Since the question about the frequently misunderstood subject of how torque
is delivered to the wheels is a common one, I thought I'd make this a "Sticky".
All four wheels are driven and pulling equally if all four tires have equal traction. If all four wheels have equally good traction, then all four will pull equally well. So even if your Jeep
does not have a locker or limited slip differential, each of the four wheels will receive approximately 25% each of the torque from the engine, when traction is equal under each of the four tires.
A standard open differential always (always!
) splits the torque 50:50 between both sides. The problem is that when one tire starts spinning due to poor traction, that reduces the amount of torque seen by either side by an exactly equal amount. So if one side starts slipping, the other side's power (torque) is reduced by a corresponding amount which usually means insufficient power to keep you moving. The engine, via the axle, can deliver no more torque than the wheel with the least amount of traction can accept before slipping.
So the short answer is 'yes', all four tires pull in 4wd. But the moment one side starts slipping, neither side gets enough power to keep you moving even if the non-slipping side still has traction.
Why does the power (torque) get reduced to both sides and not just the side that is slipping? Again, because the differential will always (!) split whatever torque it sees 50:50 to both sides (few people know that). Why does the amount of torque get reduced at all? Because the engine only develops torque when it is working into a resistance. Run an engine with a torque meter connected to it and watch what happens to the amount of produced torque as resistance to the engine is varied. When the engine is working into zero resistance and just running free, it produces nearly zero torque. Apply a braking action to the engine and the amount of torque will increase in direct proportion to the amount of resistance it is seeing. The more braking force applied to the output shaft, the more torque the engine produces.
So because the engine only produces torque when seeing resistance, a spinning tire reduces the amount of power (torque) the engine produces because the engine is only seeing the nearly zero amount of resistance produced by the spinning tire. This works the same way with 2wd and 4wd, it's just that with 4wd, you have more of a chance that at least one of the two axles will have enough traction to allow the engine to produce enough torque to keep you moving. Which is why you get stuck in the first place... when one side is spinning, there is insufficent power being delivered to the other side to get or keep you moving.
And all of this is why a limited slip differential (LSD) can help since it helps to "couple" (via a clutch or a gear-based device) the resistance the side with good traction is seeing to the other side with poor traction, increasing the amount of resistance seen by the engine... thereby increasing the amount of torque delivered to both sides. Which is why the simple technique of stepping on the brakes a little when a tire is spinning can often get you unstuck... because it too increases the resistance the engine is working into which increases the amount of torque that is delivered to the wheels. Or if a rear tire is spinning constantly, pulling the parking brake up a couple clicks can often help by acting as a poor-man's limited slip differential. Pulling the parking brake up a few clicks when you already have a limited slilp differential will help improve its operation. A key drawback to a LSD whether it be clutch or gear based is that when one tire is spinning up in the air, it doesn't usually help create enough torque for the other tire still on the ground to keep you moving. Without you helping it by stepping on the brakes or pulling the parking brake handle up if it's a rear tire that is spinning, a LSD just doesn't do much for you. On flat terrain they can help a lot, but on uneven terrain when both tires can't always be on the ground, a LSD is not very helpful at all. That's a situation where a locker reigns supreme over a LSD.
What does a locker do? It mechanically locks the left and right wheels together so when one turns, they must both turn at the same speed. This arrangement prevents one side from spinning uselessly while the other side does nothing. Automatic lockers are always locked but they unlock automatically to allow the outside wheel in a turn to 'ratchet' faster as the outside tire must do through the turn. Once the turn is completed, the locker re-locks both sides together. A manual locker is one that doesn't lock the left and right sides together until it is actuated either via a push-button or lever. An ARB Air Locker is air-pressure actuated, others like the Detroit Electrac are electrically actuated. The Ox Locker is actuated via a lever and cable. Manual lockers have an advantage for on-road driving in that when unlocked, they act like an "open" axle... i.e. one without a locker or limited slip differential... which means it drives like an unlocked vehicle
until the locker is activated via the push-button or lever.
Part-Time and Full-Time 4x4
A part-time 4x4 system called Commandtrac is in all Wranglers together with low-end Cherokees
and Liberties. A part-time 4x4 system locks the front and rear driveshafts together inside the transfer case so they drive the front and rear axles together in lock step. Because they are locked together, the front and rear tires must rotate at the exact same rpms. However, the front tires must rotate faster than the rear tires during any turn so a part-time system fights that... which makes a part-time system inappropriate on a paved road because the high level of traction on a paved road prevents the tires from slipping which would otherwise allow the front and rear tires to grudgingly rotate at different rpms. Offroad this is not a problem since the poor traction of an offroad trail allows the tires to slip as needed. But when they try to slip/rotate at different rpms on a high-traction surface, the entire drivetrain is stressed which is bad for it. This problem is called "wind-up".
In reality however, the front and rear axles really don't even turn exactly the same RPMs when you're in 4wd so you still get "wind-up" if you drove in 4wd on the street even if you drove in a perfectly straight line. Why? Because 1) you can't drive in a perfectly straight line and 2) the front and rear axle ratios are usually .01 different from each other. Like a 3.73 and 3.74, 4.10/4.11, etc.. Why the .01 ratio difference between the front and rear axles? Because the front and rear axles usually have different ring gear diameters which makes it nearly impossible for the gear manufacturers to economically make the front and rear axle ratios exactly the same. And no, they are not made .01 different on purpose to make the front or rear pull more when in 4wd, that is an old wive's tale.
Finally, a full-time 4x4 system like Selectrac is available on Grand Cherokees
, Cherokees and Libertys couples the front and rear axles together, but they are not mechanically locked together like they are with a part-time 4wd system
. The front-to-rear axle coupling can be done via either a differential like the Selectrac system uses (just just like what is in the center of an "open" axle) or a fluid (viscous) coupler. The benefit to a full-time 4wd system is that because the front and rear axles are not mechanically locked together, the front and rear tires/axles can rotate at different rpms from each other. This allows a vehicle with a full-time 4wd system to drive in 4wd "full time" on a paved road without problem since there is no 'wind-up' problem to harm the drivetrain. You cannot get a full-time 4x4 system in a Wrangler from the factory.
Hope this helps!