AWD, 4Hi, 4Lo - What is the difference and when to use
MikeB
non-4x4 crew cab now and the V6 seems to be ok for everything I do. I'm
tending to go with the 4.7 automatic with the AWD option trans.
So what are the differences in the 3 different drive modes? I know the 4Lo
is for the offroad mud and limited going. But what is going on when 4hi and
AWD are used. Also, is the limited slip really need with the AWD?
MAX340
>tending to go with the 4.7 automatic with the AWD option trans.
I would go with 4wd and kill the AWD, I'll explain why.
>So what are the differences in the 3 different drive modes? I know the 4Lo
>is for the offroad mud and limited going.
Yes, 4lo is for the tough stuff where you want a bit more torque and a bit more
control over its application, and where gorund speed is not a concern.
>But what is going on when 4hi and
>AWD are used.
AWD uses a center differential. 4WD uses a locked drive system. What this means
is..... If you were to jack up the back end of the truck and spin the tires,
one tire goes forward, the other backward. this is an open differential. In
driving, you experience it as a single wheel spinning while on ice and you go
nowhere. This same condition can happen if you have AWD and a center
differential. One axle will spin and the other will not, meaning you are not
using both axles. If one axle is spinning and it is an open differential and
not limited slip, you could be locked in AWD and driving... one wheel. Not very
effective, is it? So...
>Also, is the limited slip really need with the AWD?
I would say absolutely. Now, back to the AWD/4WD differences...
If you get 4WD, the transfer case locks the front and rear driveshafts
together, making it act as one drive unit, with nothing able to turn without
turning the other end. This means you will NEVER have less than two wheels (one
on each end) driving. This assumes you have an open differential in each axle.
If you have a limited slip in the rear axle, you will never have less than
three wheels driving.
The advantage to AWD is that it can be left engaged, with no ill effects on dry
pavement. If AWD cannot be disengaged, it will likely use more fuel to drive
than a 4WD system that can be disengaged. 4WD cannot be left engaged on dry
pavement, since turning will tend to cause stress on the driveline which could
break parts.
Personally, I have little time for AWD, its not the big advantage many claim it
to be, as it does not have the ability to drive more than one axle at a time in
a slippery (meaning one wheel is spinning, the rest cannot pull the vehicle)
situation. If you are getting ttraction, there is no need for more drive
wheels. If you are not getting traction, then what good are two more spinning
tires? Can AWD get you through stuff that 2wd cannot? Possibly. Is it as
effective as 4WD? No.
I would get limited slip either way.
Max
reality.exe not found--------> universe application terminated.
ho...@nospamextremejeep.com
Subaru or RAV-4, a single-mode system with a 60/40 or worse torque split
that's basically fwd. However, in the case of the Dodge, the AWD being
referred to is actually a full-time 4wd system, which is different, IMO.
What was left out below is that the Dodge system is actually full-time 4wd
with the option of locking the diff in the tcase making it part-time. The
NV233HD part-time t-case has the following modes:
Operating Modes 4WD Low, Locked; Neutral; 2WD; 4WD High, Locked
The NV244HD full-time t-case has the following:
Operating Modes 4WD Low, Locked; Neutral; 4WD; 4WD High, Locked
You basically lose the ability to have 2wd with the full-time option and
get full-time instead. Fuel mileage difference between the two is
inconsequential, all componants are rotating at all times regardless of
mode, all 2wd is doing is disconnecting the front driveshaft from the
t-case. Given the option, I would take full-time over 2wd. The comment
about 1wd even with full-time is technically correct, but realistically
it's pretty unlikely. That 1 spinning wheel would be spinning at 4 times
the speed it normally would, which takes quite a bit for force.
Take a normal 2wd axle, both sides off the ground, and turn the
driveshaft. Both wheels turn at the same speed. Apply pressure to one of
them until it stops and notice the other wheel is spinning at twice the
speed. By holding both front (or rear) wheels so they can't spin in a
full-time system, you're sending equal torque but double the speed to the
free axle, now hold a wheel on that free axle, you've doubled the speed
again. Given low-traction situations, I'd say one wheel spinning is
pretty unlikely given the physics of it all, and even if it did happen,
you have the option of locking the center diff and splitting power 50/50
front and rear.
