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Old 25th October 2009, 22:19   #91
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Default Centre and Axel Differential Lock

@Samurai,

Here what I understand from the reading

The Differential lock can be of

1. Centre Differential lock
2. Individual axel differential lock

Most full-time four-wheel drive vehicles have a centre differential located between the front and rear prop-shafts to prevent wind-up caused by the different rotation speeds of wheels on sealed surfaces

Differential locks on individual axles

An axle diff lock prevents differential wheel speeds on that axle, preventing wheel-spin on opposite wheels.

Limited-slip differentials (LSD)

A limited slip rear differential does the same and gives the same advantages as a lockable differential but, as the name suggests, the advantage is limited. There is some slip, which can be an advantage and a disadvantage

Like LSD, we have Vacuum/Pneumatic differential locks,Post-delivery differential locks,Automatic locking differentials.

So as per my understanding Fortuner has a centre Differential lock of type LSD with no individaul axel differential lock
1. Centre differential/viscous coupling
Located between the front and rear prop-shafts in full-time four-wheel drive vehicles only, this component distributes the power to the front and rear prop-shafts. Because the front and rear wheels rotate at different speeds when turning a corner this component must permit a differential in rotation speeds. A differential unit is fitted between the front and rear prop-shafts to do this. For off-road driving this differential can be locked, preventing differential rotation, locking
the shafts together. This differential lock must not to be confused with differential locks found on axles as the job they do is entirely different.

A viscous coupling, in brief, does the same job as the
centre differential but locking is done automatically.
A full-time four-wheel drive with its centre
differential locked is the same as a part-time four-wheel
drive vehicle engaged in four-wheel drive, the front and
rear prop-shaft are attached, as if it were a solid shaft.
These shafts drive the wheels via the axle differentials

2.Axle differentials
These components, one on the front and one on the back, distribute power from the prop-shafts to the wheels. Again, because of the different rotation speeds of the wheels when the vehicle turns, this differential permits this speed differential. (Hence the name differential)
All wheeled vehicles have differentials for this purpose. Axle differential locks are fitted to some vehicles.

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Last edited by Rudra Sen : 26th October 2009 at 14:20.
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Old 25th October 2009, 23:15   #92
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Dear Prajeesh, I really don't know what to say.
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Old 26th October 2009, 09:25   #93
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Quote:
Originally Posted by Samurai View Post
Dear Prajeesh, I really don't know what to say.
May be I didn't understand fully. I just quoted above from a book called complete guide to 4x4. So you can correct me. I am new to the technical details of 4x4. If I am confusing all, I am really sorry for the inconvenience and wasting your time
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Old 26th October 2009, 10:31   #94
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You guys are getting a bit confused with the explanations.

A limited slip differential INHIBITS slip which an open differential ALLOWS. You don't add an LSD to enable slip, its to curtail slip.

In a AWD platform the central diff (open) alone allows the front and rear set of wheels to rotate at speeds in variance to each other.

In an offroad/wet situation you have a condition where say the front set of wheels start losing traction. Now left open the diff will transfer all the motive power to the front which already have lost traction. You don't want that. Hence you opt for a LSD which limits the slip and by virtue of a clutch mechanism starts transferring power to the opposite set of wheels which retain traction.

The best remedy obviously is to mechanically lock the differential, meaning power is transferred in a 50:50 ratio. The main downside to this is it creates a bind in the drivetrain when wheels are not allowed some variance in rotational speed on account of terrain related slippage. That is, on tarmac a locked centre diff will damage the diff.

The torsen limited slip differential is not a regular LSD and does not work off clutches but a complex planetary gear setup. Its more robust and not prone to wear. Its a more expensive device to offer and luxury cars like the Audi Quattro etc use it. The Fortuner has the same from what I gather in specs of the car sold abroad

Hope this helps
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Old 26th October 2009, 11:29   #95
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All,
Thanks for your valuable explanation especially from DKG and Samurai etc.. I am getting more and more knowledge from you guys. Even I started looking for more details

Hi Here is a video which explain the Differential in a very good way with real lab demo. Please go to time 1:50. It is real worth to watch how the differential work.




