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Hill-Holder is a name for the mechanism invented by Studebaker that holds the brake until the clutch is at the friction point, making it easier to start up hills from a stop in manual transmission automobiles. It was first introduced in 1936 as an option for the Studebaker President. Another name for the mechanism is a hill hold control (HHC).
As a trade name, it was only available on older Subarus, and is currently available on the Subaru Forester and the Subaru Impreza. Hill-holder works by holding the brake in position while the driver sets-up and activates the first gear to move the car forward from a complete stop, without fear of roll-back.
In laymans terms, the hill holder function works by using two sensors, in concert with the brake system on the vehicle. The first sensor measures the forward facing incline (nose higher than tail) of the vehicle, while the second is a disengaging mechanism.
When a driver stops their vehicle on an incline where the nose of the car is sufficiently higher than the rear of the car, the system is engaged when the driver's foot is depressing the brake pedal, and then the clutch pedal is fully depressed. Once set, the driver must keep the clutch pedal fully depressed, however, they may remove his or her foot from the brake pedal. To disengage the system and move the car forward, the driver selects first gear, feathers (applies) the gas pedal, and slowly releases the clutch pedal, which, at a point in its travel, releases the braking system allowing the car to proceed.
Hill-holder works best for those who are inexperienced with manual shift techniques, or in situations with heavy traffic in steep hilly conditions (ex. San Francisco, etc.).
However the same technique can be accomplished by a driver through the use of the manual parking brake lever, coordinated with the braking, clutching, shifting and acceleration.
Similar systems are or were in use by Alfa Romeo, Fiat (including the new Fiat 500), BMW, Lancia, Mercedes-Benz, Volkswagen, Saab, smart ("Hill Start Assist"), Subaru, Cadillac ("NoRol") and Stutz ("Noback").
References
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