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Unread 10-24-2011, 02:26 PM   #1
DANBJEEP2012
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Limited slip differential and 4 wheel drive ?

I have a 2012 4 door Sahara auto trans. 321 rear end. With (out ) a Limited slip differential . A frind told me that because my Jeep did not have a Limited slip differential . When the 4 wheel drive is off is reallt one wheel drive and when the 4 wheel drive is on it is really 2 wheel drive. One wheel in the front drives the Jeep with one wheel in the back. Is this true ? can some tell me what a Limited slip differential really dose and how it affect the off road capibilities of a Jeep ? please Thank you

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Unread 10-24-2011, 03:14 PM   #2
jstrubberg
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I'm not sure the 2012 axles are really limited slip. They may be open differentials.

Your friend is giving you about half the story. One tire per axle has to slip or you couldn't drive your rig on anything other than loose soil. Limited slip and open diffs are two ways to allow that slip to happen.

Short story, all 4WD vehicles can have just one tire turning on each axle. Otherwise, you wouldn't be able to steer. The Jeep (and a few others) give you the option to add a locker to the rear axle to lock in both wheels under certain circumstances.
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Unread 10-24-2011, 03:27 PM   #3
TomF
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Do a search of this forum there is tons of stuff here, including some explanatory videos.

But the basic concept is that the wheels on a car going around a turn move at different speeds - the inside wheel on the curve turns fewer times than the outside wheel. If both wheels were drive wheels locked to turn at the same speed, they would eventually break the axle or shred the tires.

So the usual solution is an open diff that has one drive wheel at all times and allows the other wheel to travel at a different rate of revolution in a turn.

There are different ways to enable BOTH wheels on an axle to supply drive when going straight:
1) Limited slip - of numerous different types, acts automatically.
2) locker either automatic, or manual selection (electronic or pneumatic or mechanical)
3) spool - both wheels locked full time for a drag racer that only turns to get in position before going in a straight line.

Search and ye shall learn.
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Unread 10-24-2011, 03:40 PM   #4
secretspy711
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Another way to look at it: Open differentials basically let the vehicle send power to the wheel that is EASIEST to turn on a particular axle. So hypthetically, if in 2WD mode on your transfer case, and the left wheel is really wedged against some rocks and the right wheel is on a sheet of ice, the right wheel will just spin and the left wheel will do nothing. This is what your friend means by "1WD". So, if you put your transfer case in 4WD and the same situation is happening with the front axle as well, you will only have 2 wheels that are spinning, hence this is what your friend means by "4WD = 2WD" if you don't have lockers.

My own friend once told me that you don't want limited slip diffs on a jeep, but I can't remember why.

following that, having one locker = 3WD
and if you have lockers on front and rear = true 4WD... but not very useful for turning.

Your friend is just being a smartass and trying to impress you. It is NOT like some go-carts that literally drive only one wheel all the time. Power can go to any wheel, it just depends on the circumstances.
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Unread 10-24-2011, 03:50 PM   #5
jstrubberg
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Then, to complicate things even more, Jeeps use a brake lock differential. If the computer senses a wheel turning much faster than the others (indicating it's slipping) it will apply the brakes to that wheel and force the differential to turn the opposite wheel on that axle.

Works really well, until you NEED the wheels to spin...
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Unread 10-24-2011, 03:54 PM   #6
TomF
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Here are the basics, this is a classic:

http://youtu.be/K4JhruinbWc
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Unread 10-24-2011, 03:58 PM   #7
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Quote:
Originally Posted by secretspy711 View Post
Another way to look at it: Open differentials basically let the vehicle send power to the wheel that is EASIEST to turn on a particular axle. So hypthetically, if in 2WD mode on your transfer case, and the left wheel is really wedged against some rocks and the right wheel is on a sheet of ice, the right wheel will just spin and the left wheel will do nothing. This is what your friend means by "1WD". So, if you put your transfer case in 4WD and the same situation is happening with the front axle as well, you will only have 2 wheels that are spinning, hence this is what your friend means by "4WD = 2WD" if you don't have lockers.

My own friend once told me that you don't want limited slip diffs on a jeep, but I can't remember why.

following that, having one locker = 3WD
and if you have lockers on front and rear = true 4WD... but not very useful for turning.

Your friend is just being a smartass and trying to impress you. It is NOT like some go-carts that literally drive only one wheel all the time. Power can go to any wheel, it just depends on the circumstances.
Quote:
Originally Posted by jstrubberg View Post
Then, to complicate things even more, Jeeps use a brake lock differential. If the computer senses a wheel turning much faster than the others (indicating it's slipping) it will apply the brakes to that wheel and force the differential to turn the opposite wheel on that axle.

Works really well, until you NEED the wheels to spin...


To expand on the good explanations above...
Consider this. The open differential always splits the torque to both wheels 50/50.

On pavement, when you accelerate, both tires get equal torque, and they equally propel the vehicle forward.

With one tire on pavement, and one on ice, they still get equal torque.
The problem is, the one on ice may only take 50 ft/lbs to spin it. The other tire will get 50 ft/lbs, too, and that may not be enough to propel the vehicle forward.

Now, apply the brake to the spinning tire, so it takes 200 ft/lbs to spin it, and the other tire will get the same 200 ft/lbs. That may be enough the propel the vehicle forward.

An open differential allows a differential in speed.
An open differential always splits the torque equally.
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Unread 10-24-2011, 04:02 PM   #8
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Before fretting too much, you should know that your Jeep has enhanced rbake lock differential capability, as noted above. You can read up on it here with a search I'd guess, and I think its in the manual.

