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Unread 04-05-2006, 02:34 AM   #1
Sairys
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Clear up some random confusion caused by the FAQs

Ok, I know... probably a dumb question, but i've been full of stupid comments lately, so this would just flow.

Anyway, here it goes.

I was reading that FAQ about lockers. Now I understand they're supposed to lock the axles so that both sides spin even if one doesn't have traction. Now I know that the Full time 4x4 splits the torque and the what nots, so that when one tire is spinnin' like a top, the other one stands still and you get stuck. On the Part time (Command Trac tcases), You don't have that problem.

That said, is there still some power difference, or is there no use for lockers on with Part-time 4?

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Unread 04-05-2006, 03:03 AM   #2
saxj
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in full-time 4wd, you have a third open differential in the tcase. So when 1 wheel spins, the other three stand still. When you put it in part-time 4wd the centre differential is locked. So when one wheel spins, you still have power to the other axle. In "severe" cross axle situations, one wheel on each axle doesn't get sufficient traction and both of those wheels will spin.

With a locker in one of the axles, the situation changes quite a lot. If the tcase is in part-time, as long as either wheel on the axle with the locker has traction (and enough torque), the vehicle will continue to move, regardless of whether the wheels on the other axle have traction or not. This is not true in full-time however. If one wheel spins on the unlocked axle, the other wheels still stop!
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Unread 04-05-2006, 11:08 AM   #3
Jason, aka: Jeepin.com
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saxj is correct. The type of 4wd system deals with the transfer case, not the axles, so your understanding of how SelecTrac vs CommandTrac works is a bit off. The transfercase sends power to the front and rear axles, but not to the left ro right wheels -- that's what a locker or limited slip is for.
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Unread 04-06-2006, 04:50 PM   #4
syzygy
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Let me try to clarify a little more. Actually I think what I say might conflict slightly with what's been said above, so correct me if I'm wrong.

The power to move the vehicle gets transferred from the engine through the transmission, through the transfer case, along the driveshafts to each axle's differential, and finally to the wheels.

I'll start with the differentials. Each axle's diff decides which wheel on that axle will get power. There are three basic types of differentials.
  1. Open. In this case, power gets transferred to whichever wheel is easier to turn. (That is, the one with the least traction.)
  2. Limited slip. Power gets transferred to both wheels, but one wheel gets more than the other. The wheel with the least traction gets the most power, and will spin faster than the other in slippery conditions.
  3. Locked. Both wheels get the same amount of power and spin at the same speed regardless of traction.
Tranfer case modes: Similar to differential types except instead of choosing which *wheel* power goes to, the transfer case mode determines which *axle* power goes to.
  1. 2WD. Power goes to the rear axle only.
  2. Neutral. Neither axle gets any power.
  3. Full-time 4WD. Only available on some transfer cases. Basically limited slip mode for the transfer case-- some power to each axle, whichever axle is easier to turn gets more. Front and rear tires will spin at different speeds in low traction situations.
    • NOTE: This is where I differ from saxj, who states that full-time leaves the transfer case as an open diff. My understanding is that this mode is actually limited slip. The components required to achieve this limited slip mode are what makes transfer cases with full-time 4WD slightly more prone to breakdown than those that don't have it.
  4. Part-time 4WD. "Locked" mode for the transfer case: both axles get the same amount of power, and willl spin at the same speed.
To find out what will happen with a particular combination, think through the powertrain with your selected modes. A few examples:
  • 2WD, open diffs.
    • Transfer case: rear axle only.
    • Open rear diff: one wheel on rear axle- the one with the least traction.
    • open front diff: not relevant -- no power to this axle
    • This is one wheel drive. (Whichever rear tire has the least traction gets power.)
  • 2WD, limited slip diff.
    • Transfer case: rear axle only
    • Rear axle limited slip diff: Some power to each wheel, more to the one with less traction.
    • front axle diff: not relevant.
    • This is two wheel drive, both rear wheels spinning, but at different speeds.
  • 2WD, locker.
    • Transfer case: rear axle only
    • Locked rear differential: same power to each wheel regardless of traction.
    • front diff: irrelevant
    • This is two wheel drive, with both rear axles spinning the same speed.
  • FULL-TIME 4WD, open diffs.
    • Transfer case: "limited slip": some power to each axle. More to whichever axle resists less (ie, the one with the least overall traction.)
    • Open rear diff: power to least traction rear wheel
    • Open front diff: power to east traction front wheel
    • This is two wheel drive-- one on each axle, wheels can spin at different speeds.
  • Part-time 4wd, open diffs.
    • Transfer case: "locked": equal power to each axle.
    • Open rear diff: power to least traction rear wheel.
    • Open front diff: power to least traction front wheel.
    • This is two wheel drive -- one wheel on each axle, both at the same speed.
etc.
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Last edited by syzygy; 04-06-2006 at 04:54 PM.. Reason: formatting
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Unread 04-06-2006, 05:36 PM   #5
howardusa
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Quote:
Originally Posted by Sairys

