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Old 16th August 2013, 04:36   #166
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Default Re: Driving all four wheels: how is it done?

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Originally Posted by SS-Traveller View Post

On-demand or automatic 4WD is a full-time system that lets the vehicle operate in 2WD (either front or rear) until the car's system judges that 4WD or AWD is needed. It then automatically routes power to all four wheels, varying the ratio between front and rear axles as necessary. Usually a slipping wheel activates the system, however, some of the more sophisticated systems use software that switches the system to 4WD or AWD during specific driving conditions -- BEFORE a wheel begins to slip.

On-demand 4WD vehicles are not recommended for serious off-road driving. Although all four wheels can be powered at all times if so required, neither can the driver lock a centre differential, nor is there a low-ratio set of gears available to take on those steep slopes.
Forgive my ignorance, but my understanding of 4x4 is pathetic and fortunately I came across this thread and being interested in 4x4’s, this is one of my favorite threads for the insights it provides.
I am interested in road biased AWD set up’s or Real time 4x4 as I would call it and for ease of understanding will go by basic AWD’s with only TCS to limit other mechanical/electronic influences.

Considering a scenario of AWD on slippery low grip surface like going uphill on loose wet moss covered gravel and no so deep slush in heavy rain, (I had tense moments in regular 2WD’s in these situations).

For instance can someone explain how it works for the earlier Hyundai Tucson sold in India (2006 model) which I believe has a very basic AWD set up with limited off-road mechanical and electrical assistance?
Since it does have a TCS and AWD, how does the TCS help it?

If TCS applies brakes and going by the common set up for AWD’s/road vehicles having open differential, if there is no traction, the wheels slip and TCS will momentarily apply brakes to the slipping wheel,
which will build up resistance,
SO will it transfer torque to the opposite wheel
OR all the remaining wheels
OR diagonally opposite wheel?

Also considering open differentials for an AWD (something like Tucson) is the transfer of torque to the non-slipping wheel initiated electronically as open differentials follow the path of least resistance hereby increasing slip and TCS will do the opposite, limit the wheel slip.

Also since the most normal set ups are front biased and one of the front wheels is slipping, does it automatically transmit power to the rear wheels?

And if one of the rear wheels too is slipping, being open differentials I should assume one would be stuck, however does TCS help here by building resistance by braking.

Does the Tucson have a center clutch type limited slip differential? I don’t think it has LSD’s in the front & rear.

My understanding of TCS is it takes feedback from the ABS unit and applies individual braking. Since the Tucson’s unit is reactive and not proactive once it senses slip it will thereafter automatically limit wheel slip,
how then will a LSD work?

So for the AWD to work should TCS be off? I believe that must be why there is a TCS on/off switch on the dash of the Tucson.

I have a similar doubt about TCS on turns as all the wheels will have different speeds, so does TCS offer a window of difference between all the wheels?
If I understand correctly TCS does not judge traction, it only judges difference in speeds, hence traction control cannot help build up traction.

I am surprised as to why such an interesting thread has stalled, so many 4x4 addicts and I believe there should be a wealth of information to share here. Parallelly I will do some research and if I get some answers will post here.

I appreciate one and all for shedding light on such a deep but easily misunderstood topic.

Last edited by s_pphilip : 16th August 2013 at 04:38.
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Old 16th August 2013, 12:12   #167
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Default Re: Driving all four wheels: how is it done?

That's quite a detailed query! A lot of your questions can be answered by this page (http://www.thetruthaboutcars.com/201...-differential/).
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...can someone explain how it works for the earlier Hyundai Tucson sold in India (2006 model) which I believe has a very basic AWD set up with limited off-road mechanical and electrical assistance?
The 4wd system used in the Tucson is essentially similar to that (among vehicles sold in India) of the Sante Fe, XUV500, X-Trail or CR-V - all of which owe their roots to the Fiat Panda 4wd with transverse engine driving the front wheels, with drive transfer to the rear wheels through a clutch pack controlled by some mechanical or electrical/electronic mechanism, as and when required.
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Since it does have a TCS and AWD, how does the TCS help it?
TCS (traction control system) will help any car (even 2wd / FWD / RWD) to find grip when accelerating on a slippery / loose surface, not just a 4wd / AWD...


