[SS-Traveller]

Originally Posted by Sutripta

Good luck. You are a braver man than I.

Thanks! Let's find out how good or bad my fundamentals are!

Originally Posted by SS-Traveller

So what on earth is a Torsen limited slip differential??? I'll talk about it in the next post.

Forget Torsen for the moment - Torsen is not related to Sushmita Sen, and there's no reason that you should be so greatly interested in it. Let's first talk about limited slip differentials (LSD) in general, what they do, and their different types.

Once you've jacked up one of the driving wheels of your car, and then engage gear and start to drive, the wheel that is lifted off the ground does all the spinning - and the car goes nowhere. That's because your car has an "open" differential, and all the engine power is being transferred to the wheel with the least resistance. An LSD prevents just this from happening, i.e. if at least one of the wheels has traction, the car can move forward (or backward). (Of course, jack up both wheels, and both will spin freely even with LSD, and the car goes nowhere!)

Now LSDs are not standard fitment in all cars, because they're expensive, and in 99.99% of day-to-day driving, a car doesn't need them. But for vehicles venturing onto slippery surfaces frequently, or in very high-performance cars (especially when going round a corner), one of the driven wheels can lose grip. LSD then helps to transfer some of the power from the engine onto the other wheel, and keep the vehicle moving.

So how do LSDs work? There's the fun of it all! They're more complicated than your average FNG (or even A.S.S. technician) can figure out, and as a result they stay miles away from it. But broadly speaking, LSDs are of two types:

1. Torque sensitive - using different methods to work:
____A. An arrangement of gears which does not allow one wheel to spin too fast, before the other side gets its share of turning force;
____B. Clutch plates which are pressed against each other by the freely rotating wheel;
____C. Cones - a solid cone fitting inside another hollow one.

2. Speed sensitive - using these methods:
____A. A viscous coupling with a special "oil" in it, which becomes thicker and stickier when heated up by friction;
____B. A pump and clutch pack - electronics govern a pump, and once sensors tell the system that one wheel is spinning free, the pump is made to engage the clutch - this transfers power to the other side of the diff.

Now for Torsen. It's a patented design of torque sensitive LSD working through an arrangement of gears, manufactured by the Gleason Corporation. Torsen is a contraction of Torque-Sensing. TORSEN and TORSEN Traction are registered trademarks of JTEKT Torsen North America Inc (formerly Zexel Corporation, formerly Gleason Power Systems). There are broadly 3 types of Torsen LSDs (based on their design) and more details can be had from the company's website: Torsen General Frequently Asked Questions page.

[4x4addict]

To add my 2 cents:

In a Full T ime 4WD vehicle like the Toyota Fortuner/Land Cruiser/Mitsubishi Pajero the Center differential needs to maintain a 50:50 torque split between the front and rear axles. This can be done manually by engaging the center differential lock. When the manual lock is not engaged the differential need some way to maintain the torque transfer between the front and rear differentials.

For example let us take a Pajero and Fortuner where both rear wheels are slipping hopelessly in slush but the front wheels have traction. The center differential needs to sense that the rear wheels are slipping without any traction and transfer the power to the front differential. In automatic mode the Pajero uses a Cheaper Viscous Coupling Unit (VCU) and the Fortuner uses a more expensive and robust Torsen Limited Slip system to attain the required objective of providing power to the front differential. If either off these systems are not used, then a center differential pretty much makes a vehicle 2WD (either front or rear only) unless it is manually locked.

Before Pajero fans bite me, the reason I call the Fortuner system more robust is because it actually uses gears rather than a clutch pack to transfer power. Remember your clutch wears out but how often do you change the gears in your differentials?

If you like to read more about these to Center Diff Systems you can contact uncle Wiki:

Viscous coupling unit - Wikipedia, the free encyclopedia

Torsen - Wikipedia, the free encyclopedia

Please also read this article that gives the pros and cons of the VCU system with Torsen type system.

http://en.wikipedia.org/wiki/Limited_slip_differential

[SPIKE ARRESTOR]

Attaching a file which gives a brief on the various types of 4wd, first let us set the basics right and then we can take it further. Sutripta da, you there na?

Spike

[Samurai]

There are couple more threads that address the complex topic of LSD and differentials.

https://www.team-bhp.com/forum/4x4-t...-lsd-suvs.html (Center Differential and LSD in SUVs)
https://www.team-bhp.com/forum/4x4-t...ks-vs-lsd.html (Diff-Locks vs LSD)

[SS-Traveller]

Originally Posted by ex670c

I would like you to elaborate on the above statement.

Part Time 4WD and Full-Time 4WD in comparison to a FWD or RWD are definitely invincible in All no-road situations.

Sorry, Arka, I missed this post earlier. To elaborate: Part-time or full-time 4WD systems, especially those with a dual-range transfer case offering low-ratio drive, certainly are more invincible (but obviously not totally so) in no-road conditions. But all-wheel drive / automatic 4WD / on-demand 4WD systems without low-range gears are not as good at tackling no-road situations as part-time or full-time 4WD vehicles. When the not-so-discerning owner of a CR-V / Tucson / Aria learns that his car has AWD - which he equates with full-time 4WD - he dares to venture off-road, only to be sorry for it. (No doubt, though, on-demand 4WD would still do better than a 2WD of comparable specs.) Hope you'll agree with me on that.

Originally Posted by ex670c

The Transfer Case has been under development as long as the automobile itself.

Some automatic 4WD / AWD vehicles with the primary driven wheels being on the front axle, do not have a separate transfer case - I think the CR-V is a case in point. To quote from Spike's attached document:

In AWD systems based on front-wheel drive (the majority), the transfer case usually is done away with and the power is divided to the front and rear by the transaxle — a combination of the transmission and front-axle differential employed in most front-drive vehicles.

