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2. Pros and Cons of Dual Range VS AWD


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33 replies to this topic

#1 cole098

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Posted 25 November 2007 - 02:25 PM

Any opinion of which is a better performer in snow and Ice, which one works better for getting unstuck in deep snow ??? Which one provides better traction ????

#2 nipper

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Posted 25 November 2007 - 05:24 PM

Nothing works on ice unless you have studded snow tires, then everything works.

It really depends what you want to do with the vehical. Manual Dual range 4wd is cheaper and simpler to produce then an AWD system.

Either system is good for snow. If you have snow mixed with non snow surfaces, (especially on curves) you want AWD.

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#3 Sushi

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Posted 25 November 2007 - 05:39 PM

After installing AWD on my Loyale, I could never go back to part time 4wd. It is fun to drive on either snow or pavement now! And you save the 4wd button for those very deep areas....

#4 daehttub2000

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Posted 25 November 2007 - 11:52 PM

The choice between AWD versus the old-fashioned “locked hub” hi/lo 4WD depends on a few things. I think that the increasingly common addition of a limited slip rear differential to AWD cars have relegated the old-fashioned hi/lo 4WD systems to pasture (literally). Besides pulling tree stumps, tandem horse trailers in the mud, or climbing cliffs, I don’t see any advantages to the old system unless you like stopping, getting out, and locking your hubs in the mud/snow:lol:

But seriously, IMHO, the new AWD systems with limited slip diffs are safer and more affective unless (as Nipper pointed out) you always drive in a straight line and never turn. Think of your old Red Flyer wagon and what happened when you turned/pulled around a corner too fast. The fixed axle causes the outside wheel on each axle to skid and lose traction. Remember those downhill wipeouts? If you were brighter than I was when I was a kid, probably not…:rolleyes:

However, AWD without limited slip front and/or rear differentials can leave you stuck or sliding down a slick hill where the old 4WD’s would grind it out. Think of it as having two separate 2-wheel drive systems when all 4 wheels are slipping on the ice and snow, the Achilles heel of the standard Subie AWD system.:dead:

Now all we need are standard front AND rear limited slip diffs for us AWD gearheads:cool:

#5 nipper

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Posted 26 November 2007 - 12:04 AM

However, AWD without limited slip front and/or rear differentials can leave you stuck or sliding down a slick hill where the old 4WD’s would grind it out. Think of it as having two separate 2-wheel drive systems when all 4 wheels are slipping on the ice and snow, the Achilles heel of the standard Subie AWD system.:dead:

:cool:


Well it depends on how cheap the AWD system is. Ones without ciscous couplings or clutch packs can leave you with one wheel drive.

With clutch packs and viscous couplings you get 2wd (one each axle). You get a poor mans limited slip by applying light brake pressure to the rear wheels (parking brake) or all 4 (brake pedal). This puts drag on the spinning wheel and transfers that torque to the non spinning wheel.

Viscous LSD's are really poor replacements for good old fashioned mechanical LSD or Posi rears.

:)

nipper

#6 BlueTrain

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Posted 26 November 2007 - 01:28 AM

After installing AWD on my Loyale, I could never go back to part time 4wd. It is fun to drive on either snow or pavement now! And you save the 4wd button for those very deep areas....


not sure i understand this post. you have AWD in your loyale, but also have a 4wd button to push for when it gets really deep?

#7 scoobyclimbs

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Posted 26 November 2007 - 09:38 AM

Having done alot of wheelin in deep snow..I'd rather have 4wd..with a lo- range...less chance of spinning a tire...and losing traction especially on steep hillclimbs...At least the folks in OZ get a lo-range with AWD on new gen stuff..

AWD is great for street/rally style drifts..in gravel or under a foot of snow...However , once you get offset your done...without momentum...to get over obstacles...good luck getting your LSD to kick in intime...

but offroad..... 4wd is King hands down....LSD is ok but a locked front/rear diff. rules...lincoln lockers..baby..:headbang:

Besides how deep of snow are we talkin'..a foot or more...clearance is more an issue...Most of the time if ya get stuck in deep snow your gonna dig yourself out...since the snow is highcentering your vehicle....I use a telescoping snow shovel...works great..
A cumalong...or winch is your best friend.....always bring lots of shackles /straps./tree saver..Usually a couple minutes of diggin the snow out and your ready to go..

