Forester 2018 Subaru Forester

[martinbo]

Subaru has done it again. They show you enticing concept cars and let you down with production version. Had this been a "facelift" I would have said okay, not bad, and moved on. On the plus side, the interior looks clean and durable. There is gear lever, nothing like the newer electronic dial or buttons or gimmicky things nowadays. More pluses: still a boxer 4 engine and tried and tested AWD. Surely Subaru could have extracted a bit more power from that 2.5L and keep it NA. That is probably my biggest disappointment with it.

Yes indeed, Subaru manages to achieve the exact opposite of Land Rover (whose end-products transition remarkably closely from their concept progenitors) and end up creating production cars which couldn't be more disappointingly distant from its concepts cars if they tried. It's probably becoming an industry joke. For what it's worth, I showed the wife (the owner and driver of the Forester) pics of the new car and she was decidedly unmoved. Doesn't move the game on in the slightest.

 

[Beemer B773ER]

For what it's worth, I showed the wife (the owner and driver of the Forester) pics of the new car and she was decidedly unmoved. Doesn't move the game on in the slightest.

I thought she would have said something along the lines of "Your hairline has changed more than that thing!"

 

[martinbo]

I thought she would have said something along the lines of "Your hairline has changed more than that thing!"

Nah - that's what my daughter would've said.

 

[CrunchSlaughtered]

Maybe this is a language barrier thing. But in technical terms, you're wrong. Go have a look around - xDrive and other such systems are able to vary the amount of torque to both axles. BMW press info even states that 100% of the drive can be directed to the front axle.

BMW xDrive all-wheel drive system explained | Practical Motoring

How Does XDrive Work?
There are now two variants of xDrive – an on demand system which drives the front wheels and rears when required, as seen in the latest X1s, and what I consider to be the ‘real’, original xDrive which works differently, and I’d say properly for an all wheel drive vehicle. The rest of this article refers to the original xDrive.

This starts with a 40:60 split of the torque from front to rear. This is instantly good news, as most all-wheel drive systems merely power the front wheels and let the rears trail, driving them only when the computers detect a loss of traction (with the exception of Subaru). Those systems are never, ever as effective as the likes of xDrive because if all wheels are driving there’s less chance of slip in the first place, and for enthusiast drivers, there’s the sharper handling of a rear-drive biased car because the front wheels have more traction available for steering.

Interestingly, this 40:60 split also means the cen...

What is stated here is not correct. When the system is engaged the torque to each axle is determined by the available traction and not by the clutches. If the rear axle has traction you can never go under 60%, only when it looses traction the clutch pack acts like any locked LSD and it sends the torque (which is different than rotational speed) to the wheel with traction, but the percentage is defined by the traction not by the LCD. The open differential always sends the same torque to both ends on the expense of varying the speed of the semi-shafts and every LCD or clutch pack does the opposite - varies the torque on the expense of equalizing the rotational speed of the shafts. So to send 100% of the torque to one axle means that the other has 0 traction and it doesn't consume any torque, but not that the system varies the torque in that way. If the axle has 0 traction the system can not send for example 30% of the torque to it, it just stays 0%

If what you wrote was correct than it would not exist any kind of differential or LSD and everybody would use this device as a rear differential with torque vectoring

 

[Beemer B773ER]

Nah - that's what my daughter would've said.

Ouch! ..and I just got two of them. Double

 

[martinbo]

What is stated here is not correct. When the system is engaged the torque to each axle is determined by the available traction and not by the clutches. If the rear axle has traction you can never go under 60%, only when it looses traction the clutch pack acts like any locked LSD and it sends the torque (which is different than rotational speed) to the wheel with traction, but the percentage is defined by the traction not by the LCD. The open differential always sends the same torque to both ends on the expense of varying the speed of the semi-shafts and every LCD or clutch pack does the opposite - varies the torque on the expense of equalizing the rotational speed of the shafts. So to send 100% of the torque to one axle means that the other has 0 traction and it doesn't consume any torque, but not that the system varies the torque in that way. If the axle has 0 traction the system can not send for example 30% of the torque to it, it just stays 0%

If what you wrote was correct than it would not exist any kind of differential or LSD and everybody would use this device as a rear differential with torque vectoring

You're rambling on about what? Post links to facts. x-Drive and 4-Matic longitudinal AWD.
You called me out and said the BMW and Mercedes systems were not permanent AWD systems. Now go find the facts which say they're not. If you don't I will delete your posts - that's my job.

