31 March 2011

How a differential works in a car's transmission


  1. Pretty nifty piece of engineering there. Now I know how a differential works :)

  2. Yes, same for me. I believe this also applies to why you can't replace just one partially-worn tire. I had to toss a donate a quite good one to charity when the opposite tire got a sidewall puncture and had to be replaced; I was told the new tire and the good old one would have differences in diameter great enough to damage the transmission.

  3. Cool video, thanks for sharing.

    Actually, the differential can compensate for tire diameter/circumference differences from uneven tire wear. It's only on four-wheel drive vehicles that you really have to worry about tire wear differences (or perhaps in "Positraction" type 2WD drivetrains). That's because traction-control features in 4WD drivetrains are more expensive and more susceptible to tire diameter differences.

    A simple differential system will just spin the tire with least resistance, which gets you nowhere when the traction is too little to move the car. Traction control drivetrains apply some force to all driven wheels and work much better on ice, snow, and other slick surfaces. At the expense of tire and mechanical wear, of course.

  4. Stan, tire salesmen are never to be believed. As a fellow English major, it's a dear lesson I have learned over the years. They rival used car dealers. Your road hazard warranty will never be honored, your puncture can never be repaired ("I could lose my job if something happened to you in a wreck...") and the cheapest tires they sell won't fit on your car. Ever.

    There is no damage to the transmission from tire size differences. The transmission has no idea what size each tire is. It just outputs torque to the drive shaft.

  5. Pete, is that true for my Subaru with all-wheel drive?

  6. An excellent explanation of how the differential works. I remember subscribing to an extension course in engineering from the University of Michigan when I was about 12 or 14 years old which explained all of this as well as planetary gears in general. While it was very interesting, it was not as clear as this. Probably because there was no film associated with the course.

  7. I finally understood how an open-differential worked from opening one up in an RC car and observing how it worked.

    You DO NOT WANT tires of too much difference in diameter installed on either side of a differential (open or limited slip). When you are driving with 2 equal tires, there is no slip involved and your spider gears that allow the differential action stay in place. Normal turns in a road only create moderate slip for short periods of time (relatively).

    Install different tire sizes and the differential is working all the time, spinning little gears that weren't meant to run continuously. Most differentials do not have active cooling. The heat will build up until you cook the grease... thereby ruining those gears that lose the needed lubrication from running non-stop.

    This goes thricely so for a subaru which has 3 open differentials on a manually equipped car (front, rear, center) and 2 differentials with the transmission actively controlling the third on an automatic car.


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