Using four wheel drive. - Jeep Compass Forum
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post #1 of 24 Old 03-02-2019, 10:34 AM Thread Starter
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Using four wheel drive.

Just bought a 2018 COMPAS Limited and although the dealer explained the system am still confused. What does the 4wd lock button do and when exactly do you use it. Does it serve the same function as 4wd low? And does switching from auto to snow mode put vehicle in full time four wheel drive? We had a Ram 1500 previous and it was 4wd high or 4wd low, so not confusing to use. Many thanks for the help
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post #2 of 24 Old 03-02-2019, 12:31 PM
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It distributes power to the rear wheels in varying degrees. Each notch on the knob sends more power to the rear wheels until its 50:50.

I was told that AWD means if the rears need power it will go there, but frankly in normal/AWD mode I don't think there's much power going to the rear wheels at all -- its basically FWD. I've had my fronts spinning and not moving at all. Moving the knob to add more power to the rear got me going. To be honest, that wasn't a real world situation, I was trying to climb a snowbank to see how the AWD would respond.

When you come right down to it, in most cases FWD is enough, even driving in significant snow. AWD & 4wd are just things to raise the MSRP and their profit margin.

If you've got a hunting lodge out in the sticks, try making it up that country road in AWD. Your Compass will probably do it. But if you want to feel like a real man, then turn that knob, square your shoulders, throw your head back and feel that extra traction (lol). Or just leave it in AWD/FWD and you'll get there, but without the testosterone boost.

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post #3 of 24 Old 03-02-2019, 02:04 PM
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Boy that last post, terrible.

I'm a man, I can explain it better.... 4wd and rock mode is for those times when you have to get to the top of a mountain to release bald eagles into the wild which you have trained to kill terrorist from hatchlings.

Dont listen to that beta crap, be awesome. If you want to really learn how to use it, go off roading!
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post #4 of 24 Old 03-02-2019, 03:24 PM
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partyflipflop sounds more like a party girl than a macho man. Just saying how it looks.

No eagles around here this time of year. The rivers are all frozen so they head to the coast where they can fish in the open water.

If you really want to do serious off-roading, buy a Wrangler. I've got one of those in my stable, too.
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post #5 of 24 Old 03-02-2019, 10:37 PM
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"4wd lock" locks the power transfer unit (PTU) so some power is sent to the real wheels. By default, power is split 50/50 but if the traction control detects and slippage on any wheel, it will cut the power to that wheel and send the power to the wheels with more traction. As an example, if front wheels start spinning, it can make the ratio 30/70 and send the majority of the power to the rear. On each axle, break system can further divert power going to either left or right wheel, depending on which has more traction.

Jeep is very unclear about the capabilities of the system, but according to the description of the UK based company that produce the system, up to 100% of the power can be sent to any axle. So the car can effectively become anything from front wheel drive, all wheel drive or rear wheel drive. I cant think many condition where it would need to be rear wheel drive but people with Renegade in EU that have the app which allows you to monitor the power going to the axles/wheels reported up to 95% power going to rear wheel when cursing on sand.

Putting the car on other settings like snow, sand, rock etc, changes the software based constrains of the traction control. For example, snow mode tolerates tire slippage and doesnt cut the power to the slipping wheel. So that if you got stuck in snow, it allows tires to spin while you try to break free. Rock mode on the other hand tolerates almost no tire spinning since spinning tires during rock climbing can cause you to lose control or traction and therefore it is dangerous. So basically each of these setting fine tunes the traction control for better performance on the given surface.

On auto mode with 4wd lock not engaged, the car is effectively front wheel drive. This saves gas since it disengages the rear axle and reduce the overall friction. Even then, if the traction control detects wheels are sniping, PTU will engage and send power to the rear wheels. Amount of power send to rear wheel with again depend on how much traction wheels have. I assume after a certain amount of time, if slippage is no longer detected, PTU will disengage and car once again becomes FWD.

4wd low is only available on trailhawk trim. Although these cars dont have a 2 speed transfer case that you normally find on a car with low range gear, Trailhawks do has a different final drive ratio. Activating 4wd low will force the car to stay in 1st gear ( a gear that is not normally used since the car starts at 2nd gear) and modify the throttle sensitivity in such a way that it gives more throttle control in slow speeds. So in a way, it simulates how a "mechanical" 2 speed transfer case operate when in 4wd low.
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post #6 of 24 Old 03-03-2019, 02:39 AM
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Thanks, Tripod. That's the best explanation I've seen anywhere.
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post #7 of 24 Old 03-03-2019, 12:50 PM
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Always lock out the traction control if you're actually needing 4wd
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post #8 of 24 Old 03-04-2019, 04:55 AM
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There is some confusion and misunderstanding a lot of people have about the AWD system... I don't claim to have all the answers and full understanding either but...

