Do all 4 tires need to be replaced when just 1 goes bad?

[nobody >]

Pardon my ignorance, but normal front and rear differentials, not the
locking sort, seem to have no problems running for the life of most cars,
and to tell somebody you can steer your car but, not constantly or else your
front or rear diff. would burn up, would be silly. Why is it Subaru does
not use one of these style differentials in the center, if it is serving
essentially the same purpose, and doesn't seem to wear out so quickly based
on usage? Sure, it might take a tiny bit of engineering to use that sort of
differential, but maybe someone else can enlighten as to the reason not to?

You are talking about an "open" center differential. That doesn't work
at all for AWD or 4WD. One tire (any corner) spinning will take *ALL*
torque, leaving the rest powerless. Not only that, that damned wheel
will spin 4 times faster than normal.

 

[Carl 1 Lucky Texan]

I called the Subaru shop today and asked them. They said to use a
15-amp fuse. Of course, they also said that FWD mode is only for when
doing diagnostics or when towing but I've seen many folks discuss
staying in FWD mode all the time.

This comes up here or in a coupla other soob forums 2-3 times a year. No
one seems to be able to find a benefit to keepimg an otherwise 'normal'
soob in FWD mode. You still haul the same weight and have the same
frictional losses.

Carl

 

[Carl 1 Lucky Texan]

The owner manual actually says that all 4 tires must be the same
circumference (or diameter since a reduced circumferance would mean
also a diminished diameter)? I'll check again but don't remember the
book saying anything about that.

http://www.subaru.com/common/faq/tech_info.jsp


Carl

 

[Carl 1 Lucky Texan]

So then why isn't there a problem under normal use? After, all off/on
ramps to the highways are right turns. The left wheels travel further
than the right wheels. The front wheels travel further than the rear
wheels as the rear wheels don't track the front wheels. Doesnt' this
cause a problem?


Is this significant for someone who does most of his driving on highways?

I learned recently that UPS has a computer program that organizes the
routes of the delivery vans. They try to make all turns right turns as
left turns burn gas while waiting for traffic to clear. Apparently this
does save them significant amounts of fuel.

How would this impact a Subaru?

Al

No one is saying there's a problem. ALL (as far as I know) AWD systems
(and 4WD systems when 'engaged')have the potential to incur damage when
'torque bind' forces develop in the drivetrain while operated on dry
(non-slip) pavement-type surfaces. The problem can be more/less severe
depending on many different factors.

When you turn - I suppose there could be a slight inclination for the
center diff to detect rotational difference front-rear and engage a
little. But that issue will go away quickly. BUT, if you have a smaller
sized tire on one axle, the CD detects slippage CONSTANTLY - even moving
straight down the highway (this is probably the most damaging actually,
surface roads with lots of turns and a little gravel, leaves, trash in
the gutters MAY present opportunities for 'torque bind' to be relieved
with a little tire slippage occasionally) and there is no opportunity
for the torque bund to be relased, as could happen on a dirt, gravel,
mud or snow covered road.

I have heard of old 4WD vehicles left locked and getting so bound up
when someone tried to parallel park they became immovable, a jack had to
be employed to release the drivetrain!


Carl

 

[B A R R Y]

I learned recently that UPS has a computer program that organizes the
routes of the delivery vans. They try to make all turns right turns as
left turns burn gas while waiting for traffic to clear. Apparently this
does save them significant amounts of fuel.

How would this impact a Subaru?

The same as it does UPS. <G>

 

[y_p_w]

When you turn - I suppose there could be a slight inclination for the
center diff to detect rotational difference front-rear and engage a
little. But that issue will go away quickly. BUT, if you have a smaller
sized tire on one axle, the CD detects slippage CONSTANTLY - even moving
straight down the highway (this is probably the most damaging actually,
surface roads with lots of turns and a little gravel, leaves, trash in
the gutters MAY present opportunities for 'torque bind' to be relieved
with a little tire slippage occasionally) and there is no opportunity
for the torque bund to be relased, as could happen on a dirt, gravel,
mud or snow covered road.

At least with the 4EAT systems, I don't think there's a "center
differential" per se. However - I read a few Endwrench articles that
attempt to describe the system. The part I did understand is that the
speed of the front and rear driveshafts are constantly monitored and
torque distribution adjusted as needed.

http://endwrench.com/images/pdfs/4EAatPh2Win04.pdf

 

[Carl 1 Lucky Texan]

At least with the 4EAT systems, I don't think there's a "center
differential" per se. However - I read a few Endwrench articles that
attempt to describe the system. The part I did understand is that the
speed of the front and rear driveshafts are constantly monitored and
torque distribution adjusted as needed.

http://endwrench.com/images/pdfs/4EAatPh2Win04.pdf

You're right, it is an adjustable wet clutch pack I guess - functions
like a variable center diff but bears no mechanical resemblance to a
'real' differential.

Carl

 

[Vanguard]

This comes up here or in a coupla other soob forums 2-3 times a year.
No one seems to be able to find a benefit to keepimg an otherwise
'normal' soob in FWD mode. You still haul the same weight and have the
same frictional losses.

Except maybe in FWD mode then I don't have to replace the older rear
tires because they are probably less in circumference than the new front
tires. I'd like to procrastinate as to when I need to buy the 2 rears
until maybe I feel more comfortable buying all four.

 

[Carl 1 Lucky Texan]

Except maybe in FWD mode then I don't have to replace the older rear
tires because they are probably less in circumference than the new front
tires. I'd like to procrastinate as to when I need to buy the 2 rears
until maybe I feel more comfortable buying all four.

I understand - here's a link to a very long, exhaustive thread about the
issue;
http://www.ultimatesubaru.org/forum/showthread.php?t=48865

Carl