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  #1  
Old 17 Aug 2008
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2WD - Differential Choice

Hi,

New on here but a one time PBC veteran and I'm thinking of building a 2WD expedition vehicle based around a RWD estate. I know that I could pick up a Disco at great value that would be better and easier but fancy doing something different. Capable durable touring rather than extreme off road competence with a route across north africa, Morocco/Algeria/Tunisia/Libya/Egypt being in the plans for the longer term but perhaps more when I'm actually ready to go

With a vehicle choice already in mind(looking at diesel options for the powertrain elsewhere since I know diesel is the best route to go but can't decide on OE or retro options) what I'm looking to research is the differential - with the models I'm looking at there is a factory locking option which can be picked up cheaply second hand or several aftermarket LSD choices. So the question is for the odd foray onto the soft stuff which is better, standard, locking or LSD?


Thoughts appreciated please on this subject!

Cheers!
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  #2  
Old 17 Aug 2008
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Locking if you just steer straight, limited slip if you want to go round corners and normal if you like digging and sand matting.

Put it this way, let's say you decide to go off road into the sand. In a 4wd you'd probably engage 4wd or lock up the centre diff if it's one of those with permanent 4wd. But you don't have that, so you engage your rear diff lock. Then you realise a) you can't turn corners and/or b) when you do you dig your self into the sand.
With an LSD you don't get stuck as often as with a normal diff, but you can't get yourself out as easily as with a difflock, but not far off. But you can also drive around corners as normal.

So -
Diff locks are good in a straight line to get you out or to keep going.
LSD's are good for normal driving but once you're stuck there is no more switches you can press.
Normal diffs are fun, but not when it's hot and you're stuck for the umpteenth time that day.

Or get something like on the back of my Pajero - a locking LSD - best of both worlds.
But to state the relatively obvious - with the wrong tyre pressures, whatever you have, you'll struggle. The right tyre pressures trump anything by far. You'd be surprised how far you can get in normal vehicle, without gadgets, but with the right pressures and at the right time of day/year.

I'm willing to be corrected on any of that.
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  #3  
Old 17 Aug 2008
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You can steer with a locked diff, just not much, about 100m turning radius!!!

I've recently been reading about car travel in the 1920s and 1930s in Australia - diff locks; "whats that", 4x4; "whats that"; wide tyres; "whats that". The one common thing about their vehicles is the large diameter on their wheels/tyres and wooden spokes.

About 15 years ago our local council insisted that vehicles driving on the beach had to be 4x4, before that we drove Kombis and a host of other 2WD vehicles through the dunes and onto/off the beach. Its amazing how an incoming tide concentrates the mind on good driving.
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  #4  
Old 18 Aug 2008
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Should just make the statement I'm looking toward a locking differential over a locked one. So the theory being it locks solid at times(akin to Detroit Locker) when it's needed over being welded, so the steering issue isn't concerning me.


Cheers!
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  #5  
Old 18 Aug 2008
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I do understand you are talking about a diff that can be optionally locked, not one that is permanently welded.
But the steering issue is the same for both - a locked diff is a locked diff whether permanent or done optionally.

If, when you get stuck, your route out is straight forward/backward until you reach firm ground, then there is no problem. But if you need to steer until you can stop, then it's not so easy.
Unless, that is, you can engage and disengage your diff lock on the move, which I don't think you can, and you don't need to stop to do so, I can't with my factory fitted one.

Imagine you approach an area of soft sand. In a 4wd you'd engage it then go as normal, whether you stop or not before the approach depends on the 4wd system.
But you have 2wd with a difflock. If you stop and engage it to get through, then you'd struggle to steer well.
If you have to stop again to disengage it you have to pick your spot otherwise you run the danger of just getting stuck again before you've cleared the obstacle and you've lost your momentum.

Turning corners with a locker, on soft stuff, causes one of the rear wheels to try and dig down plus the front wheels, when turned, will plough a furrow as the car just wants to go straight on.
None of it helps and it defeats the object. Sometimes it takes more grip to do that than just to be in normal 2wd

On sand a difflock is works well enough if used for a short period of time to extricate yourself, or through a short section of really soft stuff - but both in a relatively straight line. But you usually need to stop to engage and disengage it and speed and steering are affected.

Difflocks work well where axle articulation is a problem, at slow speed, where grip is usually reasonable enough. This is when you are crossing rocks, run offs etc where one wheel lifts and you have to drive on the other one.

I'm not aginst them, I have one, but they do have limitations.
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  #6  
Old 20 Aug 2008
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turn on, tune in...

So you have a choice something like this?

1. Open (normal)
2. Limited slip (e.g Quaife)
3. Activated locker (e.g ARB)
4. Automatic locker (e.g Detroit)

The automatic lockers will mechanically lock and unlock, with accompanying ticks and clicks, as driving dictates (like you say, steering isn't a problem). Activated lockers are manually operated by a switch on the dash. The limited slip differentials use a clutch system to distribute torque as required, but never actually lock.

