Offroad in an 80 (Diff Locks)

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This is probably a really dumb question but I have searched and can't really get the answer I need.

My 80 is fitted with three locking diffs-
Front-center-rear.

I cannot lock either the front or rear without first locking the center, now is this because there is a technical/mechanical reason why or is it simply the way the car was designed?

I understand what a locking differential does but what I don't really understand is how to use them.

I've been reading Sahara Overland and on page 173 Chris Scott talks about going up rocky ascents and that in a full time four wheel drive such as the 80 if the traction is good on dry rock you should keep the center diff unlocked. Which I think I understand why but....

Am I right in thinking-

If you lock either the front or rear diffs on the axle you've locked both wheel will turn at the same speed so in slippery/loose surfaces or if you've got a wheel off the ground you can send power to both wheels.

What does locking the center diff lock do? Does it split the power equally? Does it mean that front and rear axles turn at the same speed but only the wheel with the least resistance?

What does transmission wind up actually mean and how can I detect and avoid it?

I'm guessing this is all really basic stuff which is probably why I can't seem to find a good answer but I would like to know for sure how to operate the 80 correctly.

Thanks!
 

Chris

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I think the best way to answer your question would be to simply copy your thoughts into a new post and claim them as my own.
You are spot on. You have to lock the center diff, yes, before you can lock back and front. Why? Well if you locked the back on its own, then all the drive would go to the front wheels because they'd have less grip now than the locked pair. OR you get no drive to the front and all to the rear, which if they were the ones on soft ground would mean you'd go nowhere. Right? The CD is just the same as the front and rear diff only going along the vehicle not across it.

Wind up is simply torsion (tension) in drive components that make life awkward. Drive needs 'slack' in it so that it can function, move about, engage / disengage etc. With wind up, you have basically run out of slop and now things have got a bit tight. Go too tight and you might snap something.

See, you knew that.

Chris
 

pugwash

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there are a very few occasions where you would use the rear locker without the fron- particularly with very very steep descents- the rear wheel can sometimes lift which causes it to contra rotate and shift power to that wheel. Locking the rear axle can help stabilise.

Can't work out the benefit of having an unlocked centre diff on slick rock (unless its really smooth)- the risk of wind-up is far better than making a very steep climb- lifting a wheel (and thus spinning all the power through it)- and thus destabilising the truck.
 
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chriscolleman

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Have a read through your operation manual.
I kid you not.

When traction stops at one or more wheels engage diff locks.

You can write a whole book about diff locks, but that about sums it all up.

The most logical sequence is the toy one.
First the center, then the rear and last the front.

Never activate lockers with wheels spinning, and look on lockers as something to be used at crawling speed.
 

pugwash

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Always post this link when we start talking about diffs- it explains perfectly how a diff works in far better than words- stick with it! http://www.youtube.com/watch?v=yYAw79386WI
Thought you might like a more in depth reply! Not promising an "expert" opinion, but am reasonably qualified through experience over the past decade or so.
James Rothwell said:
This is probably a really dumb question but I have searched and can't really get the answer I need.
My 80 is fitted with three locking diffs-
Front-center-rear.
I cannot lock either the front or rear without first locking the center, now is this because there is a technical/mechanical reason why or is it simply the way the car was designed?
Designed- if you lock the front or rear without the centre being locked you will in effect force ALL The power to the unlocked axle and in to the wheel with no resistance- it will just spin in any sort of offroad situation. There are situations where locking the front only can be a help, but they are quite rare (more of which later). Locking the front presents a much greater risk or breakage than the locking the rears (CVs are weaker etc), amd oddly ime lockign the front doesn't add as much traction as when you lock the rear. Hence you can only lock the front when the rear is locked.
In reality its only the mechanical action of the swicth that prevents you locking the front instead of the rear- if the rear doesn't lock (because of a fault)- the front still will. It wouldn’t be hard to engineer switching so that the front and rears can be locked independently- the wiring diagram for the LC e-lockers is on the net in a few places.
James Rothwell said:
I understand what a locking differential does but what I don't really understand is how to use them.
Lockers at their most basic are a way of utilising maximum traction at all wheels- they are more effective than traction control and a full posi-locker is usually more effective than an LSD- mainly because there is no delay in traction. in addition a difflock can, in certain situations, improve stability of the vehicle in extreme situations- side slopes, climbs and descents. At the same time it can also de-stabilise a vehicle in extreme situations. Knowing how a locked wheel will effect traction is key, getting it wrong can be disastrous.
One thing to mention is that lockers have one huge drawback- they allow you to get MORE stuck that you otherwise would with open diffs. Yes they will get you further through an obstacle, but if you still end up getting stuck, you will just be stuck further from help (and either further up the hill, or deeper in the gloop). in a competition enviroment this really doesn't present any problems- getting stuck is part of the fun. In an overland situation getting yourself MORE stuck that you need to be could be life threatening if you don't know how to self recover.