Tires...I run AT tires on my outback and have broke trail in almost 3ft of powder....without chains...!
Don't forget to air down your tires ....I'll air down to 15psi or less...for serious traction..

Unless it's icy.....I haven't had to chain up yet...(with street tires ..yes)

#8 Sushi

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Posted 26 November 2007 - 11:23 PM

not sure i understand this post. you have AWD in your loyale, but also have a 4wd button to push for when it gets really deep?


Exactly as you said. awd most of the time and 4wd (locks the center diff) when its really deep or the awd slips. Also 4wd makes it easier to drift at low speed I've found.

#9 GeneralDisorder

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Posted 26 November 2007 - 11:27 PM

Exactly as you said. awd most of the time and 4wd (locks the center diff) when its really deep or the awd slips. Also 4wd makes it easier to drift at low speed I've found.


Actually, you have Full Time 4WD with a center diff locker. It's neccesary to differentiate between the the AWD systems (Viscous Clutch Pack), and the FT4WD systems (center locking diff) as they are functionally quite different.

For example:

With one wheel in the air (off a cliff, on ice, whatever), FT4WD will not move a single inch without locking the center diff.

AWD will drive away as if nothing was wrong. The viscous pack insures that some power is always availible to the wheels with traction. Think of it as a very rudimentary center LSD (it's not a differential though).

GD

#10 Sushi

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Posted 26 November 2007 - 11:38 PM

Ahh, that is a good point. I was somewhat unsure of the difference between ft4wd and awd. So with ft4wd is it as necessary to use tires of exact diamter on all sides as it is with awd?

#11 nipper

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Posted 26 November 2007 - 11:57 PM

Yes for the same reasons you need to on a awd car.

on manual 4wd (M4WD) if you have differnt tires on one axle, the car will want to turn towards the smaller tire.

Now if you have size X on the right frt and rear, along with size Y frt and rear, everything will cancel out.

M4WD should only be used on slick surfaces. That allows for some wheel slip, and keeps you from getting torque bind.

http://auto.howstuff...wheel-drive.htm

and to explain the differnt AWD systems


http://www.autozine....raction_4wd.htm

#12 GeneralDisorder

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Posted 27 November 2007 - 12:30 AM

So with ft4wd is it as necessary to use tires of exact diamter on all sides as it is with awd?


That restriction is *mostly* for the AWD Automatics. The auto's have a design limitation in their center viscous coupler that causes them to wear more rapidly when used with tires of differeing sizes. This will eventually cause "torque bind".

I have never heard of adverse effects from running disimilar tires on an AWD 5 speed manual. Not that it's good for them, but they aren't as sensitive to it.

The FT4WD should not be affected at all really since there is no viscous coupler in them. The center diff will not be harmed by different sizes of tires, as it's sole purpose is to allow the front to travel at different speeds than the rear - just as the front and rear diffs allow the left and right tires to travel at different speeds.

GD

#13 nipper

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Posted 27 November 2007 - 12:34 AM

That restriction is *mostly* for the AWD Automatics. The auto's have a design limitation in their center viscous coupler that causes them to wear more rapidly when used with tires of differeing sizes. This will eventually cause "torque bind".

I have never heard of adverse effects from running disimilar tires on an AWD 5 speed manual. Not that it's good for them, but they aren't as sensitive to it.

The FT4WD should not be affected at all really since there is no viscous coupler in them. The center diff will not be harmed by different sizes of tires, as it's sole purpose is to allow the front to travel at different speeds than the rear - just as the front and rear diffs allow the left and right tires to travel at different speeds.

GD



There are posts here (not many) of vscous coupling (AWD 5sp) failing due to mismatched tires, or driving on a low tire.

Its a mixed bag. The autos will warn you nefore they have damage, the manuals its a silent death. The automatics there are things you can do to remedy the situation, manuals its no warning.

nipper

#14 GeneralDisorder

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Posted 27 November 2007 - 12:41 AM

There are posts here (not many) of vscous coupling (AWD 5sp) failing due to mismatched tires, or driving on a low tire.