Quote:
...is the transfer of torque to the non-slipping wheel initiated electronically as open differentials follow the path of least resistance hereby increasing slip and TCS will do the opposite, limit the wheel slip.
TCS is a purely electronic system using sensors on all wheels.
Quote:
Also since the most normal set ups are front biased and one of the front wheels is slipping, does it automatically transmit power to the rear wheels?
Wheel slip (loss of traction) on front wheels will activate TCS, as well as activate the clutch pack to transmit torque to rear wheels.
Quote:
And if one of the rear wheels too is slipping, being open differentials I should assume one would be stuck, however does TCS help here by building resistance by braking.
TCS is said to work better than LSD is some instances.
Quote:
Does the Tucson have a center clutch type limited slip differential? I don’t think it has LSD’s in the front & rear.
The Tucson, AFAIK, does not have a centre diff at all, never mind with or without LSD. The clutch pack transmitting power to the rear wheels is allowed to slip and prevent transmission wind-up.
Quote:
I believe that must be why there is a TCS on/off switch on the dash of the Tucson.
...traction control cannot help build up traction.
This should answer your question: http://blog.nwautos.com/2010/10/why_...n_control.html , http://www.ehow.com/how_5272801_disa...n-control.html
and

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Old 16th August 2013, 12:17   #168
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I appreciate one and all for shedding light on such a deep but easily misunderstood topic.
To summarise from the beginning, for basic 4x4 running gear:
A 2wd car needs some kind of device to allow for the driven wheels to rotate at different speeds, as they need to while the car is turning. Because the inner wheel has to turn less than the outer wheel while the turn is being made. This device, in mechanical form, is the differential. Were it not for this device, on very slippery/slushy roads, the system will cope by having the outer wheel spin to accommodate the extra revs it must make. But on surfaces where the tyre adhesion to the road surface is strong, the transmission will not cope, and will, literally, break up.

A 4wd car supplies power to both axles, both of which need a differential for the same reason as in a 2wd car. This is how the low end 4x4 cars are built. The Gypsy - and even the Safari. Works very well on slushy roads. But when turning, the two axles are also rotating at different speeds. Because the front wheels together describe a circle of larger diameter than the rear wheels. These different axle speeds can be managed without the transmission breaking, because before the stress builds up on it to a level that can come close to breaking it, the wheels and therefore the axles are able to have different rpms from each other by easily sliding in the slush to the extent necessary to achieve the rpm differential. If you engage 4wd on such cars on roads where the tyre/road surface adhesion is high - as on good tarmac - in a short time, the pressure of the need of the tyres/axles to rotate at different speeds, but unable to do so, will be sent back to the transmission, which will then break.

Once you are sure you understand this fundamental part well, you can go on to discovering what a center differential is. Lots of information on the net!

In turn, I have a question of people that have user experience of modern, electronics loaded 4x4 systems.
In a mechanically engineered 4x4, with a center differential, even when it is in locked mode, if just one tyre is resting on a very slippery surface - hard ice for example - the car will be rendered immobile. All the power will go to that tyre since it is the one that offers the least resistance, and that wheel/tyre will spin furiously on the ice driven by all the engine power, with no forward motion of the car. The only mechanical way around this is to lock the axle differentials as well - as the Force Gurkha allows you to do. But it is a rare car that comes with axle differential locks.
My question - is this well known Achilles heel of 4x4 cars completely eliminated by the current electronically controlled 4x4 systems?

Last edited by Eddy : 16th August 2013 at 17:08. Reason: Merged
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Old 16th August 2013, 17:06   #169
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All the power will go to that tyre since it is the one that offers the least resistance, and that wheel/tyre will spin furiously on the ice driven by all the engine power, with no forward motion of the car.
If the centre diff is locked, power will be transmitted to the other axle too.

