A line lock is a device that allows the front brakes to lock independently of the rear brakes, via a switch. The device is an electric solenoid that controls a valve which allows the brakes to be controlled individually. This allows the front brakes to be locked and the rear brakes to be open, and allows the driver to spin the rear wheels without wasting the rear brakes. This method is referred to as line lock and is popular among drag racers.
Cars with manual transmissions allow drivers to control the amount of power transferred from the engine to the wheels by manipulating both clutch and throttle inputs. As a result, cars can accelerate quickly without losing traction. However, launching a vehicle requires simultaneous gas pedal and clutch pedal inputs, preventing drivers from manipulating the brakes with their feet. Without brake input, drivers staging their car at the starting line would roll if the clutch was slightly engaged and red light. The line lock prevents cars from rolling through the staging line by allowing the driver to control the clutch and accelerator, using foot input and braking with the hand. In an automatic transmission vehicle, a line lock allows the engine to rev up to its peak power RPM and launch hard from the starting line. Many modern sport-oriented cars now offer optional line locks for drag and show purposes, such as offered on the 2017-18 Dodge Demon, 2019 Hellcat Redeye and 2015 Ford Mustang.
2018 Dodge Demon
Dodge Demon Line Lock Display
Although less common, rolling burnouts can be achieved with modified line locks by proportionally increasing the front brake pressure and the throttle to keep the car at a steady speed. This makes the rear tires slip and spin faster than the front tires and burnout.
Line locks can also be installed on the rear brake lines. When the line lock is activated, fluid pressure is prevented from being transferred to the rear, allowing the rear wheels to spin freely while the front brakes can be controlled with the brake pedal. This allows one to creep at varying speeds while maintaining a burnout. Achieving such a burnout requires manipulation of both throttle and brake inputs. The burnout is initiated by manipulating the clutch with the left foot and both the brake and gas pedal with the right foot. The brakes must be held while revving up the engine, then the clutch is released while continuing to hold the brakes. Once the rear tires start spinning, the left foot is used to control the brakes while the right foot continues to control the throttle. The only advantage for such a setup is the ability to maintain a burnout at relatively low wheel low speeds, which reduces wear on the rear tires. This method can be used to increase tire temperature to provide better traction prior to a drag run, but it is also often used purely for show, such as in drifting exhibitions or simply for the personal enjoyment of the driver
After market kits, such as the Hurst Roll Control, can be purchased for as little as $234 for drag racing.