To the original poster: All you really need to know is that the AWD system is more or less completely automatic and you really don't need to mess with it in day to day driving ever, with one exception: If you actually are driving in a real snowfall, then putting it in snow mode really is the best thing to do. Everything else / all the other modes are basically just software marketing gimmicks that are fun but useless to the average road-going person.


As for the rest of you trying to help the OP out here, I know you mean well but some of you have a lot of things wrong.

I don't want to pick on Tripod but his post definitely had the most, erm, errors... I'll just try to stick to the big ones...

Quote:
Originally Posted by Tripod
As an example, if front wheels start spinning, it can make the ratio 30/70 and send the majority of the power to the rear.
No it can't. Max 50% to the rear. Front axles cannot be disengaged on this system.


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Originally Posted by Tripod
On each axle, break system can further divert power going to either left or right wheel, depending on which has more traction.
Yeah kinda but don't forget torque is applied to (or you might say, used up on) the braked wheel as well so... its not like all the torque can be sent right or left, just roughly the same amount as the braked/slipping wheel can hold back. Thats why these things cut power going up a steep hill with one wheel off the ground, the computer knows when the torque is going to exceed the brake's capacity and cuts engine power back when you approach that point. This is demonstrated in a lot of videos like when the TFL guys drove one up their "gold mine hill".


Quote:
Originally Posted by Tripod
according to the description of the UK based company that produce the system, up to 100% of the power can be sent to any axle.
GKN has systems that can send full power to either axle. The system in the Compass and Renegade is not one of those systems, however. Compass can put 100% power to the front, but not the rear.


Quote:
Originally Posted by Tripod
So the car can effectively become anything from front wheel drive, all wheel drive or rear wheel drive.
No it can't, see above.


Quote:
Originally Posted by Tripod
people with Renegade in EU that have the app which allows you to monitor the power going to the axles/wheels reported up to 95% power going to rear wheel when cursing on sand.
I don't know what the software is showing them but I do know first-hand that software can show people whatever it feels like showing them to make them feel good, regardless of how accurate it is. Could be 95% of "available traction" ie that it is showing them which wheel is slipping the least and doing the most tractive work, but it is simply impossible for this system to send more than 50% rear-wards and from thereon that power is split left/right only inasmuch as both wheels have traction or brake capacity to combat slippage.


Long story short, its a complicated system that is far more limited than Jeep lets on. They prefer that people dream up it can do things it can't (like posted above), than to come out and say what it really can and can't do (which is much less impressive). Its more than good enough for average on-road driving, even in bad weather, and like I said before its almost entirely automatic so a person really not need worry about the gimmicks and software modes at all, other than that snow mode really is helpful moreso in its throttle dampening more than anything it does with the AWD power split.
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post #9 of 24 Old 03-04-2019, 07:41 AM
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Originally Posted by arudlang View Post
There is some confusion and misunderstanding a lot of people have about the AWD system... I don't claim to have all the answers and full understanding either but...

To the original poster: All you really need to know is that the AWD system is more or less completely automatic and you really don't need to mess with it in day to day driving ever, with one exception: If you actually are driving in a real snowfall, then putting it in snow mode really is the best thing to do. Everything else / all the other modes are basically just software marketing gimmicks that are fun but useless to the average road-going person.


As for the rest of you trying to help the OP out here, I know you mean well but some of you have a lot of things wrong.

I don't want to pick on Tripod but his post definitely had the most, erm, errors... I'll just try to stick to the big ones...



No it can't. Max 50% to the rear. Front axles cannot be disengaged on this system.




Yeah kinda but don't forget torque is applied to (or you might say, used up on) the braked wheel as well so... its not like all the torque can be sent right or left, just roughly the same amount as the braked/slipping wheel can hold back. Thats why these things cut power going up a steep hill with one wheel off the ground, the computer knows when the torque is going to exceed the brake's capacity and cuts engine power back when you approach that point. This is demonstrated in a lot of videos like when the TFL guys drove one up their "gold mine hill".




GKN has systems that can send full power to either axle. The system in the Compass and Renegade is not one of those systems, however. Compass can put 100% power to the front, but not the rear.




No it can't, see above.




I don't know what the software is showing them but I do know first-hand that software can show people whatever it feels like showing them to make them feel good, regardless of how accurate it is. Could be 95% of "available traction" ie that it is showing them which wheel is slipping the least and doing the most tractive work, but it is simply impossible for this system to send more than 50% rear-wards and from thereon that power is split left/right only inasmuch as both wheels have traction or brake capacity to combat slippage.


