open front & rear diffs with computer traction control is not same as locking diffs - Page 3 - Jeep Renegade Forum
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post #21 of 32 (permalink) Old 01-03-2019, 03:27 PM
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Sorry but you are wrong here, when a differential is “locked” the torque goes equally to both wheels, both wheels turn at the rate of the wheel with most traction (slower wheel). That is where the term “locked” comes from. It is for this reason a locked differential, or locked transfer case as well, cannot be used on hard surfaces roads, because in a turn the outside wheel is forced to revolve faster than the inside wheel, when locked this becomes impossible so the outside wheel skids. To address this limited slip differentials were invented, many of which are a form of mechanical inboard brake (using clutch Plates) on the differential but they are a compromise as is the electronically braked differential that we have.
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post #22 of 32 (permalink) Old 01-03-2019, 03:43 PM
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I had a 1973 Dodge Dart with a standard open rear differential, if you jacked the rear of the car up and held one of the wheels with your hand, when you put the car in gear the other wheel would spin. No torque at all applied to the wheel you are holding. Now if brake were to be applied to the spinning wheel torque proportional to that force is returned to the other side.
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post #23 of 32 (permalink) Old 01-03-2019, 04:30 PM
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I’d also like to add that the system limits torque by reducing the injector output. You might have the accelerator floored but the computer overrides your input to determine how much fuel the engine receives to keep the torque output within its designed limit. The system also considers wheelspeed in the equation. More torque can be applied to a wheel/drivetrain already in motion than one at standstill without damage. So if you are completely stopped the system will reduce initial available torque even more than if the the drivetrain was turning. This is why in a mud situation it’s able to power through if you keep momentum. If you stop then there will be even less power available to get going again.
Keep in mind that the total engine output, particularly the diesel, of this vehicle is more than some V-8 engines during the 70’s and 80’s. It is not a question of how much torque the motor can make, it’s a question of how much torque the computer will allow to the drivetrain.
Great post! Couldn't have said it better.


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post #24 of 32 (permalink) Old 01-03-2019, 06:15 PM
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Originally Posted by Fingers View Post
Sorry but you are wrong here, when a differential is “locked” the torque goes equally to both wheels, both wheels turn at the rate of the wheel with most traction (slower wheel). That is where the term “locked” comes from. It is for this reason a locked differential, or locked transfer case as well, cannot be used on hard surfaces roads, because in a turn the outside wheel is forced to revolve faster than the inside wheel, when locked this becomes impossible so the outside wheel skids.
Sorry, but you are wrong. Locked differentials transmit equal movement to each side, not equal torque. An open differential is the opposite - it allows unequal movement but in the process delivers equal torque.

For an open diff with both wheels off the ground virtually no torque is applied because the wheel(s) spin freely. Holding one with your hands just makes the other rotate twice as fast. If only one wheel is off the ground and spinning and you try to hold it (trying to is Darwin Award material) then some torque, equal to your strength, would be generated on both wheels.
This is why when a car with open diff is stuck in the mud/snow with one wheel spinning you put your mat or whatever under the spinning wheel, not the one that is still gripping.

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post #25 of 32 (permalink) Old 01-04-2019, 01:10 AM
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From Car and Driver:
The humble open center differential*—simple, reliable, cheap—has been driven to near extinction by electromechanical alternatives that offer more control and greater efficiency. An open differential, a variation of the common planetary gearset found in automatic transmissions, splits a single torque input (the transmission) into two outputs (the front and rear axles) but allows them to rotate at different speeds. Yet open diffs have no means of limiting the speed variation between the two outputs, so torque is free to follow the path of least resistance. Hence, it’s possible for a vehicle to become stuck with one wheel spinning furiously while the others remain stationary. Most modern vehicles compensate with a cheap but effective combination of software and existing hardware that uses the brakes to create a reaction torque at the slipping wheel, closing the path of least resistance and thus increasing the torque applied to the wheels with more traction.

https://www.caranddriver.com/feature...-drive-system/
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post #26 of 32 (permalink) Old 01-04-2019, 01:31 AM
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I think we are disagreeing over semantics the equal movement you describe produces torque when the wheel is braked, “reaction torque’ as it is described in the above C&D article, the torque then flows to the “path of least resistance” via the open differential (the un-braked wheel). Since that wheel has traction the torque supplied allows the vehicle to move.
Regardless of how you want to describe how or where the torque exists or what constitutes torque the fundamental point here is that the computer in this vehicle will reduce engine output so the torque does not exceed the capacity of any structure in the driveline system, the differential itself, the clutchpacks or the individual CV joints. This is the important point because there are many who think the vehicle is stuck because the engine has insufficient power to turn the wheels when in deep mud, rocks, sand ect...
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post #27 of 32 (permalink) Old 01-04-2019, 05:14 AM
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I think we are disagreeing over semantics
To a large extent, yes.
That Car and Driver item is not a bad description though the idea of torque following the path of least resistance is interesting. Torque is a force and always exists as action and reaction - you can't push with any force on something that doesn't resist. If a slipping wheel only needs X torques to spin it then X torques is what you'll get on the other side of an open diff.

My main point is that with open diffs there is no way to apply more torque to drive-train components by braking individual wheels than in normal operation, such as flooring it when the lights go green, as torque is always distributed equally. The caveat to that is that in normal driving 1st gear/low range is not used - when it is used the available torque is substantially increased, which is possibly why management of engine output is needed at such times.

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post #28 of 32 (permalink) Old 02-19-2019, 05:59 AM
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The system also considers wheelspeed in the equation. More torque can be applied to a wheel/drivetrain already in motion than one at standstill without damage. So if you are completely stopped the system will reduce initial available torque even more than if the the drivetrain was turning. This is why in a mud situation it’s able to power thro
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post #29 of 32 (permalink) Old 02-19-2019, 06:00 AM
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On steep hills, the vehicle (like a Jeep Renegade TrailHawk) appear to run out of power and can no longer move forward - even when there are no spinning tires.
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post #30 of 32 (permalink) Old 03-06-2019, 04:16 AM
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On steep hills, the vehicle (like a Jeep Renegade TrailHawk) appear to run out of power and can no longer move forward - even when there are no spinning tires.
The computer is programed to cut power to save drivetrain components from failing.

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