how do power-balancing diffs work?

[5lab]

I'm hoping someone on here can solve a question for me..

I know how a diff works. I also know how a limited slip diff works, and why it'll send 100% of power to the wheel with the most grip (by essentially locking the diff)

What I can't figure out is how these (center) diffs that send 80% of the power to the front, 20% to the rear, except when they need to send more... work. I guess they could have 2 viscous couplings with variable blades to adjust the power distribution, or use braking (such as the 'ediffs') - but neither of those sound like very good options. Am I missing something really obvious?

Cheers in advance

H

[Petrolhead_Rich]

I think:

Computer detects slip, diverts power the other way.

How it does this depends on the diff, some are as simple as putting brakes on the side that is slipping, others have other more advanced systems.

I'm sure someone will be along soon with linkys and diagrams...

Edit to add more info:

Wiki Haldex (used on VW and Audi)

Wikipedia said:

A pre-charge pump charges the hydraulic system to allow the coupling to engage quickly

So its a hydraulic coupling that is controlled electronically!



VW Haldex Information Page

Edited by Petrolhead_Rich on Friday 12th November 14:42

[5lab]

thanks!

so I can see how that gives 0% power, or 100% power, but how does it give 15% or 30% or whatever? just letting the clutch slip? (doesn't sound like it'd last very long?)

[Petrolhead_Rich]

5lab said:

thanks!

so I can see how that gives 0% power, or 100% power, but how does it give 15% or 30% or whatever? just letting the clutch slip? (doesn't sound like it'd last very long?)

Might be like a torque converter on an auto??

[sparkybean]

Petrolhead_Rich said:

5lab said:

thanks!

so I can see how that gives 0% power, or 100% power, but how does it give 15% or 30% or whatever? just letting the clutch slip? (doesn't sound like it'd last very long?)

Might be like a torque converter on an auto??

I would assume exactly this. Being a fluid clutch, there is nothing that wears while 'slipping'. It is controlled by changing the amount of fluid in the casing. More fluid = More lockup.

[Mr2Mike]

5lab said:

thanks!

so I can see how that gives 0% power, or 100% power, but how does it give 15% or 30% or whatever? just letting the clutch slip? (doesn't sound like it'd last very long?)

A standard plate-type LSD is essentially a slipping clutch and they last a reasonable length of time. The clutch in the centre diff doesn't slip significantly until one end of the car loses traction.

[thinfourth2]

Worm gears and black magic

But mostly black magic