Explain the vacuum operated lockable differential !!

[landdani]

Hello, can any one explani to me please what is the vacuum operated lockable differential ( at the bottom the gear bar) ?
I have a 1984 seriesIII stage onw 109 inch , I don`t know any thing about that vacum, I have only seen the official name in this web page at the Technical Index page .

[map1275]

Any diff allows one side to spin faster than the other (diff action). This allows road wheels on one axle to turn a corner (one wheel spins faster than the other). The outer wheel of the turn travels further than the inner. Otherwise a linked axle would eventually break from stress (wind-up).

Same principle but in this case what allows one diff to spin at a different speed to the other? A third diff in the middle.

What happens if one road wheel gets stuck in a bog? The one in trouble spins madly and the one with traction doesn't turn (diff action).

What happens if one diff spins faster than the other....
If you can lock that diff temporarily, you can force drive through to the other side.
Your owner's handbook explains the principle, operation and use quite well.

[disco2hse]

Just to add to that. When one of the wheels, left or right, lose traction the power is output to that wheel, causing it to spin. The other wheel remains stationary.

Since the Stage 1 is permanent 4WD the transfer case has a centre differential that allows the front wheels to turn at a different rate to the rear. This prevents the transfer case from binding up. The downside is that in off-road situations the front or rear wheels can lose traction with the result that the wheels that have lost traction get all the power and spin.

The purpose of the centre diff lock is to prevent the front or rear from losing all traction by locking it. So if the front, say, loses traction then the rear will still keep turning.

This provides good off-road traction except if one wheel on the rear and one on the front lose traction. For example, the front left and rear right, or front left and rear left, etc. Such as if you get cross axled in deep ruts.

The only solution to that scenario is to have front and/or rear diff lockers, but those are expensive.

The centre diff lock should only ever be used in soft ground. Never on tarmac or concrete. Mostly binding up will occur when turning but can happen even going ahead in a straight line if the tyres are not all of the same diameter.

On the other hand, any time you are going through sound ground you should have the centre diff lock engaged. The only time I have it disengaged is when going over a sidling (driving across the side of a slope).

[choc-ice]

So without the diff lock, if any one wheel loses grip, will the car stop?

[stirlsilver]

Ok, I quickly drew up some diagrams to help visualise how this all works... It can be a bit hard to describe in text.


Say you are stuck, and all each of your tyres has various levels of traction. In this case the tyre with the least traction is shown as number 1 (say it is off the ground), the one with the highest traction is shown as number 4 (say it is solidly placed on a rock) and 2 & 3 are somewhere between these two extremes.

If you were in this situation with your central diff unlocked this is where your power would go:



Straight to the tyre that is in the air (number 1) all the other tyres will have no drive at all. You won't be going anywhere here.

If you were in the same situation again and you engaged your center diff lock this is where your power would go:



Since tyre number 1 is in the air it does nothing for you here, but now power is being delivered to the rear to tyre number 2 which is probably on the ground but if it doesn't have enough traction to push or pull you out it will just spin at the same rate as number 1. Tyres 3 & 4 which have more traction but no power will be delivered to them.

Say you have installed a lockable differetial to your front diff. It's quite a different story:



You now have positive drive on tyres 1 (essentially useless), 2 & 4, this will probably get you out... since we have said that number 4 is on a nice solid rock and it now has drive... but if you are badly stuck this may not provide enough traction.

If you have really gone nuts and fitted two lockers to your car this would be the story:



All tyres will have drive, and in this configuration you have A LOT of traction, you would have tyres 1 (spinning uselessly), 2, 3 & 4 pulling. If this doesn't get you out... you better hope you have a winch or a few cars to pull you...


Hope that helps!

[disco2hse]

Good one Stirling

Displays it quite well.

So without the diff lock, if any one wheel loses grip, will the car stop?

One more thing that needs to be added to what Stirling was saying. There is a a general rule in off-roading: "Go at slow as possible, but as fast as necessary."

That means in those situations where you might get stuck, say in mud or soft sand, make certain you have the vehicle in the correct gear and at the right speed to give you the momentum to get through it. The same applies to going up hills.

If you are going over rocky ground, then you need to go as slow as possible to avoid breaking things and give the tyres a chance to get a good grip on the nard rocky surface.

So, will you just stop if your tyres start to spin? Not necessarily. If you lose momentum, then yes it is probable. But if you have enough momentum, even with wheels spinning, you are more likely to get through it.

HIH

[landdani]

thank you all.
Dear map1275, I dont have any manuel, nor any document related to the car, and that`s way I am asking youall for a help.
Do I have a (A third diff in the middle) ?
Dears disco2hse and stirlsilver, tank you extra for the illustration.
Let me see if i get it:
A) Their is only 1 diff lock in the stage1, it is the (vacuum operated lockable differential ), and it only works to separate the front weels apart from the rear ones.
B) it is better to have 3 diff locks, but only in off road & no sharp corners & for a short trip .
Do correct me all please.
By the way stirlsilver, my bruther licke your car.

[disco2hse]

Hi Dani,

Well,

1. There are three differentials, two on the axles and one on the transfer case at the rear end of the gearbox (this is called the centre diff). Only one has a lock, the centre differential.
2. You can put diff locks onto the front and rear diffs, but this requires machining and is quite expensive. It is not something you would do yourself unless you are very good with machining parts, welding and so on. And if you have access to a good workshop. Perhaps if you are someone like Stirling.
3. You should only ever use the diff lock off-road, in soft ground. It should never be used on paved roads, concrete, etc.
4. The explanation about turning corners is related to what happens when the diff lock is engaged and you are on paved roads, concrete, etc.
5. Once off-road you can use it all day long, and you can turn tight corners if you like.

[landdani]

Dear Alan, tank you.
No one is like Stirling.
No, I am zero in mechanic.
How can I get a stage1 manual?

[disco2hse]

Here's the thing. There are a few Stage 1 owners manuals available secondhand. But they are mostly too expensive and do not really provide the required information for maintenance of your vehicle. For that you need at least:
- The "Series 3 - Repair Operation Manual", which can be downloaded from this site.
- The "Series 3 Parts Catalogue inc V8 ", which is there too.
- The "Land Rover V8 Supplement", which you need to buy, possibly from Amazon.com
- The Haynes "Land Rover Series 2, 2A and 3 1958-85 Service and Repair Manual", which you need to buy, possibly from Amazon.com.
- There are also the RAVE CDs which contain important information that directly relates to your machine, which is ex-army. The wiring in military vehicles is not the same as civilian models. These can be bought or downloaded free, Google for them. The cds relate to particular models so make certain you are getting the information for yours.
- This is a good site for information http://www.fourfold.org/LR_FAQ/index.html and so is this http://www.expeditionlandrover.info/index.html#contents

[landdani]

Thank you Alan, I will try to find them.
I hope that I did get the Idea wrigt , So, If I want to seperate the front tiers from the rear ones I will just pull out that small stick ?

[disco2hse]

Hi Dani,

You should only ever have it out when off-road or on soft ground. At any other time it must be in.

Out, front and rear are linked. In, front and rear are separated.

Alan

[Mik]

Here's a pretty impressive and very educational video about differentials - from a time long ago

http://www.youtube.com/watch?v=Em1zQToa_a8

[landdani]

Very good

[landdani]

Hi, I did try the diff lock.
Ironicly, I was at a situation where both the right reare tier and the left front tier were spining at the same time.

please give me your remarks.