If the diff on that axle has a clutch pack type of limited slip diff, power will be transmitted to the wheel on the other side too.

Doesn't always necessitate electronics.
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Old 16th August 2013, 17:43   #170
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If the centre diff is locked, power will be transmitted to the other axle too.

If the diff on that axle has a clutch pack type of limited slip diff, power will be transmitted to the wheel on the other side too.
Yes, but what if there isn't a limited slip on that axle - it isn't all that common. Fortuner doesn't have any limited slip diff. The Pajero SFX has it, but not on the front axle. What if any of the tyres of the Fortuner is on the ice? What about the Gypsy, for that matter?

But my question was to the electronic gizmo laden cars, is this issue addressed in them?
PS: Ignore the first part, I figured out what you meant in the case of no centre diff or lockable center diff cars.
The latter question stands.

Last edited by Sawyer : 16th August 2013 at 18:02. Reason: PS
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Old 16th August 2013, 20:26   #171
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Endeavour: Part-time 4x4 with Center Transfer case, Torsen LSD in the rear - Meaning that you would need to lose traction in 3 wheels to come to a halt. Even if one rear wheel is in the air you can use the throttle-brake technique to get some power to the rear wheel that has traction to get out.

Fortuner: Lockable center Torsen differential that will just act like a Transfer case once locked, and also the India version of the Fortuner has No LSD in the rear. Meaning that once the center diff is locked, you would need to lose traction in only 2 wheels to come to a halt.

If all the above is true then shouldn't the Endeavour possess the best off road hardware?

- Vishnu
Wow, thank you SS-Traveller for this excellent thread and sharing information to clarify my doubts, you keep me hooked.
I didn’t get a chance to research and review the videos due to time constraints, but I plan to dedicate time on Sat/Sun to closely go through them.

But my original question is more on the lines of what Sawyer has asked,
I want to understand how TCS helps an AWD in different situations and I used the Tucson as an example as that is the kind I want to understand,
A real time 4x4 which steps in when it senses “wheel slip”, which of course will be limited by TCS having an opposite effect,

So since there isn’t an LSD which comes into play due to the speed difference in wheels,
I assume the AWD is not initiated mechanically but electronically, that is 'sensors' trigger the transfer of torque to the other wheels.

So I am still not clear by this statement of yours,
"If the diff on that axle has a clutch pack type of limited slip diff, power will be transmitted to the wheel on the other side too."

How will it happen as TCS will cancel the speed difference.
Though I accept, I am yet to review those videos which I will do tomorrow, maybe I shall get a better understanding and I may be jumping the gun now.

Also in the quoted post for an Endeavour you would need to loose traction - for 3 wheels, and for the Indian Fortuner - for 2 wheels, to be stuck in the slush scenario in my earlier post.

Won’t the Tucson do better in such scenario’s as electronic interference's negates slip and automatically transfers power to all 4 wheels.

So as Sawyer asks "My question - is this well known Achilles heel of 4x4 cars completely eliminated by the current electronically controlled 4x4 systems?"

I want a basic simple set-up explained of an AWD with only TCS (Tuscon kind) and avoid getting into details/electronics of expensive cars like Audi's as they will be better but more complex with sensors analyzing every parameter.

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Old 17th August 2013, 11:27   #172
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But my question was to the electronic gizmo laden cars, is this issue addressed in them?
Not adequately experienced in driving gizmo-laden 4wd's off-road, so cannot really comment. From what I understand of the Freelander2 / BMW X5/6, these don't use 4L gearing. There are loads of electronics though, and it is said that with loss of traction on 3 wheels (such as putting them on rollers), the electronics can still get the car moving with the help of any one wheel. How that behaves in the real world, I have no exposure to.
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Originally Posted by s_pphilip View Post
But my original question is more on the lines of what Sawyer has asked,
I want to understand how TCS helps an AWD in different situations and I used the Tucson as an example as that is the kind I want to understand,
A real time 4x4 which steps in when it senses “wheel slip”, which of course will be limited by TCS having an opposite effect,

