4WD vs AllTrac
John Hight
through the snow and ice. I've looked at a couple of pickups,
4Runners, etc., and they all are quite a bit less comfortable than
what you can get for the money in a sedan or wagon. So I'm also
looking at 1-2 yr old (used) Subaru (4wd) wagons and Corrolla
(AllTrac) wagons.
Toyota's AllTrac apparently is always engaged. This would imply to me
that it is a sustantially different technology than 4WD. Can anyone
comment (or give me a reference) on the difference between 4WD and
AllTrac.
Anyone care to comment on the usefulness of Subaru/Corolla wagons 4WD
in the snow and ice compared to the 4wd action you get with
4Runner/Patherfinder/Trooper/Pickup?
Andrew Duane
>Toyota's AllTrac apparently is always engaged. This would imply to me
>that it is a sustantially different technology than 4WD. Can anyone
>comment (or give me a reference) on the difference between 4WD and
>AllTrac.
I'll jump in here, since I'm in the same boat.
In this context, 4WD implies something that gets "shifted into".
The gears used are in the transfer case, and are simple mechanical
gears that engage power to the other axle. With a setup like this,
the two axles are mechanically coupled and locked together. This
means that you cannot stay in 4WD on dry pavement for long. What
will happen is that the tires can't slip to account for the slight
difference in their speeds, and "wind up" a torque on the transfer
case gears. Eventually ... SNAP! In our 4WD ambulances (and friends'
4WD trucks) there are big warnings about this.
On AllTracs (and Quattros and Syncros and ...) the two axles are instead
coupled with differentials like the ones that allow the two tires
on an axle to turn at different speeds. The differentials allow the
slippage to balance the rotation speeds, letting the car be in 4WD
mode all the time. I don't know about the Toyotas, etc., but the VWs
and Audis have controls to lock various differentials for extra
traction. They come with a warning not to leave them locked for long
on dry pavement. This is for the same reason as 4WD trucks.
P.S. : I call this "all-wheel-drive" to differentiate if from 4WD.
I hope this cleared it up a little.
Andrew L. Duane (JOT-7) w:(508)-685-7200 X122
Samsung Software America decvax!cg-atla!samsung!duane
1 Corporate Drive uunet/
Andover, MA. 01810 du...@samsung.com
Only my cat shares my opinions, and she's all-wheel-drive.
Sean Reifschneider
>that it is a sustantially different technology than 4WD. Can anyone
>comment (or give me a reference) on the difference between 4WD and
>AllTrac.
>
>Anyone care to comment on the usefulness of Subaru/Corolla wagons 4WD
>in the snow and ice compared to the 4wd action you get with
>4Runner/Patherfinder/Trooper/Pickup?
I got a Subaru a while ago for the snow season (as my RX-7 is just too
nasty in the snow). It seems to work pretty well for the snow/ice, as
well as the little bit of off-roading I've done in it (nice to have a
beater car). We've only had a little snow here so far this month. I'm
waiting until we get several feet of it to really form an opinion on the
usefulness of the car.
The differences... Basically a 4-Runner... will be better with the
real deep snow. The bigger tires, and height of it will give it
an advantage. I think my Subaru will do just fine around here. It's about
a foot of groun clearance.
I think the difference between All-Trac, and the method used in most of the
subarus is that the center differential is like a positraction limited slip
rear differential (This is speculation). My '81 Subaru has a transfer case,
which is a set of gears that you can engage/disengage the rear axel with,
meaning that it is only to be used on slick roads (as there is no slip
in the case).
Honda's station wagon has the All-Trac type AWD, as I recall. I think the
Subaru Legend (nice car!) also has this type available. One car you might
look at is the Justy AWD ECVT. I drove one of them during the last major
snow storm of last year, and it worked very nicely. Small car, though.
Hope this helps.
Sean
--
From the desk of Sean Reifschneider. Isn't Amiga UUCP great? Thanks Matt.
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Paul Orland
>Anyone care to comment on the usefulness of Subaru/Corolla wagons 4WD
>in the snow and ice compared to the 4wd action you get with
>4Runner/Patherfinder/Trooper/Pickup?
#opionion_on
Probably the most significant difference between 4WD/AWD cars and
wagons vs. trucks like four-runner/Pathfinder/etc. is ground
clearance. If you are sticking to paved, plowed roads, this will
not be much of an issue. Even 4WD in a car wont be much help if your
"plowing" the snow in front of you with your air dam or
your differentials.
The trucks also usually come with more aggressive tires with better
bite on snow and ice.
AWD is nice in that it allows you to have the safety advantage of
4WD all the time without the extra wear associated with running
conventional 4WD on hard dry surfaces.
- Paul.
