How Differential Gear Works

By Shamus Posted Saturday Aug 22, 2009

Filed under: Movies 44 comments

How can a single drive shaft turn two wheels at different rates? I knew what a differential did, but I couldn’t picture how. This movie gives a great visual illustration of the problem and the solution.

Skip to the two minute mark to get to the good part.

Link (YouTube)

Of course, a differential is a long understood technology, but it’s beautifully elegant and seeing it revealed in its simplicity is a lot of fun.


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44 thoughts on “How Differential Gear Works

  1. MuonDecay says:

    I had always been fairly perplexed by the idea of a differential myself, and that’s the first time the mechanics of it finally clicked into place in my head.

    Thanks for sharing!

  2. Andy says:

    A brilliant explanation of a complex system.
    I love the way they build up the system from a simple collection of sticks, increasing the number of spokes along the way, to a box of awesome gearage.
    I am sure there is an analogy here with the way that programmers take a complex problem and reduce it to a series of simpler solvable problems, only in reverse.

  3. Zaxares says:

    I’m not a mechanic by any stretch, but it was quite interesting to see it all in action. It’s quite amazing how marvellous science is at times.

  4. Mark says:

    I’m awed by the CVT (continuously variable transmission). (Links to videos at the bottom of the article.)

    Also, why anyone want to skip past military officers doing motorcycle tricks?

  5. Peter H. Coffin says:

    Now we need one for locking differentials…

  6. Nihil says:

    My own fascination is with the viscous coupling unit. That one is pure brilliance.

  7. Tesh says:

    That’s the best car ad I’ve ever seen, and now I understand how the differential works, too. Many thanks, Shamus!

  8. Simon_Says says:

    Bloody brilliant!

  9. Henebry says:

    That was a fantastic video. Not sure why, but those old educational films from the mid-century always strike me as clear and informative. They don’t try to jazz up the presentation with a lot of nonsense. But the key difference with the films of today is that the creator isn’t anxious about coming across as an authority.

  10. McNutcase says:

    I already understood differentials, but had no idea how to explain them. Now I know to just link that video.

    I just wish I knew how automatic gearboxes worked, beyond the usual “Very well, thank you”. They’re the only part of my car that’s still magic…

  11. Neil says:

    If you want to see something cool, look at a diagram of either a Dual Clutch Transmission (like the Porsche Doppelkupplungsgetriebe), or a planetary gearset (like in Toyota’s hybrid powertrain). The planetary gearset is especially cool because it works off the same principles as the differential, but it can also input power to or from any one of 3 inline shafts.

  12. My very first vehicle when I was 19 years old was an old ’76 Jeep with giant tires and a 4″ body lift.

    I was constantly having problems with the differential and the u-joint. (Ask me about the time I removed the drive shaft while wearing a pink dress.)

    That was a really informative video. Thanks for sharing!


  13. TehShrike says:

    Leslee Beldotti: what about the time you removed the drive shaft while wearing a pink dress?

  14. Logicaly_Random says:

    @McNutcase: Automatic transmissions are incredibly complicated, but the from what limited understanding I have of them, they consist of a series of planetary gears, all with a single sun gear. That one sun gear is shifted back and forth by a hydraulic ram, which is controlled by a computed that senses the speed the car is going, and what gear the car is in, and the RPM’s of the engine, and shifts when appropriate.

  15. Tausney says:

    This all reminds me of a lego car i had when i was a kid. As well as the differential problem i could’nt work out how a 4×4’s front wheels could be powered, turned and have suspension all at the same time. When i got the car, (monstourous set btw) i was well chuffed to find all that out AND learned how a gearbox worked. My spawn are going to be getting lego for many a christmas’, great brain food.

    After a bit of head scratching and searching later, the set i had was the 8880 Supercar. If you want to know differentials hands on (the car has 3!) i would really recommend it.


  16. Palette says:

    (Ask me about the time I removed the drive shaft while wearing a pink dress.)

    OK, Leslee, I am asking! :D

    Bonus points if it was a poofy chiffon dress and you were wearing heels at the time. ;)

  17. Telas says:

    Very cool. I could have used this years ago…

    For extra credit: Do you know why the shafts at either end of a driveshaft have to be parallel to each other, but not the driveshaft? (Unless you have a CV joint.)

  18. RPharazon says:

    I loved this video, and I have always wondered how exactly they make these kinds of turns work. Of course, I’m not a car person by any stretch of the imagination. I didn’t even know that this was called a differential gear thing.

    Ask me about airplanes, however, and I can explain pretty much anything.

    Lastly, I loved the end. The passengers just got out of the car and started walking on the rolling wheel blocks for little to no reason at all. The past rocked.

  19. Markus says:

    Tausney: Thank you very much, now I have to get one of those for myself! Maybe the wife will accept it if I tell her that I have to practice before I can start doing Legos with our son (2.5 months)

  20. Maddy says:

    More spokes! More spokes!!

  21. Kirin says:

    That was a really great video. I had never thought much about differentials so I was wowed at how it simplistically does it’s job.

  22. Ravens Cry says:

    A wonderful explanation of a complex issue. Thanks Mr. Young for sharing that.