So since there isn’t an LSD which comes into play due to the speed difference in wheels,
I assume the AWD is not initiated mechanically but electronically, that is 'sensors' trigger the transfer of torque to the other wheels.
I don't understand your question. Why would TCS have an opposite effect to 4x4 engagement?
Quote:
So I am still not clear by this statement of yours,
"If the diff on that axle has a clutch pack type of limited slip diff, power will be transmitted to the wheel on the other side too."
You need to know how a clutch-pack type LSD works vs. how a Torsen LSD works. For a given axle with one wheel on (very free running) rollers, the other wheel can pull out the car if the diff is equipped with a diff lock or a clutch type LSD - but not with an unlocked Torsen. That is the limitation of Torsen diffs.
Quote:
Won’t the Tucson do better in such scenario’s as electronic interference's negates slip and automatically transfers power to all 4 wheels.
On snow or ice, the Tucson can perhaps outperform an Endeavour in finding a little bit of grip and moving out. On a steep slope with moderate amounts of grip, an Endy in 4L will climb the slope without burning clutch whereas the Tucson's gearing and engine power will limit its abilities. Something like a XC90 with a 4.4L V8 climbs slopes in 2nd gear (it doesn't have 4L), or a Touareg V10 can handle without engaging 4L, that ordinary cars like Gypsies and Thars will struggle to negotiate in 4L in 2nd gear.
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I want a basic simple set-up explained of an AWD with only TCS (Tuscon kind) and avoid getting into details/electronics of expensive cars like Audi's as they will be better but more complex with sensors analyzing every parameter.
The Tucson's setup is not as simple as you think. There's plenty of use of sensors to keep those wheels turning. For example, at 40 km/h, the Tucson will disengage 4x4 based on sensor inputs, to prevent transmission damage and clutch pack overheating & premature wear.

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Old 17th August 2013, 17:39   #173
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it is said that with loss of traction on 3 wheels (such as putting them on rollers), the electronics can still get the car moving with the help of any one wheel. How that behaves in the real world, I have no exposure to.
That was the answer I was looking for. Maybe someone here has the real world experience as well and will chip in.
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Old 17th August 2013, 18:20   #174
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I don't understand your question. Why would TCS have an opposite effect to 4x4 engagement?
You need to know how a clutch-pack type LSD works vs. how a Torsen LSD works.
Thanks SS-Traveller, that I believe should be my main area of research today as that's where my confusion comes from, as I type this I have typed in "clutch-pack type LSD" in Google and will try to figure out, I may reach you if I have trouble understanding :-). Thanks again.

Currently this is what I understand from a traditional LSD set-up, for a LSD to work there should be difference in wheel speeds (one spins wildly faster), only then the LSD will engage, but in the first place won't the TCS kick in earlier and quicker to negate the wheel spin and thereby negate the speed difference by applying brakes to the faster wheel, thereby not allowing the speed difference to let the LSD come into play.

Maybe a clutch-pack type LSD is different and I have got something mixed up or maybe here it does not require wheel spins to activate mechanically but has electronic sensors to trigger the transfer of power once it detects wheel spins.

I am also going through those videos you shared earlier. Thanks.

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Old 17th August 2013, 19:05   #175
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...won't the TCS kick in earlier and quicker to negate the wheel spin and thereby negate the speed difference by applying brakes to the faster wheel, thereby not allowing the speed difference to let the LSD come into play.
The query above is not the same as...
Quote:
Originally Posted by s_pphilip View Post
A real time 4x4 which steps in when it senses “wheel slip”, which of course will be limited by TCS having an opposite effect,
So since there isn’t an LSD which comes into play due to the speed difference in wheels, I assume the AWD is not initiated mechanically but electronically, that is 'sensors' trigger the transfer of torque to the other wheels.
Between wheels on the same axle:
TCS makes use of LSD practically redundant. I cannot offhand point to any car that has TCS as well as LSD.

