[adisag]

Originally Posted by lal79

Consider a scenario, one wheel from both front and rear axle doesn't have traction. Power will flow to the wheels with least traction. In this scenario, one wheel from front and rear axle doesn't have traction, and those wheels only will spin and the wheels having traction won't spin. Now you have one wheel each from both front and rear axle spinning freely and you are stuck. This will happen even for a 4X4 with Open differential(regardless of whether there is a low ratio transfer case).

I managed to get my Yeti stuck in a unlikely position a few days back, the front right tire went into a ditch, and the weight of the car was directly on the chassis. The rear left wheel was airborne due to the weight distribution.

Basically, only two diagonally opposite wheels (front left and rear right) had grip, the other two were incapacitated. Given that my Yeti is a 4wd, what should have happened? Shouldn't the two wheels with grip been able to pull the car out of the ditch?
What ended up happening was that the two wheels in the air were spinning, but there was no torque going to the two wheels with grip. :O The car was stuck. Another thing I noticed was that every time the RPM would hit ~2000 RPM, the power would cut-off for a few seconds.
Finally got the car out by getting a friend to put some weight on the rear left side. The moment that wheel touched the ground, the car jumped out (in reverse gear) with utmost ease.
The whole scene from getting into the ditch to getting out took maybe 5 minutes, so I did'nt have the presence (or the time) to document the situation for better understanding. The next morning, I got the car onto a car lift to double check the 4wd works, all 4 wheels spin on acceleration, so I guess everything is in order (?)

The Yeti in question is a 2010 Ambiente 4x4 (No Off Road Button), I did try this with the traction control on as well as off, similar results in both conditions, but since we were out in less than 5 minutes, I may have missed some critical details.

Thoughts would be appreciated - is this normal behavior?

[roy_libran]

Originally Posted by adisag

Basically, only two diagonally opposite wheels (front left and rear right) had grip, the other two were incapacitated. Given that my Yeti is a 4wd, what should have happened? Shouldn't the two wheels with grip been able to pull the car out of the ditch?

And this is exactly why Lockers are such a lovely thing. Either that, or insane Articulation, that is.
Yes, your Yeti is fine, if it behaved this way.

[.anshuman]

As Roy commented it is perfectly normal for the Yeti Ambiente with 2 x open axles to behave like this.

The situation would have been have handled better with Electronic Differential Lock equipped (part of ESP package) Elegance variant, though nowhere as effective as the manual lockers but good enough to divert some power back although in jerks by braking the free spinning wheels.

[gragusha]

Originally Posted by .anshuman

As Roy commented it is perfectly normal for the Yeti Ambiente with 2 x open axles to behave like this.

The situation would have been have handled better with Electronic Differential Lock equipped (part of ESP package) Elegance variant, though nowhere as effective as the manual lockers but good enough to divert some power back although in jerks by braking the free spinning wheels.

I was wondering how effective will it be to try driving through the brakes in this situation - basically press on the brake pedal and the accelerator at the same time. Will this provide torque to the wheel with traction?

[.anshuman]

Originally Posted by gragusha

I was wondering how effective will it be to try driving through the brakes in this situation - basically press on the brake pedal and the accelerator at the same time. Will this provide torque to the wheel with traction?

I have not suggested to use the brakes, the EDL function in the ESP package will do that for you automatically without any manual intervention.

[gragusha]

Originally Posted by .anshuman

I have not suggested to use the brakes, the EDL function in the ESP package will do that for you automatically without any manual intervention.

Hi - understand that - but I was thinking of a simpler system like a Fortuner - I understand that the rear diff is open in that. In this case what will be possible solution for getting out (apart from putting more weight or trying to stuff rocks etc under the free wheeling wheel... does applying brakes and pressing accelerator pedal at the same time help at all?

Also ESP may not work well in automatically applying brakes to the slipping wheels, as in the above example. In that situation, weight had to redistributed for giving traction to the slipping wheel.

Thanks for the help.

[dhanushs]

Originally Posted by gragusha

does applying brakes and pressing accelerator pedal at the same time help at all?

