From: dingbat@codesmth.demon.co.uk (Andy Dingley)
Newsgroups: rec.autos.4x4
Subject: Re: need advice on AWD used suv
Date: Tue, 09 Jan 1996 06:16:32 GMT

mtsjej@panther.Gsu.EDU (Jeffrey Johnson) wrote:

>1. i obviously understand what AWD is

That's nice - I don't.

I don't like using the terms "AWD" and "4WD". They're not well enough,
or consistently, understood to be useful. I prefer the more obvious
"full time 4WD" and "part time 4WD". These are self-evident
descriptions of what they actually do, so there's no scope for
controversy. Part-time 4WD drives like a 2WD vehicle (usually RWD)
until you engage the lever to make it just like a permanent 4WD
vehicle.

The critical questions, those which affect how you drive them, are
what sort of centre differential and front hubs you have. It's almost
entirely true to say that permanent 4WDs have some sort of centre diff
and that part-time 4WDs have no center diff (that is to say, a
permanently locked centre diff).

If you're not familiar with diff action, read it up in any car tech
text. The principle is just the same, whether it's in an axle or a
central transfer case.

If you have no differential action front-to-back, then you can't drive
on dry pavement without suffering "torque wind up". The effects of
cornering lead the front wheels to travel slightly further than the
back wheels. As there is no diff to even out this action, then a large
torque exists through your prop shafts and you will eventually break
something expensive (I've personally smashed a diff due to this)
On vehicles with permanent all wheel drive and no centre diff (some
military vehicles and airport fire tenders are the only examples I can
think of) it's standard practice to bounce them over a boulder or
railway sleeper every few miles to allow this wind up to release
itself. You can't have permanent 4WD on the road without a centre
diff.

If your part-time 4WD happened to have a centre diff, it would have no
problem. In fact, you could easily throw the lever away and call it
permanent 4WD.

Centre diffs come in different sorts; locked and open (as we've just
mentioned) but also lockable and viscous.

Lockable are about the most common on permanent 4WDs. They have an
actuating lever to move between open and locked. When in locked, they
behave like a part-time 4WD with no centre diff, and will have all the
problems about wind up on dry pavement. Diff locks are normally
engaged only for short periods whilst crossing particularly messy
ground. Most off-roading doesn't need it engaged, most of the time.

Viscous diffs (including other sorts of torque sensing mechanisms,
such a Torsen or Knight-Mechadyne) are probably the best to have. They
have a range of _gradual_ degrees of locking, rather than the "all or
nothing" approach of a lever. They're also fully automatic and don't
have a manual lever.

Transfer boxes usually have two ratios and it's common for a single
small stick to operate both ratio shift and diff lock control.


Axle diffs may also be lockable. British practice is to fit open axle
diffs both front and back, whereas American practice is to offer
limited slip or lockable diffs as a factory option (don't ask me what
the GM brand names are - I'm just a dumb Limey). At this stage of the
game you don't really need either, so don't worry about it.

You might hear about "air lockers". This is just a lockable axle diff
worked by an air cylinder, as it's more convenient than a mechanical
linkage. Centre diffs are mounted to the chassis in a convenient
place, so they're usually activated directly by a lever. Some older
vehicles had a vacuum servo to activate the diff lock, with a small
electrical switch as a driver control. These have something of a bad
reputation for reliability - a simple lever is much more direct.


Yet another difference between permanent and part-time 4WD is in the
front hubs. Permanent 4WDs use CV (constant velocity) joints, like a
FWD car. These allow easier steering action and smoother cornering. As
part-time 4WDs only use 4WD for short periods off road, they can make
do with the cheaper UJ (universal joints), as used in your propshaft.
Steering with UJs feels as if the suspension isn't working quite
right. It's perfectly safe, but you wouldn't want to cruise down a
motorway like that.


Freewheel hubs are a fuel-saving feature on part-time 4WDs. Even in
2WD mode, the front wheels are still driving the internals of the axle
around, wasting energy. Freewheel hubs allow you to disconnect the
wheels from the axle when running in 2WD, so you no loger need to turn
the front axle and its diff around. They come in  two sorts; manual
and auto. Manual are generally stronger and more reliable, but require
you to get out to switch them engaged before you can use 4WD. In
practice this isn't a problem; you leave them disengaged for weeks,
then engage them before you set off on an off-road trip, switching
between 2 & 4WD from within the cab as needed. All it costs you is a
little extra fuel. It's often possible to buy freewheel hubs at a
later date and fit them yourself.

Axle disconnects are a relatively modern idea; akin to freewheel hubs
for a permanent 4WD. It's just a fuel saving feature.


Almost all part-time 4WD vehicles need you to stop before switching
between 2 & 4WD. This is also true about engaging diff locks, and less
commonly true about changing transfer box ratio. Always stopping
anyway isn't such a bad idea, and is less graunching on the box.


There are few maintenance implications for any sort of transmission
system. Change the fluids when indicated by the service manual (and
this might be as often as monthly, if you're regularly going wading)
and you won't go far wrong. Most servicing difference are due to
different likelihoods of severe damage, not due to different amounts
of routine maintenance needed.


>     88 range rover 90k miles -- hear these can hard to maintain

These are easy to maintain. '89 was the year when they became
extra-complicated, although US-spec models probably gained the
assorted electric fripperies a few years earlier.  Neither would be a
maintenance problem, unless you get a later model with air suspension

The '89 viscous centre diff is an all-round Good Thing. Better to
drive and more reliable in service.

--
Andy Dingley                                dingbat@codesmth.demon.co.uk

The Internet has given us all mountains of information at our fingertips.
...and all most people want to do is pick their noses with them.

From: w.j.markerink@a1.nl (Willem-Jan Markerink)
Newsgroups: rec.autos.4x4
Subject: Re: Diff between 4WD AND AWD
Date: 13 Jul 1996 15:37:42 GMT

In article <4s68ss$821@news.greatbasin.net>,
   mcmasuda@heather.greatbasin.com (Matt Masuda) wrote:

>: AWD has a center differential that allows it to be used on dry
>: pavement without the risk of breaking parts.  This also means you can
>: run slightly different size tires on the front and back without
>: changing gears or destroying parts.

If you apply 'slightly' to different worn tires....yes.
Anything more will kill a differential with gears in the long run. They are 
not designed to rotate continuously. Anything that is ment to rotate 
continuously has bearings. The spider gears inside the diff don't have such 
bearings.... 

From: dingbat@codesmth.demon.co.uk (Andy Dingley)
Newsgroups: rec.autos.4x4
Subject: Re: Diff between 4WD AND AWD
Date: Sun, 14 Jul 1996 10:10:06 GMT

The moving finger of w.j.markerink@a1.nl (Willem-Jan Markerink) having
written:
>Anything that is ment to rotate continuously has bearings. 
> The spider gears inside the diff don't have such 
>bearings.... 

All true, but the thing that kills most spider gears isn't actually
bearing wear, but gear tooth wear. Spider gears are so cheaply made
that they're not just made with plain bearings, they're shaped by
simple forging rather than machining. The surface finish is so poor
that slight wear in them soon produces significant play and noise from
the diff.