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complex question for all you guru's

Old 03-11-2015, 09:48 AM
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Default complex question for all you guru's

Hey VaDriven,

Wondering if anyone can explain how you can have torque split and the same angular velocity in an AWD car?
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Old 03-11-2015, 01:15 PM
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Default Re: complex question for all you guru's

Angular velocity of the car (yaw rate), or angular velocity of the wheels, or angular velocity of the prop shafts? Angular velocity of what?
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Old 03-11-2015, 01:24 PM
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Default Re: complex question for all you guru's

How exactly does a posi-trac rear-end on a Plymouth work? It just does.
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Old 03-11-2015, 01:24 PM
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Default Re: complex question for all you guru's

Yeah, more info is needed.

edit: I'm going to take a shot in the dark and say your are wondering how you can have torque split at the center differential if the front driveshaft RPM is the same as the rear driveshaft RPM?

Last edited by JDM74; 03-11-2015 at 01:27 PM.
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Old 03-11-2015, 01:27 PM
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Default Re: complex question for all you guru's

Originally Posted by DOOM View Post
How exactly does a posi-trac rear-end on a Plymouth work? It just does.
It just does. Somewhat crudely, but it works. But it's no mystery how it works.
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Old 03-11-2015, 01:29 PM
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Default Re: complex question for all you guru's

Originally Posted by JDM74 View Post
edit: I'm going to take a shot in the dark and say your are wondering how you can have torque split at the center differential if the front driveshaft RPM is the same as the rear driveshaft RPM?
The answer is that torque doesn't care about RPM. See, easy.
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Old 03-11-2015, 01:37 PM
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Default Re: complex question for all you guru's

Originally Posted by Fabrik8 View Post
The answer is that torque doesn't care about RPM. See, easy.
Duh, but I don't think that was the question. I think he is trying to ask how can a AWD car send more torque to the front or the rear of the car if all 4 tires are spinning at the same speed, i.e. the center diff is not "diffing".
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Old 03-11-2015, 03:11 PM
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Default Re: complex question for all you guru's

Well, that's an easy answer, but might be a difficult thing to wrap your head around. I know it tends to bend my brain a bit when I start to animate things in my head and visualize how it works.

Cars that have a center differential with a static torque split (under normal, non-slip conditions) have a different gear ratio for the front driveshaft and rear driveshaft, so with a planetary differential that would be different input/output gearing. You would likely say "well that would make the shafts run at different speeds" but that isn't correct. The difference in gearing changes the torque multiplication between the front and rear wheels, but the front and rear wheels are moving the same speed, so any differences in planetary gearset motion are internal to the differential.
This obviously changes when there is a difference in grip between the front and rear wheels, which means that there is now a slip condition if one set of wheels is overpowered by torque, and therefore a relative speed difference between the front and rear shafts. Then the torque split would change depending on the type of center differential and what it has for lockup characteristics, especially if you add clutches and other elements like that into the equation.

If it's not a planetary differential, so for example a crown gear differential, it gets much more brain bending because it becomes a case of gear diameter that gives the static torque split, and there is no change in gear ratio (tooth ratio). It's the application distance (moment) that is different between the front and rear shafts. If you see a picture of it, it's brain warping because it doesn't look like the torque split should actually work.

Last edited by Fabrik8; 03-11-2015 at 04:36 PM.
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Old 03-11-2015, 03:45 PM
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Default Re: complex question for all you guru's

The real question is can jet fuel melt steel beams? The answer is no.
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Old 03-11-2015, 07:10 PM
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Default Re: complex question for all you guru's

Originally Posted by DOOM View Post
How exactly does a posi-trac rear-end on a Plymouth work? It just does.
Why are boobs good?
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