Anti-Squat Calculator

In rear-wheel drive vehicles, heavy acceleration shifts weight heavily to the rear axle, collapsing the rear suspension. This is called "squat" and consumes the suspension travel required for absorbing irregularities. It also changes the balance of the vehicle during power transfer. Anti-squat geometry compensates for this by properly aligning the trailing arms on a rear suspension so that the driving force itself pulls the vehicle up when accelerating, preventing the rear end from sagging down.

This calculator allows you to calculate the amount of compensation for squat. By entering the anti-squat line angle, the distance from the rear axle to the center of gravity and the height of the center of gravity a percentage can be calculated. Conversely it is possible to enter a target percentage and leave one parameter of the geometry free which will then be calculated by the calculator.

What is anti-squat?

Anti-squat is a property of the geometry of the rear suspension that acts independently of springs or motor. When a vehicle with rear wheel drive accelerates, the reaction force acting on the contact patch creates a moment which tries to rotate the rear end downwards. By orienting the control arms in the right direction some of this load will not be absorbed by the spring but transmitted as an upward force to the vehicle.

The angle of this line, viewed from the side, determines the effect. The line runs from the contact area of the rear tire to the pivot point of the suspension. The pivot point is a virtual anchor point around which the rocker arms rotate when looking at the vehicle from the side. The steeper the line, the greater the anti-squat effect. This value indicates what percentage of the squat is absorbed by the geometry.

A value of zero means that the geometry has no effect and all weight transfer is absorbed by the springs. A value of one hundred means that the rear end will remain level during acceleration. Values over 100% mean that the geometry creates a lift effect, and the rear actually rises under acceleration. Drivers in drag racing intentionally use this effect to press the tires into the pavement at launch.

Instructions for using the calculator tool:

Enter three of the four fields and the calculator will automatically calculate the remaining value. Typically the anti-squat line angle, wheel to CG distance, and CG height are entered to find the anti-squat percentage. Each field has a units menu so you can mix metric and imperial or switch between degrees and radians for angles.

If you are starting with a target value, enter the desired anti-squat percentage and leave one of the geometry input fields blank. If you leave an input field blank for which you want to calculate a value, the calculation tool will output the corresponding angle, distance or center of gravity height that produces this value.

Anti-squat formula:

This percentage is determined by a simple relationship between three measured values.

Here the Greek letter theta represents the angle of the anti-squat link, d is the horizontal distance from the rear wheel to the center of gravity and h is the height of the center of gravity. The tangent of the angle is the slope of a line that passes through the moment point. Multiplying this value by the distance gives the upward force generated by the geometry. Dividing this force by the height of the center of gravity allows it to be compared with the vehicle's tendency to lift up.

Let's calculate this for a real vehicle. Let's say the center of gravity is at 500mm, the angle of the bar is 15 degrees and the distance from the rear wheel to the center of gravity is 1300 mm.

The tangent of 15 degrees is approximately 0.2679. Multiply this by a distance of 1300 mm, divide by a height of 500 mm and multiply by 100 at last.

So this geometry compensates for the sag when accelerating by about 70%. The larger the angle of the bar and the larger the distance is, the higher this value. Conversely, the value decreases if the center of gravity position in the denominator is higher. Since distance and height form a ratio, the units cancel out. If you use the same units, you get the same result whether you calculate in inches or millimeters.

The importance of percentage:

There is not just one "correct" anti-squat value but a range that is suitable for the respective vehicle and purpose of use. The following table shows roughly the typical behavior in the respective areas:

Drag racing cars often have values over 100% as they need to press the rear tires into the ground at launch for traction. Race cars usually use values between 50 and 80%, sacrificing some traction off the line in order to get a rear end that can better absorb bumps going around corners. Off-road and desert racing vehicles have even lower values, sometimes well below 50% so that suspension can work freely over rough terrain.

The effects of anti-squatting:

The function of the anti-squat is not limited to stabilizing the vehicle's tail end. It supports the tail during power transfer and keeps the contact surface pressure on the rear tires constant, thus maintaining a flat contact surface. This is especially useful when accelerating out of curves, where traction is needed. If the anti-squat effect is too low, the tail sinks in, the roll values change and the vehicle goes through to the end of the shock absorbers. If it's too strong, the tail becomes stiff during driving and can spring up when passing over unevenness, even if the gas pedal is held open.

It is important to note that the calculated value only applies for a certain position within the spring travel. As the suspension arms change angle and the pivot point moves, anti-squat effectiveness will typically decrease in the later part of the spring travel. So this should be considered more as a starting guide rather than an absolute property. Racing teams often plot curves showing how anti-squat changes over the entire spring travel.

The anti-squat effect also has aspects that have nothing to do with the spring itself. A common mistake is to use stiffer springs or stronger dampers in an attempt to reduce squatting. While this will change the rate at which the vehicle squats, it does not affect the degree to which the geometry resists squatting. Since the anti-squat effect is determined by the angles and positions of the suspension arms, you must alter the geometry to affect it.

Anti-squat, anti-dive, anti-lift.

The anti-lift effect can help maintain front wheel contact with the ground on acceleration with front or all-wheel drive vehicles.

Measurement of input values.

Measure the angle of the axle. Extend the rear suspension arms to cross and find the instantaneous center of rotation when looking at the suspension from the side. Draw a line from the contact area of the rear tire to this point, and measure the angle between that line and the ground. For a single wishbone, the instantaneous center of rotation is where the pivot pin for the arm attaches to the vehicle frame, so the axle angle is the same as the angle of the arm itself. In a CAD environment this task can be done on a drawing while in the car it will be determined by measuring the attachment position.

The wheelbase to CG distance is the horizontal distance between the rear axle and the center of gravity (CG) and for most vehicles will be slightly more than half the wheelbase length. The CG location is a parameter often intuitively estimated but since it appears in the denominator of many calculations, it's worth measuring accurately. A common method is to place the vehicle on a four-wheel scale, raise one end by a known amount, and read off the weight distribution. Measure with actual specifications including fuel, driver, and ballast in the vehicle.