The long wheelbase of the landcruiser, coupled with a really very strong drivetrain, as well as suspension that is really very good at keeping wheels in contact with the ground means leaves you with a vehicle with frankly stupendous traction. once drove a course in high range that LR90s with lockers all round were struggling with! (of course you pay for this traction with the size of the vehicle- you don't get something for nothing in the offroading world).
James Rothwell said:
I've been reading Sahara Overland and on page 173 Chris Scott talks about going up rocky ascents and that in a full time four wheel drive such as the 80 if the traction is good on dry rock you should keep the center diff unlocked. Which I think I understand why but....
its a transmission wind-up thing- actually you should adjust your use of any locker to the surfaces you are on- there will definitely be times when you will need every diff on dry rock.
James Rothwell said:
Am I right in thinking-
If you lock either the front or rear diffs on the axle you've locked both wheel will turn at the same speed so in slippery/loose surfaces or if you've got a wheel off the ground you can send power to both wheels.
that's pretty much correct. An axle locker will force both wheels on the same axle to turn at exactly the same rate at the same time. Obviously this reduces your ability to turn corners which is a huge problem when you get to a corner- if you have the front locker in you will find the wheel almost impossible to turn. You wil have to unlock the front. With something like an ARB that use air pressure to force the locker in and out (ie its very quick), you can flick the locker in and out very quickly which makes utilitising the front locker much easier than in a Landcruiser which can be abit slow to disengage.
James Rothwell said:
What does locking the center diff lock do? Does it split the power equally? Does it mean that front and rear axles turn at the same speed but only the wheel with the least resistance?
Indeed you have true 2 wheel drive with the centre diff locked- 2 wheels will always turn when you are in gear. In cross axled sitauations this often leads to loss of forward momentum as diagonal wheels just start to lock up!
James Rothwell said:
What does transmission wind up actually mean and how can I detect and avoid it?
As someone mentioned above it involves removing the slack from the drive train. But it’s a little more complicated than this. Imagine a car turning round a corner- the outside wheels have to travel further thatn the inside. Differentials allow the wheels to turn at different speeds (yeah I know we’ve covered this)- lock the diff and suddenly the wheels are trying to turn at the same speed. With the axles locked one wheel wil tend to skid, chirp or tweet as it rotates slower or quicker under engine load, than the ground speed is dictating.
With the centre diff locked you now have 4 wheels travelling at slightly different speeds- but you also have more driveline between the differential and the ground. This means you can drive at speed on the motorway for example, and as long as you travel in a dead straight line you will be fine- as there are variations in rotational speed you start to get all the bits travelling at different speeds- eventually all the slop is taken up in the system, the its of metal then start to take load (as things like halfshafts are surprisingly elastic). Eventually something can explode dramatically. If you have transmission wind up it also tightens up the diff- you can’t unlock it to release the tension unfortunately!
James Rothwell said:
I'm guessing this is all really basic stuff which is probably why I can't seem to find a good answer but I would like to know for sure how to operate the 80 correctly.
The key to lockers is knowing when to use them. Frankly I would have a centre diff lock in 99.9% of the time you are on low traction ground (to include gravel, mud, snow,sand etc)- there is no point not to, and every reason to.
As to axle lockers- the key to them (as with all 4x4 driving) is to read the ground ahead- and well ahead. The proper use of a locker is to have them working just before you need them- you don’t need to stop to engage them, and indeed it’s a bad idea to come to a halt- if you need lockers then you can bet you’re bottom dollar that a bit of momentum will be more useful.
So you see some ground ahead that looks marshy or difficult, if its your first time out and you are with friends with suitable kit, I STRONGLY urge you to try and get stuck- you may be surprised at how good the vehicle is!
Otherwise, as you come to the start of the obstacle make sure the rear locker is engaged (ie the rear locker symbol has change from flashing to solid), enter the obstacle and keep enough power up to keep wheels spinning at a reasonable speed- you need to keep momentum up, so continue to add power until such time as you start to get a serious reduction in momentum or serious wheel spin.
At this point one tip is to “blip” the throttle continuously- it works like a crude traction control system, but can be extrememly effective (as can waggling the steerig wheel from side to side). If you think neither these techniques will help (or you’re running out of time/speed), then flick the front locker in. The key to this is to ensure you are OFF the power and the revs are falling- this allows the locker to engage freely and not under load. Remember when the front engages you effectively have no steering so make sure you have the steering in the correct position (you will sometimes need a spotter to help here).
If you still come to a dead halt, you can be reasonably firm with the throttle and try dropping into reverse and then first- this often gets enough momentum up to get a bit further- ey is mechanical sympathy – cruisers are strong but nothing Is indestructible. Remember you are getting further into the obstacle by doing this- what you don’t want is to get so far in that you can no longer get out!
As soon as you can disengage the front locker again, with the rear following shortly thereafter. You have now crossed your first bog with the use of lockers- leaving lesser vehicles far behind.
To follow- different types of locker, off camber situations, climb/descents, common problems.
Please note that a lot of this info is “my” best practice. Others will disagree, and offer very strong arguments! What you really need to do is find a controlled environment, get some training, and most importantly as much experience as you can!
 

Andrew Prince

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Great post, Jim! Or should I say essay :clap:

This should be made into a sticky methinks!
 

chriscolleman

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If you want to experiment with your lockers, try and find a short ditch. Just deep enough that it can swallow a wheel.

Then try driving it at an angle that puts front right and rear left tire into the ditch.
First gear low and just use idle to cross the ditch.

When movement stops, roll down your windows and have a look at what your tires are doing.
Then apply diff locks.

Next try and find a short hill about waist high and attack it at the same angle.
Again low gear and just more than idle revs.

These are by far the most fun experiments with lockers.
And you'll get a great idea how much suspension travel your cruiser has got.

After you've had fun with your lockers, go buy a decent 12V compressor and use the ditch and hill to figure out how lower tyre pressure equals more traction.

A good note is to do this at your own pace and not hurry yourself.
 

HauptRenate

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Mar 14, 2010
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Cor blimey! that IS news to me! I had NO IDEA that you have to engage Central diff lock first before you can lock RR-FR....
Am I thick or what? -DOH!!
Renate
 
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