Brake lock differential is no replacement for a true locker, or, imo, for a limited slip set up, but it is a hell of a lot better than nothing.

Here's a quick run down on how to get your Jeep moving even before the BLD assists: Your Jeep is in 4wd and not getting traction. One tire on the front axle and one on the rear are spinning and you can't get her moving. Whether you have a manual or an auto, get lightly on the brake and the gas and apply brake. You may need to apply gas as well. Modulate the brake and gas as required to get moving. Being smooth is important so that you don't get the tire with traction slipping too.

The braking action will essentially force torque to the tire not spinning to get you moving.

In essence, any differential is a torque splitting device, forcing torque to both tires. The trick is delivering enough torque to the non moving tire to make it begin to turn. The engne will deliver only sufficient torque to keep itself turning (and that may take some gas too.) It needs load and fuel to develop additional torque. So, in a situation where you have poor traction, only sufficient torque to turn the easiest to turn tire will be delivered with an open differential, and that is the rub, the non-slipping tire may not have engough delivered torque to begin to turn. A locker forces 1/2 of available torque to one tire and the other half to the other tire on that axle, so it's the tire with the best raction that determines how much torqu is delivered, not the slipping tire, as with an open diff. A limited slip delivers only enough torque to somewhat even out the differential in one tire's rpm's vs the other's. Using the brake/gas trick to transfer torque is sort of using a manual limited slip.

JPK
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Unread 10-24-2011, 04:28 PM   #9
JLC08JK
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The above posted comments and explanations are great
Here is another write-up that explains it very well > http://www.jeepforum.com/forum/f27/w...ad-4wd-242949/
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Unread 10-24-2011, 04:37 PM   #10
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Quote:
Originally Posted by ronjenx View Post
With one tire on pavement, and one on ice, they still get equal torque.
The problem is, the one on ice may only take 50 ft/lbs to spin it. The other tire will get 50 ft/lbs, too, and that may not be enough to propel the vehicle forward.
Simply the BEST explanation for the weakness of open diff that I have ever read! (now can you do the math AND explain why stagger is used on locked axles in a sprint car!)
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Unread 10-24-2011, 04:53 PM   #11
panthermark
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Your friend is both right and wrong... (and ignoring brake lock diff for now)

With an open diff...
On dry pavement in 2WD while going in a straight line.....both wheels get equal power.

On loose pavement in 2WD....the wheel that spins the easiest (aka the LEAST traction) will get power.

If one wheel is on pavement, and the 2nd wheel is on really slick ice or in the air...that 2nd wheel will spin like crazy while you just sit there...hence the 1 wheel drive comment. The 1st wheel may not get enough torque to move you (this is where BLD or LSD or even lockers or 4WD come into play).

The same is true for 4WD...but you would have to be in a position where you have two wheels on dry pavement, and two wheels in their air or on slick ice or something. But unless you are out climbing rocks....or manage to come to a complete stop with two wheels on dry pavement and two wheels on slick ice coated in melted butter....it should not be a problem.
Open diff = "full slip diff"

Mechanical LSD transfers some of that torque from the free spinning wheel to the wheel going slower (which would be the wheel with traction)...thus allowing your one wheel to give you some momentum. While not quite accurate, it is really easy to think of it like this. If one wheel is getting 100% of the power (because it is in the air) with an open diff.....an LSD system may limit the difference between the two wheels (on the same axle) to no greater than 3:1 power....meaning that at most, only 75% of power may go to the free spinning wheel...so you wheel on pavement will get 25% of the power and you will move forward. This is why they are great on pavement and for snow roads....but some weaker LSD are not so great on tough trails....especially compared to Lockers. A 4WD Jeep with open diffs up front, and LSD in the back can still get stuck if two wheels get off the ground. One wheel on the ground will be an open diff (no power) and the other wheel on the ground (LSD) may not have enough power to pull you through.

LSD is a partially open diff = "limited slip diff" (It allows some slippage so you can turn corners, but no 100% open)

Lockers lock to both wheels (on the same axle) together...which means a 50/50 split no matter what. In 4WD....all 4 wheels are getting power and turning at the same speed. Great for super off-roading. Only two wheels on the ground? No problem. But they are useless for day to day driving on dry pavement unless you never plan on turning. They are either on (locked) or off (open diff)...meaning that it is kind of an "all or nothing" situation.
Lockers = "No slip"
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Unread 10-25-2011, 09:12 PM   #12
drroy
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here is a quick vid that shows the how open diff can become "1wd" and the advantages of a locking rear (at 45 seconds in)

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Unread 10-25-2011, 09:35 PM   #13
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here is a quick vid that shows the how open diff can become "1wd" and the advantages of a locking rear (at 45 seconds in)

http://www.youtube.com/watch?v=q-rQTHMVAuw
That's fine off road, but what happens when it locks on a slippery road?
There is a high probability you will end up in the ditch, or in the other lane.
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Unread 10-25-2011, 09:40 PM   #14
Charles
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Originally Posted by drroy View Post
here is a quick vid that shows the how open diff can become "1wd" and the advantages of a locking rear (at 45 seconds in)

http://www.youtube.com/watch?v=q-rQTHMVAuw
Wow, that's about as good as the explanation gets. The only thing to add is that a locker that turns on after wheel slip (reactive) is as good on some surfaces because you need the torque immediately to climb the surface.
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Unread 10-25-2011, 09:41 PM   #15
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Not sure about ending up another lane or ditch. I have the g80 on my Tahoe and its great in the snow, minimizes the need for 4wd
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