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Planetary gears on passenger vehicles?

  1. Jan 15, 2013 #1
    so, I'm a mechanical engineering student, fairly new at it though. I have a really strong real world grasp of mechanical principles, and really enjoy doing things like auto repair (consequently, my schedule is often interrupted by folks that know this fact, and want cheap auto repair) so with that in mind, please bear with me in this post, as with any others I may make in the future, should I use an incorrect term, or have a difficult time articulating my ideas.

    now, I was watching the modern marvels episode about monster trucks. They mentioned the fact that they use planetary gears on the each wheel since they are so large. which got me thinking; if this helps drive the wheels easier, why would the same not be true of passenger vehicles? If a planetary gear can be used to make each wheel turn easier, would this not improve the efficiency of the vehicle thus improving the gas mileage? just a thought :uhh:
  2. jcsd
  3. Jan 16, 2013 #2
    You've got to think about the different factors involved. Rather than just give you the answers I'll ask a few leading questionsm, as you'll be able to answer this yourself.

    What does the planetary set in the wheel actually do?
    Why would this be needed on something with very high torque outputs and high inertia (of the body and wheel)? (Think driveshafts)
    Does a passenger car have this requirement?

    Also first and foremost passenger cars have to be comfortable and cheap. What would the consequences be of replacing a single final drive with two/four planetary reduction sets in each wheel?
  4. Jan 16, 2013 #3
    makes perfect sense. Thanks much! I've got just one more question, only slightly related, but I assume anyone looking at this thread would be able to answer with ease; When measuring the radius of a gear, would you measure to the tip of a gear tooth, or just to the edge of the circle that the teeth lay on? or perhaps both depending on what youre measuring for?
  5. Jan 18, 2013 #4
    The important dimention in gears is "pitch diameter", which is the effective diameter of that gear. It cannot be directly measured, however. Just calculated from other parameters.

    It IS also true for passenger cars. That's why they have gearboxes. You pick a lower gear for starting and for going uphill, as it makes the wheels easier to turn. A higher gear for when you want to move fast. But it's not necessarily planetary gears that make turning the wheel easier, just any gears in general. Pulleys and levers also accomplish the same thing.

    Although I do believe that automatic cars DO indeed have planetary gears.
  6. Jan 18, 2013 #5

    jim hardy

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    As did the Model T Ford transmission from about 1908 to 1928. It shifted by 'bands' which the driver operated with his foot.

    Here's a great link:
  7. Jan 18, 2013 #6
    You've missed the point. Tractors and monster trucks have a single stage planetary gear-set on each wheel as an extra reduction ratio, on top of the rest of the transmission.

    It's done because you need the torque at the wheel but can't/don't want to package a transmission and driveline that can take the full torque.

    All automatic gearboxes are based on planetary gear-sets and engagement clutches.
    AMT and DCT aren't true autos.
  8. Jan 18, 2013 #7


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    Having a plnetary gearset at the wheel reduces the stress on the axleshafts and transmission through gear reduction- something you need to worry about when you're using 66 inch tires. Most passenger cars don't use them because they're a reduction in total efficiency, "standard" axle shafts can take the torque of a standard engine/transmission combination.


    That being said, a very few 4x4 vehicles do have gearsets at the wheels (known as portal axles) to help reduce driveline forces and increase ground clearance to the axle shaft- Hummer H1's and Unimogs for example:

    Hummer portal axle cutaway:

    Unimog portal axle:
  9. Apr 17, 2013 #8
    awesome! Thanks for all of the fantastic input everyone! much appreciated.
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