Between front and rear axles:
There can be complete loss of ground contact or complete loss of traction on one axle - and TCS or LSD cannot do anything there. Both wheels on that axle spin free. The 'on-demand' 4x4 system (such as in the Tucson) then engages power/torque to the other axle, to get the car moving again. TCS working on that other axle can help in such a case., whereby just the one wheel with traction pulls out the car onto firmer ground.

Hope this clarifies things.

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Old 17th August 2013, 19:17   #176
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The query above is not the same as...

Between wheels on the same axle:
TCS makes use of LSD practically redundant. I cannot offhand point to any car that has TCS as well as LSD.
Well I meant to ask how the TCS helps a 4x4 right from begining, maybe I wasn't able to word my doubt well. Sorry about it, but in that process I got more questions answered.

if I am not wrong, I believe lot many SUV's have TCS along with on-demand 4x4, and since they separately work on opposite parameters was wondering how they are supposed to work together, maybe that's why there is a TCS on/off switch.

I assumed that maybe for such (eg. Tucson, I believe it has TCS) the power transfer to rear axle/ other wheels is electronically triggered.
I take Tucson as an example as I think it does not have other electronic aids apart from TCS and on-demand 4x4, most others now have lots of electronic features.
If not for challenging terrain like climbing hills/boulders, I realize AWD - softroaders have reasonable capabilities, way back I used to feel AWD cars are only a marketing gimmick not providing a real difference from a normal car as it does not posses real 4x4 qualities, but 'you tube' videos and recent reading (I got stuck in slush in a Qualis last monsoons on a far-away internal road, since then the eye-opener) proves otherwise which has got me interested, I would like to know more, I was in the sedan camp till then.

Elsewhere while researching now, I read that Tucson is always at a constant 90:10 power transfer ratio, so the rear gets a constant 10% of the power irrespective and could go up to 50% in adverse conditions.

As you said earlier I noted after watching 'you tube' videos that TCS helps best in snow/ice, that's another topic I want to understand 'how' and I shall do a bit more research before asking this question, I watched a video now where a test driver wasn't able to climb an icy slope with TCS off, but could easily climb the slope with TCS on for a FWD car.

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Old 17th August 2013, 22:14   #177
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That was the answer I was looking for. Maybe someone here has the real world experience as well and will chip in.
There are different levels of AWD systems. The better ones are capable of transferring more torque to wheels that have grip by braking the slipping wheels. The best ones are Landrover, Subaru, jeep, Toyota. Other systems transfer very little torque to wheels with grip as they rely on viscous coupling LSDs to transfer torque from slipping wheels. Examples of vehicles with these are the skoda yeti and Honda CRV.

Here's a video in YouTube comparing the different AWD systems:

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Old 18th August 2013, 07:27   #178
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Here's a video in YouTube comparing the different AWD systems:
Brilliant examples. Back in 2004, I chose a CRV over the Forester, and have always regretted that decision. The Forester would have been a keeper. A car with 4x4 that was useful in all conditions, from highways to tricky conditions, where the all wheel drive was a real handling aid at all times.
The CRV was always in front wheel drive mode on roads, so the 4x4 was no aid to handling in the ghats, for example. The one time I saw it in slush, it was a sight to see. Engine revving frantically, and the car was lurching along, and crabbing sideways too. Every time the front wheels started spinning, the rear wheels took over, but the instant they did and the front wheels stopped spinning, all the power went back to the front, and the front wheels promptly started spinning again, restarting the cycle.
Just one thing in the video that seems wrong/incomplete - the Toyota example of full time 4wd failing the test. If the center differential had been locked, I see no logical reason why it would not have succeeded. But I noticed at the end that the video is a Subaru promo, so that might be the reason.
It would have been interesting to see how an Audi Quattro would have fared in the test.