Long story short, its a complicated system that is far more limited than Jeep lets on. They prefer that people dream up it can do things it can't (like posted above), than to come out and say what it really can and can't do (which is much less impressive). Its more than good enough for average on-road driving, even in bad weather, and like I said before its almost entirely automatic so a person really not need worry about the gimmicks and software modes at all, other than that snow mode really is helpful moreso in its throttle dampening more than anything it does with the AWD power split.
It actually can disengage the front axle. There is a whole discussion on jeep renegade forum on that subject. To my understanding, it is done at CV joint level where front wheels are disengaged from the CV joints. So the front wheel become free spinning. I would think you cant theoretically send 100% of the power to the rear since some power is still used to spin the "free" CV joints. But according to the discussion I have given below, from the renegade forum, people has seen on 90% of power going to the rear axle on their jeep skills app (which I assume 10% is wasted on disengaged CV joints) If you watch the video made by jeep below, you can also see it is showing 100% of torque being sent to the rear (It shows it at ~2:14 mark for sand and once again for mud later on). Please watch the video with audio as they very clearly say this as well. It also shows the disengaging happening at CV joints.

https://www.youtube.com/watch?time_c...&v=WHYIDliUxFQ


The monitoring software I was talking about is Jeep Skills, which is unfortunately not available in the US due to different OBD systems being used in EU and US. If you put the car in sand mode, by default power distribution starts at 40/60 front/rear ratio.


Here is the discussion on the subject from renegade forum;

https://www.jeeprenegadeforum.com/fo...h-using-4.html

Quote:
It's got a single speed power take off/transfer case up front. That works like a single speed power take off.

It's a bit different because it apparently integrates the CV joints into it. Which saves weight. The CV joints work like CV joints.

They say it is "disconnecting", they also say they can send up to 100% of torque to the rear wheels. (not 100% of available torque BTW, some people mention that up thread, it's not what the video says). Speculatively, I'd interpret those two things to imply they can disconnect the front CV joints. Since both the rear and front drvie have to go through the PTO, I don't see why that would be out of the realm of reason. However the GTK animation would seem to indicate it is 100% of available torque which may be close to 100% but not quite if your front wheels are free spinning. (uk folks with the app have pics of up to 90, but nothing higher I think(.

Mostly you drive around in FWD mode. When doing this, the drive shaft is disconnected form the rear wheels. The animation indicates that it should be disconnected from both the fornt and rear and most of the rear diff bits are NOT spinning when th rear is disengaged.

The rear diff uses a multi-plate wet clutch to connect to the PTO This can be modulated to permit something less than 100% of available torque through. Given they claim you can run 100% of torque to the rear wheels that would be varying between 0% and 100%. When fully disengaged, the wheels are spinning only a small portion of the guts of the rear diff according to GTK.

When you shift between FWD and AWD, it looks from the animation like a dog clutch in the rear diff connects the axles to all the internal spinny bits and then the clutch mechanism starts spinning up the drive shaft. Once it is going fast enough, a dog clutch in the PTO is electronically activated, and connects the drive shaft to the power train and the powered bits in the PTO.

So why do they say you have to be going below 3mph to engage 4 low? Because 4 low is 1st gear. You can't go very fast before you are out of it, and it's holding 1st gear. Beyond that I suspect you won't do damage, it just simply won't permit you to synchronize and engage. Hence the blinky light if you try it (I have inadvertently tried it not at high speeds though, more like 7-10mph). I don't think they can get the dog clutches synced at the gearing of first gear.



As for the don't exceed speed XXX while in 4wd low, with a 2 speed transfer case, you shouldn't because you can overpsin things and make them very unhappy. With the renegade's system, i just don't think you can. low is 1st gear and it'l only go so fast before you hit the rev limiter or it forces a shift. You may have to concern yourself with coasting downhill speeds though if the system doesn't disengage and thus driving the engine past redline. So that might be the reason for the warning with the GTK system. I don't think they can get the dog clutches synced at the gearing of first gear. More than doubling the rotational mass on the system suddenly at high speed with those ratios is also probably bad probably "bad", if their algorithm just said screw it on waiting on synch and just smashed the dog clutches together. They are a dog clutch, but only so big, and you can still destroy the teeth with enough load.

Looking at GTK's info in more detail, it looks like the rear half is very haldex like, but rather than use a motor to drive oil pressure to engage and disengage things, they appear to be using direct gearing to an electric motor. The front half is pretty neat, it's a very compact PTO that transfers power from the transmission to both the front and rear wheels by integrating the front CV shafts into it. It too uses dog clutches moved about by direct electric motor gearing from the looks of it rather than a viscous coupler or wet plate clutch.
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post #10 of 24 Old 03-04-2019, 12:55 PM
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. . . other than that snow mode really is helpful moreso in its throttle dampening more than anything it does with the AWD power split.
Which the human brain ought to do. If a vehicle is spinning in the snow, more power is only going to make more tires spin. A spinning tire is not getting traction -- that's why it spins.

