We may never know who invented the differential, but the technology is at least 3,000 years old. Differentials, or diffs, are based on the basic principle that, when cornering, the outside wheel turns faster than the inside wheel, covering more distance. A solid drive axle would bend and break or scuff the tires.
Several types of car differentials have been developed: Some are designed to simply allow for wheel speed difference, while others are designed to counteract it or accentuate it.
The open differential allows for differences in wheel speed or wheel slip but nothing more. The drive shaft pinion gear drives the differential drive gear. A pair of differential bevel gears drive a pair of driven bevel gears, which connect to the axle shafts to drive the wheels. On a typical dry road, the open differential allows the outside wheel to rotate faster than the inside wheel. When traction is good, power from the engine and transmission is transmitted proportionally to each wheel — 50/50 when straight, variable during turns.
The only problem occurs when a drive wheel loses traction, such as on ice or gravel. In this case, the open differential allows all the torque to go to the wheel with no traction, driving the vehicle nowhere. Open differentials are found on most vehicles in the world, but not performance or off-road vehicles.
In 1932, finding that the open differential couldn’t drive through hard corners — the inside wheel loses traction in high-speed turns — Ferdinand Porsche developed the limited-slip differential. Under normal driving conditions, straight roads and typical corners, the limited-slip differential acts like an open diff, allowing for wheel speed differences. However, under heavy acceleration and hard cornering, clutches or plates in the limited-slip differential prevent the differential from sending all torque to the wheel with the least resistance. This enables the race car to power through high-speed, high-power corners. Limited-slip differentials are found on many performance vehicles and some ostensibly off-road vehicles.
Sometimes, any wheel slip is too much, which locking differentials address. Locking differentials, or lockers, can be an extension of limited slip, using clutches and springs to activate a locking mechanism, sending equal torque to each wheel, regardless of the traction availability. Selectable locking differentials use air, electricity or cable. Detroit lockers or click lockers offer excellent locking and torque transfer, but aren’t really differentials, as they completely disengage an axle in turns. Locking differentials of varying types are found on off-road vehicles and some performance vehicles.
Torque-vectoring differentials are the most advanced and most complicated types of car differential, accentuating the differences in wheel speed as a vehicle corners. Using electronically actuated clutches and a separate controller, torque-vectoring differentials forcibly slow the wheel on the inside of the turn, sending torque to the outside wheel, powering the vehicle through the turn. Also referred to as active differentials, they modulate torque delivery on demand, resulting in a dynamic driving experience and better cornering performance. Torque-vectoring differentials are usually found on rear-wheel-drive and all-wheel-drive performance vehicles, and some vehicles mimic this by modulating brakes on the inside wheel.
Whatever you drive, know that the differential can affect traction and stability, and maintain it regularly to extend its life.
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Photo courtesy of Wikimedia Commons.
Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.