My personal opinion is to go with whatever option makes the most sense for
you and your location. If you drive in snow or patchy snow a lot during
the winter, get the full-time option. You just set it and forget it. If
snow is something you rarely see then the cheaper system would probably
make more sense.
Enjoy...
MAX340 <max...@aol.comq> wrote:
> Max
--
--
Hogan Whittall
'47 CJ2A - 302, C5, 5.38's, yada, yada
'98 XJ Classic - BFG MTs, RS9ks
__________________________________________________________________
This is formal notice under California Assembly Bill 1629, enacted
9/26/98 that any UCE sent to my email address will be billed $50
per incident to the legally allowed maximum of $25,000.
__________________________________________________________________
MAX340
>referred to is actually a full-time 4wd system, which is different, IMO.
Wrong. In order to have full time 4WD, the transfer case MUST have a center
differential, for two reasons. First, in a turn all four wheels describe a
different radius of that turn, and thus all rotate at different speeds. You
MUST allow this to happen in Full time 4WD, since the vehicle will end up
trashing something in the driveline from the inherent stress of different shaft
speeds if coupled directly together. Now, assuming it doesn't break parts
immediately, it WILL jump around in a very odd manner (to the uninformed) on
dry pavement, IF there is no center differential. Try this in a Jeep Cherokee.
We have one, it has 2H, 4H FT, 4H PT and 4L.
>What was left out below is that the Dodge system is actually full-time 4wd
>with the option of locking the diff in the tcase making it part-time.
Then it is NOT an AWD system, and my definition of an AWD system would not
apply to it.
>Fuel mileage difference between the two is
>inconsequential, all componants are rotating at all times regardless of
>mode, all 2wd is doing is disconnecting the front driveshaft from the
>t-case.
Incorrect. The Central Axle Disconnect means the axle no longer drives the
front driveshaft. This is a bulge on the axle housing itself on the right side
of the vehicle.
>The comment
>about 1wd even with full-time is technically correct, but realistically
>it's pretty unlikely.
Once again, incorrect. As soon as you enter a slippery situation, the torque
will find the path of least resistance by using the choices the central
differential and the open axle differentials allow. That means that if ONE tire
is on ice, and you have open differentials and a central differential, it will
spin the ONE tire on the ice.
>That 1 spinning wheel would be spinning at 4 times
>the speed it normally would, which takes quite a bit for force.
Once you break traction, there is no need for huge gobs of torque.
ho...@nospamextremejeep.com
>>However, in the case of the Dodge, the AWD being
>>referred to is actually a full-time 4wd system, which is different, IMO.
> Wrong. In order to have full time 4WD, the transfer case MUST have a center
> differential, for two reasons. First, in a turn all four wheels describe a
> different radius of that turn, and thus all rotate at different speeds. You
> MUST allow this to happen in Full time 4WD, since the vehicle will end up
> trashing something in the driveline from the inherent stress of different shaft
> speeds if coupled directly together. Now, assuming it doesn't break parts
> immediately, it WILL jump around in a very odd manner (to the uninformed) on
> dry pavement, IF there is no center differential. Try this in a Jeep Cherokee.
> We have one, it has 2H, 4H FT, 4H PT and 4L.
I have a Cherokee and a '47 Willys, see my .sig, and you didn't get my
point about AWD vs. full-time, I guess. To me, AWD and full-time 4wd are
similar in that both are/can be engaged all the time. The actual systems
between something like a Subaru's AWD and a Jeep's SelecTrac full-time are
VERY different, which was my point. I never disputed the fact that
there's a center diff, I disputed the Dodge system being called AWD.
>>What was left out below is that the Dodge system is actually full-time 4wd
>>with the option of locking the diff in the tcase making it part-time.
> Then it is NOT an AWD system, and my definition of an AWD system would not
> apply to it.
Which was why I made the post. :) Your comments on AWD were right, but
they didn't apply to what the original poster was asking about. He
wasn't clear on the terminology and misapplied the term AWD. ;)
>>Fuel mileage difference between the two is
>>inconsequential, all componants are rotating at all times regardless of
>>mode, all 2wd is doing is disconnecting the front driveshaft from the
>>t-case.
> Incorrect. The Central Axle Disconnect means the axle no longer drives the
> front driveshaft. This is a bulge on the axle housing itself on the right side
> of the vehicle.