The ETC is practically useless if trying to start on a snow covered hill. I did some experiments with my FWD Patriot and all ETC did was cut power to one wheel so the other tire pulls the vehicle in one direction -- it totally overrules the steering. The only solution is to stop before sliding off the road. The only alternative at that point is to back down the hill.

I learned to drive in snow before we had all these gizmos. I always got there.
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post #11 of 24 Old 03-04-2019, 04:35 PM
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Which the human brain ought to do. If a vehicle is spinning in the snow, more power is only going to make more tires spin. A spinning tire is not getting traction -- that's why it spins.

The ETC is practically useless if trying to start on a snow covered hill. I did some experiments with my FWD Patriot and all ETC did was cut power to one wheel so the other tire pulls the vehicle in one direction -- it totally overrules the steering. The only solution is to stop before sliding off the road. The only alternative at that point is to back down the hill.

I learned to drive in snow before we had all these gizmos. I always got there.
It actually doesn't cut power to one wheel, at least it is not how it is working with 2nd gen compass. It applies brakes to the spinning wheel until the spinning wheel and the wheel that has traction has the same rotational speed. This basically allows the power to be diverted to the wheel with traction. But I dont think it would apply enough brake power to completely stop the wheel with no traction, it just does this until both tires have the same rotational speed. If the car is 4wd, it can also do this with the rear axle and the rear wheel.

I think the situation you experienced with car being pulled to the direction of the tire with traction is not due to ETC. I experienced exactly what you described on my Grand Cherokee WK that has mechanical locked differential. It is just how locked differentials work, regardless of it being brake powered like in 2nd gen compass or mechanical like Grand Cherokee. The reason why the car is pulled to the side with traction is because the other tire has not traction. So even if you installed a mechanical locked diff that has perfect 50/50 power split to each wheel, the car will still be pulled to the side with traction. The tire that lost traction simply cannot hold that side of the car in place, so the wheel with traction will pull the car towards it. On a 4wd car, it can actually also happen with the rear wheels, where you can see and feel the rear side of the car being pulled to one direction (or if you accelerate rapidly on ice or snow, the back can fish tail due to this happening). I think this is one of the reason why on 2nd gen compass, as soon as this happens, ETC also cuts down the throttle.

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post #12 of 24 Old 03-04-2019, 05:00 PM
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Quote:
Originally Posted by Tripod
It actually can disengage the front axle. There is a whole discussion on jeep renegade forum on that subject.
No, it can't, and the post you quoted from the renegade discussion backs me up on that. They don't know what they are talking about either, but the one accurate part is

Quote:
However the GTK animation would seem to indicate it is 100% of available torque which may be close to 100% but not quite if your front wheels are free spinning.
^^^ That exactly. The front CV axles are always being driven, period. They do not disconnect. The PTU locks the rear driveshaft to the CV axles' assembly via dog clutch.

Lets keep the complexities of the RDM clutch out of the discussion for a moment and just assume the RDM variable clutch is at 100% lockup. In this condition, at least one rear CV axle is bound 1:1 to at least one front CV axle. I say "at least one" because there are open differentials front and back.

Leaving out the additional complexities of the open differentials and their side-to-side splits, lets assume left-to-right things are dead even on each axle but different front to back, so say front tires on glare ice and rear tires on tar. The PTU is locked and the RDM is at 100% lockup. In this situation, yes virtually all the usable torque "goes to the rear" but the front wheels are still being driven, they are just being driven nearly uselessly as they have no traction. So your thinking is close when you say "which I assume 10% is wasted on disengaged CV joints" its actually X percent wasted on front wheels that have no traction. They (the front CV axles) are engaged and they cannot slip because of the 1:1 PTU, there is no differential between the front and rear axles so you can't slip the fronts unless the rear has also broken traction (still working under the assumption that the RDM variable clutch is at 100% lockup).

I can't really get more into this now though, other busy things to do today.

Edit: One last piece of 2 cents, though, in regards to the video:

As an engineer myself, I know that it is all too common for sales/marketing to mis-speak or mis-represent or mis-understand the product that I have built when they turn around and start selling or demo-ing it. It happens all the time and very, very easily. In this case, it seems to be a misunderstanding that stems from them saying "the torque" when they mean "the available torque", and on top of that they are either accidentally or purposefully skimming over the finer details, probably in an effort to simplify the explanation for people (which is fine), but hurts the overall technical accuracy.

No matter that they suggest, it is impossible for this system to drive only the rear wheels and not the fronts. If the fronts have no traction and the rears do, then its kinda technically correct to say all usable/available torque is being used at the rear axle but the front is not disengaged, and as soon as the front tires DO have any sort of traction then that reduces what is being used at the rear axle.

I'm having a hard time boiling it down to something that is simple and easy to remember. How about "The rear axle can use up to whatever input torque that the front wheels can't use (due to lack of traction)."

