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Old 8th August 2016, 16:53   #121
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Default Re: Center Differential and LSD in SUVs

Yes you are right.
It is full time in 4H
once shifted forward on the fly 4HL mode locks the central diffrential.
Shift to nuetral main gearshaft first and go to 4LL the low transfer case will engage and the central diff will be locked always in low mode.
Even if you dont shift the gear to nuetral engaging the transfer case gear to nuetral will shift the main gear to nuetral on its own,

It depends on country or model. In most cases if there is no rear diff lock provided on the dashboard assume that a full time 4WD Toyota SUV will have an LSD in the rear diffrential.
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Old 7th May 2018, 12:13   #122
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Default Re: Center Differential and LSD in SUVs

Quote:
Originally Posted by Sawyer View Post
For some clarity about what LSD essentially is, a paragraph from Wikipedia
Quote:
Originally Posted by Samurai View Post
A center differential can ease this stress, but center-diff is not usually found in part-time 4WDs.
Thanks for your valuable inputs to this thread, gentlemen! I have been trying to better understand Super Select II, the 4WD system employed by Pajero SFX. I am quite well-versed with the mechanics of the conventional part-time 4WD system which comes with the likes of Safari, Gypsy, etc. An interesting variable introduced by the SS II is the center diff.

Things I am clear on, about the SS II:
  • There is a center diff, which - when open - allows for 4WD to be used on tarmac at highway driving speeds, essentially by allowing for the difference in rotation speeds of front and rear drive shafts, primarily while turning - this is the 4H mode. The key advantage I get here is better handling on, say, wet highways or dirt tracks.
  • The center diff is oblivious of (or maybe it is not even active in) the 2H mode (when in use)
  • Front diff is an open (regular) one
  • Rear diff is LS - will continue to provide torque to the wheel with better traction, to an extent, despite the other wheel losing traction

Please correct me if I am wrong on any of the above.

Now for things I am NOT clear on:
  • What is the type of LSD in the rear axle? Is it clutch-pack based or something else?
  • Is the center diff LS as well? If so, what type?

An interesting scenario - Let's say I am driving in 4H (center diff open) on tarmac and cut over into a dirt track. Here, one of my rear wheels suddenly loses traction and starts spinning. Would the 4H fare any better here than the 2H mode? My understanding is that if not for the rear LSD, all 4 wheels would get the same torque as the spinning one (nearly 0), rendering the car motionless, unless I lock the center diff. Can someone please validate this understanding?

Last edited by cool_dube : 7th May 2018 at 12:26.
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Old 8th May 2018, 00:26   #123
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Default Re: Center Differential and LSD in SUVs

Quote:
Originally Posted by cool_dube View Post
Now for things I am NOT clear on:
I won't comment on Super Select II since I am not too familiar with that system.

Quote:
Originally Posted by cool_dube View Post
An interesting scenario - Let's say I am driving in 4H (center diff open) on tarmac and cut over into a dirt track. Here, one of my rear wheels suddenly loses traction and starts spinning. Would the 4H fare any better here than the 2H mode? My understanding is that if not for the rear LSD, all 4 wheels would get the same torque as the spinning one (nearly 0), rendering the car motionless, unless I lock the center diff. Can someone please validate this understanding?
Yes, your understanding is correct. Any open differential will ensure both sides gets equal torque. So if one side demands little torque to get that wheel spinning, equal amount of torque is sent to the other side, which may result in other wheel not turning. Locking a differential can send unequal torque to the wheels if the traction for each wheel is not equal. But the speed of rotation will be equal.

If not for the rear LSD, an open center diff does result in the situation you describe. So having an open center diff without rear LSD is not good.
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Old 8th May 2018, 10:22   #124
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Default Re: Center Differential and LSD in SUVs

Quote:
Originally Posted by cool_dube View Post

Now for things I am NOT clear on:
  • What is the type of LSD in the rear axle? Is it clutch-pack based or something else?
  • Is the center diff LS as well? If so, what type?

An interesting scenario - Let's say I am driving in 4H (center diff open) on tarmac and cut over into a dirt track. Here, one of my rear wheels suddenly loses traction and starts spinning. Would the 4H fare any better here than the 2H mode? My understanding is that if not for the rear LSD, all 4 wheels would get the same torque as the spinning one (nearly 0), rendering the car motionless, unless I lock the center diff. Can someone please validate this understanding?
Mr. Dube, I believe for the Super Select II 4WD system; there is a viscous coupling present in addition to the centre differential which acts in conjunction with the centre diff therefore in 4H mode your torque split is 33/67 (front/rear) in normal conditions (this can go up to 100% to either axle if slippage occurs). Please note that although the center differential is not locked but is active in 4H mode, the viscous coupling also kicks in case of slippage and therefore the scenario that you’ve described, you’re better off in 4H than 2H mode.

In my limited understanding center differential cannot be an LSD as that would defeat the purpose of locking the front and rear differentials.

As for the rear axle of the Pajero it I believe is an LSD (Torsen) that uses the helical/worm gears and not a clutch pack; though I must add that I came across this info quite some time ago while reading about the Toyota 4WD systems so not a 100% sure about this. Mitsubishi experts; chime in, please.
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Old 8th May 2018, 11:11   #125
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Default Re: Center Differential and LSD in SUVs

Quote:
Originally Posted by Samurai View Post
If not for the rear LSD, an open center diff does result in the situation you describe. So having an open center diff without rear LSD is not good.
Thanks, Samurai. Further to the current situation, let's say one of my rear wheels is spinning and the rear LSD kicks in, I presume it would send some torque to the other rear wheel and the car should be able to move, right (of course, assuming that the other wheel has traction)? Also, would this end up transferring any torque to the front axle at all (in 4H i.e. open center diff)?

