Question on How Subaru Symmetrical AWD Works

[Soul Shinobi]

Let me just say up front that I don't know a whole lot about cars, but I've learned a good bit from reading this forum, the Subaru Global website and Wikipedia (I don't really have any knowledgeable friends ).

There's a concept I'm having trouble wrapping my head around that if I'm not mistaken has to do with differentials, the likes of which I just barely understand.

I was reading Wikipedia and was around the end of the Acura TL page and noticed that in 2009 it will receive SH-AWD (Super Handling All Wheel Drive). Clearly a proprietary system (with a lame name, no less), I looked at the Wikipedia page on it. It seems to brag combining "front-rear torque distribution control with independently regulated torque distribution to the left and right rear wheels." Now clearly this SH-AWD differs from Subaru Symmetrical AWD and if you read the bit on it (found here) it's clearly very different.

What really caught my attention is the independent left-right torque distribution. Allow for me to cite the Subaru Global web site: "That's where the technology of our torque distribution system sets Subaru apart from conventional 4-wheel drive set-ups. Evolved from the very outset specifically for Symmetrical AWD, it actively distributes engine torque to individual wheels." That was found here. What I want to know is if this is anything special compared to Subaru Symmetrical AWD, and I stress the SH-AWD system's left-right torque distribution being independent.

Now, obviously Subaru's system does differ from the 4-wheel drive systems found on trucks because trucks with selectable 4WD usually lack differentials ... someplace. I've clearly reached the extent of my knowledge, this is where you guys come in. In summary, is the SH-AWD that Honda wants to use anything special, and moreover, is my Legacy's AWD already more advanced in concept (based on only what I quoted about SH-AWD, not including all the details on the Wikipedia page)? And perhaps to clarify anything else, how exactly does Subaru Symmetrical AWD work beyond what their global site tells us?

[Splinter]

The main difference between Subarus system and other AWD systems is that subarus have a longitudally mounted engine (crankshaft is perpendicular to the direction the wheels turn) whereas other manufacturors use a transverse mount (crankshaft spins on same axis as the wheels)

This means that for the front wheels on other cars, one axle has to be longer than the other. Longer axle = more room for the physical material of the axle to twist before the wheel begins turning.

On a subaru, all the axles are the same length, so they all twist the same amount before the wheels begin to turn.

[n2x4]

On a subaru, all the axles are the same length, so they all twist the same amount before the wheels begin to turn.

Ahh, I never thought about that myself. So even Subaru front wheel drive should be better than other FWD too, since both axles are equal length?

[Splinter]

I believe so

[entirelyturbo]

Yep, that's why Subaru FWD cars have basically no torque steer.

[PhyrraM]

Axle length has more to do with torque steer then traction. It's really just a by-product of Subaru's longitudinal engine orientation. It is also cheaper to manufacture two of the same part instead of two different parts.

Most Subarus (minus autos and DCCD) have a very simple and elegant solution. The one thing to keep in mind when talking about any differential is that a standard (open) differential will alway try to send the torque to the side (in this case "side" can be front or rear also) with the least restriction. When all 4 wheels are on the same surface and your going relatively straight the traction is just about even, so torque distribution is pretty even also.

Most every Subaru has an open (normal) front differential. This means that the majority of torque goes to the wheel with the least traction.

Many Subarus also have an open rear differential also. Some have a differential with a viscous coupling in the rear. Manual 5-speeds also have a viscous coupled center (front/rear) differential.

What the viscous coupling adds to the equation is that when the differential tries to send more torque to one side and that one side starts to spin there is a speed difference between the two sides. This speed difference heats the fluid in the coupling. The now heated fluid acts to lock the two sides together bringing the different speeds closer together. This is completely passive. But it does require some slip to function. Also, this is just the basics of normal 5 speeds. Anything DCCD or with an Automatic has a different center differential.

Many other systems, including Honda's/Acura's, use an active system. They use sensors, computers and electronic clutches to do the same thing. Depending on how smart the computer is this can be proactive, redistibutng the torque before any slip can occur. But even the best computer can be fooled every now and again.

I guess the short and long of it is....Subarus use a simple, but well proven AWD system. Many other makers are using more complex systems.

[Soul Shinobi]

Hmm, that's a shame, I'm a bit disappointed. On the Subaru Global site they brag that the system is proactive rather than reactive (found here). I think that in truth this applies only to the few models of theirs that have a torsen differential. Is the differential in the picture on that page any particular type?

EDIT 1: that page displays a limited slip differential as stated, can an LSD also be of the torsen type, or does it mean the same thing? Can an LSD be of a viscous coupling type?

EDIT 2: the flash demonstration on the page I showed in the first post (found here) shows more power going to the wheels that are not slipping. Would this be only with more complex models, not the basic?

[PhyrraM]

LSD refers to any kind of differential that attempts to equalize torque. This applies to Viscous, clutch, Torsen, etc..

All types of "passive" LSDs are reactive. The Torsen type is reactive the other way, it, by nature, sends torque to the wheel with the most traction. (Don't ask me how, I don't really know). But, even with a Torsen, a wheel completely loses traction all torque will go there. Torsen is good for road race, but not really accepted in rally, for example.

Personally, I prefer the simpler passive/mechanical way. Less likely to get it fooled. Less likely to break. Easier to troubleshoot/maintain. More predictable under all circumstances. Drivers still drive Subarus. Some of the more Electronic Brands (such as the RL) can feel intrusive when your having fun behind the wheel. The AYC Evos, seem to have gotten it mostly right, but I've heard that some still have quirks.

[Subtle]

The older 4eat automatic tranny essentially delivers about 90 % of the torque to the front most of the time.

Upon acceleration in first or second the computer controlled clutch pack sends a maximum of 50 % to the rear before any wheel spin, and is proactive.

The Turbo Wagons have a viscous lsd at the rear which when one wheel begins to slip it sends torque to the other wheel, and is reactive.

The Torsen sends torque to the wheel with traction and is v. expensive.

[555BCTurbo]

The Turbo Wagons have a viscous lsd at the rear which when one wheel begins to slip it sends torque to the other wheel, and is reactive.

No they don't


The only SS's with LSD's were the 91's (therefore no Touring Wagons)...other than that it was a very rare special order item

[Soul Shinobi]

Personally, I prefer the simpler passive/mechanical way. Less likely to get it fooled. Less likely to break. Easier to troubleshoot/maintain. More predictable under all circumstances. Drivers still drive Subarus. Some of the more Electronic Brands (such as the RL) can feel intrusive when your having fun behind the wheel. The AYC Evos, seem to have gotten it mostly right, but I've heard that some still have quirks.

I can totally empathize with that.

[entirelyturbo]

The only SS's with LSD's were the 91's (therefore no Touring Wagons)...other than that it was a very rare special order item

Not sure if this is what you meant, but rear LSDs were only available as an option on 91 turbos. No other BC/BF/BJ Legacy had them, at all.

And the old 4EATs move traction around more often than just every once in a while. About the only time they're at 90/10 is when going straight at a constant speed.

The 6-speed DCCD is really just a manual transmission with the clutch packs out of a 4/5EAT, and the driver has the ability to control the split, unlike the 4/5EAT which controls itself.