I'm trying to understand how the engagement of the clutches is varied. I see the power is transmitted through the center input shaft and is transmitted through the four pinion gears, but I don't understand how the engagement of the clutch(s) is controlled based on which axle has more/less traction. Its it a centrifugal mechanism that is not clearly illustrated, or maybe even the shape(conical?) of the pinion gears them selfs? One point I have heard is the front and rear Crown gears are of a different diameters and the diameter determines the torque split(Some what illustrated, larger diameter being the rear axle which statically gets 60% assuming even traction).

One question I would like to answer from all this is, what would happen if traveling at a non-insignificant speed (45 Mph +), and the rear axle was stopped momentarily (imagine hitting the Emergency brake), what results could you expect?

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  • Great Question, look forward to seeing some good responses hopefully. Commented Feb 10, 2016 at 21:20

1 Answer 1


This set up is a quazai differential/planetary gear set. This can make it difficult to see how it functions.

  • Power comes in from the transmission and into the pinon gear carrier and pinon gears. The pinon gears then transfer power to the front and rear crown gears. While the speed of the front and rear axles is the same the pinons do not rotate on their axis. This guarantees that the differential housing is also rotating at the same speed.
  • When the front axle, for example, starts to spin faster because it lost traction the front crown will spin faster. Because the speed of the two crowns is not the same the pinon gears start to spin on their axis. The spinning causes the pinon carrier to spin faster which also causes the differential housing to spin faster. Because the differential housing has a set of clutches attaching it to the crown it will drag the slower crown along trying to speed it up. This sends power from the wheels that slip to the wheels that grip.

If you hit the parking break while driving at high speed and lets say lock up the wheels the front wheels will just roll. Because you shouldn't be hitting the gas pedal in the process the speed difference will get sent into the transmission which will be free wheeling.

  • So does the pressure applied (clamping force) to the clutches not change? It just has a pre-load? From descriptions it sounded like it had a way of detecting slip/loss of traction. Sounds like from your description, it's more of a passive operation than active. A little like a slipper clutch. On the e-brake, I assume if the car is in gear(manual, transmission clutch engaged), the differential clutches will be forced to slip?
    – mmmlll lis
    Commented Feb 11, 2016 at 0:04
  • @mmmllllis Yes, it is a passive device. If it's a manual the clutches would be forced to slip. I was thinking automatic.
    – vini_i
    Commented Feb 11, 2016 at 0:12
  • How is the pre-load applied? Does it maintain load throughout the life of the clutches or does it decrease as the clutches wear?
    – mmmlll lis
    Commented Feb 11, 2016 at 0:21
  • @mmmllllis The differential housing is a cylinder with the end plates screwed in. That's how i assume they preload is applied. Yes as the clutches wear there will be reduced performance. Here is a nice video that shows how the assembly breaks down. youtube.com/watch?v=xZ9x9jHkTrg
    – vini_i
    Commented Feb 11, 2016 at 0:37
  • Yeah that video is one of the best illustration I was able to find. One thing I noticed is the lack of what appears to be a spring which is shown in the image in my OP. Guess a repair manual will give a definitive answer. Thanks for the replies!
    – mmmlll lis
    Commented Feb 11, 2016 at 15:44

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