4xe Regen Braking - Rock-Trac HD Full Time 4WD Sys vs 2wd etc.

[jahysea]

Been wondering about regen braking on the 4xe. In a normal vehicle the front brakes do most of the work. If you are driving a 4xe in 4wd, great then the front wheels can generate max regen. I am getting a Rubicon, which I think all come standard with Rock-Trac HD Full Time 4wd. The owners manual supplement seems to indicate the vehicle can be placed in 2wd but that might be for non Rock Trac models.

If you are in 2wd you obviously cannot regen using the front wheels as they will not be spinning the front driveline back to the electric motor to run it backwards as a generator. Also likely the front pads need to engage to provide stopping power pretty quickly. Computer might need to make some very fast decisions. Does anyone know about this? If you have to leave the vehicle in 4wd to get max regen braking it seems you lose a lot of efficiency powering the front wheels moving forward. We have had a Volt but it was FWD and an AWD Tesla so in both cases the front wheels were available for Regen.

My comments above cover both max regen and other, any kind of regen is going to need the drivetrain engaged to produce regen.

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[greensprit]

Based on my experience with my Volvo PHEV (XC90 T8) I don’t think it will be an issue. On the Volvo the gas engine only drives the front wheels and the electric only drives the rear wheels. When in pure EV mode all power and regeneration only occurs on the rear axle. Regen is pretty effective at slowing the car down, and if you move into a more aggressive braking the computer instantly engages the front brakes - it’s totally seamless and feels very smooth. Hopeful the jeep software is as good. The only issue I’ve had is on snowy roads. The regen on the rear on slippery surfaces creates some issues, Volvo solves this by offering snow mode which cures the problem. On the jeep, putting it in 4wd should also solve the problem.

 

[Arterius2]

Been wondering about regen braking on the 4xe. In a normal vehicle the front brakes do most of the work. If you are driving a 4xe in 4wd, great then the front wheels can generate max regen. I am getting a Rubicon, which I think all come standard with Rock-Trac HD Full Time 4wd. The owners manual supplement seems to indicate the vehicle can be placed in 2wd but that might be for non Rock Trac models.

If you are in 2wd you obviously cannot regen using the front wheels as they will not be spinning the front driveline back to the electric motor to run it backwards as a generator. Also likely the front pads need to engage to provide stopping power pretty quickly. Computer might need to make some very fast decisions. Does anyone know about this? If you have to leave the vehicle in 4wd to get max regen braking it seems you lose a lot of efficiency powering the front wheels moving forward. We have had a Volt but it was FWD and an AWD Tesla so in both cases the front wheels were available for Regen.

My comments above cover both max regen and other, any kind of regen is going to need the drivetrain engaged to produce regen.

I don’t think you understand fully how regenerative braking works on the JLs.
(Hint: it’s not in the brakes)

Pretty sure regenerative braking works for both 2wd and 4wd, it works regardless if front drive shafts are connected because the regen is happening in the electric motor which is located in the engine bay which is connected to the transmission, so it works like engine braking.

 

[Mike DeSimone]

Been wondering about regen braking on the 4xe. In a normal vehicle the front brakes do most of the work. If you are driving a 4xe in 4wd, great then the front wheels can generate max regen. I am getting a Rubicon, which I think all come standard with Rock-Trac HD Full Time 4wd. The owners manual supplement seems to indicate the vehicle can be placed in 2wd but that might be for non Rock Trac models.

If you are in 2wd you obviously cannot regen using the front wheels as they will not be spinning the front driveline back to the electric motor to run it backwards as a generator. Also likely the front pads need to engage to provide stopping power pretty quickly. Computer might need to make some very fast decisions. Does anyone know about this? If you have to leave the vehicle in 4wd to get max regen braking it seems you lose a lot of efficiency powering the front wheels moving forward. We have had a Volt but it was FWD and an AWD Tesla so in both cases the front wheels were available for Regen.

My comments above cover both max regen and other, any kind of regen is going to need the drivetrain engaged to produce regen.

I have a 2012 Fisker Karma which is RWD (2 motors tied together on the rear axle.) Regen works fine- Default on startup is no (very little) regen; I always bump it to max as soon as I start to drive. I also had a 2013 Tesla Model S which was RWD- they didn't have dual motors back then. Regen on that car was even more aggressive than the Fisker, and again no problems.

The biggest difference that I can see is that those cars had the motor(s) driving the rear wheels directly via a fixed reduction gear. On the 4xe the coast-down energy has to go back through the transmission to drive the motor. It's a bit more of an engineering challenge due to multiple ratios being available. I imagine that the engineers at ZF spent quite a bit of time and testing on routing the torque through the trans, which would require unique programming for each vehicle type, matching the amount of motor drag with gear selection, 4wd engagement and road speed.

I'm going to be interested to experience it when I finally have mine.

 

[jahysea]

I have a 2012 Fisker Karma which is RWD (2 motors tied together on the rear axle.) Regen works fine- Default on startup is no (very little) regen; I always bump it to max as soon as I start to drive. I also had a 2013 Tesla Model S which was RWD- they didn't have dual motors back then. Regen on that car was even more aggressive than the Fisker, and again no problems.

The biggest difference that I can see is that those cars had the motor(s) driving the rear wheels directly via a fixed reduction gear. On the 4xe the coast-down energy has to go back through the transmission to drive the motor. It's a bit more of an engineering challenge due to multiple ratios being available. I imagine that the engineers at ZF spent quite a bit of time and testing on routing the torque through the trans, which would require unique programming for each vehicle type, matching the amount of motor drag with gear selection, 4wd engagement and road speed.

