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open front & rear diffs with computer traction control is not same as locking diffs

open front & rear diffs with computer traction control is not same as locking diffs - and the difference is not even close !!!

I assume many of you have watched some videos of a Jeep Renegade (or any other 4x4 with open differentials with computer traction control) climbing a hill , where one tire in the front and one tire in the rear are up in the air or touching the ground with almost no traction. 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.

Here is the reason why:
A 4x4 vehicle with front & rear locking differentials can apply all of the torque to any tire. Thus if 1 tire has traction and 3 tires have no traction, the one tire with traction can get all of the torque to turn the tire to move the vehicle.
However - A 4x4 vehicle with open front and rear differentials can not apply all of the torque to any single tire (even with computer traction control applying full locking brake action to any tires in the air). With this type of a vehicle (like a Jeep Renegade TrailHawk), when the computer applies a traction control brake force to a slipping tire, the remaining tire with traction will now have 1/2 of the available maximum torque to turn the tire. This condition with one front and one rear tire in the air with computer traction control effectively results in a engine to torque ratio of half - and it thus behaves similar to having a 2x taller gear ratio.

Thus - if you have a choice of locking front & rear differentials verses computer traction control , and you absolutely need the torque pulling power, then always use a locking differential.


So, a Jeep Renegade TrailHawk with a 20:1 crawl ratio with one front and one rear tire in the air actually has the torque equivalent of a 10:1 crawl ratio. This explains why in some videos where a Jeep Renegade appears to run out of power when climbing a steep hill where one or two tires are in the air.

Now if the Jeep Renegade TrailHawk had front & rear electronic locking differentials (in addition to computer traction control) , the vehicle would possibly be a much more capable vehicle off road.
 

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Yea , I am several years late.
I've only had my Renegade for one week now , so to me , all of these Renegade forum topics are new to me.
I just started reading everything I can find on the Renegade and watching lots of Renegade off-road videos. And - one thing I have noticed is often when a Renegade has some wheels in the air while climbing a steep hill, that it appears to run out of power without all four wheels spinning. This is a condition I have not experienced when I previously drove my FJ.
One thing for sure, I really like the easy 30+ mpg (almost 40) I get with my Renegade when I am driving 40 miles to work.
 

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Hello Tom Jones,
I hope that you are enjoying your Renegade, and you are doing the right thing: study and try to understand you driveline/transmission. Only understanding it's architecture you will be able to explore it to it's full capability.

Your logic is right..... with traction control based on electronic breaking of sliping weels, you can loose up to 50% of torque. But I would like to share with you some thoughts on that:

1) Most part of the situations where the renegade appears to run out of power, the driver is using the wrong drive mode: they select the "rock" mode instead of "sand" with low gear. The rock mode is programmed to avoid wheel spin at any cost, in order to protect the tires over sharp rocks and sudden accelerations when traction is gained again. By doing so, it limits the capability when trying to climb a hill. The Sand mode shall be tried in these situations!

2) The apparent lack of torque (in extreme situations) on the American Renegade Trailhawk, is limited to North American vehicles. On Europe and South America, the Trailhawk has always plenty of torque because the engine is a Turbo Diesel with 260 Lb.Ft (the 2,4 Tiguershark only has a maximum of 175 Lb.Ft). This is more 50% of torque than the Tigershark... and at much lower rpms (1750rpm instead of 4500?).

3) For "pure" off road, locking diferentials in a driveline like the one used on the Wrangler Rubicon are clearly better.... but a good AWD like the Renegade has some advantages most of the days (for the most common driver):

3.1 - My wife (or anyone else) can, by mistake, select 4WD and drive all the day on the motorway and town... that the transmission will not be damaged. On a part time 4x4 if you forget to disengage 4x4H and drive for long time, you "wind up" the transmission and can cause serious damage.

3.2 - When there is ice or light snow, it is much more safe to drive in 4WD.... you can do it on the Renegade, but not on a part time driveline like the one on the Wrangler. In fact, the Renegade electronic is so sophisticated, that on AUTO mode, if the temperature goes bellow 3ºC (close to freezing point), it automatically engages 4WD to increase safety. Also, if some other conditions are met, like rain (detected through the activation of the wipers), it automatically engages 4WD to increase safety.

3.3 - Preemptive 4WD engage. The Renegade engages "preemptively" the 4WD everytime it thiks that maybe I will need it. For instance, every time I stop on a road junction, it engages 4WD and if I need to "floor" the accelerator I will have torque in 4 wheels - on a part time 4x4 I'm limited to 2 Wheels.

