remebered this during a discussion on another gtir forum..Might be of interest to some of you.
credited to krazeguy
OK!!!! I have finally figured it out once and for all. Now ill try to explain this as best I can. This actually works in a VERY simple yet sophisticated way. There is just your average run of the mill differentials that controls the torque split. On your average diff you have a bunch of spider gears. For those that don't know the spider gear on one side controls the left wheel and the spider gear on the other side controls the right wheel. When your driving along in a straight line these gears spin together at the same rate, but when you turn, the spider gear on the inside starts to give way and allow the outter side to rotate more.
So most people know the workings of an open diff. Now the problem with an open diff is that if your driving in a straight line and you loose traction, one wheel will get all the torque leaving the other one with nothing. To combat this some diff's have a spring type clutch between the spider gears, this locks the two spiders together but still allows either spider gear to 'fall behind' a little when turning a corner. These are reffered to as a Limited Slip Diff.
Open
LSD
Another type of Limited Slip Diff, does away with the spring system between the spider gears and makes use of a Viscous Coupling. (VLSD) The way this works is that you have the right spider gear controlling the right wheel, and the left spider gear controlling the left wheel. But connecting to the left spider (/axle) is a little unit. This unitr connects directly to the Final drive (/diff casing) and inside the unit is a series of plates connected to the casing, then another series of plates connected to a gear. Between these plates is a Viscous fluid which thinkens as friction between the plates increases and heats the fluid. So if you loose traction in a straight line (or round corners for that matter) and the right wheel is spinning, the plates in the viscous coupling (which are connected to the final drive gear) start spinning, heat the fluid and bind up with the other plates which are connected to the left spider gear, which in turn, 'catches up' the left wheel so both wheels are spinning ... which of course means more traction.
Now the T4/GTIR 4WD system takes the VLSD principle and instead of using it to control left and right wheels, It controls Front and Rear. The final drive gear connects to the VLSD, the right spider gear connects directly to the transfer case which drives a reduction gear (about 4:1 by doing a rough test) which then goes to the rear wheels. To meet up to another VLSD to control Left and Right rear wheels. Then you have the Left spider gear to control the front wheels. There is an axle running through the right spider gear to the left one which goes through the transfer case to spin the Front diff. The front diff controls torque between Left and Right front wheels. (This diff is open, so if you are on grass and drop the clutch only 3 wheels would spin. The two rear wheels and one front wheel.)
I might have lost some in that last paragraph. But as I was saying the left spider gear controls the front diff, now if that 'Centre' diff was open then when you lost traction only the front or only the rear wheels would spin. Which would mean its not really a very good 4WD system. But the centre diff is VLSD so as the right spider gear spins (rear wheels) then the left spider gear (front wheels) has that little viscous coupling along with all its plates and fluid to help bring the front wheels up to speed.
Right, so hopfully you got that. Then you might be wondering why the T4/GTIR promotes under steer while going round a race track, or on the dyno etc. This is probably because of the reduction gear in the transfer case. case that transfers the to the rear wheels. So as the left spider gear (front wheels) catch up and viscous coupling is all bound up then the front wheels start getting more direct power and the rear wheels get slightly less simply because of the gearing in the transfer case.
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