Torque Split & description of the MT/AT AWD system (Merged)

PigSti,
I have both a manual and auto Forester and there is no doubt the manual is more sure footed in slippery conditions.
I spend a lot of time at Mt Buller in the snow season and this year I am using the auto instead of the manual. I have a 4wd access permit so I can park in the village. Using the hold function as you say locks out first gear and therefore reduces the chance of wheel spin. I am not convinced that when hold is activated that the torque is distributed 50/50. The auto has what I would describe as a happy tail compared to the manual.
With the power function I suspect most of the torque is going to the front. I have noticed that the wheel wear on the manual is even between the front and rear whilst on the auto the front tyres wear quicker.
That's my two bobs worth.
 
You're right about the tyre wear, the fronts disappear at twice the rate of the rears.
It would be great if there could be a way to tap into the OBDII and get a 0-50% digital read out of what the torque split is happening between the front and the rears real time! Could there be a module read out for this in OBDII?
 
torque split

We drive 4wheel drive cars, but how much torque does each wheel get ?

Well, it depends what differentials we have and the surface we drive on.

In a straight line with open diffs, each wheel will get 25% of torque…

Now lets imagine a Subaru with welded center and rear differentials and lifting the rear left wheel, how much torque will the rear right wheel get…25% or 50%

I think its 50% but am waiting your thoughts and answers :rolleyes:
 
Of course, the 90:10 is not bogus. It comes straight from Subaru. But it refers to the earlier 4EAT OBs and Foresters. Others claim that it is really 80:20 in testing. Others again think that 90:10 may have been the case early on (like my 95 Legacy), 80:20 subsequently. But whatever the details, the 60:40 split on 4 EATs is a relatively recent development.

PAY ATTENTION TO CHANGE OVER TIME :evil:;)
 
As for the person who asked about torque distribution above:

No, you won't get what you want. I am unsure as to the maximum transfer my VTD can provide to either axle, but assuming 60% rear and one rear wheel in the air, I do not get 30% on the other one: the wheel braking function is not equivalent to full lock. In addition, the Subaru systems transfer torque all over the place in the blink of an eye, there is no such thing as fixed ratios on a slippery off-road terrain, I think. As for the limited slip: I would rather have the wheel braking, thanks. Thus the tears shed on forums after the LSrD was dropped were a result of misunderstanding. What matters, at the end, is that with the VTD I have never experienced traction problems, be that in mud or slippery desert terrain.

My angles and my clearance stop me well before I need a full lock.

The old AWD system on the 1995 Legacy made that vehicle feel much closer to an ordinary FWD than to my 2013 OB. Neither is made for the off-road, but the latter's traction is so awesome that one gets really irritated by the lack of adjustable height suspension.
 
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jf1sf5 said:
Sorry for the new thread but my question was not only about Subaru AWD specs but more general.
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I know nothing about the others except that I do not see them where I go: unless we talk approach dirt roads, in which case they do not enjoy the sight of me for long;).

I think what marks Subaru's systems is the continuous transfer of torque as the system "hunts" for traction. That is why it is important, when traction is lost, to keep one's foot on the gas: not too much, so one does not "burn" tires, but no taking the foot off either, so that the system can do its job.
 
I am ok about what you say, my question was just more mathematics/physics, I could compare it to a bicycle/tricycle (no rear diff on the tricycle !)

When accelerating with your bicycle, 100% torque goes on the rear wheel... on the tricycle, 50% torque goes on each rear wheel…

Now if you lift one rear wheel of the tricycle, what will be the torque split ratio ?
 
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100% less wind resistance, etc of the lifted wheel.

From my Batphone
 
MiddleAgeSubie said:
As for the limited slip: I would rather have the wheel braking, thanks. Thus the tears shed on forums after the LSrD was dropped were a result of misunderstanding. What matters, at the end, is that with the VTD I have never experienced traction problems, be that in mud or slippery desert terrain.
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I would prefer both! IMO dropping the rLSD was merely to save money, rather than it not being needed. With the VTD/VDC, a rLSD isnt as critical, but having both would certainly increase rear grip, it would also reduce power loss due to less braking being applied on the lifted wheel
 
NachaLuva said:
I would prefer both! IMO dropping the rLSD was merely to save money, rather than it not being needed. With the VTD/VDC, a rLSD isnt as critical, but having both would certainly increase rear grip, it would also reduce power loss due to less braking being applied on the lifted wheel
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Lol, sure. But it is not needed: Subarus are built for the bad weather and the off-pavement, not for the off-road, after all, even if we take them there. If the company cared one little bit about off-road we would be getting an adjustable height suspension or a slight factory lift option (1"), a proper tire option with a full size spare, and a factory skid plate option. Oh, and angles would have been a bit better. It is true that Australia gets better vehicles than the US, or at least it used to be true with low range and full size spare, but are Subarus really marketed as off-road there?
 
jf1sf5 said:
Now lets imagine a Subaru with welded center and rear differentials and lifting the rear left wheel, how much torque will the rear right wheel get…25% or 50%

I think its 50% but am waiting your thoughts and answers :rolleyes:
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Assuming you're on even ground (apart from the rear left being lifted), yep 50%. But in reality if you are lifting wheels the ground will be uneven and if you have welded the centre too then you'll have all sorts of reactions going on in the driveshafts as they are being forced to turn at the same speed by the welded diffs while the uneven ground will be trying to turn the wheels at different speeds.

jf1sf5 said:
Now if you lift one rear wheel of the tricycle, what will be the torque split ratio ?
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We're talking a 2 wheel drive tricycle with an open diff here? In that case, it's still 50:50, as it always is with an open diff, period. However, the total torque itself will be basically zero as the lifted wheel has nothing to react against (apart from wind resistance as Ratbag referred to earlier, and the wheel's own inertia) so your wheel on the ground gets 50% of basically zero, which is (drum roll) basically zero.

If your tricycle has a locked diff then then it's 0:100, in favour of the wheel on the ground (except for the negligible effects of wind resistance and inertia on the lifted wheel).

NachaLuva said:
With the VTD/VDC, a rLSD isnt as critical, but having both would certainly increase rear grip, it would also reduce power loss due to less braking being applied on the lifted wheel
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When braking (by VDC and similar systems on other makes) occurs on a lifted wheel, you are not losing power - quite the opposite. The braking of the wheel provides something for its driveshaft to react against, thereby meaning the diff can send a (useful) corresponding amount of torque to the opposite wheel which hopefully does have enough traction to get you moving.

You're right generally though in that having an LSD (a clutch type or, particularly, a torsen) in combination with VDC would make a nice combo, with the VDC not having to work so hard. Having a VLSD with VDC however would not be so helpful as a viscous coupling only offers a small amount of resistance until there is a significant difference in driveshaft speeds, at which point the VDC would have stepped-in anyway - I suspect this is why they deleted the rear VLSD on VDC equipped vehicles.
 
I have often thought that the whole torque split descriptions are largely irreverent for an offroad vehicle (and I mean 4wding not rallying or similar where it is more important), what is important is the systems ability to change from the normal torque distribution to some other distribution.
Say we have a vehicle with 50:50 Front rear torque split and open diffs as soon as one wheel lifts off the ground or is in something slippery essentially 100% of power (in reality pretty much no torque at all) goes to that wheel and with the 50:50 split means that no torque goes to any other wheel. so in essence we have a one wheel drive.
What is really important is the systems ability to shift from whatever the front rear torque distribution is to 0:100 and 100:0 when wheels start to slip and the rate of slip required for this to happen, specs that are very rarely quoted.
After all an open diff is technically 50:50 left right torque split and it is damn useless offroad.
 
I am no engineer and I lack any actual knowledge of how 4wd and AWD systems operate. However, from what I read I have to conclude that the only way to get what you want is a true locker.

I think that in all other cases the question is how much really goes where.

I do not understand your 50:50 example. The center differential is not open so a 50:50 vehicle needs either 2 front or 2 rear wheels to have traction. Hence the diagonal spin that plagued early OBs and Fories.

I am clueless as to how much torque my 3.6 VTD can move around, but the wheel braking system surely works and helps. Traction problems are virtually unknown to me. I think I need a really slow going, really steep AND rocky hill to be stopped due to lack of real locker. I recently got out of a steep and really loose wash with a couple of sizable rocks without any real problems. I did not go over the first one right away, but all I had to do was to turn the wheel a bit. That is it.

For me traction is a non-issue--and our terrain in the US southwest is as rough as any. Angles and clearance stop me well before I can feel the lack of locker.

Thus the bottom line is that however torque moves around, which supposedly happens in fractions of a second, it works very well.

There is literally no comparison between my 95 Legacy and my 13 OB.
 
MiddleAgeSubie said:
I do not understand your 50:50 example. The center differential is not open so a 50:50 vehicle needs either 2 front or 2 rear wheels to have traction. Hence the diagonal spin that plagued early OBs and Fories.
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The center differential in a first gen manual forester is essentially 'open' when there is only a low speed difference between the front and rear wheels. That is why you can jack up one wheel and let the the clutch out with the engine at idle in 1st gear low without the car going anywhere. Its only when you accelerate a little that the centre diff start to change the 50:50 torque split to a ratio with higher torque sent to the end of the car with both wheels on the ground that forward movement is actually obtained.
 
Wow, I did not know that. I never took my 95 Legacy wagon off-road. A few dozen miles of well-maintained gravel roads is all it ever saw.
 
Dedman said:
I have often thought that the whole torque split descriptions are largely irreverent for an offroad vehicle (and I mean 4wding not rallying or similar where it is more important), what is important is the systems ability to change from the normal torque distribution to some other distribution.
...
What is really important is the systems ability to shift from whatever the front rear torque distribution is to 0:100 and 100:0 when wheels start to slip and the rate of slip required for this to happen, specs that are very rarely quoted.
Click to expand...

True that the ability to change to 0:100 or 100:0, and do so quickly, is the ultimate answer. However there is some benefit in having it already distributed somewhat (say 50:50, 60:40 or whatever, rather than 100:0 or 0:100) under normal conditions so as to reduce the likelihood and impact of wheelspin the the first place.

MiddleAgeSubie said:
However, from what I read I have to conclude that the only way to get what you want is a true locker.
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I'd actually prefer to have an open centre diff and manual locker rather than a viscous LSD (same for rear and even front for that matter) but this does rely on the driver knowing how and when to use it. The automatic systems that Subarus use - such as centre viscous LSD, MPT or VTD, and also VDC for cross-axle control - don't require user intervention. They don't perform quite as well as having manual fully lockable diffs though.

Dedman said:
The center differential in a first gen manual forester is essentially 'open' when there is only a low speed difference between the front and rear wheels. That is why you can jack up one wheel and let the the clutch out with the engine at idle in 1st gear low without the car going anywhere. Its only when you accelerate a little that the centre diff start to change the 50:50 torque split to a ratio with higher torque sent to the end of the car with both wheels on the ground that forward movement is actually obtained.
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Excellent example and description:). The current model manual Foresters and Outbacks still use a viscous centre LSD and would behave similarly, aside from VDC intervention, if any (though the 'phase 2' centre is a little different in details - see: https://forums.jdmvip.com/Transmiss...earchInfo_Thread_Subaru_Subaru_General-2746-t).

Nice vid here too of VDC in action preventing 'diagonal spin': [ame="https://www.youtube.com/watch?v=HuxyT2qy0W0"]Subaru Forester 2013(2014 US) Diesel MT Steep uphill and Diagonal Test - YouTube[/ame]
 
I meant rear manual full lock, yes. I do not know if there is a single stock vehicle with front locker (maybe the Wrangler Rubicon, if that).

As for the center, I do not know how exactly the planetary gear one works, but it gives me all I need.

The video above is good, except for labeling that hill as "steep." It is a hill, but it is hardly steep:)

But, again, to me Subaru traction issues are a thing of the past, or at least so far as mine goes. Now I want an adjustable-height suspension:)
 
Factory front diff locks are not common, but not unheard of either (eg some Landcruisers, Merc G Class). I think we are already seeing less and less of manual locks though as VDC and similar systems, as well as so-called 'e-diffs', take over.

Good to hear the current Subaru traction systems are working well for you though.

Back on the original topic of the thread though, what is vital to note in these discussions but is not well understood is that a fully locked (or welded) diff, or something that approximates it like a fully engaged MPT or VTD, does not mean a 50:50 torque split. An conventional unlocked (open) differential provides a 50:50 torque split. A locked diff or coupling means the rotation speed, not the torque, of the driveshafts is equal - the torque split then can (and will) vary according to the external conditions the drivetrain is subjected to (eg loose surfaces, lifted wheels, cornering...)
 
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