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View Full Version : Center Diff Mod........



4inchMAF
01-19-2009, 10:34 PM
im want to learn more about this:

*after installing the washers, do you notice any difference when just driving normally? or is it just around turns at high speed?


*also, what is the power split between the front and rear stock (its not actually 50/50 is it?)? and is the power split "locked in" always?




my Mercedes CLK is rear wheel drive and i love it.....

thanks for the input guys!!!!

Poopie
01-19-2009, 10:52 PM
Run a search. been covered pretty in depth a few times. Answers all your questions and more.

My opinion: useless unless you are really pushing your car on the track. Grippy tires would be a better investment since they improve all around performance from cornering to braking. Its not a bad mod, but money could be spent else where.

goodhews
01-19-2009, 10:56 PM
http://www.audizine.com/forum/showthread.php?t=206325&highlight=center+washers

about everything you could wanna know

4inchMAF
01-20-2009, 05:18 PM
i read a LOT of stuff about it already, but the questions i have up there are a few things i never got a solid answer on :(

djwimbo
01-20-2009, 05:38 PM
High speed cornering is improved. Strightline performance will not change at all, fuel mileage is no different, and no it's not a 50/50 split.

... well, in a straight line, it's split 1:1. Around a corner it's split 2:1. The modded diff is a 4:1.
If you know or understand how a Torsen Differential works, then you can understand the effects of the TBR(torsen bias ratio). If not, let me know and I'll try to explain in as much detail as I can.

Torque multiplication has one problem, multiplying by zero, still equals zero. To take full advantage of the diff mod, you would need an LSD in the rear.

anmagro
01-20-2009, 06:18 PM
What would the LSD allow?

4inchMAF
01-20-2009, 07:14 PM
If you know or understand how a Torsen Differential works, then you can understand the effects of the TBR(torsen bias ratio). If not, let me know and I'll try to explain in as much detail as I can.


explain away my friend!!! [:)] or if you know of a link?


thanks man!!!

bassed
01-21-2009, 09:18 AM
im want to learn more about this:

*after installing the washers, do you notice any difference when just driving normally? or is it just around turns at high speed?


*also, what is the power split between the front and rear stock (its not actually 50/50 is it?)? and is the power split "locked in" always?




my Mercedes CLK is rear wheel drive and i love it.....

thanks for the input guys!!!!

Unless it's wet, snowy etc... there is not a noticable difference, the difference is on the track with how you apex. You can basically point and shoot with the 4:1. Unless you track this is absoltley pointless for you to do IMO.


What would the LSD allow?


80% Rear bias.

dreVanHalen
01-21-2009, 10:08 AM
i thought an LSD would more or less "lock" the diff so the outer and inner wheels spin at the same speed as opposed to a bias between the front and back.

djwimbo
01-21-2009, 10:12 AM
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 [drive]

I'll come back in a bit to add to that, I need to grab some lunch

Poopie
01-21-2009, 10:12 AM
if one of the tires lift the 80% of the power going to the rears ends up spinning one tire. Thats why you would want an lsd

djwimbo
01-21-2009, 10:13 AM
one more thing to remember, the Torsen Differential we have for our center diffs is a form of an LSD.

bassed
01-21-2009, 10:19 AM
if one of the tires lift the 80% of the power going to the rears ends up spinning one tire. Thats why you would want an lsd


*Ding*

UCFQuattroguy
01-21-2009, 04:40 PM
my 2cents:

Depending on your setup, you won't even need to lift a rear wheel to be "jonesing" for a rear LSD. There are many instances where all 4 would be planted as I feed power for corner exit, but there'd be enough weight x-fered away from the inner-rear that it'd actually start to spin. The acceleration would hesitate under power, then when enough weight was x-fered back to the inner-rear the whole car would sort of "jerk" back into place and off you go. There was one quicker increasing radius turn where the car would be very "floaty" as I fed the throttle (think of a pseudo crab-walk), but when I started to unwind the throttle the car would plant itself.

4inchMAF
01-23-2009, 11:19 PM
anyone know of a good link to learn more about how a torsen diff works?

UCFQuattroguy
01-25-2009, 07:51 AM
do a search on google for "torsen white paper"

djwimbo
01-25-2009, 11:47 AM
do a search on google for "torsen white paper"

Like this....
http://flashoffroad.com/features/Torsen/Torsen_white_paper.pdf

The differences are that instead of the ring gear driving the assembly, the output shaft of the trans drives it. The Torsen in our trans is longitudinally mounted and the power is transferred out to the front and rear differentials, instead of directly to the axle/wheel.

ghost6303
01-25-2009, 11:47 AM
http://www.torsen.com/products/T-2R.htm

btw that was a nice explanation dj

djwimbo
01-25-2009, 12:02 PM
btw that was a nice explanation dj

well thank you. The sad part is that I haven't gone on to explain the Torsen.

When I learned how the Torsen worked, I didn't have the opportunity to read the "white papers" on it. I sat down with the diff in front of me and pondered. I even went for a walk to visit one of my instructors that is a Mechanical Engineer. You know what he told me? "A Torsen? I know what it is, but why would you expect me to know how it works?" He babbled on about their uses, but nothing about how it worked. So I walked away and decided to learn on my own, since he had no idea.