[KorbenR]

I'm considering getting myself an xDrive car and I want to better understand the torque application parameters.

So I read about a 40% baseline. Is this indicating that just under normal driving conditions with no wheel slip at all that there's 40% being applied to the front?

Maybe put it another way, lets say the road is wet, and you're stopped. You turn off DTC, crank the wheel the left and gun it, what will it do? Is there not yet any drive power going to the front so the rear tires will spin and the car and flip around. Or is there already power being applied to the front and the car will just turn and accelerate?

I hope this question makes sense.

[Billfitz]

IMO just drive it and don't worry about it. I'm totally unaware that it's there at all, until it snows and I go through it like there's no snow at all. As for the technical aspect, it's electronically controlled, not mechanically. It detects slip and the car corrects for it usually before you to even notice that it happened.

[visionaut]

It’s rear-wheel biased, but the fronts pull from a stop, which is why the Xdrive has a faster 0-60.

[KorbenR]

Originally Posted by visionaut

It’s rear-wheel biased, but the fronts pull from a stop, which is why the Xdrive has a faster 0-60.

Hmmm, I consider that a negative. I don't want the front doing anything until it's needed.

[Skyhigh]

The basic principle is that xDrive is a full-time AWD system with dynamic torque-distribution default set to 60% rear, 40% front.

There are two (main) exceptions from the above:
1. Low speed manoeuvring uses primarily or even exclusively RWD drive
2. High speed (>180 km/h) also uses 100% RWD

The two extreme points of the torque distribution are front:back 50:50 to 0:100, meaning the maximum torque which can be mechanically sent to the front is 50%, minimum 0.

There is another thread where the term "virtually" has been beaten to death (some sources claim 100% torque can be "virtually" applied to either axle). This is "virtually" true only in the case that both rear wheels lose traction entirely, in which case clearly 100% of the momentarily available torque is used by the wheels with traction - in this case the front. The key here is to understand the secret meaning of "virtually", as realistically the front wheels can only get a max of 50% of the drivetrain torque. This is a straight-forward mechanical limitation (clutch fully engaged).

To conclude - xDrive is totally worth it for:
- Winter (snow/ice) driving
- Fast acceleration from stand or low speeds
- Overall stability during sporty driving (no corner too sharp)
- Overall better drive performance

Downsides:
- Added weight
- Increased drive-train losses
- Increased fuel consumption (due to the two above)
- Slower acceleration when sufficient traction is there (e.g. 100-200 acceleration suffers in comparison to RWD)
- Increased maintenance burden

I would not have bought a BMW without xDrive and I do not regret that decision.

Also keep in mind - you can always disable xDrive if you really want to (e.g. by using tools like xDelete), but you can never add xDrive.

Hope this helps.

[KorbenR]

Originally Posted by Skyhigh

Hope this helps.

Very much, I really appreciate it.

Originally Posted by Skyhigh

I would not have bought a BMW without xDrive and I do not regret that decision.

Nor would I, non-AWD car isn't even within the realm of reasonable options for me. What I'm trying to do is understand as well as I can the differences between the AWD system in my current car and the BMW system.

My current setup has a transfer case near identical to the BMW. The rear is directly driven and front applied via a clutch. The biggest difference is that in my current setup the normal condition is RWD. The front is only there for use as needed. There's a mechanical disconnect in the front axle to reduce drive train losses in conditions where it's unlikely AWD will be needed. This connects automatically when it's cold out, wipers are on, traction control is in partial off, or transmission is in manual mode. It then still only applies the clutch to send power forward after it senses rear slip. This makes for a car that the vast majority of the time behaves like a RWD. While the lower drivetrain losses are nice what I really appreciate about this is the effect on handling. I can whip it around like a RWD, then the front kicks as needed.

It appears I have more control as well, using steering wheel buttons while driving I can command the car to RWD only, or preemptively lock the front disconnect. This is done by aftermarket gizmo, but doesn't require a phone or flash, just menu buttons.

The biggest things I don't have compared to a BMW is primarily a manual transmission and more advanced engine. Power yes, efficiency, not so much.

So as you might imagine this:

Originally Posted by Skyhigh

There are two (main) exceptions from the above:
1. Low speed manoeuvring uses primarily or even exclusively RWD drive
2. High speed (>180 km/h) also uses 100% RWD

Is very helpful, most of all #1. I'm assuming that BMW does that because the speed variation between front and rear is high under those conditions. Worse even the front must turn faster than the rear and neither of our AWD systems can do that.

However does that mean that at slow speeds the car will be tail happy. If one does what I describe in the 2nd part of my first post will it spin around? The duality of being able to spin the rear and flip it around just before the AWD kicks in is one of my favorite things about my current setup. I don't think I'd be willing to give that up for a manual transmission.

Hope this makes sense what and why I'm trying to understand.

[Skyhigh]

As already suggested - best thing to do is to test-drive it.... digging into the theory will not give you definitive answers as you obviously expect a very specific behavior (which will also heavily depend on tires, DSC settings etc.)

