Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Automotive Confused about automotive differentials

  1. Aug 15, 2004 #1

    I tried readin up on HSW and another resource about how differentials in automobiles work... and i am pretty confused.


    at that site i found good diagrams of it. I understand why the "planet and sun" gears arent spinning when its going straight.

    but I do not understand why they do spin when its turning and why the two wheels get different speed...

    could anyone be kind enough to clarify this for me?

    thanx in advanced!
    Last edited by a moderator: Apr 21, 2017
  2. jcsd
  3. Aug 15, 2004 #2

    :yuck: The drawing that you have the link to is really confusing due to the fact that you can't see the relationship of the spider and side gears. The ring and pinion gears do not drive the axles directly. The power is divided according to the restance applied to the wheels. So, the less resistance, the more torque is applied to that wheel. Since the ouside wheel on a turn has less resistence, it gets to turn faster. Also, if you have a car in the air, you can hold one wheel while the other one turns freely. Don't try this with a limited slip differential though.
    Dirt track race cars have either a locked differential or a "spool". There are no spider or side gears in a spool and the axles are driven directly. The axles turn at the same time as would be the case of a solid axle. These cars are very difficult to drive on a paved surface because the inside wheel will slide and the car will want to go straight and are not practical for a passenger car.
  4. Aug 16, 2004 #3


    User Avatar
    Science Advisor
    Gold Member

    Differentials are inherently difficult to visualise on paper or in text. There is a fantastic simulation of one at http://www.howstuffworks.com (search for differential). Alternatively, build one out of Lego or Meccano! By far the best way to understand.
  5. Aug 16, 2004 #4


    User Avatar
    Science Advisor

    Ok, the ring and pinion are there just to get the direction of rotation to be 90 degrees different. The case of the differential is spinning, in a car with a transaxle there is no ring and pinion so the spinning differential case is all that matters.

    The gears inside the differential being stationary you understand, good.

    The gears inside the differential spin to overcome an outside force applied as torque on one of the axles. So if you have a differential in mid-air and spin one of the tires the other will rotate in the opposite direction. The gears inside react to the torque on one side and apply it to the other. So one side might be +10RPM and the other side -10RPM, both sides always equal.

    When a car is turning in a circle, the diameter of the circle is different for each tire. This difference is distance is going to apply force to each tire, the inside is going to be slowed by the force and the outside tire is sped up by the force. The differential gears equalize this force between them, just like if spun in midair. So if the axle was spinning at 50RPM and the speed was the same in the corner the inside might be 40RPM and the outside 60RPM. The same -10RPM and +10RPM relationship as earlier but noW with a net 50RPM rotation speed. When back in a straight line there is no difference in force applied to the tires so the gears no longer spin.

    That help?
    Last edited: Aug 16, 2004
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook