Team-BHP - Center Differential and LSD in SUVs

Quote:

Originally Posted by DKG (Post 1548998)

You guys are getting a bit confused with the explanations.

maybe you guys need a better looking teacher (aka Marisa Tomei). THe youngsters here might think she is old but to me she looks hot for someone who is born December 4,. 1964. I guess Brooklynites age gracefully.:D

http://www.youtube.com/watch?v=Y-L_b...eature=related

lol:

here is the text...
And why not? What is positraction?

It's a limited slip differential which distributes power equally to both the right and left tires. The '64 Skylark had a regular differential, which, anyone who's been stuck in the mud in Alabama knows, you step on the gas, one tire spins, the other tire does nothing......

Enjoy! all you engine-vengineites!

Hi All,

Had a query regarding front differential on 4WDs.

As I understand, a differential has 2 purposes :

1. transferring power to the axles.
2. differentiating the speed of wheels while a vehicle is negotiating a turn.

If a car doesn't have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on the car: For the car to be able to turn, one tire would have to slip.

Now, in case of a 4 wheel drive vehicle, an additional differential is made available on the front which serves the same purpose.

Now if the differential is able to allow the wheels to rotate on different speeds, why is it not advisable to use a part time 4 wheel drive on 4H mode on tarmac ? even if the front axles are engaged, the differential would anyways allow slipping of wheels hence no strain would occur.

I might sound noob but just wanted to get this clarified.

Quote:

Originally Posted by Sabareesh (Post 2275676)

As I understand, a differential has 2 purposes :

1. transferring power to the axles.
2. differentiating the speed of wheels while a vehicle is negotiating a turn.

If a car doesn't have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on the car: For the car to be able to turn, one tire would have to slip.

Now, in case of a 4 wheel drive vehicle, an additional differential is made available on the front which serves the same purpose.

Now if the differential is able to allow the wheels to rotate on different speeds, why is it not advisable to use a part time 4 wheel drive on 4H mode on tarmac ? even if the front axles are engaged, the differential would anyways allow slipping of wheels hence no strain would occur.

I might sound noob but just wanted to get this clarified.

Hi Sabreesh,

Transmission Wind-Up.

http://www.team-bhp.com/forum/4x4-te...ng-thread.html

Regards,

Arka

Quote:

Originally Posted by Sabareesh (Post 2275676)

Now if the differential is able to allow the wheels to rotate on different speeds, why is it not advisable to use a part time 4 wheel drive on 4H mode on tarmac ? even if the front axles are engaged, the differential would anyways allow slipping of wheels hence no strain would occur.

Simble!:) The front and rear differentials allow the wheels to turn different speeds. That also means front and rear axles are turning in different speeds. But in 4WD, the transfer gear will turn the front and rear drive shafts at the speed. Therefore, the drive shafts take the brunt of this difference.

A center differential can ease this stress, but center-diff is not usually found in part-time 4WDs.

This thread has been pretty useful in understanding the difference between full time / part time 4WD systems and need for center differential. I now understand why a Safari / Scorpio 4WD cannot be driven continuosly in 4WD mode unlike a GV/Fortuner/Pajero.

Regarding Pajero / Pajero Sports which has a center differential , but a fulltime 2WD mode as well , can someone guide whether it is using auto-locking hubs (like a Safari) or front axle disconnect (using electric actuators as in the case of Safari Storme / Aria)?

Also is the center differential an integral part of the transfer-case (like the gearbox/differential in a transaxle)?

What I am sure about: Fortuner has open diffs front and rear, that cannot be locked even if you want to do so. The Centre diff can be locked by you, if you desire to have it so, thereby rendering it non functional. By doing so, all the power will never be diverted to just one axle.
Note that a lockable differential and LSD are two different things.

The Pajero rear differential has an always on LSD; I have no idea what the Fortuner has by way of LSD - limited slip differential. In the Pajero, the effect is for each rear wheel to always have some power up to a certain point. Beyond that point, it allows all power available to rear axle to reach a spinning wheel on it.

The Pajero also allows 2 wheel drive and is about as state of the art as purely mechanical 4x4 gear gets, both in terms of function and reliability, and is part of the reason why it can still hold its own against cars equipped with modern electronic gizmos when the going gets tough.

I am not sure if the Pajero that was sold in India after the SFX had an equally capable drive, the Super Select II.

Quote:

Originally Posted by navin (Post 1670041)

maybe you guys need a better looking teacher (aka Marisa Tomei).

I just noticed this; and the movie, My Cousin Vinny, is in frequent play on one of the movie channels just now. She won an Oscar for this performance, and any time I get the chance to do so, I make sure to see once again her performance on the witness stand where she is the most knowledgeable person on autos in the court room - it is a classic scene. Very capable and attractive actor too, part of the charm of the movie to a great extent.

Quote:

Originally Posted by desertfox (Post 4019618)

The full time 4WD A/T Fortuner has an LSD in the rear.
The Toyota Prado full time 4WD also has a similar set up.

Are you talking about Fortuner AT 4x4 currently on sale, desertfox? If yes, then I don't think it has LSD in the rear. 4x4 AT Fortuner has same diff setup as 4x4 MT earlier had, centrally lockable Torsen LSD.

Quote:

Originally Posted by Tushar (Post 3641469)

The Fortuner is equipped with full time 4x4, Torsen (torque sensing) limited slip differential and a dual ratio (high / low) transfer case. The diff is lockable too.

Its not mentioned in our TBHP official review, however I would like to be corrected if Fortuner AT has rear LSD.

I always thought, for full time 4x4, LSD in centre is required!

For some clarity about what LSD essentially is, a paragraph from Wikipedia:

"The main advantage of a limited-slip differential is demonstrated by considering the case of a standard (or "open") differential in off-roading or snow situations where one wheel begins to slip. In such a case with a standard differential, the slipping or non-contacting wheel will receive the majority of the power (in the form of low-torque, high rpm rotation), while the contacting wheel will remain stationary with respect to the ground. The torque transmitted by an open differential will always be equal at both wheels; if one tire is on a slippery surface, the supplied torque will easily overcome the available traction at a very low number. For example, the right tire might begin to spin as soon as 50lb-ft of torque is placed on it, since it is on an icy surface. Since the same amount of torque is always felt at both wheels, regardless of the speed which they are turning, this means that the wheel with traction cannot receive more than 50lb-ft of torque either, which is far less than is required to move the vehicle. Meanwhile, the tire on the slippery surface will simply spin, absorbing all of the actual power output (which is a function of torque provided over time), even though both wheels are provided the same (very low) amount of torque. In this situation, a limited-slip differential prevents excessive power from being allocated to one wheel, and so keeps both wheels in powered rotation, ensuring that the traction will not be limited to the wheel which can handle the minimum amount of power."

Cars can have any of the differentials with/without LSD. LSD equipped differentials of the kind referred to above can be used on any kind of road surface without drive line damage.

All that a full time 4X4 needs to work without damaging the drive line is an open centre differential, and having this lockable on demand confers additional benefit on poor surfaces, but causing drive line damage if kept locked on tarmac. Those that include an LSD there as well offer further benefits. And there are many kinds of LSD out there, with proprietary names making things more difficult to keep up with.

Quote:

Originally Posted by desertfox (Post 4020183)

The lever in both these vehicles is 4H - 4HL which means central diff locked in 4H mode, N nuetral and then a shift forward and right to 4LL which means Low transfer case engaged and central diff locked.
The rear diff is LSD in all full time 4WD M/T or A/T except FJ Cruiser which has a rear diff lock option.

In this region Asia and Middle East this is the case in other countries specifications may vary.

You mean to say while centre diff in Fortuner is lockable, it is the rear diff which is LSD? Pardon my stupid question but I am still not clear. :confused:

Quote:

Originally Posted by Sawyer (Post 4020192)

Don't you mean it is locked in 4HL mode only? One can drive either in 4H or in 4HL mode, via the lever?

Fortuner has 4 modes: 4H-4HL-N-4LL

Centre Diff is locked in 4HL and 4LL modes only. 4H is full time 4WD with open centre diff. :thumbs up

See the attached image from TBHP Review.