Easy enough until you start talking about the left-and-right splits from open diffs

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post #13 of 24 Old 03-04-2019, 05:09 PM
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No, it can't, and the post you quoted from the renegade discussion backs me up on that. They don't know what they are talking about either, but the one accurate part is



^^^ That exactly. The front CV axles are always being driven, period. They do not disconnect. The PTU locks the rear driveshaft to the CV axles' assembly via dog clutch.

Lets keep the complexities of the RDM clutch out of the discussion for a moment and just assume the RDM variable clutch is at 100% lockup. In this condition, at least one rear CV axle is bound 1:1 to at least one front CV axle. I say "at least one" because there are open differentials front and back.

Leaving out the additional complexities of the open differentials and their side-to-side splits, lets assume left-to-right things are dead even on each axle but different front to back, so say front tires on glare ice and rear tires on tar. The PTU is locked and the RDM is at 100% lockup. In this situation, yes virtually all the usable torque "goes to the rear" but the front wheels are still being driven, they are just being driven nearly uselessly as they have no traction. So your thinking is close when you say "which I assume 10% is wasted on disengaged CV joints" its actually X percent wasted on front wheels that have no traction. They (the front CV axles) are engaged and they cannot slip because of the 1:1 PTU, there is no differential between the front and rear axles so you can't slip the fronts unless the rear has also broken traction (still working under the assumption that the RDM variable clutch is at 100% lockup).

I can't really get more into this now though, other busy things to do today.


If you read that discussion to further down, it says;

"They say it is "disconnecting", they also say they can send up to 100% of torque to the rear wheels. (not 100% of available torque BTW, some people mention that up thread, it's not what the video says)."

Also video made by jeep, which is still on their website for renegade 4wd system says 100% of torque can be sent to the to the rear axle, not 100% of the available torque.
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post #14 of 24 Old 03-04-2019, 05:17 PM
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Quote:
Originally Posted by Tripod
If you read that discussion to further down, it says;

"They say it is "disconnecting", they also say they can send up to 100% of torque to the rear wheels. (not 100% of available torque BTW, some people mention that up thread, it's not what the video says)."

Also video made by jeep, which is still on their website for renegade 4wd system says 100% of torque can be sent to the to the rear axle, not 100% of the available torque.
See my edit above regarding sales/marketing easily failing to convey what the engineers tell them, and easily loosing technical accuracy as they attempt to simplify the explanations they give. It happens all the time.
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post #15 of 24 Old 03-04-2019, 07:12 PM
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It actually doesn't cut power to one wheel, at least it is not how it is working with 2nd gen compass. It applies brakes to the spinning wheel until the spinning wheel and the wheel that has traction has the same rotational speed. This basically allows the power to be diverted to the wheel with traction. But I dont think it would apply enough brake power to completely stop the wheel with no traction, it just does this until both tires have the same rotational speed. If the car is 4wd, it can also do this with the rear axle and the rear wheel.

I think the situation you experienced with car being pulled to the direction of the tire with traction is not due to ETC. I experienced exactly what you described on my Grand Cherokee WK that has mechanical locked differential. It is just how locked differentials work, regardless of it being brake powered like in 2nd gen compass or mechanical like Grand Cherokee. The reason why the car is pulled to the side with traction is because the other tire has not traction. So even if you installed a mechanical locked diff that has perfect 50/50 power split to each wheel, the car will still be pulled to the side with traction. The tire that lost traction simply cannot hold that side of the car in place, so the wheel with traction will pull the car towards it. On a 4wd car, it can actually also happen with the rear wheels, where you can see and feel the rear side of the car being pulled to one direction (or if you accelerate rapidly on ice or snow, the back can fish tail due to this happening). I think this is one of the reason why on 2nd gen compass, as soon as this happens, ETC also cuts down the throttle.
I stand corrected on the cutting power vs. braking. Indeed, all the ABS does is brake the wheel that was trying and ignore the tire that had nothing to do.

In the experiment I referenced above I was comparing snow tires. Both Patriots were FWD, one had a 5-spd Manual with General Altimax Arctics, the other had a CVT with Dunlops Grand Trek. The winner was the CVT and the Dunlops but I think I owe its "victory" as much to the CVT that enabled a smoother start as to the tires. It was also my chance to play with the ETC since I was on an isolated road with no traffic. I was starting from a dead stop on the steepest part of a hill with 4" fresh snow over ice -- about as difficult as it might ever get and still have a chance to get moving. With the ETC working, I got pulled to the sides with both vehicles so badly that it was worse than useless. Without the ESC I was able to start on the incline and navigate my way up the hill with the CVT, again, only because it could start smoother without the jerk of the clutch.

I also took our Wrangler (manual) to the same place. It was helpless in 2wd, but as expected, started easier on the hill in 4wd than the FWD Patriot. Wrangler has General Grabber Arctic LTs. All the tires were almost new at the time. I had no agenda for my experiment, I just wanted to see if there was a difference between the tires. The ESC was a by-product of my tire investigation.

