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Hey,

I tried to write you an email back, but the return address was invalid. Here's the email I wrote up for you:

I'll try to give you the short-version, and if you're interested you can look at these links for more info:

http://auto.howstuffworks.com/differential.htm
http://auto.howstuffworks.com/differential6.htm

The quick version is that when a vehicle is turning the "outside" wheels travel a further path than the "inside" wheels (the radius of the arc they travel is obviously bigger). As such, there has to be some sort of mechanism by which the left and right side wheels at either end of the vehicle can travel at different speeds. That mechanism is the differential (I won't even try to get into the specifics of how it accomplishes that, the links will help).

Now, this is fine and well on the street. However, the nature of the way a differential works means that power will "flow" to whichever wheel is the path of least resistance. In other words, the wheel with the LEAST traction will get the power. This is the opposite of what you'd want in a low traction situation (i.e. off-roading). For example, if you're doing some crawling on uneven ground or rocks and one wheel lifts into the air, that wheel will then get all the power for that end of the vehicle, and just spin freely while the other wheel which is solidly planted on the ground gets no power and can do no work to move the vehicle forward.

Limited slip differentials, which are pretty common at the back of most newer trucks and SUVs, but still pretty rare in the front, use friction within the differential (again, I'll leave the specifics to the websites) to help even the power distribution. However, in extreme situations (i.e. one wheel in the air) they still cannot overcome the massive difference in traction at each wheel, and as such the wheel in the air will still spin freely while the wheel on the ground does little/no work.

Now, a selectable locking differential acts like a normal (aka "open") differential most of the time. However, when necessary it can be "locked" to a state where it completely mechanically links the two sides of the differential together. Both wheels are physically required to spin at the same speed, regardless of traction. On pavement this would mean that the inside tire would have to actually break-traction and spin during a turn since it's traveling the same speed as the outside tire. This is hard on parts, hard on tires, and makes for "jumpy" turning. In the dirt though it's not a problem since the inside tire can easily spin a bit without you noticing. And when the going gets tough it allows whichever wheel has some traction at the time to keep you moving.

Land Rovers are some of the only SUVs you can get lockers in. Jeep Wrangler Rubicons (only the Rubicon, not any of the other models) also have lockers.

LR and some other manufacturers also use the brakes to try to combat the down-sides of open differentials. When the computer senses that one wheel is spinning significantly faster than the other wheel at the same end of the rig it will apply the brake on the spinning wheel. This increases the resistance at that wheel, and hopefully channels some of the power to the wheel with real traction. It actually works pretty well, and parallels an old off-roader's trick of riding the brakes a bit to combat a spinning wheel on an open differential equipped vehicle, but does so more effectively since it only brakes the spinning wheel.

Hope that helps,
TJ
 

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2010-2012 Range Rover MkIII / L322
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Great explanation. Now I get it.

Thanks.


Rafa
 

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Discussion Starter #3
rafa said:
Great explanation. Now I get it.

Thanks.


Rafa
No prob. I guess it works out better that I post it here anyway, this way more ppl get a chance to read it.

-TJ
 

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2006-2009 Range Rover Sport
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Also some fun things you can do with the family in a RRS with a rear locking diff:
1. Donuts in the snow and wet grass
2. Drive across a slippery hill, smack the accelerator, and have the car go backwards down the hill

Kids love that stuff. Wives, not so much.

Btw, its not that rare an option, you can get it in many Toyota, Nissan SUV's and pickups, etc.
 

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Discussion Starter #5
gooseyloosey said:
Also some fun things you can do with the family in a RRS with a rear locking diff:
1. Donuts in the snow and wet grass
2. Drive across a slippery hill, smack the accelerator, and have the car go backwards down the hill

Kids love that stuff. Wives, not so much.

Btw, its not that rare an option, you can get it in many Toyota, Nissan SUV's and pickups, etc.
Hahaha, indeed there are fun things one can do with a locked-up diff!

