# Automotive How do Center differentials work

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1. Feb 16, 2016

### Heijneker

Greetings all,

In my quest to imagine the ultimate sportscar, I began to wonder about the following:

Certain cars are All Wheel Driven (AWD) and they do so via a Center differential delivering power to both axles. Amongst those cars, some have fixed Torque Bias/Split Ratio, which doesn't alter depending on the situation. I really would like to know how this works.

I read somewhere that these center differential are planetary geared differentials, but the details of the workings were lacking. Now I wonder; how is it possible to have different power outputs at the two output shafts of this planetary gear center differential? Also, how does it allow the two axles to turn at different speeds, like when the car is turning, or as an extreme example, drifting trough a corner while the front tires aren't being overpowered but the rear ones are? (What I mean by that is, the front tires still get the same amount of torque percentage, but are able to put the power down, while the rear tires are being blown to pieces)((And what I mean by that is, rear axle turning, say, 2 times as fast as the front axle))

I couldn't find the correct term to say some of the things I wanted to say, but that is because I'm not a native speaker and am only in the first year of Engineering college. So please excuse me :)

2. Feb 16, 2016

### Nidum

Last edited: Feb 16, 2016
3. Feb 21, 2016

### Rx7man

To start with, lets take an "open" differential as a center diff for an example, coupled to open front and rear differentials.
In this case, the torque to each wheel is about 1/4 of the total, and if it breaks loose, the other wheels will not get any more torque than the slipping one.
What is a simple way of changing the bias? You can use different final drive ratios in the front and back, so if you have a 3.55 ratio in the front and a 4.10 ratio in the back, you will effectively reduce the torque on the front tires.. to get additional torque to tires that aren't slipping, you can add limited slip and locking differentials in all 3 locations. Some vehicles will apply braking to a slipping wheel to bias more torque to the ones with traction too