The point of the above post was that experience in snow trumps ETC.

BTW, both the FWD Patriots had no difficulty starting up on the hill in reverse because the torque shifted the weight down to the front tires enhancing their traction.

Last edited by Jasmine; 03-05-2019 at 02:14 AM.
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post #16 of 24 Old 03-04-2019, 07:38 PM
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See my edit above regarding sales/marketing easily failing to convey what the engineers tell them, and easily loosing technical accuracy as they attempt to simplify the explanations they give. It happens all the time.

As a thought experiment, lest assume we lifted the front end of the car, but jacks under it so that front wheel are above the surface and have zero traction and all the traction is on the back two wheels. If I went on and pressed on the gas (assuming the car doesn't lunge forward and fall of the jacks lol ), would the front wheels rotate or not?

To my understanding, they would not rotate. It is unclear to me whether this is achieved mechanically via the CV joints like the link I gave suggests, or it is achieved electronically via the traction control breaking on the front wheels and stopping the spinning. I agree that 100% of the torque will not go to the rear wheels since some of it is wasted either on free spinning the front axle (if there is a mechanical CV disengagement), or it is wasted on trying to spin front wheel that are hold by the breaks. Even if the CV thing is not present, the car should still be able to achieve what I described using the breaks. And even if 100% of the power is not going to the rear axle, majority of the power (maybe 90% if jeep skills app is to be believed) will go to the rear.

I agree with you that "brake" based torque vectoring, regardless of it being used to distribute power between front and rear axles and/or right and left wheels, is not as capable as a true mechanically locked central or axle diffs. Because, if I rev the engine to say 4000-5000RPM, engine will generate enough power to overcome the breaks and therefore the wheels will start to spin (to prevent this traction control would probably start to cut the engine power). On a mechanical diff, you can red line the engine and power will still be distributed based on how much the diffs allow. But in most cases that you would/should normally encounter with a Compass, if you cant get the car to move at ~2000-3000rpm, you wont be able to move it at +4000RPM either. Tires on the car simply do not have enough surface area to hold on to the surface when engine is putting out 150HP, the other 1-3 tires would simply start spinning regardless of the type of diff you have. A mechanically locked diff would help if you could install wider aftermarket tires with more traction, but on stock level, I think the system is capable enough for anything you can throw at it (within reasonable limits of course).

Last edited by Tripod; 03-04-2019 at 07:43 PM.
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post #17 of 24 Old 03-04-2019, 09:59 PM
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I think we are getting closer to being on the same page now.

Quote:
Originally Posted by Tripod
As a thought experiment, lest assume we lifted the front end of the car, but jacks under it so that front wheel are above the surface and have zero traction and all the traction is on the back two wheels. If I went on and pressed on the gas (assuming the car doesn't lunge forward and fall of the jacks lol ), would the front wheels rotate or not?
It absolutely would lunge forward and fall off the jacks, for starters. My impression is that most of the time the programming endeavors to start from all stops in AWD, and then once rolling along it will disengage the rear axle if deemed appropriate. On any sort of incline, it would definitely start out in AWD (part of hill start assist programming). Now even if the angle wasn't a factor and all other conditions were so perfect that it initially "tried" to start out in FWD, before the front wheels made one rotation it would start to realize what is going on and would lock in the rear axle. So either instantly or almost instantly, you go lunging off your jacks. So... don't try this!

Lets maybe picture it with rollers instead, like the TFL guys do (they have tons of videos
where they put 2-3 wheels on industrial rollers and test what happens, its pretty cool)

So hypothetically the front two wheels are on rollers and have zero traction. Got it. What happens now when you press on the gas depends entirely on the state of the AWD system.

IF you happened to be running the car in 2WD Dyno mode (which is something cool that alfaOBD can enable temporarily), THEN your front wheels would spin themselves silly on the rollers and you would be hella stuck.

IF The vehicle was in 4x4, THEN the front wheels would not spin so long as neither rear tire breaks traction. The reason being, when the PTU is engaged (and just assuming full lockup on the variable clutch in the RDM) then the front and the rear axles are now locked together with no center differential. That is probably the most important point. In this system there is no differential between the front and rear axles, there is only the variable computer controlled clutch in the RDM at the back.

Most of my hypotheticals assume the clutch is at 100% lock to simplify things, in reality it will often be some % less than 100 a lot of the time to take strain off things since there is no center diff to help ease the tensions between wheels moving at potentially different speeds in turns. The % of lockup probably depends on the steering wheel position and 12 other things, most notably the drive modes. In Normal mode it probably rarely hits that 100% mark unless slipping conditions are occurring. In mud or rock modes it maybe holds at 100% even in turns. In snow mode I suspect it never really gets above 60-80% engaged to help prevent the rear end from spinning out on ice.

