OK. - I give up.
I'm trying to understand how the centre differential in VTD equipped cars works, and it's got me stumped.
The only explanation I can find is here
http://www.subaru-global.com/about/parts/06.html.
But the explanation falls a little short of being useful.
Quote:
The VTD of the Subaru Legacy is patented technology called "double-planetary gear type variable torque distribution."
This VTD has a mechanism consisting of helical gears-- one front sun gear, one rear sun gear and three planetary gears, and one carrier.
I listened to a two-hour explanation with the use of a model about these center differential gears. This explanation was much like an explanation of gears for kindergartener. As a result I think I was able to understand how they work. However, when I look at the movement of the gears of the center differential, the metal gears appear to move freely just like a living thing.
|
The way that
I think that I understand it is that the power input to the entire "double-planetary gearset" is via the helical gear on the coupled planetary gearsets and that the two outputs are via the two sun gears.
Because the planetary gears (and the meshed ring and sun gears) are different sizes, the torque split between the front and rear sun gears is going to be unequal (65% rear/35% front in the SVX). This would mean that the natural output shaft speeds would be different too.
But because the front and rear wheels will be coupled, via the roadwheels to the road surface, the output shafts will normally be forced to rotate at exactly the same speed. This discrepancy is taken up by allowing the ring gear to freely rotate.
Have I got it right so far?
If I have, then here's the kicker!
When the two speed sensors for the front and rear output shafts differ by some unknown percentage,
(let's say 5% for argument's sake) then the Transmission Control Unit presumably applies a brake via a brake band to the ring gear to progressively lock the front and rear driveshaft's rotational speeds together. You can imagine that the greater the discrepancy in the output shaft speeds is, then the harder the brake would be applied.
BUT, surely this means that the transmission is now forcing the front and rear roadwheels to rotate at grossly different speeds, and therefore forcing the brake to be applied even harder!!!
This cannot be right can it?????
I've obviously misunderstood something somewhere.
Has anybody found a better explanation somewhere, or can explain where (or if) I'm going wrong?
I suspect I am too old for kindergarten.