 

[CrunchSlaughtered]

Ok, I had to look for a while. Otherwise the statement that I made is not based on information that I have read but on experience of having two times to DIY repair the transfer case of E70 X5 and its from where I figured it out how it functions. I think the following texts explain exactly how the system works. The first quote is concretely for the xDrive and please look at the comment No 2, the second quote explains the difference between automatic all wheel drive and full time all wheel drive. The fact that the old X5 is mentioned in the full time all wheel drive sections is because it has also an integrated planetary gear set that was removed in the models that followed:
1.Xdrive
2.All Wheel Drive Explained

I find this discussion very constructive, but if you find it trolling please remove my posts

 

[Giannis]

@CrunchSlaughtered

I'm sorry but you are fundamentally wrong, which calls for a long write-up. Do you happen to have any background in engineering?

 

[CrunchSlaughtered]

T

@CrunchSlaughtered

I'm sorry but you are fundamentally wrong, which calls for a long write-up. Do you happen to have any background in engineering?

Take your time, I can wait.

 

[martinbo]

T

Take your time, I can wait.

Let's first recap before you make a hardworking real engineer like Giannis waste his time.

You refuted my statement that BMW's xDrive AWD is permanent. This means that you say it isn't.

Nothing you have linked to in those two (very old) pages contradicts my statement. Not one thing.

There is a permanent mechanical link between the front and rear axles on both BMW and Merc systems. You are saying it is not permanent and you are wrong. This is quite possibly because English is not your first language and you are confused by what the word permanent means in the context of the plethora of AWD systems out there.

Your reference to the old E70 xDrive transfer coupling is outdated - modern multi-clutch pack couplings are designed for high usage rates; certainly cornering in an xDrive or 4Matic on a twisty road will not burn the clutches out.

In summary the information you have given is insufficient to support your statement that xDrive and the like are not permanent AWD systems.

 

[CrunchSlaughtered]

Than

Let's first recap before you make a hardworking real engineer like Giannis waste his time.

You refuted my statement that BMW's xDrive AWD is permanent. This means that you say it isn't.

Nothing you have linked to in those two (very old) pages contradicts my statement. Not one thing.

There is a permanent mechanical link between the front and rear axles on both BMW and Merc systems. You are saying it is not permanent and you are wrong. This is quite possibly because English is not your first language and you are confused by what the word permanent means in the context of the plethora of AWD systems out there.

Your reference to the old E70 xDrive transfer coupling is outdated - modern multi-clutch pack couplings are designed for high usage rates; certainly cornering in an xDrive or 4Matic on a twisty road will not burn the clutches out.

In summary the information you have given is insufficient to support your statement that xDrive and the like are not permanent AWD systems.

Just one question. The differential makes the same job regardless if it is a central one in an all wheel drive car of if it is the rear one in a rear wheel drive car. The transfer case in a Xdrive BMW consists mainly of two wheels and a chain, with the size of the wheels determining the split ratio and a clutch pack with its actuator. This system is not very expensive and if it can permanently drive the two axles and in the same time varies the torque split, why is this system not used as a rear differential in a RWD car?

 

[martinbo]

Than

Just one question. The differential makes the same job regardless if it is a central one in an all wheel drive car of if it is the rear one in a rear wheel drive car. The transfer case in a Xdrive BMW consists mainly of two wheels and a chain, with the size of the wheels determining the split ratio and a clutch pack with its actuator. This system is not very expensive and if it can permanently drive the two axles and in the same time varies the torque split, why is this system not used as a rear differential in a RWD car?

You're harping on about my use of the term center differential to what end? Are you trying to catch me out? The term differential is commonly applied to all forms of central couplings tasked with distributing drive front to rear without the driveline binding. I understand the fundamental differences between a traditional axle open differential and a center "diff" in its many forms. You're leading where with this? Nowhere. Full stop.