I'm going to be interested to experience it when I finally have mine.

Interesting Mike, thanks. Agree re: transmission/transfer case/etc involvement in regen, very different model from a motor basically on the axle itself. I didn't think the rear drag was enough to be effective in many stopping situations, helpful to hear from someone that has had a RWD electric car. Supposedly 70% of all braking on average comes from the front so I am a little surprised and curious to see how that feels.

Still curious to see (or hear) how the Rock Track drivetrain works re FT vs PT 4wd. I can certainly live with all time 4wd, I've had an Audi and my wife's Tesla that are AWD but both are more carlike.

If there is a careful dance between computer controlled front braking and rear regen one has to wonder what happens when we go to 37's, regear, change vehicle weight with bumpers and plating etc. My JK has a ridiculous Electronic Skid Protection feature that gets completely confused by larger tires and a lift, starts randomly applying individual brakes on windy roads, I've had to shut it off entirely with a kill switch. 4xe will not be so easy to modify I suspect.

 

[Sboden]

Interesting Mike, thanks. Agree re: transmission/transfer case/etc involvement in regen, very different model from a motor basically on the axle itself. I didn't think the rear drag was enough to be effective in many stopping situations, helpful to hear from someone that has had a RWD electric car. Supposedly 70% of all braking on average comes from the front so I am a little surprised and curious to see how that feels.

Still curious to see (or hear) how the Rock Track drivetrain works re FT vs PT 4wd. I can certainly live with all time 4wd, I've had an Audi and my wife's Tesla that are AWD but both are more carlike.

If there is a careful dance between computer controlled front braking and rear regen one has to wonder what happens when we go to 37's, regear, change vehicle weight with bumpers and plating etc. My JK has a ridiculous Electronic Skid Protection feature that gets completely confused by larger tires and a lift, starts randomly applying individual brakes on windy roads, I've had to shut it off entirely with a kill switch. 4xe will not be so easy to modify I suspect.

From the pictures I've seen, the awd system is like my trucks. You don't have to run in full-time all wheel drive as there is a 2wd selection as part of the system.

 

[Jocko]

Going along with what @Arterius2 said, regenerative braking has nothing to do with the brakes. I thought I read it was done by the starter/generator motor on the front the engine rather than the main drive motor, but regardless, the concept is the same.

Basically, it involves flipping the way the electric motor is used. To drive, you apply electricity to the motor and that creates magnetic forces which create propulsion. But the cool thing is that it works in reverse. The force of the spinning shaft in the motor can generate electric current in the motor coils if you reverse the flow. So that allows for the momentum of the Jeep to generate some electricity back into the system.

Sorry if that's not the greatest explanation ever. Might be worth a quick Google of how dc motor/generators work for a better explanation with a cool diagram. Heh.

 

[Arterius2]

Going along with what @Arterius2 said, regenerative braking has nothing to do with the brakes. I thought I read it was done by the starter/generator motor on the front the engine rather than the main drive motor, but regardless, the concept is the same.

Basically, it involves flipping the way the electric motor is used. To drive, you apply electricity to the motor and that creates magnetic forces which create propulsion. But the cool thing is that it works in reverse. The force of the spinning shaft in the motor can generate electric current in the motor coils if you reverse the flow. So that allows for the momentum of the Jeep to generate some electricity back into the system.

Sorry if that's not the greatest explanation ever. Might be worth a quick Google of how dc motor/generators work for a better explanation with a cool diagram. Heh.

That's a great explanation, way better job than mine.

 

[JandS]

I have a 2012 Fisker Karma which is RWD

IMO, the Karma and the Rapide are the most beautiful 4 door built in the past 50 years.

Cheers!

 

[dudemind]

IMO, the Karma and the Rapide are the most beautiful 4 door built in the past 50 years.

Cheers!

It certainly is, and that's one of the reasons why Fisker, as a company, failed so miserably. Henrik Fisker is an legendary car designer; among other things, the guy is partly responsible for the absolutely stunning design language that Aston Martin has been using for the past couple decades (and still is). But he had no business running an entire automotive company.

Creating a vehicle is always a compromise between between the designers, engineers, accountants, real-world regulations, etc. It's why so many mind-blowingly gorgeous concept vehicles become so incredibly, unrecognizably ugly by the time the actual production units roll off the line -- paper sketches rarely translate to feasible reality. Among the large automakers, Infinti displays some of the most hilariously egregious examples of this discrepancy between concept and production vehicles.

At the helm of his own company, Fisker was notoriously unwilling to compromise with reality on many aspects of his dream concept sketch: the Fisker Karma. The mentality was very much "this is the final design, make it work". This, of course, was an engineering/production nightmare, as solutions to make the vehicle actually work under the pretty packaging were at once unreliable, costly, and subpar from a performance perspective. Delays and cost overruns followed naturally, and necessary compromises on quality followed shortly after that. The Karma is so pretty because it's basically a car that shouldn't actually work based on modern production vehicle standards.

For what it's worth, he's still at it with his new effort, the Fisker Ocean (along with a few other planned vehicles). A WSJ article from last year described him as having learned his lesson and being more mature about making the necessary compromises to successfully run a vehicle manufacturing operation, and I guess the way-uglier-than-the-Karma Ocean shows that, so far. We'll see how it plays out.

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