Can someone explain wheel spin in 4WDs and how Full Time 4WD really works on sand?

[Shutterbug Lurker Find all posts View Profile Close Shutterbug]

Hi,

I'm constantly told different versions of how my 97 Prado's Full Time 4WD system works. I mostly use low 4WD on sand. So can someone offer
a simple explanation of how the wheels drive when bogged. Like do some slip more than others depending on the condition of the bog etc?
Adding to the confusion I've seen many explanations on the Net but they always say that Full Time 4WD doesn't offer 2W Drive -- when, of course, the Prados actually do offer 2WD. Thanks.

Signed

Dazed and Confused

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

A differential allows the two wheels to spin at different speeds, the power is applied
to both wheels and for this power to be transfered to both wheels both wheels must
have traction, if you lift one wheel off the ground that wheel will spin and no power
can therefore be transfered to the other wheel. This is normal two wheel drive with
no lockers or limited slip diffs etc.

All time 4wd, drive is spread over 4 wheels, if you lift any wheel off the ground
and the car won't move unless it has a automatic the centre diff lock in which case
it will then send power to the axle that has traction.

If you lock the centre diff then drive is shared equally between front and rear
diffs, if you for example lift one front wheel off the ground it will spin all drive
is effectively then passed to the rear diff and rear wheels will drive the car.

Same if you lift both front wheels off the ground, if you lift one front wheel
off the ground and one rear wheel off the ground they will spin and car won't
move.

Again assuming no limited slip diffs or lockers.

[krypto Avid PP Poster! Find all posts View Profile Close krypto]

In answer to part of your question, the Prado does not have 2WD as power is always going to all 4 wheels, it's just that the front and rear wheels are not locked together when the center different is unlocked.

I generally use low range on sand because it puts less strain on the engine and means you don't bog down if you hit super soft sand as happened to me on the weekend.

[Anth120playdo Banned Find all posts View Profile Close Anth120playdo]

The simple operation of a diff, if one wheel spins, the other side has no drive.
Your vehicle has 2 diffs, one front & one rear.
So if your spinning one front & one rear you have a no wheel drive.
You may have an LSD in the rear , it's probably worn & has trouble sticking to stop spin, so it's almost like a standard open diff.

This is where lockers work.
If your front or rear diff is locked it always has drive to each side.
If your centre diff is locked, it's locked front to rear.

When you have front & rear lockers only then you have a true 4wd.
Then you spin wheels, that's traction problems lol..... All 4 on the ground & spinning evenly, vehicle going backwards! Get the blood pumping.

[MRW82 Member Find all posts View Profile Close MRW82]

Your vehicle has 3 diffs, one front & one rear, and a centre diff in the transfer case.

Fixed.

[Shutterbug Lurker Find all posts View Profile Close Shutterbug]

Thanks for all the replies. I'm especially surprised to learn that the Prado is driving on all 4s even in when 4WD is not engaged. So in my simplistic lingo:
All wheels are normally engaged in normal drive but if one wheel (anywhere) has no traction then the whole car has none. So engage 4WD and if one wheel has no traction, the wheels at the other end will still engage unless one of them has no traction. And the LSD (if working) will lock the rear wheels only -- to a certain extent?

Also is the 90 series basically a rear wheel drive -- or when all wheels are engaged in normal do you call this 'all wheel drive' ?

Cheers

[Anth120playdo Banned Find all posts View Profile Close Anth120playdo]

Fixed.

I was trying to be simple & not technical, but Thankx anyway.

Thanks for all the replies. I'm especially surprised to learn that the Prado is driving on all 4s even in when 4WD is not engaged. So in my simplistic lingo:
All wheels are normally engaged in normal drive but if one wheel (anywhere) has no traction then the whole car has none. So engage 4WD and if one wheel has no traction, the wheels at the other end will still engage unless one of them has no traction. And the LSD (if working) will lock the rear wheels only -- to a certain extent?

Also is the 90 series basically a rear wheel drive -- or when all wheels are engaged in normal do you call this 'all wheel drive' ?

Cheers

Not quite, if one wheel looses traction & it's opposite has no drive.
Eg: left hand front is spinning, right hand front has no drive. Then you are rear wheel drive.
Same story, but visa versa for the rear.

If you have one spinning at front & one at rear, you have nothing.
If you were to have the centre diff locked (the 3rd diff between the front & rear diffs)
This changes things, likely it would then be one front & one rear were spinning the same speed.
But, it may be enough to have got you through. It makes sure the split is locked & smooth.

Yes your LSD understanding is correct.

Yes all wheel drive.

[stu#71 Junior Member Find all posts View Profile Close stu#71]

This might help
http://www.lcool.org/technical/diffs/diffs.html

[r4ndll Avid PP Poster! Find all posts View Profile Close r4ndll]

Sorry. Does the 95 have a Torsen centre diff? I'm pretty sure that with an unlocked open centre diff, and if you don't have traction control, when one wheel loses traction your're going nowhere.

Skip this video to 1:10

http://youtu.be/EBmw6Wpfe6M


If it's got a Torsen diff and no TC, then the weakness of the Torsen is that it can't lock fully when no traction is sensed in one of the axles. So you get both wheels spinning and no drive to the other pair
https://youtu.be/zmCgHdfyYl0

Locking the centre diff ensures that drive is split 50/50 front and rear and you avoid the issue above unless you cross axle

[glen_ep Addicted PP Member Find all posts View Profile Close glen_ep]

The 90's transfer case does not have a torsen or any other "smart" type of load sensing mechanisms, it has a double planetary gearset which can be locked.

In H mode it simply sends power along the path of least resistance, which is initially more towards the rear output shaft but will very quickly change to where ever a tyre has lost traction, lifted, or an axle/shaft has broken. Worst case scenario; 1x tyre spinning, any location.

Engage the transfer's centre lock and it's a simple 50/50 split front/rear. Worst case scenario; 1x front tyre AND 1x rear tyre spinning.