Originally Posted by
anmagro
What would the LSD allow?
The LSD would be the most efficient and reliable way to harness the added power supplied to the rear wheels. The rear(and front) diffs are "open" differentials, meaning they apply torque to both wheels until the traction threshold is breached. This is why RWD cars will burn out with one wheel (right rear). The torque being transferred through the differential to the axles/wheels/rotating parts, will be equal on both sides. Speed and traction will not be equal.
Example that has happened to me very recently...
I parked along a street where they have odd/even parking. Due to the city's poor plowing and residents not following proper parking, there are large piles of hard compact snow along the driver's side of the car when parallel parked.
While exiting my parallel parked position (driving over at least an 8-10" high mound 2' wide and the length of my car), the suspension became fully compressed on the right front and left rear, the left front and right rear were fully expanded and I think close to being off the ground. I have the center diff mod, but I do not have an LSD. With the car accelerating from a stop, to going through the above described procedure, the car hesitates to pass the peak of said snow/ice mound.
Here's the problem. The two corners that are fully expanded have close to zero traction, where the corners that are compressed have all the traction. For this example we'll say that the compressed corners have 95% traction, and the expanded corners have 5% traction... or really "Traction potential".
The open differential can only apply 5%(of available power/torque) to the wheels that have the most traction, because it only has 5% traction on the wheel with the least amount of grip.
Adding in an LSD, Locker, God forbid a Lincoln Locker, etc. would change the differentials properties. I'll explain a locker first...
A locked differential is designed to identify the differences between an axle making a turn and driving straight, for the sake of this argument we'll say it's locked and "knows" it's going straight.
It will provide 50% of available torque/power to both rear wheels indefinitely. In an AWD vehicle you do not want a locker though, and you do not want an LSD up front, especially if you have one in the rear.
If 50% of the power applied to the rear axle was applied to the wheel with 95% of the available traction, the car would have calmly driven over the hump/embankment/etc, but the front wheel(the one with the 5% traction) would have still spun.
Basically, the center diff(if you're going straight) applies 1:1 power to front and rear axles. So 50% of available power is applied to the rear axle. The rear axle splits power 1:1(50:50) with a locker. So, ~25% of engine power could be applied to the wheel with the most traction.
25% would also be applied to the rear wheel with the least amount of traction, but both the rear wheels would be driven at the same speed, with the same power applied.
An LSD would function rather similar EXCEPT that an LSD is designed to allow for slippage, so the likelihood of actually applying 25% of power to the wheel with the most traction would be slim. It would probably be closer to 18-20%.
However, the LSD is much more street friendly, and your friends won't have to ask you, "WTF is wrong with your car? Every time you take a corner, it clicks and vibrates."
Now the fun part... I won't explain Torsen differentials right now, maybe later. I need to eat before I start that discussion.
Hopefully you understand some of the theory and how the rear half of the drivetrain works, or would work...
The front, we hope (unless
maybe building a drag only car), will remain an open differential on an AWD car.
The center diff in an unmodded form has a bias ratio of 2:1. So the differential will multiply torque by the axle with the most traction.
The modded form will be close to 4:1.
What this means: If the rear axle is capable of transferring/consuming 100 lb/ft of torque, then the center differential will multiply that amount of torque to the front wheels, until they reach an equilibrium. When the reach the equilibrium, the diff acts as a 1:1 split.
This would basically be a launch/straight/drag race. Pretty much, don't take that for 100% fact, keep reading...
This works in the reverse fashion as well. If you enter a corner (applying pressure and increasing traction on the outer front wheel) the diff will "sense" the speed and torque variation, and multiply the amount of torque applied to the front axle, to the rear axle. By "over driving" the rear axle the rear of the vehicle will attempt to increase speed while the front axle decreases speed. ... OVERSTEER
I'll come back in a bit to add to that, I need to grab some lunch
or if you know of a link?
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