Good to know. But just to clarify - there's no VC to fail in a FT4WD, so Sushi is fine to run mismatched tires, while us poor AWD folks have to watch what we put on.

GD

#15 bulwnkl

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Posted 27 November 2007 - 11:06 AM

This is a really interesting thread. I was about to cry foul on several of the posts describing the difference between FT4WD and AWD until I remembered that everyone is talking about Subarus specifically and not 4WD as used in most vehicles on the planet.

So, to the OP, I'd take AWD like my XT-6 had it if only it was still available: open conventional center differential (no silly viscous anything) with the ability to manually, mechanically lock it. Then, I'd add dual-range if it was available. That's nearly the best combination IMO because you have low range for off-road and open diff for zero possibility of torque bind ever. The only thing better might be the planetary center (or that concept, since I don't think I understand that system having never seen a good schematic with an explanation of function) to bias torque rearward most of the time and only bring it forward as necessary, but I'd like that with dual range, ability to lock it up tight mechanically, and a manual transmission, please!

#16 GeneralDisorder

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Posted 27 November 2007 - 08:05 PM

So, to the OP, I'd take AWD like my XT-6 had it if only it was still available: open conventional center differential (no silly viscous anything) with the ability to manually, mechanically lock it. Then, I'd add dual-range if it was available. That's nearly the best combination IMO because you have low range for off-road and open diff for zero possibility of torque bind ever. The only thing better might be the planetary center (or that concept, since I don't think I understand that system having never seen a good schematic with an explanation of function) to bias torque rearward most of the time and only bring it forward as necessary, but I'd like that with dual range, ability to lock it up tight mechanically, and a manual transmission, please!


That's the RX transmission. FT4WD, Diff Lock, and D/R. Low is only 1.2:1 though instead of the 1.59:1 of the 4WD D/R as the RX transmission was designed for rally. Basically it's a 10 speed FT4WD box - it's not suited to crawling as the diff ratio is also only 3.7.

GD

#17 Sushi

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Posted 27 November 2007 - 11:49 PM

Nice to know I can run different tires after I made a special effort get a matching set for the transmision.
I suppose using very different tires would have an adverse effect on any of the 3 diffs though, similar to pinion failure caused by running a temp spare on the back of a rwd car.

I have studied the schematics for the ft4wd xt6 5spd tranny, and I would like to see how it compares to an Impreza one, does anyone have a link or pdf for that?

#18 Gloyale

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Posted 30 November 2007 - 12:45 AM

AWD will drive away as if nothing was wrong. The viscous pack insures that some power is always availible to the wheels with traction. Think of it as a very rudimentary center LSD (it's not a differential though).

GD


Ever actually take one apart? It is so a differential. It's a limited slip Differential, but still has spider gears on a pinion just like any other diff. Viscous pack is attached to it much like the clutch type LSD used in rear ends (except obviously a viscous pack instead of mechanical clutch disc stacks)

#19 Gloyale

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Posted 30 November 2007 - 12:57 AM

That restriction is *mostly* for the AWD Automatics. The auto's have a design limitation in their center viscous coupler that causes them to wear more rapidly when used with tires of differeing sizes. This will eventually cause "torque bind".

I have never heard of adverse effects from running disimilar tires on an AWD 5 speed manual. Not that it's good for them, but they aren't as sensitive to it.

GD


This is contadictiory and somewhat incorrect.

Firstly, Automatics DO NOT have a viscous coupler/differential. They use hydraulic, electronically controlled, clutch pack. Just like the clutches used for the automatic trans gears. When it gets pressure it is an absolute direct coupling to the rear axles. However, they do not like mismatched tires still (altough autos can be put into Front Wheel Drive)

Now the Manual trannys DO have a viscous diff. And there is no way to disable it. Put tires of any signifigant size difference on one and it will cook the viscous pack. You won't notice any problems till you need the traction and you get nothing but One wheel spinning. Or it will fail in the other manner and lock and you'll have massive torque bind trying to turn into that parking space.

