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Tuesday, December 06, 2005

 

The Myth of One Wheel Drive

I'm starting another category of posts: Mechanical Mythology. Here, I will examine common claims and misconceptions about how cars work. In classic Snopes fashion, I'll list the myth, its status, and then a detailed explanation.

Claim: An open differential is only one wheel drive.

Status: False.

You've probably seen a car stuck in the mud or snow with one wheel spinning and the other wheel standing still. It's easy to assume that the drivetrain must only be connected to one of the wheels. The reality is much more complex. A normal drivetrain drives both wheels at the driving end (or all four wheels if you're talking an all wheel drive car), but they are not connected with a rigid shaft.

Since the wheels turn at different speeds when going around a corner, a car needs to use a system of gears that permit the driving wheels to turn at different speeds. The cheapest way to accomplish this is to use an open differential, which is a system of gears that sends equal torque to both drive wheels. If you put both drive wheels on a two wheel drive in the air and run the engine, both wheels will turn unless the brake on one wheel is stuck.

The problem with an open differential is that it can only send equal torque to both wheels, except for some very slight differences caused by friction. And it cannot send more torque to either wheel than the wheel can put to the ground. Consequently, if one wheel slips, the differential can only send it a very small amount of torque. However, it is not able to send any more torque to the opposite wheel, since the torque must be equal. Consequently, an open differential is either two wheel drive - or no wheel drive.

Limited slip differentials can often send torque to only the wheel that has traction while sending no torque to the other. It may seem counterintuitive, but a differential that locks up and moves both tires instead of one is temporarily acting as a one wheel drive system.

If both wheels have equal traction, an open differential does not have to spin just one wheel. Gary Donovan's 10 second Plymouth Reliant demonstrates that you can smoke both tires and lay two patches of rubber even with an open differential.

Howstuffworks.com has an article on what's actually in a differential, complete with animated drawings.

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