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Last edited by bblost : 26th October 2009 at 11:41. Reason: corrected youtube URL
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Old 26th October 2009, 11:43   #96
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DKG, looks like you mistook my statement.

Slip is not the same as difference in wheel rotation. Slip is the difference in torque. The very purpose of the differential is to allow difference in rotation while delivering the same torque to both wheels. And rotational torque is the least force required to the roate a wheel. If one wheel goes up in air, it needs almost zero torque to rotate it. So the drive shaft will deliver the same almost-zero torque to the other wheel in order to maintain same torque to both wheels. This is because open diff doesn't allow any slip.

Open diff: Delivers same torque to both wheels no matter what, while allowing difference in rotational speed of individual wheels.
Fully Locked diff (or no diff): Delivers same torque to both wheels, and allows no difference in wheel rotation.
Diff with limited slip: Allows difference in torque (which we call slip) and difference in rotation of individual wheels.
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Old 26th October 2009, 12:54   #97
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I have always understood the word slip to refer to wheels slipping when traction is lost. If you feel that is not the right application of the word then do please elaborate. Its possible you are using some other meaning which makes your statements sound odd. If you agree on the above definition of the word then the following observations need to be relooked at

Quote:
Originally Posted by Samurai View Post
1) A fulltime 4x4 needs a center differential with limited slip (known as LSD), so that it allows slip between front and rear axles.
Sharath are you sure about this? I think that statement is totally inaccurate. In a open differential you have no control over slippage and hence you introduce LSD devices which limit slip not allow it !

Quote:
A fully open or fully locked differential doesn't allow any slip between the front and rear axle.
Not entirely true. An open diff has no control over slip and allows plenty of it !!

Quote:
Without the center LSD, you can't drive the fulltime 4x4 on tarmac without damaging the driveline.
Not true again. With an open centre diff you can drive on tarmac (please explain why you don't think so)

Once again LSD is only to limit slippage of wheels not allow it !

Quote:
2) The front or rear LSD will allow slip between left and right parts of the given axle.
No Sir it limits slip. Its the open diff which allows for slippage
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Old 26th October 2009, 14:03   #98
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A very interesting aspect of the functioning of the open diff is in relation to traction.

One would normaly think that in a car the amount of torque applied to the wheels is the same as what the engine puts out regardless of the traction at the wheels. Nothing is further from the truth.

In a open differential the amount of torque applied to the wheel is a derivative of the traction the wheels enjoy. Lets say one wheel has its traction compromised, say due to ice or sand. The open diff ends up applying only as much torque as the wheel requires to rotate, and not more. So if with minimal torque a wheel is able to spin, the diff unirformly applies it to both wheels. However the effective lowering of torque renders a car immobile.

So to get motive power at the wheels its crucial in an open diff to have traction at both wheels. The more traction at the wheels, the more torque gets applied at the wheels.

If you brake a wheel that is spinning freely the diff encounters more traction (albeit false) and ends up applying more torque to counter the load. This helps move the other wheel with traction effectively. So in an offroad situation if we can selectively apply the hand brake to either rear wheel you can move out provided the other wheel has traction.

For this reason alone an AWD car employs the use of LSD or a Torsen diff in the centre to prevent the car from becoming either a rear wheel drive or front wheel drive in slippery conditions. But on regular tarmac an AWD car with open diff at center works just fine and poses no problem to the drivetrain.

Last edited by DKG : 26th October 2009 at 14:14.
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Old 26th October 2009, 14:18   #99
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Quote:
Originally Posted by DKG View Post
I have always understood the word slip to refer to wheels slipping when traction is lost. If you feel that is not the right application of the word then do please elaborate.
The term slip applies to both, but meaning is not the same in both cases. I am entirely writing this from my current understanding, no google research here.

Consider doing slow circles in a vehicle (front and rear diffs open) on dry tarmac with the steering locked to one side. The wheels facing the center of this circle rotate lot lesser than the wheels on the outer side. In fact, the rear inside wheel will be hardly turning, practically pivoted you can say. Right now the difference between the roational speed of left and right tyres are at the maximum. Would you call this slip? Not really.