So to your hypothetical, I think if you tried it in snow mode the front tires might spin a bit since the clutch in the RDM probably won't go to 100% lock, but I don't know for sure that it wouldn't at 0 MPH. It could be setup to do 100% until 10 MPH is reached or something like that.

Its all in the programming that way, but the hardware is pretty much set in stone. I sure wish I had a set of rollers like the TFL guys, they have not done those tests on a Compass or Renegade or Cherokee yet that I have seen, hopefully soon.
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post #18 of 24 Old 03-04-2019, 10:55 PM
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Quote:
Originally Posted by arudlang View Post
I think we are getting closer to being on the same page now.



It absolutely would lunge forward and fall off the jacks, for starters. My impression is that most of the time the programming endeavors to start from all stops in AWD, and then once rolling along it will disengage the rear axle if deemed appropriate. On any sort of incline, it would definitely start out in AWD (part of hill start assist programming). Now even if the angle wasn't a factor and all other conditions were so perfect that it initially "tried" to start out in FWD, before the front wheels made one rotation it would start to realize what is going on and would lock in the rear axle. So either instantly or almost instantly, you go lunging off your jacks. So... don't try this!

That is why I said this though experiment, I know it will lunge forward. It would be fun to watch this happening though lol . I guess this can be done in real world if you put the rear wheels on a dyno and immobilize the car by bolting the chassis to something so it cant lunge forward or fall back in any way.


Quote:
Originally Posted by arudlang View Post
I think we are getting closer to being on the same page now.


Lets maybe picture it with rollers instead, like the TFL guys do (they have tons of videos like this where they put 2-3 wheels on industrial rollers and test what happens, its pretty cool)

So hypothetically the front two wheels are on rollers and have zero traction. Got it. What happens now when you press on the gas depends entirely on the state of the AWD system.

IF you happened to be running the car in 2WD Dyno mode (which is something cool that alfaOBD can enable temporarily), THEN your front wheels would spin themselves silly on the rollers and you would be hella stuck.

IF The vehicle was in 4x4, THEN the front wheels would not spin so long as neither rear tire breaks traction. The reason being, when the PTU is engaged (and just assuming full lockup on the variable clutch in the RDM) then the front and the rear axles are now locked together with no center differential. That is probably the most important point. In this system there is no differential between the front and rear axles, there is only the variable computer controlled clutch in the RDM at the back.

Most of my hypotheticals assume the clutch is at 100% lock to simplify things, in reality it will often be some % less than 100 a lot of the time to take strain off things since there is no center diff to help ease the tensions between wheels moving at potentially different speeds in turns. The % of lockup probably depends on the steering wheel position and 12 other things, most notably the drive modes. In Normal mode it probably rarely hits that 100% mark unless slipping conditions are occurring. In mud or rock modes it maybe holds at 100% even in turns. In snow mode I suspect it never really gets above 60-80% engaged to help prevent the rear end from spinning out on ice.

So to your hypothetical, I think if you tried it in snow mode the front tires might spin a bit since the clutch in the RDM probably won't go to 100% lock, but I don't know for sure that it wouldn't at 0 MPH. It could be setup to do 100% until 10 MPH is reached or something like that.

Its all in the programming that way, but the hardware is pretty much set in stone. I sure wish I had a set of rollers like the TFL guys, they have not done those tests on a Compass or Renegade or Cherokee yet that I have seen, hopefully soon.
Yeah I think we are on the same page on this. I think we have different definitions of available torque vs. total torque, so that was the reason for disagreement. I think when you talk about total torque, you talk about torque generated by the engine. And since, even if the front wheels arent spinning, some torque is still applied to them (which causes a tug of war with the breaks) and therefore you say 100% of total torque cant go the the rear. So only a portion of the total torque (or total available torque) goes to the rear axle. When I talk about total torque on the other hand, I was talking about the torque that hits the road. So in my case, as long as front wheels arent spinning, they generate zero torque on the road and since only rear wheels arent spinning, they have 100% of the torque. I think both statements are correct within the given definition of total torque for each case.

So when I say the car can become rear wheel drive, what I mean is; under some strange condition where front wheels have zero traction, traction control can operate the car as if it is a real wheel drive vehicle. Like you were driving on sand and somehow front wheels dug into sand but some how rears did not. In that case, until the front wheel can at least get some traction, the car can temporarily work as rear wheel drive, where you would see rear wheel rotating while front wheels are not rotating. So if it makes sense from a traction perspective to only run the ear wheels, the car has the capacity to only run the rear wheels while keeping the front wheels stationary. But of course, while cruising on highway, there is no way to operate the car as a rear wheel drive vehicle.