That said, is there still some power difference, or is there no use for lockers on with Part-time 4?
I'm still curious as to what the answer is to the original question.

Part-time 4wd, open diffs.
Transfer case: "locked": equal power to each axle.

I assume by the above statement that is no use for lockers with the part time system.
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Unread 04-06-2006, 06:07 PM   #6
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Yes there is a use for lockers because it locks power from side to side the transfer case only does it front to back.
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Unread 04-06-2006, 08:25 PM   #7
syzygy
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Yes, yes, yes... think it through:

part-time with lockers front and rear:
-- transfer case - equal power to front and rear *axle*
-- rear locked diff -- equal power to both rear tires
-- front locked diff -- equal power to both front tires
== this equals all wheels having equal power regardless of traction--- thus if each axle has one tire in the air and one tire on the ground you will still be able to move because all tires will be receiving power.

vs

part-time with open diffs front and rear:
-- transfer case - equal power to front and rear *axle*
-- rear open diff -- power to one front wheel
-- front open diff -- power to one front wheel
== this equals two wheels with power going to them--- and they're the two wheels that slip the easiest. the part-time transfer case selection means that both of these wheels will spin at the same speed, but if there one tire on each axle is in the air in this case you will not go anywhere -- the tires that are in the air will spin and no power will go to the wheels that are on the ground!
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Unread 04-06-2006, 08:26 PM   #8
howardusa
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Quote:
Originally Posted by Bulldog2Delta
Yes there is a use for lockers because it locks power from side to side the transfer case only does it front to back.
If your locked front to back in part time won't you have the same effect? I guess I just don't fully understand exactly how this system works. When I'm in part time my assumption is that the axles are locked and all wheels are pulling. 50% to the front and 50% to the rear and that if one or more wheels become stuck the others will pull you out. Assuming of course they have the traction to do that.
Hope I did't hijack this thread. The orginal poster asked a question that I was curious about.
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Unread 04-06-2006, 10:27 PM   #9
90lerado
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http://auto.howstuffworks.com/four-wheel-drive.htm
this should clarify any confusion you have
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Unread 04-06-2006, 10:49 PM   #10
Sairys
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No worries about hijacking. Ask away. I was worried that this would be a noob questions, so I'm glad that someone else is getting use out of it.
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Unread 04-06-2006, 11:42 PM   #11
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I've always been a little confused with the 242 as well, when I put it in full time 4x4 in the winters for getting around town quicker on ice, I know it's not only 1 tire spinning. So there has to be something going on in there, is it a gear type limited slip? I know there's no clutches in there, but it can't be a straight up open diff either. VooDoo science, I tell ya.
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Unread 04-10-2006, 01:50 PM   #12
syzygy
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I believe the full-time mechanism employs a differential to do the work...

try: http://jeepforum.com/forum/showthread.php?p=1937515
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Unread 04-10-2006, 06:49 PM   #13
blackxj99
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Quote:
Originally Posted by howardusa
If your locked front to back in part time won't you have the same effect? I guess I just don't fully understand exactly how this system works. When I'm in part time my assumption is that the axles are locked and all wheels are pulling. 50% to the front and 50% to the rear and that if one or more wheels become stuck the others will pull you out. Assuming of course they have the traction to do that.
Hope I did't hijack this thread. The orginal poster asked a question that I was curious about.
HT

50% of the power goes to the front and 50% to the back so both axles have equal power. However, with an open differential both tires on that axle do not have equal power, the one with the least traction will get more power. So if one tire on each axle looses traction then you are stuck. With lockers both wheels on each axle also get equal power so all four wheels recieve the same amount of power.

The full time mode works great on slippery road conditions because all four tires have similar amounts of traction, so one particular tire isn't recieving more power.
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