14 - 4x4 operation
09-03-2014, 09:24 PM
#11
Senior Member
Thread Starter
thanks everyone! hetman
so what I get out of all the reply's in a 4wd scenario :
1) front & rear diff. are open ( only pwr to one wheel per axel @ any given time 4 lo / 4 hi )
2) ELD Locked - 2 rear wheels locked up to 25mph(4hi) or any speed in (4lo)- this setup would give you a max of 3 powered driving wheels
3) that I can lock 4hi @ any speed but probably should limit to 55 or below when engaging
4) and most important Never Trust the sales Dept. for a straight answer
so what I get out of all the reply's in a 4wd scenario :
1) front & rear diff. are open ( only pwr to one wheel per axel @ any given time 4 lo / 4 hi )
2) ELD Locked - 2 rear wheels locked up to 25mph(4hi) or any speed in (4lo)- this setup would give you a max of 3 powered driving wheels
3) that I can lock 4hi @ any speed but probably should limit to 55 or below when engaging
4) and most important Never Trust the sales Dept. for a straight answer
09-03-2014, 11:00 PM
#12
]only pwr to one wheel per axel @ any given time 4 lo / 4 hi
That's not quite correct. It was explained correctly here:
To expand on that, on dry pavement in 4hi (not recommended BTW) all four wheels are pulling equally--no wheel has preference or advantage over the other. Now put one of the 4 wheels on ice. Even though only one wheel is on ice, if you have an open diff you will pretty much loose not only the wheel on ice but also the opposite wheel on dry pavement that is on the same axle. (Note this is ignoring brake traction control/one wheel spin control which if activated would help the situation.)
09-04-2014, 12:10 AM
#13
This is not all entirely correct. Our trucks have a brake actuated limited slip that will use the truck brakes to slow down or pulse the slipping wheel, sending torque to the opposite wheel. I believe this works for the front and rear axle, even if you do not have the ELD in the rear. I saw a youtube video before I bought my truck that showed a guy without the ELD and one tire on pavement with the other tire in dirt. To illustrate how the system worked he did a burn out and both wheels were spinning, leaving rubber on the pavement and throwing up dirt from the other wheel.
09-05-2014, 12:25 AM
#14
Senior Member
To understand how the 4x4 works you must first understand how differentials work.
A true locker (such as the electronic locker in our F150s) will turn both tires at the same speed all the time, no matter what. So even with 1 tire off the ground, they will both spin at the same time.
An open differential splits the torque 50/50 between each tire. Exactly 50% to each tire. That means each tire always receives the same amount of torque. A certain amount of torque is required for the tire to turn. When a tire loses traction, there is very little torque required to turn that tire. When the torque drops on that tire, it also drops on the other tire. Often the torque drops enough that the other tire will no longer turn, even though both are receiving the same amount of torque.
From my understanding, our trucks use the brakes to act as a limited slip as described above. And it actually works very well and IMO is better than a clutch driven LSD. In fact, in the Jeep world it is well known that if you halfway engage the parking brake it will help turn both tires with an open diff. It's known as a "poor man's LSD" and I've seen it work on many occasions. The beauty about the system on our trucks (as I understand it) is that they can apply brake pressure to only one tire. That forces BOTH tires to get more torque, but doesn't increase the torque required to turn the tire with more traction.
So, in short, all 4 tires in our trucks will always have power when in 4x4, even if they don't all turn.
Here's a great write up on 4x4 systems:
http://www.jeepforum.com/forum/f27/4...nswers-130169/
A true locker (such as the electronic locker in our F150s) will turn both tires at the same speed all the time, no matter what. So even with 1 tire off the ground, they will both spin at the same time.
An open differential splits the torque 50/50 between each tire. Exactly 50% to each tire. That means each tire always receives the same amount of torque. A certain amount of torque is required for the tire to turn. When a tire loses traction, there is very little torque required to turn that tire. When the torque drops on that tire, it also drops on the other tire. Often the torque drops enough that the other tire will no longer turn, even though both are receiving the same amount of torque.
From my understanding, our trucks use the brakes to act as a limited slip as described above. And it actually works very well and IMO is better than a clutch driven LSD. In fact, in the Jeep world it is well known that if you halfway engage the parking brake it will help turn both tires with an open diff. It's known as a "poor man's LSD" and I've seen it work on many occasions. The beauty about the system on our trucks (as I understand it) is that they can apply brake pressure to only one tire. That forces BOTH tires to get more torque, but doesn't increase the torque required to turn the tire with more traction.
So, in short, all 4 tires in our trucks will always have power when in 4x4, even if they don't all turn.
Here's a great write up on 4x4 systems:
http://www.jeepforum.com/forum/f27/4...nswers-130169/