[dahoseman]

For better or worse, I may be doing some transmission work soon to my 2005 OBXT with the 5MT. While the opportunity presented itself, I was considering installing a LSD center differential. After doing some research on the 5MT and differentials, I became more confused about what the center diff is really supposed to accomplish. As far as I can tell, its only potential purpose is to allow a FWD bias, which manufacturers have determined to be a little bit safer for a typical driver. I'm wondering if anyone has ever simply fused/welded the center differential so that power is permanently split front/rear?

My thoughts:
The more I have thought about it, the less actual function I see for the 5MT viscous coupling center diff and the more I would rather that the coupling was simply fused. The coupling really only functions to "try" to keep front/rear spinning at equal speeds. I understand the purpose of other power distribution systems that can sense wheel slip and distribute to individual tires with the best grip, but it doesn't really accomplish that. As I've read and possibly experienced, it even overheats/fails if you use it under heavy slick conditions with constant slipping (one of the reasons I have AWD). As I've gathered from it's design, it doesn't have any fuel economy benefits. As the viscous coupling ensures, the rear drive train components are always spinning no matter what, along with their weight and friction, which is simply powered by the engine through the front wheels. If it is intended to reduce wear for front/rear planetary motion, I can't imagine that it would be too much of a "pressure release" for the the drive train either. There are two other differentials to allow power to find a way out without breaking anything. In most conditions, they move in lockstep anyway.

Am I overlooking a critical function of the center diff?
Hypothetically, what would be the down sides of having a fused coupling?

 

[Euchre]

You would be turning your center differential into a transfer case. When you turn, the wheels on each side - and each end - are going to turn at different speeds than each other. This is the whole reason differentials really exist. The fact that some AWD systems can use 3 differentials to manage torque delivery is really more of a bonus. Aside from this, the center viscous types are not meant to be locked up solid for a good reason.

Boiled down, don't do it.

 

[CNY_Dave]

Cornering causes the rear tires to travel a shorter distance than the front. Without some front-rear slip, simple driving will place enormous strain on the entire drivetrain.

Parallel parking and maneuvering in and out of a parking spot would literally be impossible.

The viscous coupler works quite well when it works properly.

 

[dahoseman]

Thanks for enlightening me. Excellent information.

I wasn't about to go ahead and fuse mine or anything, but I was interested in the purpose for the flimsy viscous coupling and a better system for more robust power distribution from front/rear. I know it's not an actual off-roader, but I would certainly like to have a transfer case on the Outback. I had full lockup capability on an old truck that a previous owner had modified. That was fantastic and came in handy many times.

 

[dahoseman]

If only the XT had been available with the option of the STi drivetrain <sigh>.
I have seen the swap threads, but that's more money and effort than I'm willing to invest

 

[Dajeepman]

well if the subaru can split 50/50 thats what my Jeep does in its Part time Transfercase. Its a 50/50 split but it does not allow any slip like the subaru VC does like when you try to turn.

 

[nipper]

You already have a LSD center diff

AutoZine Technical School - Traction

If you fuse the center diff, your car will be undrivable on tarmac.

HowStuffWorks "How Four-Wheel Drive Works"

You need slip in the center diff in an AWD vehical. 4WD is a different animal. Read the links.

 

[1 Lucky Texan]

^^^what he said = plus, it's Viscous LOCKING Center diff. for 50/50 split. evidently there are some viscous units made for RallyX that have quicker response than factory. probably $$$$

and, as mentioned, the DCCD from the STI would likely be prohibitively expensive to swap in.

if you want to mod the car for better off-road performance, better tires and a rear LSD would be the first place to make changes to my mind.




about halfway down this page;http://www.awdwiki.com/en/viscous+coupling/

viscous coupling integrated into the center differential

In this case, all wheels are powered at all times. Viscous coupling is integrated into the center differential. Central differential distributes power to all wheels and lets them turn at different speeds while cornering. When excessive wheelspin occurs on one of the axles, viscous coupling locks the differential and equalizes the speeds of both axles. Torque is transferred to wheels that have traction. This is a full-time all wheel drive system.

Viscous coupling can also be integrated into the rear differential.

Figure: Viscous coupling locking differential
viscous-coupling-locking-differential

Figure: Viscous coupling (left) and its installation in the rear (top right) and center planetary gear differentials (lower right)
viscous-coupling-differentials

Read more

HowStuffWorks

 

[rasterman]

Selectable transfer could be possible with the 5EAT. It's been done in the 4EAT. No hope for the MT other than the DCCD $wap.

 

[dahoseman]

Sorry about that. I reread my post. I was having a pretty serious brain fart this afternoon with how I had the Suby 5MT drive train arranged in my head. I managed to insert a "redundant" external coupling in my thought processes for some reason. Seemed really useless. Weird things happen when I get a few weeks into board study sessions.

Depending on what is actually wrong with my transmission, I almost assuredly won't be doing any aftermarket modifications to the stock setup. A gearing setup with helical front diff setup would be nice, but I'll leave it to dreams. Time and money dictate that I'm pretty well done with recreational mods for a while, haha.

In any case, I would still love to have a selectably adjustable center diff attached to dual LSD's. My winter setup Blizzak snow tires are fantastic, but there's no substitute for the safety and stability of fully adjustable distributed power delivery, especially in a car with more oomph. It seems a shame that Subaru had it developed and up for sale, but did not have it available even as an optional upgrade on the "adult" cars.

 

[rasterman]

This is where snow tires prove their edge- not AWD. Where AWD can really show off is traction on gravel, mud, dirt or sand. The grown-up cars don't go there as often.

 

[dahoseman]

I'll agree that tires make a huge difference on an individual vehicle. They are the only part of the car that touches the ground. However, the power delivery is also important. This is just my $0.02, but in my personal experience, AWD or 4WD has always trumped tire choice on my personal vehicles. At one point years ago, I used to live a little ways up an old unplowed dirt logging road in a ski town, 9400ft up. The drive up to the house was snowpacked all winter, then pure mud for a few weeks at thaw during "mud season", then gouged dirt and rock during warm months. Commuting to work was an off-road challenge in itself.

1. My old '99 Forester with mostly-bald summer tires (free off of a police cruiser at the end of summer) never had any problem getting to the house.
2. My Maxima with new snow tires often had difficulty and had to be left down the mountain at the end of the plowed road.
3. I had winter tires for my souped-up Dodge 2500 diesel 4x4, but it was able to get up the hill just fine with all-seasons. I was too lazy and unmotivated to put the winter setup on since I didn't have access to a garage large enough to lift up the truck, so I eventually sold them.
4. I inherited an old rock crawler Toyota truck while I was there that had been pulled out of the woods. It had flattened, decade-old glazed bald-ish tires when it was pulled out of the woods, which I didn't change for quite a while. However, in full lockup it was able to get up just about anything. Quite impressive.
5. My buddy had new snow tires on his rear wheel drive truck and it was hit-or-miss whether he could drive to the house or had to walk/snow shoe.
6. For what it's worth, the Land Rovers that visited seemed to be the most competent stock vehicles in adverse conditions. I think that most people that buy them outside of the mountains only use them as a glorified station wagon, though.

Anyway, I'm sure other people have had other experiences with various tires and vehicles, but those have been mine. At this point, excluding sports car choices, I wouldn't go back to a daily driver without AWD (in alpine environments). I just enjoy the feel of it and use it too much. I've been spoiled.