John Light
Real 4wd either requires you to turn it off/on in response to road conditions
or has a center differential to take up the difference in revs between the
front and back axles. They typically allow you to lock the center
differential to reduce spin in extreme circumstances.
"AllTrac" (from Toyota) and it's peers provides no user controls over the
4wd action. This is good and bad. The good news is that you get 4wd action
any time you need it, instantly. (Though most of the torque is usually
directed to one axle.) This is done with a "viscous coupling", of which I
have no understanding. The viscous coupling takes up the difference in
front/rear axle turns.
The only negative I know about AllTrac, etc. is that you have to keep the
tire wear exceptionally even. As you let the front to rear tire difference
get larger, the viscous coupling generates more heat making up the
difference. As far as I know this is only a long term wear problem, but
a large enough difference could cause catastrophic heat failure, I suppose.
Of course, mixing tire sizes is a no-no with AllTrac. I would even be a
little paranoid about mixing tire brands and styles. Tire rotation becomes
more important, and when one tire needs to be replaced, you better replace
them all.
So on my Previa AllTrac I can look at a $400-500 tire bill if one of my
half-worn tires suffers catastrophic failure. On the other hand, I love
the AllTrac action.
John Light
Walter A. Koziarz
>
>Probably the most significant difference between 4WD/AWD cars and
>wagons vs. trucks like four-runner/Pathfinder/etc. is ground
#define sarcasm ON
HUH? WHAT? Pathfinder is a TRUCK? Four-runner is a TRUCK? My G*d; whatever
I've got is out of line!!! I thought my W350 was a truck. :-) :-)
Actually, the mentioned non-trucks are Sport-Utility Vehicles (SUVs); totally
different concept, despite the delusions of their yuppie owners.
Walt K.
Joe Pfeiffer
Walt K.
Let me see if I understand this. I have a 4WD Toyota pickup. I don't
think I'll get any arguments when I call this a truck -- a little,
truck, but a truck. Now if I remove the inner wall of the bed, put a
roof over it, cut a couple of doors, install seats, and call it a
4Runner, I no longer have a truck? How about last year's version,
where the roof was fiberglass and the new doors weren't cut? SUVs are
trucks.
-Joe.
Mike Harris
At least he didn't call it his "RIG". Real men drive Rigs!
--
Mike Harris - KM4UL har...@dg-rtp.dg.com
Data General Corporation
{world}!mcnc!rti!dg-rtp!harrism
Research Triangle Park, NC
Paul Orland
>
>HUH? WHAT? Pathfinder is a TRUCK? Four-runner is a TRUCK? My G*d; whatever
>I've got is out of line!!! I thought my W350 was a truck. :-) :-)
>
>Actually, the mentioned non-trucks are Sport-Utility Vehicles (SUVs); totally
>different concept, despite the delusions of their yuppie owners.
>
I know of a rusty W350 used to haul pig manure for sale. Totaly non-yuppie.
Want to make an offer?
- Paul.
P.S. How 'bout some clarification. Is a Blazer a "real (man's)" truck? how
about a Suburban? Or only a Ramcharger because it's made by DODGE. I
wonder what Peterbuilt owners would think about your W350.
Wayne Covington
> "AllTrac" (from Toyota) and it's peers provides no user controls over the
> 4wd action. This is good and bad. The good news is that you get 4wd action
> any time you need it, instantly. (Though most of the torque is usually
> directed to one axle.) This is done with a "viscous coupling", of which I
> have no understanding. The viscous coupling takes up the difference in
> front/rear axle turns.
I don't know what is in Toyota's AllTrac system, but the viscous coupling in
the old (mid 70's) Jeep Quadratrac was a spring-loaded cone clutch. It was
trouble-prone unless the fluid was changed regularly -- i.e., annually. The
failure mode appeared to be excessive roughness of the mating steel surfaces
of the clutch. The symptom was what Jeep called "slip-stick", which made an
unmistakable chattering sound. I would expect Toyota's system to be more
reliable, but I would be diligent about following their preventive maintenance
recommendations.
Wayne Covington
wa...@hpfcqd.fc.hp.com
Dan Dunphy
viscosity silicon oil. This prevents loading up the driveline,
but won't let you exceed some defined differential front/rear
driveline velocities. It always transmits some torque to both
ends. The first automotive example I ever heard of this was the 78
AMC Eagle.
Karl Nordstrom
>I think the vixcous coupling...
> ...always transmits some torque to both ends.
I think the viscous coupling differential allows some difference in
front and rear axials speeds before the viscous oil in the diff thickens
and thus locks the axails together. In other words it is all front wheel
drive until the front wheels spin enough to cause the diff to
automatically engage the rear axial.