I wasn't aware of that on the trucks, I was going by the current Jeep
setup which hasn't had the axle disconnect for atleast a few years now.
It's simply the tcase disconnecting the front driveshaft.
>>The comment
>>about 1wd even with full-time is technically correct, but realistically
>>it's pretty unlikely.
> Once again, incorrect. As soon as you enter a slippery situation, the torque
> will find the path of least resistance by using the choices the central
> differential and the open axle differentials allow. That means that if ONE tire
> is on ice, and you have open differentials and a central differential, it will
> spin the ONE tire on the ice.
>>That 1 spinning wheel would be spinning at 4 times
>>the speed it normally would, which takes quite a bit for force.
> Once you break traction, there is no need for huge gobs of torque.
Key phrase being "once you break traction". If you drive properly you
shouldn't have a problem. In the simplest case of having a simple
differential in the tcase, torque is split 50/50 front and rear, both
axles get the same amount. A spinning/lifted tire requires pretty much 0
torque to maintain speed, so 0 torque goes to the front axle. Getting
back to wheel speed, if road speed in 1st gear is 15mph under normal
driving, the wheel speed of one spinning tire on ice with all others not
moving with be 60mph when your speedo indicates 15mph. How much torque do
you suppose it takes to get that wheel up to 60mph if it's on a slippery
spot and the other 3 tires are on pavement? There's some traction, but
not much, but I'm guessing the torque to get that wheel up to 60 fairly
fast is also enough to move the vehicle since equal torque is sent to the
front axle.
MAX340
>they didn't apply to what the original poster was asking about. He
>wasn't clear on the terminology and misapplied the term AWD. ;)
AWD and the "full time" position in the Jeep are the same basic thing.
>Key phrase being "once you break traction". If you drive properly you
>shouldn't have a problem.
Even driving properly, its unlikely you will avoid ice unless you move south.
Breaking traction on ice is easy.
>A spinning/lifted tire requires pretty much 0
>torque to maintain speed, so 0 torque goes to the front axle.
Thats Wrong. Torque will follow the path of least resistance, thus the easiest
wheel to get moving will use all the torque in an open diff situation.
>Getting
>back to wheel speed, if road speed in 1st gear is 15mph under normal
>driving, the wheel speed of one spinning tire on ice with all others not
>moving with be 60mph when your speedo indicates 15mph.
true, and with ALL the available torque going to one wheel, thats easy.
>How much torque do
>you suppose it takes to get that wheel up to 60mph if it's on a slippery
>spot and the other 3 tires are on pavement?
very little, but since ALL of it is going to the easy wheel, NONE of it goes to
the other wheels.
>but I'm guessing the torque to get that wheel up to 60 fairly
>fast is also enough to move the vehicle since equal torque is sent to the
>front axle.
WRONG. If the differential in the transfer case is open, it will follow the
path of least resistance.
ho...@nospamextremejeep.com
>>A spinning/lifted tire requires pretty much 0
>>torque to maintain speed, so 0 torque goes to the front axle.
> Thats Wrong. Torque will follow the path of least resistance, thus the easiest
> wheel to get moving will use all the torque in an open diff situation.
>>Getting
>>back to wheel speed, if road speed in 1st gear is 15mph under normal
>>driving, the wheel speed of one spinning tire on ice with all others not
>>moving with be 60mph when your speedo indicates 15mph.
> true, and with ALL the available torque going to one wheel, thats easy.
>>How much torque do
>>you suppose it takes to get that wheel up to 60mph if it's on a slippery
>>spot and the other 3 tires are on pavement?
> very little, but since ALL of it is going to the easy wheel, NONE of it goes to
> the other wheels.
>>but I'm guessing the torque to get that wheel up to 60 fairly
>>fast is also enough to move the vehicle since equal torque is sent to the
>>front axle.
> WRONG. If the differential in the transfer case is open, it will follow the
> path of least resistance.
Wrong, wrong, wrong, and wrong again. A differential does not bias
torque! An open differential will give equal torque to both sides
regardless of if one is spinning or on the ground. The wheel with the
least resistance determines the amount of torque developed, but the
spinning wheel does not get all the torque. This is the basic mechanics
of how a differential works. It allows a SPEED differential, that is all.