Note that electronic traction control appeared first time on the Land Rover Discovery 2 in 1998. Since then it improved dramatically, and now the most advanced electronic traction control found on the Land Rover Discovery IV or the Bentley Bentayga are much more sophisticated, faster and precise. Of course the Renegade don't use the "state-of-the-art" system, but it works well.

The Jeep Active Drive 1 architecture used on the Renegade Trailhawk, using a 9-speed gear box (were the first gear is so reduced that is considered Low Range, and the car usually starts in 2nd gear except if in one of the off-road modes), auto rear axle engage/dis-engage, and open diffs with electronic traction control is also being used on the Range Rover Evoque.

In fact, The Renegade Trailhawk (specially the European Turbo Diesel version) is almost a "case study", being refereed several times on the recent text book "Four-by-four driving, 4th edition" from Tom Sheppard, that presents on the first chapters the various four-wheel systems, detailing how do they work, their advantages and disadvantages, as well as future trends.
Best regards,

Bruno
Germany
Renegade 2.0MJET Trailhawk.
 

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I've been told one other benefit (feature) of the open differential/electronic 4WD system on the Renegade is that you have to replace all four tires at one time to keep the same tire circumference all the way around. You won't damage the drive train if the tires are of slightly different sizes. (Case in point, We damaged the side wall on the right rear tire on my wife's Dodge Durango. With 30,000 miles on the tires, the tread depth on the remaining three tires was below what FCA recommended as the maximum difference between tires if we had simply replaced the one damaged tire, so we had to replace all four - good news for Discount Tires. Not so good news for our bank account). If we encountered a similar situation on my Rene TrailHawk, I could get by with just replacing the damaged tire.
 

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The Wrangler has brake-diff-locking too.
Go figure.


:lol:
 

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Two totally, completely different set ups. Not even relatable. In my CJ7, I have a LockRight in the front and limited slip in the rear. The LockRight will lock both front tires and keep digging.
 

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Hi all, I noticed (or have a feeling) that my TH diesel 2.0L is loosing power/torque whenever is pushed too hard To test this, I tried the same hard hill climb twice on same occasion using the exact same handling but the 2nd time the vehicle couldn't make it up, and it felt weaker than the first time. Can this be true?? Is the system reducing power-totque due to overheating? Oh, and when is pushed too hard the “4wd serv” notification pops up also, which means 4wd system is not functioning anymore. Can anyone help? Thanks!!
 

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You are all forgetting an important consideration. “Available Torque” is determined by the engine and driveline computers. The engine computer is designed to reduce “available” torque in conditions where a wheel is being braked to distribute traction to the remaining wheels. This is done to prevent damage to the drivetrain. The computer can and will reduce available torque to almost zero if, for example, two of the four wheels are being braked and one of the remaining 2 wheels has more traction than the other. If this did not happen and full available torque was applied to one wheel in the drivetrain (braking the other 3) damage will likely be caused. The drivetrain hardware in the Renegade is identical to that in the Range Rover Evoque. It is licensed to Fiat by the same company (GKN Driveline in the UK). However the parts in the Renegade system are much less robust than that in the Evoque.
Secondly braking a wheel does not reduce available torque by 50%. Although it does reduce available torque it s something far less than that. Again it is the engine computer that drops the torque to protect the system.
If you were to disable the engine torque management the system would put all the engine can produce into the driveline. When 4 wheels have traction it will go to all four wheels. If one or more wheels lost traction and were then “braked” the remaining torque would go to the wheel(s) that have traction. The next thing likely to happen is the CV joint on that wheel(s) would be damaged.
 

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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.
 

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Thanks for your input, really enlightening! I've also experienced the situation you described with the accelerator floored but no rpm response. I guess this torque reduction/cut is really to protect the 4WD system? But then again why bother using the terrain system anyway when in most off-road scenarios you'll often encounter traction loss of at least one wheel , and this is precisely where you need the system to work and get you out...unless I haven't fully understood the protection mechanism. I'm also really puzzled as to why the 4WD system has managed to climb that loose rocky surface the first time clearly indicating that it was well within it's capability limits, but the 2nd time that followed immediately (same day-time and approach used) it failed and power was cut.
 

