Team-BHP - Driving all four wheels: how is it done?

Quote:

Originally Posted by gragusha (Post 3539702)

Well this happened to the X5 and ultimately had to be pulled out by a tractor. I guess the only thing that worked under these conditions in the LSD was the "S" part!

(How I got into this situation is somewhat stupid and embarrassing!)

I was checking online if the X5 has LSD but what I found out is, X5 doesnt have LSD but uses automatic differential braking (xDrive), which sends power to the tires that have grip by individually braking the tires that don't. But it looks like they are not responsive enough in real life situation?

Quote:

Originally Posted by gragusha (Post 3539702)

2. Rear Diff is locked

This never happens in any vehicle when you merely engage 4WD.

Quote:

Originally Posted by gragusha (Post 3539702)

1. I understand you correctly then a Scorpio just transfers the power to all the wheels. Since there is no central differential, it works like a central diff. lock?

Correct.

Quote:

Originally Posted by gragusha (Post 3539702)

2. Apart from the Jeeps, Thar etc, is there any SUV in India (in the 10-30L range) that provides rear differential lock capability?

Jeep/Gyspy/Thar don't provide rear differential lock either. However, companies like Lamda, ARB, etc., make diff locks that can be added aftermarket on Jeeps and Gypsies and Thar. Only Gurkha comes with front and rear diff locks that can be engaged manually, it doesn't engage when you switch to 4WD.

None of the below 40L SUVs have lockable diffs, not to my knowledge. As usual, aftermarket locks are available for most models.

Quote:

Originally Posted by gragusha (Post 3539702)

Sorry, if I sound stupid - I don's have deep knowledge or experience of this stuff.

Don't worry, none of us have deep knowledge, it is all relative. That is why we discuss and share.

Quote:

Originally Posted by gragusha (Post 3539702)

Well this happened to the X5 and ultimately had to be pulled out by a tractor. I guess the only thing that worked under these conditions in the LSD was the "S" part!

It is hard to comment without seeing in person.

There are some brilliant 4WD experts here and I have been following their discussions closely from some time, lots to learn, need to brush up, this is what I understood -

Quote:

Originally Posted by gragusha (Post 3539702)

1. Power is transferred to all four wheels

Just my take, I think that depends on whether the car has a center diff, then it gets equal distribution between front and rear.
With constantly locked drives like Mahindra which constantly power front and rear, in such if one of the wheels slip then again works like center differential where the other axle gets equal power to pull out.
I can stand corrected here and I would love to learn.

With the center diff too, if one wheel on each axle slips, then you are stuck again.
Here is where locked differentials or Limited Slip Diff's help, so one on each axle and power can be transferred to the other wheel, with LSD's too there are several types which like Torson, clutch type and viscous and some electronic aided types.
With permanently locked diff's you'd need free wheeling hubs else the mechanicals can get damaged due to stress.

Safari has clutch pack type rear LSD so the faster the wheel slips the more power is transferred to the other wheel, but this has power losses.
Endeavor has Torson which is a gear multiplier so the wheel with grip gets X times torque lost to the slipping wheel, unless it is free wheeling.

Pajero I guess has viscous, so the faster fluid churns, it passes power to the other side, but again has power losses.

Indian Fortuner which a lot a lay (not referring to BHPians) people think to be the ultimate 4x4 on roads, only gets a center diff, torson type. So two wheels on each axle slip and you are stuck again.
Depending on the surface and situation a Safari or even a Yeti may pass by. Of course Fortuner has brutal power to pull it out of most.

Most 4x4's and AWD's don't get rear LSD's.

This is an interesting topic, I love every morsel of information here.

Quote:

Originally Posted by s_pphilip (Post 3541114)

There are some inconsistencies in these statements.

First let us take on open center differential. It has only one purpose, to allow 4WD to be used on tarmac or hard surface. When center diff is open, even if one wheel loses traction, the vehicle will get stuck.

Let me give an example. Imagine a SUV with open center diff slowly climbing an inclining hard surface using 100Nm torque. Each wheel will be getting 25Nm. Suddenly, one wheel encounters slush, and requires only 1Nm to keep spinning. That means even the opposite wheel will get just 1Nm. That is because open diff always sends same torque to each side. That means front prop shaft is only sending 1+1 = 2Nm to the front axle. But center diff is also open, that means even the rear prop shaft will get just 2Nm, which in turn sends 1Nm to each wheel of the rear wheel. With only 4Nm total available at the wheels, the vehicle is stuck.

That is it for now, if this is understood, I'll explain further in later posts.

A clarification : taking you guys back a few posts

Quote:

Originally Posted by gragusha (Post 3539613)

First my assumptions:
1. Scorpio 4WD has two differentials and in the 4WD mode, the rear differential gets locked.