  23. jubuttib says:

    About the locking differential: There are many ways to do it, but one of the more common types uses a clutch pack to connect the two half shafts (the ones going to the wheels) together. The clutch pack allows them to spin at different rates, up to a point. If one wheel tries to spin considerably faster than the other, the clutch pack engages and locks the wheels together. This can be done in any number of ways, from simple hydraulics to electronic systems.

    And btw, Howstuffworks has a great explanation on how a manual (and automatic, though that is the kind of hydraulic withcraft I try to avoid) transmission works. The main thing that I didn’t know was that in a manual transmission all the gear pairs are always connected and rotating, but the gear lever selects which one is connected to the driveshaft.

  24. Lochiel says:

    Anyone else expect a Rickroll?

    Actually, the video was fantastic. It wasn’t till I was watching it that I realized that while I knew how a differential worked, I couldn’t have explained it.

  25. Zaghadka says:

    That was pretty cool. Thanks.

  26. Chris says:

    @Tausney: I had the same questions when I was little! I also attempted to make different car suspension geometries/drivetrains. I could never get all three for fwd, steering, suspension and drive, the closest I got was steering and suspension, which was fragile enough without trying trying to add drive also, and I couldn’t work out how to add it anyway :) I know a bit more now, I might have another go…

  27. Bob says:

    You don’t listen to Geeknights, do you?

  28. Davin Valkri says:

    How am I gonna drive a car with two independently turning wheels? The answer is spokes on pivots. And if that don’t work, use more spokes on pivots.

    –the Engineer

    I love these old things!

  29. Telas says:

    jubuttib @ 23: Not always. Some transmissions don’t have a synchromesh first gear, like the Borg-Warner T-18 in my old 1970 IHC Scout. Heck, way back in the day, synchromesh hadn’t been invented, so you had to double-clutch every shift. Thank God (and German engineers) for progress!

  30. Ryan says:

    It’s the same kind of stuff behind the South Pointing Chariot. Absolutely brilliant.

  31. McNutcase says:

    @RPharazon #18: the wheel-walking at the end was another demonstration of the “inside wheel rotates slower” thing.

    I wound up hitting howstuffworks to try and wrap my head around automatic gearboxes. I think I sort of got it, but I’d love a video more like this one to explain it to me!

    And of course, most modern cars don’t actually have a differential like the one in the video, because it’s not the same for front-wheel-drive. It’s only us rear-wheel-drive dinosaurs that have them (but I’m used to rear-wheel-drive dynamics, so I’m looking to keep my jalopy running for a while. The partial cloverleaf offramp near here is WAY more fun in a rear-wheel-drive car than a front-wheel-drive, because I can pull dumb tricks like using the gas pedal to steer…)

  32. ClearWater says:

    I like how they let the woman run on the slower inside wheel while the man had to try to keep up on the outside one. So considerate!

    @Lochiel: I was half expecting a rickroll as well at first.

  33. Rosseloh says:


    That one sun gear is shifted back and forth by a hydraulic ram, which is controlled by a computed that senses the speed the car is going, and what gear the car is in, and the RPM's of the engine, and shifts when appropriate.

    Automatic transmissions are the one thing that kills my brain when it comes to cars as well. Not that I’m a guru, by any means.

    Anyway, just wanted to mention that while nowadays computer shifters are the norm….My friend had a 1965 or so Ford Fairlane wagon that had an automatic in it (sweet car). And I think 1965 was too early for chips to be in cars? So it had to be controlled mechanically somehow. But I’m not sure, someone please enlighten me if you can.

    edit: Heh, a quick search on HowStuffWorks and I’ve answered my own question. It’s hydraulic, and when the pressure in one “system” gets too high, it triggers the next, and so forth causing the shifter to take over….a highly compacted explanation, of course.

  34. Cain G, says:

    I love these old educational pictures with the grand soundtracks and that tone of narration that’s only done in parody today.

  35. jubuttib says:

    Telas @ 29: My bad, I of course meant modern transmissions. Though a synchromesh isn’t needed to build a constant mesh gearbox, like one found in (at least some) motorcycles.

  36. Jabor says:

    The elegant solutions are always the most magical.

  37. Jens says:

    Well if you are interested in what´s going on in your ride, this may be of interest:

  38. Timmy says:

    Thats alot simpler than I envisioned it. Actually I couldn’t figure how in the world that happened. That is a very old video, but it got it right.

  39. Josh says:

    That was a great video. I thought it would a lot more difficult for me to understand. It was really well-explained. Thanks for sharing, Shamus.

  40. vede says:

    I think the black-and-white cold-war-era informational video style needs to be revived. They’re so easy to learn from.

    I can just imagine a black-and-white video with graphics on paper in front of the camera used to demonstrate how a modern CPU works.

    That’d be awesome.

  41. McNutcase says:

    I think part of why we learn so well from them is the sheer awesomeness of the accents. British ones are even better, but even the American ones have that wonderful Voice of Authority!

  42. EMK says:

    Thanks for the link! This was very interesting, plus it inspired me to look up more how-things-work videos on youtube! :D Geekpower!

  43. GeneralBob says:

    Wow thanks for sharing that

    Now time to build it in legos

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