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Discussion Starter #1
When in low range is the power split 50/50 front/rear ?

I'm guessing no since the Titan has no center diff right,
just a transfer case ??

So how is the power transmitted (how does the transfer case work, how does it distribute the torque?) from the transfer case to the front/rear axles ?


Inquiring minds want to know :upsidedow
 

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Discussion Starter #2 (Edited)
found my own answer :upsidedow on another forum


" In what is coined a "Part time" 4 wheel drive system which 4x4 trucks have, there is a differential on the front axle, a differential on the rear axle, and a transfer case.

When you are in 2HI, the only the two back tires get the engines torque (or power), where the 2 front wheels are just spinning along for the ride essentially. Now when you turn, think about how the wheels travel. For instance you are turning right. Ignore the front wheels in 2 HI, they are spinning freely, the 2 back tires travel different pathes and one is going to need to spin more times than the other to complete the turn. The differentials job in this instance is to detect that the outside wheel on the turn needs more engine power to spin it more, and send the torque that way to complete the turn. In a sense both rear tires get 50/50 of the engines torque IF you are travelling in an absolute straight line, and if you turn, they change that ratio so the outer wheel in the turn gets more of a share and the inside less of a share to manage the need to spin differently.

Now as soon as you turn the transfer case from 2HI to 4 HI or 4LO, the engine's torque is now sent 50 percent to the front differential (wheels), and 50 percent to the rear differential (wheels). If you were travelling in an absolute straight line, each wheel would get 25 percent of the engines torque evenly as distributed by their differentials to maintain the straight path. No matter what though, the 50/50 to the front/rear can't be compromised, it will always be 50/50. So the time comes when you have to make a turn in 4 wheel drive. Since the front and rear differentials do NOT talk to each other, and the 2 front wheels and 2 rear wheels travel in different pathes and require different numbers of spins than each other to make the turn, you in effect have a "conflict of interest" between the 2 differentials, 2 axles, and the 2 wheels found on each. In this case, 1 axle (and the 2 sets of wheels found on it) has to make a compromise, and it is usually the axle's wheels that have to spin more times (usually the front) that have to "slip" a bit to make up for the other axles wheels which require less spinning to make the turn. If you are on a slushy or muddy road, or somewhere where the wheels can slip to compromise, this is fine (and why you should only use part time 4wheel drive off road or in poor road conditions). If you are on dry pavement, you will put extreme pressure on your axle and wheel components because the wheels can not slip. If you do try and turn on dry pavement in 4 wheel drive, you will feel the two front wheels "jump," from extreme pressure put on them from fighting their own friction with the pavement as well as fighting the rear wheels and their less RPM spin rates in the turn. You will also need to make wider turns. This is commonly refered to as "understeer."

Here is a picture/link for more information.
http://www.4x4abc.com/4WD101/def_turn.html "
 

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Discussion Starter #3 (Edited)
" , about Center differentials.

Full time 4wd and All Wheel Drive systems have a front, rear, and as mentioned, a Center differential.
If you note above, I said specifically that in a part time 4wd system, you have a front and a rear differential combined with a transfer case (to shift into 4 wheel drive) and that no matter what, the front and rear get 50 percent of the engines torque each. This means that the revolutions that the front wheels get can NOT be sent to the rear wheels, and Vice Versa, to properly complete a turn (IF you review what I said above and what was mentioned by others, and do the math, it is the reason why you can't turn in 4 wd on pavement - as you've figured out).

The missing link though is a center differential. A center differential allows the engine to send its torque back and forth between the front and rear differentials accordingly. The bonus - this allows you to turn while still being in 4 wheel drive. In effect, on a turn what will happen is since the combined revolutions of the front wheels is often more than the combined revolutions of the rear wheels as already mentioned above, it will for instance send more torque to the front differential thanks to the center differential than to the rear differential so that the revolutions of all the tires prevents the axles wheels from fighting each other on turns.

But now you're probably confused - why wouldn't all 4 wheel drive vehicles have a center differential then?

Reason:
Think about how a differential works. Look at one axle for instance and it's 2 wheels. Imagine you are in the snow. Your right rear tire loses traction and starts to spin. What does the differential do? Instead of sending the engines torque to the wheel with traction to help you out of the snow, it instead sends MORE of the engines torque to the wheel with NO traction that is already spinning freely. Why does it do this? Because the differential THINKS that you are turning, and kicks up the torque to the already (believed by the diff to be the outside wheel) spinning wheel as if you are making a more sharp turn. In a part time 4 wheel drive system, since you CAN NOT exchange torque back and forth between the front and rear axles because there is NO center differential, this allows you to still have 50 percent of the engines torque in the 2 wheels on the diff with traction, thus pulling you out of this slippery situation.

In the same situation with a center differential (full time 4wd/All wheel drive), what would happen is the engine would send all or most of it's torque to the spinning wheel through the center diff. For instance if your right rear wheel started spinning. All the the torque from the front axle and rear axle would go to that right rear wheel.

Now what is the difference between full time 4 wheel drive and all wheel drive? Both have a front, rear, and center diff. As far as I can say, full time 4 wheel drive has hi range and lo range (IE a transfer case). All Wheel drive is just that, but with no lo range. Many, not all, full time 4 wheel drive systems will come with a locking center differential, which effectively locks the torque so that it is like a part time system in that 50 goes to the front diff, and 50 to the rear diff with no exchange as the center diff is "locked."

Many hardcore 4 wheel drive enthusiasts who tackle muddy or snow situations often utilize a locking rear or front differential as well (usually rear - rarely both - on part time 4wd systems). If they run into a tough situation, by locking the rear differential as an example, so that each rear wheel (as an example if you only had a rear "Locker") gets 25 percent of the torque no matter what in 4 wheel drive, ensures that the wheels are always spinning regardless of what the differential thinks it should do in the event that your front wheels may be be faced with tough traction situations as well. Difficult to steer so should only be used in serious situations, but believed by some to be the difference.

Then you have a sort of wanna be locking differential called a "limited slip" differential, you'll see often referred to as LSD. No we're not drug attics. A limited slip differential uses a clutch type system often times to ensure that no matter what, the wheel without traction never gains all the torque, but some is sent to the wheel with traction. I am no mechanic though, nor do I fully understand how LSD's work, perhaps someone could elaborate a bit more. But essentially a LSD is not as reliable as a locking differential, but better than just a straight differential. "
 

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ahh, the power of the search button. very nice find, thanks. :cheers:
 
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