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Agree, and I’d love to have a technical bulletin on specifically how the system works, where does it get its inputs, what are the limitations and triggers on those inputs and what effect the different drive (traction) modes have on the limitations. This way it would be possible to modify the way you take on obstacles to avoid complete loss of available torque.
These systems have been around for a long time in aviation. Here they are commonly referred to as governors. Propeller torque especially is governed in turboprop engines. Over-torque and fail the prop-shaft and you come out of the sky (instead of being stuck on the side of the road). The airplane I currently fly has a maximum torque ability of 1800 ft/lbs but it governs the prop torque to 1012 ft/lbs maximum. It will allow 1350 ft/lbs if the prop is spinning at high rpm and airplane is over 140 KIAS. It also doesn’t give you the full amount on acceleration, allowing only 800 ft/lbs until the props reach full speed. All this is to protect the propeller gearbox and propshafts. Exactly the same as what’s happening in the GKN Renegade drivesystem.
Does the system work? How many reports of broken Renegade CV joints or prop-shaft joints have you heard? I’ve not heard a single one in this vehicle, but if you go to the Jeep CJ or any other older tech 4wd forum you will find countless posts on repairing/upgrading rear ends, transfer cases, CV and universal joints that are grenaded by too much torque. Same thing on hot rod builds, if your going to build the engine, first thing to upgrade is the rear differential.
 

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Thanks for your input, really enlightening! I've also experienced the situation you described with the accelerator floored but no rpm response. I guess this torque reduction/cut is really to protect the 4WD system? But then again why bother using the terrain system anyway when in most off-road scenarios you'll often encounter traction loss of at least one wheel , and this is precisely where you need the system to work and get you out...unless I haven't fully understood the protection mechanism. I'm also really puzzled as to why the 4WD system has managed to climb that loose rocky surface the first time clearly indicating that it was well within it's capability limits, but the 2nd time that followed immediately (same day-time and approach used) it failed and power was cut.
Well I am no 4-wheeler, but I have had a similar experience in my old Grand Cherokee SRT. I can say that the torque management is there to protect the 4wd, and also the brakes if equipped with BLSD (brake limited slip differential). Playing around learning its limits and repeated spirited starts around corners was intoxicating. Doing it one time too many would result in a 1-wheel peel with seemingly no movement. Like the awd/blsd just shuts down until things cool off.

There is a look-up table somewhere in the pcm that determines the heat load on the brakes based on how much and how long the BLSD is in use was the consensus... But I wouldn't think slow rock crawling with a wheel in the air would not generate enough heat on the BLSD to effect your 2nd climb.
 

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Thanks for your input, really enlightening! I've also experienced the situation you described with the accelerator floored but no rpm response. I guess this torque reduction/cut is really to protect the 4WD system? But then again why bother using the terrain system anyway when in most off-road scenarios you'll often encounter traction loss of at least one wheel , and this is precisely where you need the system to work and get you out...unless I haven't fully understood the protection mechanism. I'm also really puzzled as to why the 4WD system has managed to climb that loose rocky surface the first time clearly indicating that it was well within it's capability limits, but the 2nd time that followed immediately (same day-time and approach used) it failed and power was cut.

You have the diesel, so you have wayyy more torque but I recently experienced how protective the torque reduction is (which is good in this case) when playing around with 9th gear on the highway. I was worried if it lug the engine if speed dropped and sure enough it only allowed enough throttle to keep the engine happy regardless of throttle input.
 

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Well I am no 4-wheeler, but I have had a similar experience in my old Grand Cherokee SRT. I can say that the torque management is there to protect the 4wd, and also the brakes if equipped with BLSD (brake limited slip differential). Playing around learning its limits and repeated spirited starts around corners was intoxicating. Doing it one time too many would result in a 1-wheel peel with seemingly no movement. Like the awd/blsd just shuts down until things cool off.

There is a look-up table somewhere in the pcm that determines the heat load on the brakes based on how much and how long the BLSD is in use was the consensus... But I wouldn't think slow rock crawling with a wheel in the air would not generate enough heat on the BLSD to effect your 2nd climb.
Got it! I wish there was some kind οf thermometer indicator provided for the 4wd system as well, similar to the coolant, transmission, oil etc. that you can check on driver screen. At least you'll know in advance and decide, or avoid getting stuck due to unexpected 4WD failure.
 

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This is done to prevent damage to the drivetrain. The computer can and will reduce available torque to almost zero if, for example, two of the four wheels are being braked and one of the remaining 2 wheels has more traction than the other. If this did not happen and full available torque was applied to one wheel in the drivetrain (braking the other 3) damage will likely be caused.
That isn't actually correct.
Just considering one axle:
When both wheels grip the diff applies equal torque to both half-shafts - half the propshaft torque to each.
f there is a locking diff and one wheel has no traction the full torque from the propshaft will be applied to the gripping wheel's half-shaft.
With a braked system with open diff the two half-shafts always get half the torque - the slipping wheel is prevented from spinning by the brake, which from the drive line's point of view is the same as being on the ground.
 
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