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Old 20th August 2013, 00:49   #179
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Brilliant examples. Back in 2004, I chose a CRV over the Forester, and have always regretted that decision. The Forester would have been a keeper. A car with 4x4 that was useful in all conditions, from highways to tricky conditions, where the all wheel drive was a real handling aid at all times.
The CRV was always in front wheel drive mode on roads, so the 4x4 was no aid to handling in the ghats, for example. The one time I saw it in slush, it was a sight to see. Engine revving frantically, and the car was lurching along, and crabbing sideways too. Every time the front wheels started spinning, the rear wheels took over, but the instant they did and the front wheels stopped spinning, all the power went back to the front, and the front wheels promptly started spinning again, restarting the cycle.
Wow, great example, I was trying to judge competence of such real time systems from pure imagination and know how and you gave a perfect example of the confusion easily created in my mind, my research is to try and understand the characteristics before hand.

I realized that the Tucson has a ESC and on demand 4x4.
So does that mean CRV is less competent than a Tucson.

This is what our good friend wiki says about TCS:

"When the traction control computer (often incorporated into another control unit, like the anti-lock braking system module) detects one or more driven wheels spinning significantly faster than another, it invokes the ABS electronic control unit to apply brake friction to wheels spinning with lessened traction. Braking action on slipping wheel(s) will cause power transfer to wheel axle(s) with traction due to the mechanical action within a differential.
All-wheel drive AWD vehicles often have an electronically controlled coupling system in the transfer case or transaxle engaged (active part-time AWD), or locked-up tighter (in a true full-time set up driving all wheels with some power all the time) to supply non-slipping wheels with (more) torque.

This often occurs in conjunction with the powertrain computer reducing available engine torque by electronically limiting throttle application and/or fuel delivery, retarding ignition spark, completely shutting down engine cylinders, and a number of other methods, depending on the vehicle and how much technology is used to control the engine and transmission.
There are instances when traction control is undesirable, such as trying to get a vehicle unstuck in snow or mud. Allowing one wheel to spin can propel a vehicle forward enough to get it unstuck, whereas both wheels applying a limited amount of power can't get the same effect. Many vehicles have a traction control shut off switch for just such circumstances."

And about ESC:
"ESC is built on top of an anti-lock brake (ABS) system, and all ESC-equipped vehicles are fitted with traction control."

And on the BorgWarner drivetrain:
"Multi-plate clutch coupling. The ITM 3e multi-plate clutch coupling is used in the Hyundai Santa Fe, Hyundai Tucson and Porsche 911. In normal conditions, it sends up to 95% of the torque to the front wheels. In the case of the Hyundais, the system can be locked in a 50:50 split so that you have a more dedicated off-road system. The system can be locked with the '4WD LOCK' button. The ITM 3e uses a multi-plate clutch coupling with magnetic activation."

So when stuck ESC or TCS may not necessarily help. In the process of realization I got some parallel question answered.

Also on Limited Slip Differential (LSD) - CLUTCH type, some explanation that may help:

http://mgaguru.com/mgtech/rearaxle/ra302a.htm

http://auto.howstuffworks.com/differential4.htm

and on Torsens

http://auto.howstuffworks.com/differential6.htm

http://mgaguru.com/mgtech/rearaxle/ra302c.htm

Also what is an "Electronic limited-slip differential", wiki explains electronic limited-slip differential will typically have a planetary or bevel gear set similar to that of an open differential and a clutch pack similar to that in a torque sensitive or gerotor pump based differential. In the electronic unit the clamping force on the clutch is controlled externally by a computer or other controller.

Any vehicle in India that uses this system?

my research is still on. Will keep updating, thanks again for this great thread. Its learning all the way.Cheers!

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Old 20th August 2013, 06:56   #180
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Originally Posted by s_pphilip View Post
I realized that the Tucson has a ESC and on demand 4x4.
So does that mean CRV is less competent than a Tucson.
I don't know or remember the Tucson, but it will probably behave in the same way as the CRV in slush. Except that I have a dim memory that it has a lock mode, if that is the case, it should do better.

For normal road driving on twisty tarmac, the 4wd thing will be just as useless as the CRV - this is based on deduction, I have never driven one.

Also, the latest generation CRVs seem to have an improved 4wd, but how much of that works or is just a marketable gimmick, I can't say.
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