Now tell me - if you're such an AWD expert and have now finally identified the key components in the xDrive transfer case (those two "wheels and a chain") please explain to me how the connection between the front and rear drive lines is anything other than permanent in current versions of xDrive and 4Matic.

 

[CrunchSlaughtered]

You're harping on about my use of the term center differential to what end? Are you trying to catch me out? The term differential is commonly applied to all forms of central couplings tasked with distributing drive front to rear without the driveline binding. I understand the fundamental differences between a traditional axle open differential and a center "diff" in its many forms. You're leading where with this? Nowhere. Full stop.

Now tell me - if you're such an AWD expert and have now finally identified the key components in the xDrive transfer case (those two "wheels and a chain") please explain to me how the connection between the front and rear drive lines is anything other than permanent in current versions of xDrive and 4Matic.

xDrive is very similar to the Haldex system in the FWD cars. The two big differences are in BMW the clutch pack and the transfer case are combined in one unit, while in the cars using Haldex they are separated and BMW uses a step motor as an actuator and Haldex system uses electro-hydraulic. Is the connection permanent with the Haldex system? Here the clutch pack is used as a connection between the lines. When it is engaged the lines are connected, when is disengaged they are disconnected.
I don't pretend to be an expert in this area and I'm also lacking engineer knowledge in the matter but the topic is interesting to me

 

[Giannis]

xDrive is very similar to the Haldrex system in the FWD cars. The two big differences are in BMW the clutch pack and the transfer case are combined in one unit, while in the cars using Haldrex they are separated and BMW uses a step motor as an actuator and Haldrex system uses electro-hydraulic. Is the connection permanent with the Haldrex system? Here the clutch pack is used as a connection between the lines. When it is engaged the lines are connected, when is disengaged they are disconnected.
I don't pretend to be an expert in this area and I'm also lacking engineer knowledge in the matter but the topic is interesting to me

It's Haldex, not Haldrex.

 

[Giannis]

The differential makes the same job regardless if it is a central one in an all wheel drive car of if it is the rear one in a rear wheel drive car.

Let's start from the beginning. What is a differential?

A differential is a mechanical component which has a circular motion of a certain rotational velocity as an input and gives out two outputs, in different directions, that may or may not have the same rotational velocity. Hence the name differential.

When taking a turn, the inside wheel travels a shorter distance than the outside wheel, in the same amount of time. And since ΔS=V*T (where ΔS is the distance traveled, V is the velocity and T is the time) the outside wheel needs to have a rotational velocity higher than that of the inside wheel. But remember, the differential's input rotational speed is the same for both wheels!

At this point, I suggest watching this highly informative video:

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[Giannis]

Now, what is the difference between an open differential, a "locked" (or in garage language, "welded") differential and a limited slip differential?

Power, just like current, flows to the path of the least resistance. Therefore, the wheel with the least grip will receive the most power. With an open differential, with one wheel on tarmac and one wheel on ice, you will never launch, unless the ice melts. But once you start moving, everything will be fine.

A "locked" or "welded" differential is just that, an open differential that is welded, so that the two outputs rotate at the same speed. Just like go-carts that come with a solid rear axle. The downside of welded diffs is that the moment you turn the steering wheel, the inside wheel will slip. Why? Because physics. The fun thing is that slip means loss of traction and combine that with a significant horsepower rating and you get massive drifts. Like maaaaassssive drifts!

And finally, what is a limited slip differential? An LSD is just like a normal diff, where power goes to the wheel with the least resistance, but up to a certain limit. Let's say that your LSD can send up to 80% of the power to one wheel. This means that if you have one wheel stuck on ice, still 20% of the power will go to the other one so that you can get going.

LSDs have evolved over the years, and some magic happens, so that modern mechanical and electronic LSD send more power to the wheel with the most grip, thus achieving maximum traction.

Are we good so far?

 

[CrunchSlaughtered]

Now, what is the difference between an open differential, a "locked" (or in garage language, "welded") differential and a limited slip differential?

