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Understanding locking differentials...

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PackerEdgerton
Posts: 168

Re: Understanding locking differentials...

Post#11 » Fri Feb 28, 2014 10:20 am

Thank you, I appreciate the education, and makes me appreciate my 300GD quite a bit more!

All the best,

Packman

PackerEdgerton
Posts: 168

Re: Understanding locking differentials...

Post#12 » Thu May 15, 2014 1:05 pm

4x4abc wrote:
PackerEdgerton wrote:
My W460 has no center differential, thus when the transfer case is in 4WD and the front and rear differentials are locked, then any there is no speed differentiation between front and rear axles. Thus, even if only 1 tire has traction, you can still move.

Did I get that right?

Thanks,

Packman


Packman,
...
Your 300GD produces roughly 340 lb/ft. First gear roughly 4:1, transfer case 2.14:1, axles 4.9:1 = 42:1 crawl ratio.
42 x 340 = 14,280 lb/ft in low range, first gear.
So, 14,280 lb/ft would be the force acting on one axle shaft, if you had (sufficient) traction on only one wheel.
Quite a lot. And thus better spread out over all 4 wheels (3,570 lb/ft ea).

nuff for now


I was re-reading this today, and I'm not sure this is correct. When we are in 4-Lo with both differentials locked, then the engine's torque is being sent equally to all wheels regardless of traction. If only one wheel has traction, then the torque is 1/4 of the total available, isn't it? It's not like all wheels stopped and the torque was sent only to one wheel. Therefore, I think the max torque (assuming the math above is correct otherwise), would not exceed 3,570 lb/ft at the one wheel, right?

Thanks,

Packman

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AlanMcR
Posts: 5543

Re: Understanding locking differentials...

Post#13 » Thu May 15, 2014 2:15 pm

PackerEdgerton wrote:I was re-reading this today, and I'm not sure this is correct. When we are in 4-Lo with both differentials locked, then the engine's torque is being sent equally to all wheels regardless of traction. If only one wheel has traction, then the torque is 1/4 of the total available, isn't it? It's not like all wheels stopped and the torque was sent only to one wheel. Therefore, I think the max torque (assuming the math above is correct otherwise), would not exceed 3,570 lb/ft at the one wheel, right?

Thanks,

Packman

100% of the torque is delivered to the wheel or wheels which still have traction. Practically, this often results in that wheel breaking free too.
This stands in contrast to the systems that use individual brakes to slow down the spinning wheels. Those systems wind up distributing torque relatively evenly to all wheels, but any torque that isn't transferred to the ground is converted to heat by the brakes.

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4x4abc
Posts: 533
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Re: Understanding locking differentials...

Post#14 » Mon May 26, 2014 6:00 pm

Packman,

Alan got it right (why wouldn't he) - if only one wheel has traction, only at that wheel torque will be created.
If all 4 wheels had equal traction, then 1/4 of the maximum torque can be created at each wheel.
The amount of torque at only one wheel with traction, depends on the amount of traction at that wheel.
100% of possible torque at one wheel is very unlikely. Maybe on coarse granite with a lot of weight pressing the tire down.
100% of torque at wheel is not desirable. A lot less is needed to move the car. And 100% is a tremendous breaking force. G axles are designed to withstand that tremendous force - Jeep axles are not.

Torque needs to be "created". The "potential" torque needs a counter force to create torque. A counterforce in form of resistance like traction. The amount of resistance determines how much torque can be created (up to the maximum the gear train is able to deliver).

If you have a potential torque of 1,000 units and the traction at all 4 wheels is greater than 250 units, a maximum of 250 units of torque per wheel can be created. If one of the wheels is on slippery ground (3 open diffs) and has only 100 units of traction, that tire would break lose and slip as you increase the gas to more than 400 units of torque (100 at that tire). If your car would need a minimum of 500 units of torque to move, you would be stuck, because you are creating only 400 units. So, even the 3 wheels not moving are getting 100 units of torque each. It's a rule in mechanics that with open differentials all wheels get equal torque. Always.

If you so will, open differentials act like safety valves - no wheel can get more torque than the wheel with the least traction. That way no excessive torque is ever created - which could twist/break some vital parts.

Once you disable the differentials (diff locks disable the differentials) the wheel with the most resistance/traction will have the most torque. That way you can create the amount of torque needed to move the car. But since now the total torque is handled by either 3, 2 or one wheel(s), one has to be very very gentle on the throttle. #1 - those tires with traction could easily overpower the traction and start slipping as well, #2 with super traction on the remaining wheels, any amount of torque higher than needed to move the car will go into twisting components. Just ask the Jeep guys or check youtube.com.

Baby talk like "power goes to the wheel with the least resistance" is just that - baby talk. Or forum talk.
Power does not move the car. Torque does.

Sorry for the rant - 4x4 stuff is not easily explained. And there is no short version.

Daewoo
Posts: 706

Re: Understanding locking differentials...

Post#15 » Mon May 26, 2014 6:59 pm

4x4 you are not ranting. You definitely understand the theory behind 4x4. Anyone can cite torque is r x f, but it takes real understanding of 4 wheeling to explain the physics like you have done above. Thank you!

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vadimivanovich
Posts: 8485

Re: Understanding locking differentials...

Post#16 » Mon May 26, 2014 9:18 pm

Hear hear! Thank you Harald!

PackerEdgerton
Posts: 168

Re: Understanding locking differentials...

Post#17 » Thu May 29, 2014 7:11 am

Thank you for taking the time to educate me. It's really appreciated.

All the best,

Packman

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4x4abc
Posts: 533
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Re: Understanding locking differentials...

Post#18 » Wed Jul 08, 2015 7:53 am

Alan,

the ABS system provides some vital parts for electronic traction control (wheel speed sensors, the computer, the ABS "distributor"). You would need to add a reservoir with brake fluid constantly held under pressure by a suitable pump. ABS decreases brake pressure on selected channels - electronic traction control increases brake pressure on selected channels.
Harald

Quilmes
Posts: 6
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Re: Understanding locking differentials...

Post#19 » Sun Jun 21, 2020 8:12 pm

Hello need a bit of help with Locking Diff, I have a 1990 300GE, It has been sitting for a while, years, I finally got it running and notice when I drive one of the locked diff lights is on red, and when I try to make a rather sharp turn, Left or Right, it seems to be pushing or trying to come to a stop, as if the front diff was locked.
Can it be that it is locked versus open. How do I resolve that, I do not want to drive it especially on dry pavement until this is fix.

Any help ideas is greatly appreciated.

Martin

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AlanMcR
Posts: 5543

Re: Understanding locking differentials...

Post#20 » Mon Jun 22, 2020 7:36 am

As you suspect, the front is locked. Don't drive this until fixed.
For diagnosis/repair, pull the parking brake (test that it works), Put the transfer case in the middle position (N). Then lift one of the front wheels. If you can't rotate the wheel freely, the locker is on.

If you are lucky, the problem is hydraulic. Open up the bleeder on the front slave cylinder. And try the lifted wheel again. It should release and the red light should go out. If not you'll need to remove the slave cylinder from the axle and inspect. Removing the cylinder will allow you to drive, but you'll need to put a plate over the locker opening in the axle.
...Alan

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