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Closer Look at the Axle Assembly

The rear axle assembly’s center piece is the differential, or rearend, and controls torque delivered to the rear wheels. The unit allows the two wheels of a rear-wheel drive vehicle to receive the right amount of power in corners or in a straight line as well, independently, while still retaining sufficient traction to turn the drive wheel(s).

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© Loraks | Dreamstime.com Differential

A high-powered muscle car drive chain will fail very quickly if the rearend does not meet demands placed on it. The outer wheel must turn more quickly to cover a longer distance than the inner one; this enables the rear wheel’s instant response to constantly changing demands without fault. The transmission delivers power via the drive shaft to a pinion gear that turns a much larger ring or crown gear inside the unit’s housing, received by a set of planetary gears where the power is then dispersed to each axle. The pinion/crown gear meeting point (hypoid gear) is the final drive ratio and happens before the momentum reaches the wheels at the end of each axle shaft.

When the pinion gear makes four revolutions, causing the ring gear to rotate one time; the unit will then have a 4:1 final drive ratio or axle ratio. The combined ratios of the transmission and rearend gears, determine the amount of torque it can produce, the top speed attained, and how quickly it can achieve speed.

The differential, or rearend, connection to the drive shaft is on the left hand side of the image below.

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© Masuti | Dreamstime.com Axle House

Ferdinand Porsche developed the limited slip differential (LSD) or “posi-track” prior to 1932 for Grand Prix racing that year and is a more refined version of the standard rearend. LSD offers dynamic advantages, providing a variable in the angular velocity applied to each of the two output shafts. The LSD does, however, increase the complexity of the differential’s operation. During the muscle car era, from the early 1960s to the late ’70s, the LSD-equipped muscle cars delivered equal torque to each wheel, outperforming all counterparts consistently by minimizing wheel spin. Imagine an icy surface with one wheel not getting traction, the standard or “open” rearend will deliver power to the wheel that’s slipping, unable to get traction, spinning freely while the other stationary, firmly planted, but powerless. With the slippage limited, a vehicle will have both rear wheels supplied with power and rotating, but the wheel that’s losing traction will not get as much power as the one providing the most traction.

Locking differentials, or lockers, are a variation that an enthusiast can manipulate to have both wheels turning, equally supplied with power on any terrain.

When both rear axles lock, then they lock into the same rotation, functionally, one piece, like the simple spool axle with power going to each wheel regardless of traction. In the unlocked position, while cornering, the locker will deliver maximum power to the wheel that can use it most.

A few choice video clips on the differential are below:

https://www.youtube.com/watch?v=K4JhruinbWc    excellent but retro

https://www.youtube.com/watch?v=gIGvhvOhLHU

http://en.wikipedia.org/wiki/Limited-slip_differential

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