Three differentials, if am right. But no lock

Quote:

Originally Posted by gragusha (Post 3539702)

Based on your post I had two questions:
1. I understand you correctly then a Scorpio just transfers the power to all the wheels. Since there is no central differential, it works like a central diff. lock?

Quote:

Originally Posted by Samurai (Post 3540486)

Correct.

Doesn't all 4WD need to have a central differential? I think when you said 'correct', you were talking about the central differential lock, which is absent in Scorpio, right?

Quote:

I guess the only thing that worked under these conditions in the LSD was the "S" part!

That was very funny!!

Quote:

Originally Posted by mallumowgli (Post 3541512)

A clarification : taking you guys back a few posts
Three differentials, if am right. But no lock
Doesn't all 4WD need to have a central differential? I think when you said 'correct', you were talking about the central differential lock, which is absent in Scorpio, right?

!

Part time 4WDs have no center diff. That is why they are called part time. If you have center diff, you can use 4WD all the time(like AWD). However scorpio 4x4 should be used on tracks which allow slip(gravel sand snow etc.,).

So when you engage 4WD in scorpio, it sends power to both the front and rear diff. I.e. if the gearbox output is spinning at X RPM
The front shaft will spin at same RPM, and ditto for rear shaft.

So if you are turning, this will cause tires to slip. On tarmac and high friction surfaces, this can cause damage.

All time 4WD with center diff have a differential at center which allow front and back shafts to spin at different speeds. So even with 4WD engaged, there is no risk of transmission windup.

Quote:

Originally Posted by Samurai (Post 3540486)

Thanks Samurai.

First my understanding: With all-time 4WDs where central differential can be locked, (available only on a few all time 4WD SUVs like XUV 500 AWD, Fortuner, Pazero Sports etc) you can lock the power to front and rear axles. These will get stuck if one front and one rear wheel has low traction. Even if the car has some LSD etc (or something equivalent) kind of mechanism in front and rear differentials (apart from central diff lock), if the need for torque is not high then you can possibly get out, else you will probably still get stuck.

In part time 4WDs (Scorpio etc) most of them again don't have rear differential (or front) lock installed - similar situation will occur.

1. I would like to know if there are any other part time/full time 4WD cars (apart from Gurkha) where the rear differential can be manually locked without any modification from factory build.

2. In OTRs where you are going over slush and uneven boulders, very often possibly only one tyre will have traction, or one on each axle - these will need rear diff lock (and possibly front as well)?

3. Just to clarify my understanding further, even in a pretty “normal” situation where you are on bad roads (say Kaza route) with loose dirt/gravel and one of your front tyres goes into a deep ditch, weight will shift to that tyre and the other front tyre will not get traction. In this situation, with uneven weight shift possibly one of the rear tyres will get most of the weight and the other will lose traction on loose dirt. So, technically in a supposedly secure AWD (specially without a central differential lock) you can get stuck in such an innocuous looking situation? I have not seen this happen, but am trying to understand the capabilities of various setups and possible solutions.

Thanks again!

Quote:

Originally Posted by mallumowgli (Post 3541550)

Oh ok. I meant rear or front

Then you are right.

Quote:

Originally Posted by gragusha (Post 3541552)

1. I would like to know if there are any other part time/full time 4WD cars (apart from Gurkha) where the rear differential can be manually locked without any modification from factory build.

None below 40L.

Quote:

Originally Posted by gragusha (Post 3541552)

2. In OTRs where you are going over slush and uneven boulders, very often possibly only one tyre will have traction, or one on each axle - these will need rear diff lock (and possibly front as well)?

Yes.

Quote:

Originally Posted by gragusha (Post 3541552)

In this situation, with uneven weight shift possibly one of the rear tyres will get most of the weight and the other will lose traction on loose dirt. So, technically in a supposedly secure AWD (specially without a central differential lock) you can get stuck in such an innocuous looking situation? I have not seen this happen, but am trying to understand the capabilities of various setups and possible solutions.

Yes, you can get stuck then, unless you had some momentum to push you through.

Quote:

Originally Posted by mallumowgli (Post 3541714)

So, when you take a Scorpio and the to-be-launched Duster, the Duster may have a slight edge, because of the central diff lock?

Locked center diff is same as not having center diff. BTW, I don't think Duster has a center diff. The lock in Duster is nothing but engaging 4WD manually.

Coming back to center diff, in my earlier post in the morning, I have clearly mentioned it. The only purpose of center diff is allow 4WD driving on tarmac or hard surface. Under offroad conditions, it gives no advantage over 4WD vehicles without center diff.