Truck 4x4 Front Wheels Spin Faster Than Rear?

I was told by a colleague today that in a truck 4x4 system (no differential
between front and rear drive axles), in 4x4 mode the front wheels are
purposely designed to spin slightly faster than the rear. Apparently this
increases stability by causing the front wheels to "pull" the vehicle along
similar to a front wheel drive vehicle.

I questioned his explanation for this, but did not really come up with an
argument.

How could they possibly do this? Would they put an extra tooth on the ring
in the front differential?
Can anyone either explain this to me better, or dispel the myth?

I've heard this too, but I only have anectodal info to support it. I have
an old 4X4 system (1971) with locking hubs etc. I cannot drive this on
pavement in 4WD because it's so screwy. The owners manual recommend against
it as well. In addition, my vehicle is extremely unstable on ice in 4WD - I
end up spinning out a lot. I believe this is due to what you describe
below.

Newer 4X systems and AWD obviously don't have this problem.

HTH,

Andy



"T H" <heppler @ unforgettable.com> wrote in message
news:HrQub.59762$1K.42124@edtnps84...
> I was told by a colleague today that in a truck 4x4 system (no
differential
> between front and rear drive axles), in 4x4 mode the front wheels are
> purposely designed to spin slightly faster than the rear. Apparently this
> increases stability by causing the front wheels to "pull" the vehicle
along
> similar to a front wheel drive vehicle.
>
> I questioned his explanation for this, but did not really come up with an
> argument.
>
> How could they possibly do this? Would they put an extra tooth on the
ring
> in the front differential?
> Can anyone either explain this to me better, or dispel the myth?
>
>

I'm far from an expert on this, but I think from what I've read in my 4x4
mags, and what I understand about the mechanics of a 4x4 system this might
answer your question:

Normally, the front and rear axle ratios should be matched. Having them
unmatched would cause lots of drivetrain parts to break unless you've got
really really low traction. I can drive my blazer and my k1500 on dry
pavement in four wheel drive, as long as I'm not trying to go around any
corners. I check perodicly to make sure the electronic shift system in my
blazer is working properly, since I had so much trouble with my 2000 S10
ZR2's electronic system. Basicly, its squirly on dry pavement because the
the front and rear axles always have to cover the same net distance with no
center diff. I'm not sure I can explain why your truck is less stable on
snow and ice in 4 wheel drive. I tend to feel the opposite with my trucks.

The times that guys (or gals too) put different ratios in front and rear
axles are when they're into some hardcore mud. From what I understand,
having the front and rear running at different speeds helps keep a truck
moving in serious mud. On trails and such though, they have to use 2wd to
get to the mud, again because unless there is significantly low traction,
different ratios will cause problems.

If I'm wrong, someone please correct me.
-NW



"T H" <heppler @ unforgettable.com> wrote in message
news:HrQub.59762$1K.42124@edtnps84...
> I was told by a colleague today that in a truck 4x4 system (no
differential
> between front and rear drive axles), in 4x4 mode the front wheels are
> purposely designed to spin slightly faster than the rear. Apparently this
> increases stability by causing the front wheels to "pull" the vehicle
along
> similar to a front wheel drive vehicle.
>
> I questioned his explanation for this, but did not really come up with an
> argument.
>
> How could they possibly do this? Would they put an extra tooth on the
ring
> in the front differential?
> Can anyone either explain this to me better, or dispel the myth?
>
>

"T H" <heppler @ unforgettable.com> wrote in message
news:HrQub.59762$1K.42124@edtnps84...
> I was told by a colleague today that in a truck 4x4 system (no
differential
> between front and rear drive axles), in 4x4 mode the front wheels are
> purposely designed to spin slightly faster than the rear. Apparently this
> increases stability by causing the front wheels to "pull" the vehicle
along
> similar to a front wheel drive vehicle.
>
> I questioned his explanation for this, but did not really come up with an
> argument.
>
> How could they possibly do this? Would they put an extra tooth on the
ring
> in the front differential?

IF this is correct, it would be done by putting one more tooth in the REAR
differential ring gear,
not in the front.

> Can anyone either explain this to me better, or dispel the myth?
>
>

"john.riedel" wrote:
>
> "T H" <heppler @ unforgettable.com> wrote in message
> news:HrQub.59762$1K.42124@edtnps84...
> > I was told by a colleague today that in a truck 4x4 system (no
> differential
> > between front and rear drive axles), in 4x4 mode the front wheels are
> > purposely designed to spin slightly faster than the rear. Apparently this
> > increases stability by causing the front wheels to "pull" the vehicle
> along
> > similar to a front wheel drive vehicle.
> >
> > I questioned his explanation for this, but did not really come up with an
> > argument.
> >
> > How could they possibly do this? Would they put an extra tooth on the
> ring
> > in the front differential?
>
> IF this is correct, it would be done by putting one more tooth in the REAR
> differential ring gear,
> not in the front.
>
> > Can anyone either explain this to me better, or dispel the myth?

Generally you run the same gears front and rear (assuming same sized tires). I know this is the case with Toyotas and when I changed gears in mine, I put the same ratio in both axle. Some makes use different types of diffs front and rear and sometimes
its not possible to get the identical ratio in both ends. For example a front diff may have a 4.10 gear and the rear a 4.11, I think the rule is to keep the ratios within 1% for most uses. I think that in some situations, like mud drag rigs, the front
ratio will be taller both because the front tires may be smaller than the rear, but also to sping the front tires faster for the above noted stability.

--
Roger

T H <heppler @ unforgettable.com> wrote:
> I was told by a colleague today that in a truck 4x4 system (no differential
> between front and rear drive axles), in 4x4 mode the front wheels are
> purposely designed to spin slightly faster than the rear. Apparently this
> increases stability by causing the front wheels to "pull" the vehicle along
> similar to a front wheel drive vehicle.

> I questioned his explanation for this, but did not really come up with an
> argument.

> How could they possibly do this? Would they put an extra tooth on the ring
> in the front differential?
> Can anyone either explain this to me better, or dispel the myth?

There was actually a thread discussing this on Outerlimits just
yesterday:
<http://www.outerlimits4x4.com/PHP_Modules/phpBB2/viewtopic.php?t=10015>

Basically, yes, sometimes the diff ratios (number of teeth on pinion/ring
gear) are very slightly different. Pulling slighty faster with the front
can help a 4wd steer straight in some situations. The difference is quite
minor though, and can be negligible if tyre sizes are slightly different
anyway (due to wear). As long as you are not driving on concrete or
asphalt or something, the difference shouldn't make any difference. The
tyres will just scuff ever so slightly.

--
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