Quote:
Originally Posted by Vikram Arya View Post
Mr. Dube, I believe for the Super Select II 4WD system; there is a viscous coupling present in addition to the centre differential which acts in conjunction with the centre diff therefore in 4H mode your torque split is 33/67 (front/rear) in normal conditions (this can go up to 100% to either axle if slippage occurs)
Thanks, Vikram. Does this imply that the VCU ends up providing a "limited slip" effect (across the 2 drive shafts/axles) in a way? Also, going by this logic, in the scenario quoted above, a slipping rear wheel should cause the VCU to send some torque to the front axle as well, thereby pulling me out without locking the center diff, right? If so, this is in contradiction to what Samurai says above.

Quote:
Originally Posted by Vikram Arya View Post
In my limited understanding center differential cannot be an LSD as that would defeat the purpose of locking the front and rear differentials.
From what I head, the Fortuner runs a LS center diff, which is torque-sensing (Torsen). I might be wrong.

Quote:
Originally Posted by Vikram Arya View Post
As for the rear axle of the Pajero it I believe is an LSD (Torsen) that uses the helical/worm gears and not a clutch pack; though I must add that I came across this info quite some time ago while reading about the Toyota 4WD systems so not a 100% sure about this. Mitsubishi experts; chime in, please.
I am clueless on this. Would love to hear back from other experts.
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Old 9th May 2018, 06:36   #126
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Default Re: Center Differential and LSD in SUVs

Quote:
Originally Posted by cool_dube View Post
Thanks, Vikram. Does this imply that the VCU ends up providing a "limited slip" effect (across the 2 drive shafts/axles) in a way? Also, going by this logic, in the scenario quoted above, a slipping rear wheel should cause the VCU to send some torque to the front axle as well, thereby pulling me out without locking the center diff, right? If so, this is in contradiction to what Samurai says above.



From what I head, the Fortuner runs a LS center diff, which is torque-sensing (Torsen). I might be wrong.



I am clueless on this. Would love to hear back from other experts.
Sir,

You’re right indeed; I stand corrected, several Toyota platforms have had a Torsen type lockable center differentials; a notable mention would be Landcruiser 100/150 & 200 series. If I’m not wrong then the Fortuner till the model year 2013/14 (not sure about the exact year when Toyota took it away) used to have a lockable center differential that was Torsen type.

Also, most of the center differentials will either be a Torsen type or will have a VCU but not both. In most cases, the center differential is lockable which of course results in 50/50 (front/rear) torque split.

Again I must mention again I’m no 4WD expert and so in my limited understanding when the VCU is integrated into the center differential the drive train acts as an AWD.

Now, this is strictly for Mitsubishi’s SS II:

So back to your scenario; the 4H is activated, the torque split is 33/67 (front/rear). Centre differential distributes torque to all wheels and lets them turn at different speeds while cornering. When excessive wheelspin occurs on one of the axles, viscous coupling kicks in and acting in conjunction with the center differential increases the amount of torque to the axle that has relatively more traction.

Yes, you’re right; the VCU, in this case, is acting like an LSD, however, the downside is that a VCU engages relatively slowly as compared to a good Torsen type unit which is reflected in the different driving characteristics of the vehicles equipped with each type under similar (treacherous) conditions.

Please note here that on account of center differential/VCU action the torque is being split between the front & rear axles . Now I also must mention here that both your front and rear differentials are still open though the rear differential is the Torsen type so that helps quite a bit.

I call the LSD, a poor man’s differential lock (not to be confused with the center differential) that would suffice in reasonably treacherous conditions but for hard core mud plugging, nothing to beat the lockers on the rear axle or better still on the front as well (À la Jeep Wrangler Rubicon).

Hope I have not confused you even more .

Last edited by Vikram Arya : 9th May 2018 at 06:48. Reason: Typo correction
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Old 12th May 2018, 23:26   #127
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Default Re: Center Differential and LSD in SUVs

Here is a small video by renowned overlanding/offroading expert Andrew St. Pierre White explaining how the full-time 4WD system works in a Land Rover Defender.


(Skip to 1min and 19secs to see the demonstration)


In the video, the Defender is running an open centre diff which can get the vehicle stuck even if it has only 1 wheel in the air.

This seems to be a typical example of a full-time 4WD without any form of LS mechanism in the center or the rear diffs. But, can anyone clarify if the Indian spec Toyota Fortuner (First gen: 2009-11) has an LSD? If yes, is it on the center or the rear differential?
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Old 14th May 2018, 03:11   #128
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Hi Sharc_biker

Allow me to clarify your concerns with my limited understanding of the subject. I am open to being corrected by more experienced off roaders.

I have a Toyota Fortuner 2015 model which has Torsen type Centre Differential. The benefits of having a torsen type differential is that it allows relative difference in rotational speed of the front and rear differentials whilst transmitting power to all the 4 wheels. This benefits the driver when one is driving fast on highways during monsoon when the roads are wet and you benefit from the additional traction.

The situation described in the youtube video will occur during off roading situations and in such cases one can be proactive and lock the center differential to prevent such situations.

In most other situations, where the vehicle has momentum, the torsen equipped center differential will transmit power to all the wheels even if one of them is slipping more than others. The way I see it, the need to lock the center differential will occur in real off road situations where the driving speed is rather slow and chances of one or more wheel losing traction is high. For most other normal driving situations, the torsen diff will work fine.

Finally, the Fortuner is also equipped with traction control which will also brake the spinning wheel and transfer power to the other wheels.

Trust this helps.

Cheers
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