But in the end, there are not many cars that can in real time switch between fwd, awd and rwd. IF you think about it, 2nd gen compass is better at this than an old wranger that can either be 4wd or rear wheel drive, but it cannot become front wheel drive to the same extend the compass can become rear wheel drive. Even if you lifted the rear of the wrangler in a similar though experiment and put the car to 4wd, the rear wheels will still rotate within the "slip" limits of the differential (of course the new ones also have the break based torque vectoring, so those can be fwd as much as compass can become rwd). The only cars that on top of my mind I can think of that can seamlessly switching between true fwd, 4wd and rwd are Land Rovers and Toyota Land Cruiser. And these cars have three torsen differentials, one at the center and 2 at each axle. They can mechanically send 0-100% of the power to any single wheel and completely take in or out both front and rear axel from the drive-train.
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post #19 of 24 Old 03-04-2019, 11:29 PM
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Yep, definitely getting closer to the same page. I will clarify though, if the rear wheels both have traction my understanding is that no break action will be required in the front wheels. The PTU seems to be driven off of something kind of what looks like the front diff ring gear, and if that is true then the front wheels cannot hold still while the rear wheels are being driven. The fronts may not have traction and may not be helping, but they are locked together and turning with the rear axle, I believe. If the PTU is driven by something else, well then I don't know, but it sure looks like its the ring gear of the front diff meaning if all else is locked up the backs can't spin without the fronts spinning also.

Gonna make me go get myself some test rollers yet, aren't ya? =D
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post #20 of 24 Old 03-05-2019, 01:55 AM
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Quote:
Originally Posted by arudlang View Post
Yep, definitely getting closer to the same page. I will clarify though, if the rear wheels both have traction my understanding is that no break action will be required in the front wheels. The PTU seems to be driven off of something kind of what looks like the front diff ring gear, and if that is true then the front wheels cannot hold still while the rear wheels are being driven. The fronts may not have traction and may not be helping, but they are locked together and turning with the rear axle, I believe. If the PTU is driven by something else, well then I don't know, but it sure looks like its the ring gear of the front diff meaning if all else is locked up the backs can't spin without the fronts spinning also.

Gonna make me go get myself some test rollers yet, aren't ya? =D
Lol I was looking to see if there were any tune shops with dynos near me. I was thinking to put the rear wheels on a dyno and fronts on a roller to test it. Though in that case, traction control can act like no wheel has traction since no acceleration will be detected by the computer and it will probably assume I am spinning all 4 wheels .
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post #21 of 24 Old 03-05-2019, 04:31 AM
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It would be cool to see what a dyno could measure at the rear wheels, would answer a lot of questions, but unless you have a buddy somewhere who can do it for cheap or for free too rich of a test for my blood.

Tell you what I maybe will spend a buck on though, if it helps, could do some logging with alfaOBD of some of the relevant data in some test situations. Here is a portion of the data that can be logged:



Would give a little insight to what the system is doing under different circumstances, if properly recorded. Would need to be sort of mapped up with some gopro footage of driving around different icy or special scenarios I suppose. Would be kinda fun to do but I don't have unlimited free time either, so we'll see.
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post #22 of 24 Old 03-05-2019, 04:24 PM
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Quote:
Originally Posted by arudlang View Post
It would be cool to see what a dyno could measure at the rear wheels, would answer a lot of questions, but unless you have a buddy somewhere who can do it for cheap or for free too rich of a test for my blood.

Tell you what I maybe will spend a buck on though, if it helps, could do some logging with alfaOBD of some of the relevant data in some test situations. Here is a portion of the data that can be logged:



Would give a little insight to what the system is doing under different circumstances, if properly recorded. Would need to be sort of mapped up with some gopro footage of driving around different icy or special scenarios I suppose. Would be kinda fun to do but I don't have unlimited free time either, so we'll see.
I wish we had the Jeep Skills app here in US. Such a simple program that would have allowed us to see the power split between the axles, engine HP and power produced in real time, etc. Jeep said they cant release the app in the US because the OBD system used in US is not compatible with it. Cant understand how a multi billion dollar company is incapable of coming up with a new software that could work with OBD system used in US, yet a 15$ OBD scanner of ebay can read these values .

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post #23 of 24 Old 03-05-2019, 07:12 PM
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I wouldn't trust what some gimmicky app like that shows anyways. They are only showing a percentage, not specifying what it really represents. This to me is a classic case of developers over-simplifying something to try to make it easier for idiots to understand, but the usefulness of the data gets watered down in the process.

I am much more excited to read some raw values and piece together the puzzle that way. More work but I will tend to trust the results.

I might have to go to the windows version of the app at some point, though, to have enough screen space to graph all the stuff I want to see. I wonder how good the android app does on a large-screen tablet...
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post #24 of 24 Old 03-05-2019, 08:22 PM
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I wonder how much info the OP really needed?
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