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General automotive: Why is the differential geared down?

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Old 04-12-2012, 10:04 PM
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Default General automotive: Why is the differential geared down?

Hi,
This is probably a stupid question, so excuse my dumbness, but why is the their a final drive ratio? Couldn't the differential be 1:1 and have all the gearing down happen in the transmission?
Old 04-13-2012, 03:54 PM
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Because then you'd need 3-4X the torque capacity in
the driveshaft and U-joints, for same rear wheel torque.
That's extra rotating mass for the required stiffness.

And you'd need a bigger pinion gear, because the ring
teeth are not that far from their limits. That'd be a heavier
pumpkin all told.

But transaxles basically do this (driveshaft spins at motor
RPM, all reduction in the unified trans / diff casing).
Old 04-14-2012, 12:22 AM
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Torque multiplication in stages. Just like jimmy said. The more gear sets you can multiply or reduce torque, the better.

That's why on big time heavy trucks like the mining trucks that are The size of a house, they have planetaries in the wheel hub itself to spread the gear reduction forces out more.
Old 04-16-2012, 10:44 PM
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Oh ok that makes sense. Thanks guys this'll be helpful for my paper. Appreciate the help.
Old 04-17-2012, 11:50 AM
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I don't see how making the final drive 1:1 and all the gears in the trans modified to make the gears still similar to what it would be would require heavier DS and ujoints? You'd still be applying the same force to the driveshaft. I think that you would, however, need a beefier transmission, but a heavier duty rear end may not be required.

Imagine what a 1:1 gear ratio ring & pinion would look like
Old 04-17-2012, 12:03 PM
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Originally Posted by HoLLo
I don't see how making the final drive 1:1 and all the gears in the trans modified to make the gears still similar to what it would be would require heavier DS and ujoints? You'd still be applying the same force to the driveshaft. I think that you would, however, need a beefier transmission, but a heavier duty rear end may not be required.

Imagine what a 1:1 gear ratio ring & pinion would look like
If all gear reduction was happening in the transmission, more force would be applied to your driveshaft and rear end. In real cars, the torque is multiplied both in the transmission gears and the rear end. If all of this torque multiplication was happening in the transmission, this would mean more stress on the DS and rear.
Old 04-17-2012, 12:32 PM
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Now that I did some thinking about it, I get it. It's like loading furniture. 2 people, 1 pushing 1 pulling is better than 1 strong person pushing. I retract my statement.




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