It does not bias torque one way or the other. If you don't believe me,
look it up.
MAX340
>torque!
Yes, it does if there is resistance to the free movement of the tire at the end
of the axle shaft. If one tire will move, and the other will not, the one that
will move will get all the available torque, the other will get none, thus the
need for limited slip and posi-traction units. If what you claim were true
regardless of reality, there would be no need for limited slip differentials.
>An open differential will give equal torque to both sides
>regardless of if one is spinning or on the ground.
Absolutely incorrect.
>The wheel with the
>least resistance determines the amount of torque developed,
So wrong as to be incomprehensible. The wheels do NOTHING to determine the
torque developed, the ENGINE does.
>but the
>spinning wheel does not get all the torque.
Wrong. If the spinning wheel is going twice the speed of the speedo reading (as
you correctly said it did), it is in fact getting ALL the torque. The ONLY way
it could double the speed reading on the speedo is to have one axle shaft
sitting still so the "spider" gears double the output RPM of the other shaft.
>This is the basic mechanics
>of how a differential works. It allows a SPEED differential, that is all.
You are right, but you fail to see the rest of the "physics" as you put it, so
that is not all.
>It does not bias torque one way or the other. If you don't believe me,
>look it up.
Look it up yourself, THEN apply reality. A differential allows a "differential"
in axle shaft speed, but in doing so, it splits the torque. Torque will follow
the path of least resistance. If one axle is held still, the torque will spin
the other axle. It is not the action of the differential, but the action of the
friction surface which each tire is sitting on that splits or "biases" the
torque.
ho...@nospamextremejeep.com
you're talking about on this subject. I have looked it up, I have studied
it, I have tested it, I know WTF I'm talking about. Sorry you don't
believe the "reality" of it, but that doesn't change the truth. If I had
the time to explain it AGAIN in greater detail I would, but I don't.
Maybe someone else will. Have a nice Monday. :)
MAX340 <max...@aol.comq> wrote:
> Absolutely incorrect.
> Max
--
MAX340
>you're talking about on this subject.
Have you actually worked on this stuff?
>I have looked it up, I have studied
>it, I have tested it, I know WTF I'm talking about.
Unfortunately, no you don't. In fact, you haven't a clue.
>Sorry you don't
>believe the "reality" of it, but that doesn't change the truth.
Your reality takes only the strict engineering veiw, reality causes other
things to happen.
>If I had
>the time to explain it AGAIN in greater detail I would, but I don't.
If you had time to explain it again, you would still be wrong.
Lets get a few things straight. The input to the differential is applied by the
driveshaft, and affects both axles equally until a turn occurs, then one axle
must have more rotation (rotational force, torque) to move more distance. If
you keep the differential from splitting the torque, as a limited slip or posi
unit might (or more extreme, a spool), you force one of the wheels to spin at a
rate that is NOT the same as its ground speed. Thus if the differential is
working properly at all, it splits the torque, it HAS to in order to do its
job. It is NOT designed to FORCE this split, but it IS designed to ALLOW this
split.
Now, if I apply a gradually increasing amount of torque to an open
differential, I will only be able to increase the amount of torque to the point
where either the vehicle moves, OR, one of the tires breaks traction. If one
breaks traction, I will NOT be able to increase the amount of torque delivered
to the other tire, since all the torque will be used to increase the speed at
which the tire that already lost traction spins.
Thus, whether you have studied it forvever and you think you know what its all
about, reality says that an open diff will allow me to drive only one wheel in
a limited traction situation. If I drive one diff that directs power to front
and rear diffs, and they direct power to right and left tires, and ONE tire
loses traction while the others hold relatively firm, only that wheel will
spin, since the diffs have involuntarily "split" the torque.
In AWD, it has to have a center diff that allows a difference in drive shaft
speeds, thus, if one tire loses traction, its stuck. If all tires are gripping,
it does increase the stability of the vehicle. If a part time or locking center
diff has one wheel lose traction, the other axle will move the vehicle, thus
two wheels will always be driven, and is better than the AWD for extreme
situations. If I put a sure grip unit on the rear axle, I have three wheels
always driven. If do the same up front, I have four wheels always driven.
But its clear to me you have little idea of the reality here, so I suggest you
go out and play with these ideas in a real truck, and let us know how ya do.