How is torque delivered to the wheels on a 4x4?

Jerry Bransford

TJ Guru
Supporting Member
Nov 9, 2015
8,339
9,324
Escondido California
#1
Since questions about the frequently misunderstood subject of 4x4 and how torque is delivered to the wheels is a common one, I thought I'd explain it.

All four tires are driven equally by the 4x4 system and all four pull equally if they all 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 an equal amount which usually means insufficient power to keep you moving.

The engine can develop no more torque than the tire with the least amount of traction can accept before spinning.


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 always (!) splits whatever torque it receives 50:50 to both sides. Why does the amount of torque get reduced at all? Because the engine only develops torque when it is working into resistance. Run an engine with a torque meter connected and watch what happens to the amount of torque produced as resistance to the engine is varied. When the engine is working into zero resistance and running free, it produces nearly zero torque. Apply a braking force to the engine and the amount of torque will increase in direct proportion to the amount of resistance the engine is working into. The more braking force applied to the engine's output shaft, the more torque the engine produces.

So because the engine only produces torque when working into resistance, a spinning tire reduces the amount of power (torque) the engine produces because the engine is working into less resistance caused by the tire spinning from lack of traction. 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 tireis 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 systems...

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!
 
Last edited:

mrblaine

TJ Guru
Supporting Member
Nov 20, 2015
2,568
2,887
Quail Valley, CA
#3
Awesome writeup Jerry!!
It is but a lot of folks aren't quite able to wrap their head around the part where torque is function of resistance. No resistance = no torque = no tire movement.

An easy way for those that struggle to visualize and understand it is with the use of a torque wrench. If you were to start turning a lug nut that you just threaded on by hand with your torque wrench set to the correct value, it won't click to indicate that the lug nut is tight until the lug nut is seated and gets about a full turn or so. Until then, just like the tire in the air, it takes very little force or torque to turn the lug nut and like Jerry said, the differential always delivers the same amount of torque to both sides equally so the side that has the least amount of resistance will determine how much the other side gets.

If it takes 5 ft lbs of force to turn the tire in the air, the one on the ground is also getting 5 ft lbs of force.

The good news about lockers is they will always deliver the engine torque being developed to the tires. The bad news about lockers is they will always deliver the engine torque to the tires.

We have had several times in JV where a rig is moving up an obstacle and two things happen nearly simultaneously that break stuff. One of the front tires will drop over the top of a rock so the back side of the tire is on the down slope and then the rear diff will get pulled up onto a diff hanger and reduce the traction on both rear tires. The other front tire is in a marginal traction situation so now you have one front tire with a extraordinarily high level of traction and the other three have very little. All it takes in that scenario is a little blip of the throttle and you'll break something on the side with the traction because the rig can't move off the diff hanger and all the torque goes to that one side.
 
#5
Thanks Jerry, it's a little more detail than Miss Vito gave me.

Vinny Gambini: And why not? What is positraction?

Mona Lisa Vito: It's a limited slip differential which distributes power equally to both the right and left tires. The '64 Skylark had a regular differential, which, anyone who's been stuck in the mud in Alabama knows, you step on the gas, one tire spins, the other tire does nothing.
 

Chris

Administrator
Staff Member
Sep 28, 2015
32,073
19,592
Salem, Oregon
#6
Thanks Jerry, it's a little more detail than Miss Vito gave me.

Vinny Gambini: And why not? What is positraction?

Mona Lisa Vito: It's a limited slip differential which distributes power equally to both the right and left tires. The '64 Skylark had a regular differential, which, anyone who's been stuck in the mud in Alabama knows, you step on the gas, one tire spins, the other tire does nothing.
Ha... what a hilarious, hilarious movie!
 

Xposure

TJ Enthusiast
Jul 13, 2018
397
159
Trussville, AL
#7
I know this is very old...but I'm new to all this. I've had a 4wd before but not with lockers and never have done anything that even came close to required them.

So I'm just going to give a little repeat to make sure I'm understanding it right. The locker locks both axles together so they will both spin at the same rate, being the lower rate since the side with traction has the lower rate. So if one tire has traction going up a rock and the other doesnt a stock tj will have the traction side barely moving as its underlay and across from it the other tire is free spinning in the air real fast. With a locker it locks them together so that the load of the rock face is being seen all the way across the differential and the it all loads up good.

I would imagine this would make it MUCH easier to snap axles wouldn't it? Your effectively putting the side that has traction under much more stress than normal, right?

What are good lockers that you can activate when your in the situation and need it?
 
Oct 31, 2015
2,613
2,420
Kootenays, BC, Canada
#9
I would imagine this would make it MUCH easier to snap axles
Which is exactly why the Rubicon comes with D44 axles front and rear.

Other 4x4s, never intended for rock crawling come with a smaller, lighter front axle because in 4WD they are sending 50% of the torque front and 50% rear. But in 2WD they send 100% of the torque to the rear so the rear axle is always heavier. ie D30/D35. Or in pickups D44/D60, D60/D70.
 
Last edited:
Likes: Xposure

Westtown Willy

TJ Enthusiast
May 10, 2018
484
671
Saltopia, PA, USA
#13
I would imagine this would make it MUCH easier to snap axles wouldn't it? Your effectively putting the side that has traction under much more stress than normal, right?