Power, just like current, flows to the path of the least resistance. Therefore, the wheel with the least grip will receive the most power. With an open differential, with one wheel on tarmac and one wheel on ice, you will never launch, unless the ice melts. But once you start moving, everything will be fine.

A "locked" or "welded" differential is just that, an open differential that is welded, so that the two outputs rotate at the same speed. Just like go-carts that come with a solid rear axle. The downside of welded diffs is that the moment you turn the steering wheel, the inside wheel will slip. Why? Because physics. The fun thing is that slip means loss of traction and combine that with a significant horsepower rating and you get massive drifts. Like maaaaassssive drifts!

And finally, what is a limited slip differential? An LSD is just like a normal diff, where power goes to the wheel with the least resistance, but up to a certain limit. Let's say that your LSD can send up to 80% of the power to one wheel. This means that if you have one wheel stuck on ice, still 20% of the power will go to the other one so that you can get going.

LSDs have evolved over the years, and some magic happens, so that modern mechanical and electronic LSD send more power to the wheel with the most grip...

Have you read my posts?

 

[CrunchSlaughtered]

Not all that you wrote is correct. When the car has an open differential the following is happening: Lets assume a certain number: the transmission sends 33 Nm at 1000 rpm to the crown and the pinion, after them the number becomes 100 Nm at 330 rpm (just a random ratio of 3,33 to 1) and here is the interesting, if both wheels have full traction they will receive 50 Nm each, while the rpms of the semi axles combined make 660 rpm. So one may be 400 and the other 260 depending on the resistance, but the torque is split 50 to 50%. When one wheel starts to loose traction it can accept for example only 10 Nm and then the differential equalizes that torque to the other wheel and it also receives only 10 Nm. The torque split between the two wheels is still 50 to 50% but only between them, not from the torque coming from the transmission (in that case is only 10% to 10%). If one wheel is in the air it receives only the torque necessary to overcome the inertia of the wheel, maybe 0,5 Nm and through the spider gears the other wheel will receive only 0,5 Nm regardless of the power coming from the transmission. The rpms are still combined 660. All the other power is wasted
The limited slip eliminates the waste of power and that's why the cars equipped with it are exiting faster from the corners. It puts almost all the power to the ground. With LSD when one tire is in the air it will receive 0,5 Nm and the other on the ground (of course if it has full traction) will receive 99,5 Nm and the rotational velocity of both wheels is the same 330 rpm each. A tire in the air can not receive 20% from the torque. It receives only that much as it can consume. The percentage of the LSD is to indicate at what difference in the rotational speed between the semi axles it is actuated and not how much torque it sends.
Open differential - always equal torque to both wheels, but it differs the rotational velocity
LCD it locks the semiaxles and the rotational velocity is the same but the torque to both wheels differs depending on the traction

 

[Giannis]

With all due respect, my friend, but you are very, very wrong.

I don't want to brag, or anything, and I'm just a random guy on the internet, but I've also got a Diploma (BEng+MEng) in Mechanical Engineering, followed by a part-time pursuit of a PhD in the local Mechanical Engineering department, in the Machine Design Laboratory. Proof of that can be provided upon request via PM. Therefore, I'm quite sure of what I just wrote!

 

[CrunchSlaughtered]

With all due respect, my friend, but you are very, very wrong.

I don't want to brag, or anything, and I'm just a random guy on the internet, but I've also got a Diploma (BEng+MEng) in Mechanical Engineering, followed by a part-time pursuit of a PhD in the local Mechanical Engineering department, in the Machine Design Laboratory. Proof of that can be provided upon request via PM. Therefore, I'm quite sure of what I just wrote!

Sorry, but nothing that I wrote is wrong. I have a lot of experience in construction equipment including wheel loaders, where I had to consult the customers with what kind of differential to buy the machine. For example in a wheel loader you may have an open differential, a proportional differential,which is similar to the open one but with clearance between the teeth so that it can simulate increased traction with up to 15%, and several types of LSD differential - LSD 20%,LSD 40% and so on.
Everything that I wrote is corect