There is absolutely no slip from the tarmac during this operation. All the powered wheels will receive exactly same torque. That means, in case of 2WD the powered axle is delivering same torque to both wheels. In case of 4WD, all 4 wheels will receive 25% torque. If it has open or locked or no center diff, both front and rear dirve shafts will receive 50% torque. Some vehicles have different torque bais, let's ignore that for now.

The above operation won't have any ill-effect on 2WD vehicle. However, the 4WD without center diff or locked center diff is already in trouble. The front axle and rear axle are obviously turning at different speeds.

The affect on:
Open center diff: Nothing, when front and rear drive shafts turn at different speeds, the torque gets re-adjusted and continues to balance the torque devliery to each drive shaft. Small question, can anyone name a 4WD vehicle with fully open center differential, without LSD option?
Locked or no center diff: Imagine two ends of rod trying to turn at different speeds. Now somebody gotta give... either the center diff should allow slip (but it is locked) or the ground should provide slippage. If ground slips, then the stress on the drive shaft will be eased. That is why one shouldn't use 4WD in locked center-diff or no center-diff in non-slippery ground.
Center LSD: Imagine two ends of rod trying to turn at different speeds. Now somebody gotta give... either the center diff should allow slip or the ground should provide slippage. In this case the LSD will slip and allow for unequal delivery of torque to different drive shafts there by releasing the stress.

After this explanation, one might wonder what is the difference in usefulness between open center diff and center LSD on hard tarmac. Nothing. A center LSD will behave exactly like open center diff on hard tarmac. So you can happily drive at highspeeds on highways like any open diff vehicle.

Difference comes when you go offroad where the ground offers slippage. A fulltime 4WD vehicle with open center diff (and open front and rear diff) will be stranded even if one wheel slips or loses traction on the ground. If one wheel needs near-zero torque to turn (due to slippery terrain), all the remaining wheels will get the same near-zero torque, stranding the vehicle.

The center LSD on the other hand will be able to deliver unequal torque to differen wheels. The exact behavior varies depending on the type of LSD.

The vehicle with locked or no-center diff will continue to deliver 50% to both front and rear drive shafts, so one wheel won't strand the vehicle. That will need one zero traction wheel in both axles.

A vehicle like Gurkha, where one can lock front and rear diffs, you can keep on going if 3 wheels are slippery, and 4th wheel will still get 25% of original torque.

Last edited by Samurai : 26th October 2009 at 14:53. Reason: oops, no center diff in Gurkha
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Old 26th October 2009, 14:49   #100
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Quote:
Originally Posted by Samurai View Post

A vehicle like Gurkha, where one can lock all 3 diffs, you can keep on going if 3 wheels are slippery, and 4th wheel will still get 25% of original torque.
Here when you say 3 diffs, it means centre differential and individual front and rear axel differential. Please correct me if I am wrong
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Old 26th October 2009, 14:54   #101
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Quote:
Originally Posted by g_prajeesh View Post
Here when you say 3 diffs, it means centre differential and individual front and rear axel differential. Please correct me if I am wrong
You are right, but I made a small mistake. Gurkha doesn't have center diff, so it is same as locked center diff. I corrected my statement.
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Old 26th October 2009, 15:14   #102
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So this is the scenario, My vehicle is doing a rotational motion or taking a circular curve,

1. the speed which the rotation of front axel and rear axel is different and this differential is provided by a centre Differential

2. There is a differential in speed of Wheels ( left(inner) and right(outer) in each of the axel(front and rear). This differential is provided by axel differential
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Old 26th October 2009, 15:43   #103
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Yes, the difference is compensated by open diff or limited slip diff.
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Old 26th October 2009, 17:14   #104
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Default Diff-Lock and LSD

Hi Guys,

Some info on Diff-Locks and LSDs

http://www.team-bhp.com/forum/4x4-te...-vs-lsd-2.html (Diff-Locks vs LSD)

Regards,

Arka
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Old 26th October 2009, 17:26   #105
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Arka is right, this thread is more focused on the working of center differential. Working of different types of LSDs should discussed in the other thread.
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