What are good lockers that you can activate when your in the situation and need it?
That is my understanding. As explained thoroughly above with the open diff when you lose traction you lose torque, so the stress on the axle goes down due to the reduction in torque seen by that axle thereby reducing the chance of breakage. By contrast with the locked axle when you lose traction the stress on the axle goes up because it’s seeing more torque & it’s all on that wheel (& therefore that axle shaft) that has traction thereby increasing the chance of breakage, particularly if it’s the only 1 of 4 tires that has good traction, as was also mentioned above. This is why I really like selectable lockers (especially for a D30/35) because I can decide when I put those smaller axles under that kind of stress, & because I have to consciously push a button I always know exactly when it’s on & what the conditions are under the Jeep (often I’ve gotten out & looked under & around to figure out what I’m doing), so I can take it really slow & reduce the chances of breakage & then the moment I’m free of whatever obstacle that required locking I hit that button again & disengage the locker & I’m back to the lower probability of breakage that comes with an open differential. In some instances if you think the risk is too high of breakage while locked that’s when it’s a good idea to go to Plan C, stop & pull out your winch controller. I’ve dragged myself up, over & through some stuff where I felt the only way to get it done with the lockers would be to hit the skinny pedal harder & harder & that’s just asking for trouble, I’m not so proud that I’ll put my axles at higher risk unnecessarily. That’s been rare, I’d say 99% of the time the lockers get the job done.
 

Xposure

TJ Enthusiast
Jul 13, 2018
397
159
Trussville, AL
#14
That is my understanding. As explained thoroughly above with the open diff when you lose traction you lose torque, so the stress on the axle goes down due to the reduction in torque seen by that axle thereby reducing the chance of breakage. By contrast with the locked axle when you lose traction the stress on the axle goes up because it’s seeing more torque & it’s all on that wheel (& therefore that axle shaft) that has traction thereby increasing the chance of breakage, particularly if it’s the only 1 of 4 tires that has good traction, as was also mentioned above. This is why I really like selectable lockers (especially for a D30/35) because I can decide when I put those smaller axles under that kind of stress, & because I have to consciously push a button I always know exactly when it’s on & what the conditions are under the Jeep (often I’ve gotten out & looked under & around to figure out what I’m doing), so I can take it really slow & reduce the chances of breakage & then the moment I’m free of whatever obstacle that required locking I hit that button again & disengage the locker & I’m back to the lower probability of breakage that comes with an open differential. In some instances if you think the risk is too high of breakage while locked that’s when it’s a good idea to go to Plan C, stop & pull out your winch controller. I’ve dragged myself up, over & through some stuff where I felt the only way to get it done with the lockers would be to hit the skinny pedal harder & harder & that’s just asking for trouble, I’m not so proud that I’ll put my axles at higher risk unnecessarily. That’s been rare, I’d say 99% of the time the lockers get the job done.
What lockers are you running?
 

Westtown Willy

TJ Enthusiast
May 10, 2018
484
671
Saltopia, PA, USA
#15
What lockers are you running?
ARB, they're really nice, they engage & disengage instantly & so far so good, no problems. Downside is they're a little pricier than autos but I really didn't want to be locked 24/7 in the rear & in the front every time I'm in 4WD, turning with a front locker on can be difficult & that's another reason I shut that thing off as soon as possible after clearing the problem. Sometimes you have no choice & have to leave it on but most of my 'stuck' issues come in steep hill climbs that are relatively straight
 

RedrumRH

WNC Asheville Area
Supporting Member
Apr 9, 2018
203
175
Mars Hill, NC, USA
#16
Since questions about the frequently misunderstood subject of 4x4 and how torque is delivered to the wheels is a common one, I thought I'd explain it.

All four tires are driven equally by the 4x4 system and all four pull equally if they all 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 an equal amount which usually means insufficient power to keep you moving.

The engine can develop no more torque than the tire with the least amount of traction can accept before spinning.


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 always (!) splits whatever torque it receives 50:50 to both sides. Why does the amount of torque get reduced at all? Because the engine only develops torque when it is working into resistance. Run an engine with a torque meter connected and watch what happens to the amount of torque produced 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 force to the engine and the amount of torque will increase in direct proportion to the amount of resistance the engine is working into. The more braking force applied to the engine's output shaft, the more torque the engine produces.

So because the engine only produces torque when working into resistance, a spinning tirereduces the amount of power (torque) the engine produces because the engine is working into less resistance caused 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 tireis 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 systems...

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!
 

Xposure

TJ Enthusiast
Jul 13, 2018
397
159
Trussville, AL
#18
ARB, they're really nice, they engage & disengage instantly & so far so good, no problems. Downside is they're a little pricier than autos but I really didn't want to be locked 24/7 in the rear & in the front every time I'm in 4WD, turning with a front locker on can be difficult & that's another reason I shut that thing off as soon as possible after clearing the problem. Sometimes you have no choice & have to leave it on but most of my 'stuck' issues come in steep hill climbs that are relatively straight
Yeah, at some point I need to go on and get lockers, I’d like too now but they are pretty darn expensive.
 

Donations

Jeep Wrangler TJ Forum's operating expenses are primarily paid by the generous contributions of members.

Donate

Latest Posts

Ride of the Month

Congratulations to ElectricWizard for winning the December 2018 Ride of the Month!

December 2018 Ride of the Month

Forum Statistics

Threads
15,572
Messages
263,982
Members
10,483
Latest member
Joshuarbelcher