#20 grossgary

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Posted 30 November 2007 - 11:23 AM

Gloyale is right - running mismatched tires is bad for any AWD or FT4WD transmission. manuals or automatics can be damaged this way. failed manual trans viscous packs make for nice candidates to turn into RWD!

although i think GD is getting at the fact that the FT4WD transmission has an "open" center diff in like say, the XT6. in this case they can probably take quite a beating, but i don't know what those limits are. towing with two wheels off the ground and two on definitely fries the center diff so there's a certain amount of difference it can not handle. whether mismatched tires reaches that threshold i do not know, but Subaru recommends matched tires here.

#21 grossgary

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Posted 30 November 2007 - 11:25 AM

Nice to know I can run different tires after I made a special effort get a matching set for the transmision.

no, you did the right thing. you do not want mismatched tires on any AWD, 4WD, FT4WD...basically any subaru unless it has FWD (either FWD or PT4WD).

I have studied the schematics for the ft4wd xt6 5spd tranny, and I would like to see how it compares to an Impreza one, does anyone have a link or pdf for that?

what are you looking for, we can probably answer your questions? are you thinking of combining them? like installing the center diff lock in the EJ stuff? that's all been answered and looked into. there are some really cool threads including transmission disassembly of various center diff locks, dual range, and EJ stuff, you might want to find those and read through them. cool stuff.

#22 Sushi

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Posted 02 December 2007 - 01:05 AM

What I'm looking for is just a internal visual of an Impreza tranny. Does it have a setup similar to the xt6 tranny? Front pinion run though hollow countershaft, center diff behind with lsd clutch pack or...? Also, is the Impreza 5spd a true 50/50 split untill something slips like the xt6? I would love to see the offical repair manual for this. Just to clarify, this is regarding a first gen Impreza.

#23 nipper

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Posted 02 December 2007 - 01:29 AM

I may be able to help there, whats a good email addy for you?

Prvt me if you wish.


nipper

#24 GeneralDisorder

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Posted 02 December 2007 - 01:31 AM

Ever actually take one apart? It is so a differential. It's a limited slip Differential, but still has spider gears on a pinion just like any other diff. Viscous pack is attached to it much like the clutch type LSD used in rear ends (except obviously a viscous pack instead of mechanical clutch disc stacks)


You are correct - it is a "differential" in the sense that it allows the front and rear to travel at different speeds. I was refering the to fact that it's not a typical open diff or LSD diff and is not designed to allow it to "differentiate" constantly as they are. It's designed more as a limited-use LSD.... if that makes sense.

GD

#25 Gloyale

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Posted 02 December 2007 - 04:15 PM

You are correct - it is a "differential" in the sense that it allows the front and rear to travel at different speeds. I was refering the to fact that it's not a typical open diff or LSD diff and is not designed to allow it to "differentiate" constantly as they are. It's designed more as a limited-use LSD.... if that makes sense.

GD


Check out This pagehttp://www.autozine....ction_4wd_2.htm

Obviously we don't have trosen diffs. So scroll down.

The second type shown is what you are describing. A simple viscous coupling, with no differential. NOTHING transfer to the rear(or front, VW vanagon) unless there is slip. The wheels that are at the other end of that coupler just roll along until there is slip to activate the viscous pack to "lock up" and send power. Because there is no actual geared differential. Notice subaru is not listed as using this type.

Look at the third type. That is what we have. A conventional, open differential, with a viscous coupler. So a long as all wheels have traction, they will all get the same amount of power. But small differences created by turns and such are compensated for just like any open diff. But when lots of slips becomes present at any wheel the Vicous pack heats up and limits the slip. Notice here it does say this is the system ued by subaru.

Here is another great page. Showing them disassembled and a teting proceedure.
http://www.mrtrally....us_coupling.htm

Posted Image
This pictue ^^^^^shows the Viscous unit with one bevel gear in his right hand. The other half, with the spiders, pinion, and the other bevel gear are in his left.

Posted Image
This diagrahm ^^^^^ is a general depiction of the Viscous unit only, not a complete subaru differential unit.




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