rockey, BMR
Many factors are at play when power is applied to the chassis, but for the sake of this discussion, we are going to focus on rear suspension geometry. Most of us are familiar with the term instant center, but for those who aren’t, the instant center is the intersection point of the upper and lower control arms if both links were extended until they intersected. This point can be changed by raising or lowering the mounting points of the upper or lower control arms.
Changing this point in relation to the car’s center of gravity has a big influence on the chassis, and how power is applied to the tires. The higher the point is, the more aggressive power will be applied. The lower the point is, the softer the hit will be. The factory S197 three-link suspension system does not allow for any instant-center adjustability, but BMR Suspension has some popular components that allow for this.
As you might expect, control arm relocation brackets allow you to change the rear lower control arm mounting location. This is effective for correcting rear suspension geometry on lowered cars when the LCA mount of the body becomes lower than the rearend side. This does a few things. Firstly, it changes the instant center location. Second, it increases the anti-squat percentage.
Anti-squat is the suspension’s mechanical resistance to compression due to forces from the engine. This means when power is applied to the rear suspension, the force of the engine is being applied to the links instead of the body of the car. This causes the rear tires to separate from the body instead of the body squatting over the tires. The benefit to anti-squat with big power Mustangs is that the tires are driven into the ground hard, which can help with traction. The negative side of more anti-squat is that the car will need rear shocks with the ability to control the aggressive axle movement. Without “enough” rear shock, the rear can react so quickly that it will literally bounce the tire off the ground, causing the suspension to unload and spin the tires.
Getting The Upper Hand
Similar to BMR’s lower control arm relocation brackets, its adjustable upper control arm mount features three positions to further increase the level of adjustability in the rear suspension. When combined with the lower control arm brackets you have 12 possible instant center positions. For the average enthusiast, this gives you the ability to correct the rear suspension geometry at almost any level of drop. For the race enthusiast, you have the ability to move the instant center location and run the amount of anti-squat of your choosing to help tune the chassis for hard launches.
Anti-squat is measured using the instant center location and the neutral line. The neutral line is a line from the center of the rear tire contact patch to the center of gravity at the front axle centerline. If the instant center falls on this line, you will have 100-percent anti squat (this is considered neutral). As power is applied to the chassis, the body and tires should not separate, and the body should not squat on the tire; all of the power should be applied to moving the car forward. In a perfect world, this is the most efficient scenario. The higher you get above the neutral line, the more the axle will separate from the body. Inversely, the less percentage of anti-squat, the more the body will squat over the rear tires.
Plotting Instant Center
Plotting your instant center can be a lengthy and tedious process. The car needs to be level and at ride height, preferably with the weight on the tires. Instant center location will differ from car to car based on ride height, tire size, air pressure, and suspension-link location. For the sake of this example, we plotted all of the possible IC locations on BMR’s 2011 Mustang GT with stock-style wheels and tires. The car is equipped with BMR’s Lower Control Arm Relocation Brackets (PN CAB005), Lower Control Arms (PN TCA019), Adjustable Upper Control Arm Mount (PN UCM002), and Adjustable Spherical Upper Control Arm (PN UTCA033).
The effective control arms lengths are measured at 18.25 inches for the lowers and 9.4 inches for the upper. You will also need to measure the center of gravity. Normally, center of gravity height is measured based on the camshaft location, but this is not applicable with overhead cam engines. There are a few different ways to measure center of gravity–but for naturally aspirated cars–we normally go by the center of the water pump pulley on Mod-motor and Coyote cars. For most power-adder applications, we normally recommend measuring from the top edge of the water pump pulley.
Once you have all of your measurements, you’ll need to plot the suspension. There are plenty of instant-center calculators on the Internet, and we used Baseline Suspension’s General IC Calculator. Simply input your measurements, wheelbase, tire diameter, and height of the center of gravity, and it will give you all the pertinent information.
So long as your measurements are accurate, the results will be accurate.
The calculator will give you the instant-center length and height, as well as the anti-squat percentage. With this information, you will be able to make an informed decision about what changes you need to make to improve your launches. It is important to know that not all instant-center and anti-squat settings will work for every setup. Since big changes (to either setting) directly affect how power is applied to the tire and the chassis, it may take some trial-and-error to figure out what will work best for your combination. Tire pressure, along with adjustable shocks, will also need to be changed to optimize any setup.