[k8ysv]

Originally Posted by Blaine59

Can I test you on my WJ's qudradrive?

It has a PC not a VC and LSD's front and back. Jeep claims it can trasfer the power to any of the wheels it wants... Now I have seen this... a WJ on a steep rock incline at moab is losing momentum and eventaully comes to stailmate with the incline. Then you see the power shifting from one wheel to the next untill it finds the best combonation of power and the Jeep continues up the rock... HUH? how'd that happen???

I'm gonna take a guess at this one:

The traction control system's computer is tied into the ABS system. Since ABS can apply braking to any wheel individually, when the traction control senses wheel slip, it applies braking to that wheel. This will encourage the opposite wheel to turn and hopefully it has more grip. The computer will try different combinations of braking on different wheels until it gets the results it wants to see, which is no wheels slipping and the vehicle moving forward.

This is not a known fact (to me anyway) but a SWAG.

[Jerry Bransford]

(from a thread on torque and how much torque gets sent through the axles under low and high traction situations)

What is really happening is that an engine cannot develop 100% of the torque it is capable of if it sees a reduced amount of traction where the tires meet the road surface. An engine can only produce its maximum theoretical amount of torque when it is working into pure resistance with no tire spinning or slippage. The instant one side (tire) starts slipping when no locker is present, the amount of torque gets reduced to that precise amount that causes a tire to start spinning. If an engine can theoretically produce 400 (or whatever) ft-lbs of torque, it may only actually produce 100 ft-lbs or even 40 ft-lbs if the tires are on a slippery surface. The maximum amount of torque that can ever be produced by an engine is that amount that starts a tire spinning. So if you're driving on a slick surface, your engine would not be able to produce its full potential amount of torque since the tires will start slipping well before that point.

So if it takes, for example, 100 ft-lbs of torque to start a tire spinning on a slick surface, then an open differential will split that 100 ft-lbs so each side is seeing 50 ft-lbs.

Torque is an interesting force. Connect an engine to a torque-measuring device and run the engine without a load connected to it and the amount of torque produced by that engine at any RPM will be nearly zero. Start applying a braking force against that engine's output shaft so the engine starts working harder and the amount of torque will increase proportionately to the amount of resistance the engine is working into. The more work the engine does, the more torque it can generate.

So if your monster 400 ft-lbs of torque engine had one wheel on a high-traction surface and the other wheel on a slick oil-coated surface, the engine would produce only enough torque to start the wheel on the slick surface spinning. Once that one side starts spinning, the engine is producing the maximum amount of torque it's going to which is just enough to start that tire spinning. Giving it more gas so the RPMs jump while that tire is spinning will still produce the same low-amount of torque. Whatever low mount of torque that engine is developing, that open axle is dividing it 50:50 left and right.

Installing a locker then allows the engine to produce more torque since the engine is then seeing the increased resistance from the tire on the other side of the axle that has good traction... remember the above example where one tire had good traction and the other tire had poor traction. With a locker, the engine sees the resistance of the side with the most traction. With an open axle, the engine is only seeing the resistance of the side with the least amount of traction.

The bottom line is that an engine only produces enough torque to cause slippage of the tires. With an open axle, it is limited by the side with the least amount of traction. With a locker, it is limited by the side with the best traction. With a limited slip differential, the torque is somewhere between that of an open axle and a locker, but much closer to the side with the open axle. A limited slip differential usually only has around a 30% torque advantage over that of an open axle vs. 100% of a locked axle.

I hope this helps clarify this often misunderstood subject just a tad.

[blackZZR]

I think what a few are missing on the torque thinngy is the difference between produced power and applied power
produced power is potential, you need resistence to get applied power

[ncb76]

This might help

[dirtbagofva]

Here is a little Pysics lesson:
If it takes 50# of torque to slip, it takes only 25 to spin. Same reason why it takes longer to stop with your wheels locked up on snow/ice/wet road.

So in the example above: You are producing 200# one front wheel spins, the torque going to each front wheel goes from 50 down to 25. Now all the torque you can produce goes to the rear less the front (200-25-25=150) If the rear has the traction to handle the extra power you go forward if not you go nowhere/backward.

[table]

Here's one i stole from someone else, forgot who:

[Broken_Soldier]

Originally Posted by Jerry Bransford

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.

Given the above quote and the nature of a differential's effect on torque with slipping tires, does this also apply to front/rear? If a full-time system got into a situation where both rear tires or both front tires had little or no resistance would little or no torque be developed on the opposite side's tires? Imagine a Selectrac funtioning, for the front/rear wheels, like an axle differential for the left/right wheels. Is this a crazy misunderstanding on my part or entirely possible?

[Jerry Bransford]

Yes that effect is entirely possible with some full-time 4x4 systems... which is exactly why Jeep's Quadradrive system has hydraulic gizmos at all four wheels to apply resistance when needed.

[Broken_Soldier]

Ah very cool. Thanks for the answer Jerry. So this means I can't brag about my t-case being locked?

[Jerry Bransford]

Originally Posted by Broken_Soldier

Ah very cool. Thanks for the answer Jerry. So this means I can't brag about my t-case being locked?

I don't know what transfer cases are available in the LIberty but I'd still think your transfer case has a part-time mode in addition to any full-time mode it may have. At the least, if it has a 4Lo mode, that is a locked part-time mode.

Any more questions really should be made in the main part of the forum, not in this sticky that we try not to get too long.

[menoche]

Nice article

I had to write something like that on the subaru forum to explaine the LSD

To add to the why torque goes down, here's a nice example:

Imagine the torque going to each wheel being water hoses.

The friction to the road is a faucet.

So you have 4 faucet and 4 hoses coming from the same water pump (your transmission).

On a nice paved road, your friction is at max (all faucet barely opened)

each faucet thus expelling a small spray but with lots of pressure right?

if you loose traction on one wheel, it's the equivalent of opening wide one of the faucet. You'll end up with a large water spray with NO PRESSURE or barely. and the 3 other faucet spray will loose their pressur thus your traction.

with the quadratrack, the system comes to the rescue by binding the water hose (applying brakes) restricting the water flow and giving back pressure to the other faucets...

I hope this helps the "visual" type of guys in here

Philip

[winkosmosis]

This thread is what taught me that both wheels on a differential always get equal torque. It clicked, and made me go "OH". That was years ago, before I even had a jeep. No idea how I had found this thread, but thanks for posting this comprehensive overview

[erichb]

Originally Posted by warsw

Jerry, I have always understood how an open differential works but, this is the best explanation I have ever heard as to why. Thanks for the post.
Randy

X2!
Way to go!

[RogueSpear2023]

Thanks Jerry, Very well explained the article if any one reads is good too, especially the part about torsion diffs never understood why they were better than a regular LSD until now. I still love my ARB lockers though.

[TopMonkey]

Menoche, thanks, I'm definately a "visual" type of guy. Now I get it