So, to try and answer your question, a fully locking diff is best for out-and-out sand traps, but for a mixture of surfaces with a road bias I reckon an LSD might be more useful in a 2WD.

LSD's work better on tarmac than a Detroit and compared to an ARB type locker there is a continual grip benefit rather than a £500 switch on the dash that might only get used a couple of times.

Or you could stay standard and pack a shovel...
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  #7  
Old 20 Aug 2008
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Detroit type lockers do seem to offer a compromise between an open and a fully locked diff, but with the drawback that the odd noise now and then takes getting used to and you don't have personal control over its operation, if that's what you want.
How reliable they are over an extended Africa trip is something I have no knowledge of. Automatic anything is something I'm always cynical about, I'm probably too much of a Neanderthal.

Last edited by onlyMark; 20 Aug 2008 at 08:19.
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  #8  
Old 20 Aug 2008
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I'd go for a rear axle with an ARB diff lock (or an elec diff lock).
Going down a slippery hill you don't really want any locking going on. So I think it's nice to have the choice.

Or fit/make fiddler brakes! Now you're talkin'

Cheers,
Matt
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  #9  
Old 20 Aug 2008
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I don't know how many of those that have denigrated Detroits have ever lived with one, but I can say that in 15 years with 4 vehicles (2 LandCruisers, a M37 army truck and a Ford F350) I've never had a problem with a rear autolocker, even on ice, which is present here 5 months a year. They work in all materials and do NOT alter turning radius; there is a tight U-turn I use every day to get to work. The "automaticity" means all the bits are internal to the diff and there are no air lines, compressor or electrics to fail.
I honestly wish the rear diff in my Unimog was an autolocker rather than selectable.
One more point: selectable diff locks do not require stopping to shift in or out, just a brief lifting of the pedal.

Charlie
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  #10  
Old 20 Aug 2008
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"and do NOT alter turning radius"
Are you saying that with a diff lock engaged it doesn't alter turning radius?
Or are you saying that when a Detroit automatic diff lock is fitted, but in normal driving, doesn't alter turning radius?

If you say the former then I disagree.
If you're saying the latter then I agree as it wouldn't be very user friendly if it did. If it's not affecting your turning radius, then it's simply not fully engaged, surely.
The point is, and it doesn't matter what flavour of diff lock you've got, if it's engaged - it affects the radius.

I wouldn't denigrate them, I just admit to not having much experience of them.

"One more point: selectable diff locks do not require stopping to shift in or out, just a brief lifting of the pedal."
I'll go for that, that's reasonable - as long as you are driving reasonably.

One other factor to consider with a 2wd - you don't tend to have a low range - do you?!
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Old 20 Aug 2008
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Have a good look around at second hand vehicles things like rear wheel drive Volvo's already had LSD fitted and my choice would be to buy something like that fantastic carring capacity built like a tank and reliable.
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Old 20 Aug 2008
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Volvo is what I had in mind, I'm sure you can get both locking and limited slip differentials for them(Wentworth models on the 940 series I'm sure have LSD as standard) very cheaply.

Only concern I have is I would prefer a LHD and ideally diesel and sadly diesel even in RHD isn't massively common and going on to the continent sees them fetching top dollar(approx double the UK equivalent).
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  #13  
Old 20 Aug 2008
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Quote:
Originally Posted by mattsavage View Post
I'd go for a rear axle with an ARB diff lock (or an elec diff lock).
Going down a slippery hill you don't really want any locking going on. So I think it's nice to have the choice.

Or fit/make fiddler brakes! Now you're talkin'

Cheers,
Matt
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Hi Matt,

What happens when you go down a slippy slope with diff locks.
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Old 21 Aug 2008
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Detroit Locker disengages on turns, so it does not affect turning radius even slightly. But it locks up as soon as the slower wheel in a turn speeds up to the faster wheel. This would only occur in a turn on a slippery surface.

Charlie
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  #15  
Old 21 Aug 2008
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[QUOTE=onlyMark;203185]"and do NOT alter turning radius"
Are you saying that with a diff lock engaged it doesn't alter turning radius?
Or are you saying that when a Detroit automatic diff lock is fitted, but in normal driving, doesn't alter turning radius?

If you say the former then I disagree.
If you're saying the latter then I agree as it wouldn't be very user friendly if it did. If it's not affecting your turning radius, then it's simply not fully engaged, surely.
The point is, and it doesn't matter what flavour of diff lock you've got, if it's engaged - it affects the radius.

QUOTE]



Surely this is only really noticable on a hard surface, like tarmac.

on sand/mud etc, the outer wheel will just "slip" or "skid" over the loose surface, and you won't notice much difference.

I certainly didn't when i had the rear ARB engaged in my 90.
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