You're right, a lot of Tacomas and 4Runners over the years have a push-button rear-locker. I actually wanted to switch out the ARBs in my 'old SASed 'Runner for the factory E-lockers because the air system for the ARBs was always problematic for me. But I sold it before I ever changed them. I didn't know Nissan was offering them now too, pretty cool.

-TJ
 

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Good stuff. I really appreciate this post. I'm selecting my RRS and my dealer is selling me on NOT getting the rear diff lock. I think this is b/c they don't order them with it.

I apologize... have tried searching for my answer and this is the best I came up with. Should I insist on the rear diff lock?
 

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So I assume older 4x4 cars (before full-time 4 wheel drive), the push button four wheel drive ones, such as the 95' full size bronco I had a few years ago have locking diff. I remember that you were never supposed to put it in 4 wheel drive on pavement unless there was snow/rain.
 

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Rhyno46 said:
Good stuff. I really appreciate this post. I'm selecting my RRS and my dealer is selling me on NOT getting the rear diff lock. I think this is b/c they don't order them with it.

I apologize... have tried searching for my answer and this is the best I came up with. Should I insist on the rear diff lock?
Will you ever *seriously* off road it? IE Baja, North Africa, Moab, Hollister, Pismo, Rubicon, etc. etc.? If so, yes.

If not, no. I've done some serious deep snow work (on road) in my RRS with the locking diffs, and never needed to lock em to get out of anything.
 

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payne said:
If not, no. I've done some serious deep snow work (on road) in my RRS with the locking diffs, and never needed to lock em to get out of anything.
The stock center and rear (if equipped) lockers are only controlled by the computer; sans selecting certain Terrain Response programs that will lock/unlock them quicker/longer than other programs. That is, unless you know something I don't. /:(
 

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TripleE said:
payne said:
If not, no. I've done some serious deep snow work (on road) in my RRS with the locking diffs, and never needed to lock em to get out of anything.
The stock center and rear (if equipped) lockers are only controlled by the computer; sans selecting certain Terrain Response programs that will lock/unlock them quicker/longer than other programs. That is, unless you know something I don't. /:(
Oh, I'm a noob. Good thing I didn't get stuck then :)

Jason
 

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I dont have the lockers, but doesnt something light up on the dash/infotainment system when they engage?
 

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payne said:
TripleE said:
payne said:
If not, no. I've done some serious deep snow work (on road) in my RRS with the locking diffs, and never needed to lock em to get out of anything.
The stock center and rear (if equipped) lockers are only controlled by the computer; sans selecting certain Terrain Response programs that will lock/unlock them quicker/longer than other programs. That is, unless you know something I don't. /:(
Oh, I'm a noob. Good thing I didn't get stuck then :)

Jason
Eh, me too; no worries. :lol:

streetfish said:
I dont have the lockers, but doesnt something light up on the dash/infotainment system when they engage?
You have a center locking differential, everyone does, but you may not have the rear. It displays itself as a padlock icon in the 4x4 screen and will color orange and show locked when it is locked.
 

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payne said:
Will you ever *seriously* off road it? IE Baja, North Africa, Moab, Hollister, Pismo, Rubicon, etc. etc.? If so, yes.

If not, no. I've done some serious deep snow work (on road) in my RRS with the locking diffs, and never needed to lock em to get out of anything.
I'd like to but I doubt it. My POV is that it's only $500 on a $70k buy. I'd hate to regret not having it over a measly $500.

Thanks for the reply!
 

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Rhyno46 said:
payne said:
Will you ever *seriously* off road it? IE Baja, North Africa, Moab, Hollister, Pismo, Rubicon, etc. etc.? If so, yes.

If not, no. I've done some serious deep snow work (on road) in my RRS with the locking diffs, and never needed to lock em to get out of anything.
I'd like to but I doubt it. My POV is that it's only $500 on a $70k buy. I'd hate to regret not having it over a measly $500.

Thanks for the reply!

Agreed! :thumb:
 
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