What Is a Torque Arm?

Lori Kilchermann

The torque arm is a suspension component that mounts on a rear-wheel drive vehicle's rear-drive axle. This arm allows the vehicle to accelerate in a straight line without rotating the rear axle. This arm also assists the vehicle in braking by applying force to the braking system. The torque arm is primarily used in what designers call a three-link suspension system.

The torque arm of a race car can be adjusted by pit crews during a race to improve the car's handling.
The torque arm of a race car can be adjusted by pit crews during a race to improve the car's handling.

The length and mounting point of a vehicle's torque arm is designed and engineered around the vehicle's instant center line. This is the point when the vehicle's suspension geometry intersect between front and rear. By mounting the arm as close to the instant center of a vehicle's suspension, the greatest amount of rotational energy is transferred into acceleration energy.

Racers have been experimenting with the mounting points of vehicle suspensions for decades. By altering the mounting location by even the slightest amount, the vehicle will react to acceleration and braking much differently. In a racing-only suspension, the mounting points are adjustable in order to tune the suspension to changing track conditions.

On a production vehicle, the mounting location and length of the suspension pieces are engineered under a compromise theory. The principal design is set up to achieve the best results over a given set of parameters. Taking all conditions of a street-driven vehicle into account, the suspension is designed to function well under a vast array of conditions.

This design practice is why some sports cars feel more like race cars than some family sedans or pick-up trucks. The intended purpose of the vehicle was examined by the engineers and the suspension was designed accordingly. In the design of the torque arm, the engineers understand that the street-driven vehicle does not need to accelerate like a drag-racing vehicle and its design reflects real world driving conditions.

Without a torque arm, the vehicle's suspension would bind as the rear axle attempted to rotate around itself. The vehicle would suffer from broken drive shaft and suspension pieces. Under the most modest braking conditions, the rear axle would again attempt to rotate as the tires slowed rotation and the suspension would again bind, causing the vehicle to skid out of control.

The arm is designed to provide adequate traction in wet weather as well as dry. Snow and mud are also taken into consideration so the vehicle will perform satisfactorily in all conditions. Race cars can be adjusted by pit crews during the course of a race to improve the vehicle's handling characteristics. The street car must be adjusted on the designing board and all performance characteristics must be engineered at the time of manufacturing.

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