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Old 01-26-2010, 11:34 AM
vrooom3440 vrooom3440 is offline
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Location: Granite Bay, CA
Join Date: Sep 2009
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Mein Auto: 330i/X3/El Camino
Originally Posted by X3emist View Post
...Quote from link above-

xDrive is the permanent all-wheel drive system from BMW: under normal circumstances, it distributes driver power between the front and rear axles in a 40:60 ratio, and changes this figure variably when the road surface or overall driving conditions change.
I have always wondered at that 40:60 ratio. It sounds good but just how does it do that?

Looking at the hardware, the system is locked up or not. There are no fluid couplings or anything like that involved. Cannot really run any kind of gear ratio in the transfer case without causing lots of binding.

Which leaves freewheeling differentials in front, rear, and middle and application of brakes to transfer power from one wheel to another.

So that 40:60 ratio suggests to me that the front brakes are always being applied to shift 10% of the power to the rear. Does not sound very fuel efficient to me.
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