Nevertheless here some additional info:

Originally Posted by KorbenR

My current setup has a transfer case near identical to the BMW. The rear is directly driven and front applied via a clutch. The biggest difference is that in my current setup the normal condition is RWD. The front is only there for use as needed. There's a mechanical disconnect in the front axle to reduce drive train losses in conditions where it's unlikely AWD will be needed. This connects automatically when it's cold out, wipers are on, traction control is in partial off, or transmission is in manual mode. It then still only applies the clutch to send power forward after it senses rear slip. This makes for a car that the vast majority of the time behaves like a RWD. While the lower drivetrain losses are nice what I really appreciate about this is the effect on handling.

I know some manufacturers have what you describe (VW/Audi, in some of their models, for example). There are however some clear advantages of full-time AWD vs on-demand AWD. The benefits of the first have diminished as electronics became faster and faster, but they are still there. E.g. during sporty driving, full-time AWD keeps you planted, whilst on-demand AWD rather brings you back on track once traction is lost.

BMW's xDrive still feels very much RW-driven though, due to the rear-bias. Ironically, even newer Quattro systems use 40:60 torque distribution and are full-time AWD.

First and foremost however, you need to understand one fundamental thing which has been beaten to death in various threads in this and other forums - opening the clutch in the transfer case to make the car RWD does NOT reduce drive-train losses (in any appreciable way)!! This is based on simple physics! All AWD components are still there, all the extra weight is still there, the rotational mass is unchanged!! So with front-wheels disengaged from the engine, you do NOT have lower losses!
The only insignificant difference comes from the friction within the transfer case which is now minimised/eliminated, but it is such a small contributor, that it has no meaningful effect.

I can whip it around like a RWD, then the front kicks as needed.

You can absolutely do that with xDrive too, in Sport+ mode! Just as you describe it.

It appears I have more control as well, using steering wheel buttons while driving I can command the car to RWD only, or preemptively lock the front disconnect. This is done by aftermarket gizmo, but doesn't require a phone or flash, just menu buttons.

This is a feature available on M models only. Even with xDelete, you cannot do that on the fly.

The biggest things I don't have compared to a BMW is primarily a manual transmission and more advanced engine. Power yes, efficiency, not so much.

I personally do not understand the desire for a manual transmission compared to a state of the art 8 speed automatic, which is nearly as fast as a DCT (especially when flashed with xHP). I have been driving manuals all my life (as most people in Europe) and hated automatics... however the modern sport automatics and the ability to still change gears manually, with a quick flip on the back of the steering wheel, are a completely different story.
The F36 is my first automatic car and just like xDrive - I don't regret it for a sec. It in reality gives everything a manual does + more.
I fully respect people who drive manual for a reason, but I am getting a feeling it is just a hype in the states, due to manuals being exotic.
But that's offtopic anyway.

[KorbenR]

Originally Posted by Skyhigh

As already suggested - best thing to do is to test-drive it....

True, but I need to find one where they'll let me screw around some on a test drive. It's on my to-do list but I want to understand as much as I can ahead of time.

Originally Posted by Skyhigh

(which will also heavily depend on tires, DSC settings etc.)

DSC settings is something I'd like to understand better as well. I've done a fair bit of reading on it but some details aren't clear.

From what I've read there normal DSC mode, a quick press of the button gets DTC mode that will allow some wheel spin, and a long press will turn it off. From what I understand DTC will still only allow some spin, it'll still pull power to limit spin. However off, does that turn everything off, no ABS, no electronic diff lock, etc. Or do those other things still stay on?

Originally Posted by Skyhigh

full-time AWD keeps you planted, whilst on-demand AWD rather brings you back on track once traction is lost.

I'd say it's more of a question of how much do you want it to let you take it over the limit before it steps in. I'm not a fan of "drifting" just for the sake of drifting, I think it's dumb. But there's absolutely times where I want some slide and I want the car to let it happen.

Originally Posted by Skyhigh

the rotational mass is unchanged!!

I'm honestly not sure how accurate that is. To be clear in mine there's two places the front drive is disconnected. The clutch in the transfer case of course. But also a mechanical disconnect at the front axle. When this is disconnected the rotation of the front wheels can't apply any load to the front differential, driveshaft, etc.

How much of an actual difference does this make, that I don't know. I'd have to assume though that they wouldn't incur the cost of putting it in the car if it didn't help. I should do a test drive with it locked and unlocked and see if I notice a difference in MPG.

Originally Posted by Skyhigh

You can absolutely do that with xDrive too, in Sport+ mode! Just as you describe it.

Does a '16-'18 manual xDrive 340i have this Sport+ mode?

Originally Posted by Skyhigh

But that's offtopic anyway.

Not at all, it's very on topic, it's something I go back and forth on. Modern autos are fantastic, most of all when one has paddles, modes, and can get in and program them to do exactly what you want. However there's a level of control a clutch provides that just can't be done with even the best auto. In every other way the modern auto is superior. There's just times though where that control really matters and I miss it.