Yes, it'll help. The wheel with traction now, has torque needed to overcome the braking and get the free wheel spinning.

[Sheel]

In 4H at very low speeds and steering at full lock, does it provide some resistance? It gets sluggish, low, crawling speeds.

[Tejas@perioimpl]

Originally Posted by Sheel

In 4H at very low speeds and steering at full lock, does it provide some resistance? It gets sluggish, low, crawling speeds.

Yes, your steering would want to return to centre. On paved roads in 4H (locked), and while turning you are effectively dragging one wheel or hopping it (called chirping also).

[dhanushs]

Originally Posted by Sheel

In 4H at very low speeds and steering at full lock, does it provide some resistance? It gets sluggish, low, crawling speeds.

Yes. Drivetrain wind up is the reason. Technically, you shouldnt be driving on surfaces with good traction, in 4WD with center diff locked. If at all you are driving, hold the steering wheel freely. Dont push it.

[shashanka]

Originally Posted by Sutripta

This is a topic meant for experts. As a not foolish (I hope) layman, do you think I should rush in!? .....Lets start with the basics. Maybe the experts can answer a simpler question for us: -
If you were the vehicle designer, what would be your thought process which lead you to choose one of the drive configuration from all those (RWD, FWD, Full time 4x4, part time 4x4 .........., and options of LSDs and MLDs) available.
Regards
Sutripta

Sutripta, as another aspirant to the discussion, I'd say the primary control would be the cost of the sytem and the target customer.
Thereafter being given a free hand, I'd prefer the full-time AWD with the three Torsen diffs to take advantage of its inbuilt limited slip capabilities. Without the bother of external hydro-electrics(Haldex) or the speed sensitive mechanicals of the Eaton type.

The Torsen takes great advantage of the relative/partial irreversibility of the worm/wormwheel drive and incorporates it into its limited slip function. The one drawback of the Torsen is its relatively higher power absorption and of course its higher cost.

But as I mentioned earlier, the target customer is important, as witness the Quattro's almost total domination of the rally scene at the time of its introduction.

[shashanka]

Originally Posted by Samurai

I could be totally mistaken here. But I remember reading this long time back. A torque sensitive LSD won't work if one ........... free wheel under brakes may need 20% of available torque to keep spinning, there by transmitting 5X or 100% of available torque to the other side.

Spot on, Samurai - this is the reason for the name, Torsen. It doesn't really sense wheel spin or slip. The degree of torque on the spinning wheel is sensed and if the internal bias of the torsen diff. is 5X (depends on the pitch of the teeth of the meshing worm-to-worm internals), then five times the torque is sent to the stuck wheel - which may not be enough to get that wheel moving. Therefore the application of the brakes to multiply the available torque.
I wonder why this relatively simple method - EBD I mean - has not been incorporated with the function of the standard diff. as a cheaper (though less effective perhaps) way of getting around this whole wheel slip issue.

Surely, with the kind of software development expertise that is available in the country, it can't be an insurmountable problem to integrate the two functions - the differential and the EBD.
Leaving aside the issues of robustness, durability, reliability etc, surely it would put the benefits of limited slip performance - of a kind - within the reach of enthusiasts with limited means !

[Sutripta]

^^^
If you have brake control over individual wheels, do you need a LSD?

Regards
Sutripta

[roy_libran]

Originally Posted by Sutripta

^^^
If you have brake control over individual wheels, do you need a LSD?

Regards
Sutripta

I think the answer is No. But, usability would be adversely impacted, would it not?
Imagine the jugglery you'd need to do the moment you're unstuck!

[shashanka]

Originally Posted by roy_libran

I think the answer is No. But, usability would be adversely impacted, would it not?
Imagine the jugglery you'd need to do the moment you're unstuck!

Yes, absolutely - it would probably need a fair amount of orientation time (days perhaps) to familiarize oneself and start applying the brakes at the right time to avoid trouble.

An EBD with the necessary input bandwidth (to encompass commonly faced situations) and response time could provide a cheaper, simpler, tho' less competent, alternative (to the massive overkill of the full Quattro system) for the average motorist with average means - me, in other words !