[Skyhigh]

Originally Posted by KorbenR

DSC settings is something I'd like to understand better as well. I've done a fair bit of reading on it but some details aren't clear.

From what I've read there normal DSC mode, a quick press of the button gets DTC mode that will allow some wheel spin, and a long press will turn it off. From what I understand DTC will still only allow some spin, it'll still pull power to limit spin. However off, does that turn everything off, no ABS, no electronic diff lock, etc. Or do those other things still stay on?

There isn't really much I can add to what you will find by googling... here is also a nice thread with a good analogy by someone

https://www.2addicts.com/forums/show....php?t=1214851

Quite honestly I don't dare to drive with DSC off... I have no need to and would rather have someone help me out when in need. Driving with DTC however can be fun. Again not something I do daily, but sometimes it does boost the adrenaline to have a little bit of controlled anarchy

I'd say it's more of a question of how much do you want it to let you take it over the limit before it steps in. I'm not a fan of "drifting" just for the sake of drifting, I think it's dumb. But there's absolutely times where I want some slide and I want the car to let it happen.

To me, that is simply DTC (which you get automatically in Sport+ mode)

I'm honestly not sure how accurate that is. To be clear in mine there's two places the front drive is disconnected. The clutch in the transfer case of course. But also a mechanical disconnect at the front axle. When this is disconnected the rotation of the front wheels can't apply any load to the front differential, driveshaft, etc.

How much of an actual difference does this make, that I don't know. I'd have to assume though that they wouldn't incur the cost of putting it in the car if it didn't help. I should do a test drive with it locked and unlocked and see if I notice a difference in MPG.

Can't speak for your car, obviously.
In BMW's world however, the only way to disengage the front wheels is the TC. This does not reduce the drive-train losses (apart from the insignificant friction in the TC itself), since both the overall weight and the rotational mass are unchanged. Physics, really.

Even if some part of the drive-train is disengaged from rotation entirely in your case, I sincerely doubt you will see any appreciable benefits in terms of efficiency, since that would be a small percentage of the friction, whilst you still carry the weight of it all.

Does a '16-'18 manual xDrive 340i have this Sport+ mode?

Yes, of course. But when I refer to Sport+ mode, what I really mean is DTC, which is automatically engaged in Sport+. You can however also engage it manually in Comfort mode.

Not at all, it's very on topic, it's something I go back and forth on. Modern autos are fantastic, most of all when one has paddles, modes, and can get in and program them to do exactly what you want. However there's a level of control a clutch provides that just can't be done with even the best auto. In every other way the modern auto is superior. There's just times though where that control really matters and I miss it.

I do understand what you mean. As I said - I, like most Europeans, have been driving cars with sticks my entire life! Currently my daily car (Audi) is also a MT car. And every time I have had the opportunity to drive an automatic (typically rentals), I have experienced and felt that exact lack of perceived control and lack of predictability you mention and which is provided by a manual transmission by nature. With a MT, you always (sub-)consciously know which gear you are in and what response to expect hitting the throttle, based on the vehicle speed. With an old-school automatic, you never know what the response will be, occasionally it is the correct one, more often however it completely misreads the intent. But mainly - it is unpredictable and slow in reading your intent and in some situations - even a hazard! On top of everything else, shifting speed has not been dramatically faster than (experienced) manual shifting either.

But here come the new, modern ATs, which have 3 fundamental advantages over the old/classic ATs:
- Much more intuitive, with an element of AI (learning driver's individual style)
- Speed of shifting is XX times faster than a MT
- Manual mode, in which YOU get to define which gear you want to be in at any given time and change gears as YOU see fit, just like on a MT, except XX times faster and with both hands on the steering wheel!

Therefore I said - you get all the advantages of a MT + all the advantages of an AT. The only thing you don't get is the mechanical stick, which seems to be a fancy hype in the states

This said, due to the intuitiveness and speed of shifting in automatic mode, I drive in automatic (including sport-automatic) 90% of the time. I go over to manual mode usually during spirited mountain driving, when it is only me who can anticipate slope, turns and traffic in an expedient and sport-driving oriented way.

[visionaut]

Skyhigh has pretty much nailed it, nice explanations IMO.

Also agree you need to TRY IT. Lol

I’m also a longtime 3-pedal guy who loves the Sport Auto transmission, Adaptive suspension, and X-Drive in my F36 THP.

I found this table extremely helpful in understanding all of the configurability and integration BMW puts at your disposal with the different drive modes, and DSC/DTC, the engine & the trans, etc. It’s a bit older, but still relevant to newer models..

Hope it helps!
.

[HighlandPete]

Originally Posted by KorbenR

I don't want the front doing anything until it's needed.

I'm assuming you are looking at something in the F-series range of models. AWD is engaged pretty much all of the time. Except the exclusions already commented on.

The latest G-series do have some changes, where RWD is more dominant and AWD is only on demand. More so in Sport and Sport+ modes. The ATC13-1 transfer box is revised to allow an efficiency mode and transfer box drivetrain losses are lower. Also the latest xDrive models are not the 60:40 ratio of the older designs, but 50:50.