Originally Posted by SPIKE ARRESTOR

Attaching a file which gives a brief on the various types of 4wd, first let us set the basics right and then we can take it further. Sutripta da, you there na?
Spike

Thanks, Spike - you're a genius! At one stroke, with one document, you've actually said all that this thread was trying to convey! And Sutripta-da is the real mover-shaker behind this thread!

@Samurai: Thanks for moving this thread to the right section!

[Sutripta]

Originally Posted by SS-Traveller

Thanks! Let's find out how good or bad my fundamentals are!

Forget Torsen for the moment

Question is not your understanding, but your lucidity at explaining.
Great decision not to start with the Torsen.

Now LSDs are not standard fitment in all cars, because they're expensive, and
the clutch based designs wear out and need maintainence.
.

____B. A pump and clutch pack - electronics govern a pump, and once sensors tell the system that one wheel is spinning free, the pump is made to engage the clutch - this transfers power to the other side of the diff.

Wasn't there a non electronic version? Worked on the principle that the two shafts were connected to the rotor and housing of an oil pump. Shafts at same speed -> pump not working -> no pressure?

Originally Posted by SPIKE ARRESTOR

Attaching a file which gives a brief on the various types of 4wd, first let us set the basics right and then we can take it further. Sutripta da, you there na?

Spike

Why are the experts picking on me. I'm getting paranoid
I agree that one (me) should get the basics right. Starts with my original question.

OT:- How does a cycle rickshaw work?

Regards
Sutripta

[fazalaliadil]

SS_traveller,
A very interesting and educational topic going on here. I am learning.
Thanks,

[Samurai]

I have a question to the experts. Even in our humble Jeep/Gypsy, one can move from 2H -> 4H -> 2H while the vehicle is moving. Let's assume the hubs are locked or have a flange. Why don't we call this shift-on-fly?

Why only SUV which can do the same are known to have shift-on-fly?

Is auto-locking hub qualifying it to be called as shift-on-fly?

[Ragul]

Originally Posted by SS-Traveller

When a vehicle negotiates a turn, the front wheels travel greater distance than the rear wheels, as seen in the diagram below.
Attachment 438354

In the diagram you posted (I recall seeing the original page and being similarly confused) A+B looks much less than C+D - at least visually! This contradicts the statement above, or should I say confuses as the visual assessment challenges the notion of front wheels traveling more than the rear.

Anybody else agree with my perception of the visuals, or is it just me that is visually challenged?

Thanks

[SS-Traveller]

Originally Posted by Sutripta

Why are the experts picking on me. I'm getting paranoid
OT:- How does a cycle rickshaw work?

Experts picking on another very modest expert on Team-BHP, who refuses to share his gigantic knowledge base!

To answer your (not quite) OT question (let's see if I get this right):

The cycle rickshaw has one wheel directly connected to the axle, while the other is connected via a free-wheeling hub. The axle in turn is connected to the chain through another freewheel. The wheel directly connected to the axle moves the rickshaw, while the other one is allowed to speed up / slow down on its own by the free-wheeling hub. Lift up the wheel with the free-wheeling hub, and the cycle rickshaw can still drive away - but not if one lifts the wheel on the other side.

[Sutripta]

^^^^
Re: cycle rickshaw.
One driven wheel. So no differential. And it works! Zillions on the road.
So now we want to design an autorickshaw. What do we do, and why?

Regards
Sutripta

[SS-Traveller]

Originally Posted by Sutripta

____B. A pump and clutch pack - electronics govern a pump, and once sensors tell the system that one wheel is spinning free, the pump is made to engage the clutch - this transfers power to the other side of the diff.

Wasn't there a non electronic version? Worked on the principle that the two shafts were connected to the rotor and housing of an oil pump. Shafts at same speed -> pump not working -> no pressure?

That is the Gerotor pump. One more type that I wasn't aware of!

This works by hydraulically compressing a clutch pack. The gerotor pump uses the housing to drive the outer side of the pump and one axle shaft to drive the other. When there is differential wheel rotation, the pump pressurizes its working fluid into the clutch pack area. This provides a clamp load for frictional resistance to transfer torque to the higher traction wheel. The pump based systems have a lower and upper limits on applied pressure, and internal damping to avoid hysteresis. The newest gerotor pump based system has computer regulated output for more versatility and no oscillation.
(Limited slip differential - Wikipedia, the free encyclopedia)

[SS-Traveller]

Originally Posted by Samurai

Let's assume the hubs are locked or have a flange. Why don't we call this shift-on-fly?

Why only SUV which can do the same are known to have shift-on-fly?

Is auto-locking hub qualifying it to be called as shift-on-fly?

What I understand about it...

Older-style part-time 4WD systems sometimes required drivers to stop and lock the front hubs before engaging four-wheel drive (like the earlier Gypsys with their manual locking front hubs). At least some part-time 4WDs now have front ALH (auto-locking hubs) (like the Safari & Scorpio) and the ability to "shift-on-the-fly" (at speed) from 2WD to 4WD. However, there is usually some limit on the speed at which this can be done. ALHs sometimes require the driver to stop and back up to fully disengage the front hubs, but do not necessitate his getting out of the car.

Ideally speaking, therefore, your CJ without free-wheeling front hubs should qualify as a "shift-on-the-fly" system. And you don't even have to reverse to disengage front hubs!

[DirtyDan]

Drivetrain windup gives testimony as to just how much traction tyres really give you on pavement. There is always a difference in speed between front axle and rear axle in a 4 wheel drive vehicle even on a straightaway. Either the front drags the rear or the rear pushes the front. When front and rear are hard locked the result is friction. The friction will not make the wheels scrub or slip on a hard dry surface. Instead, friction will be realized in the drive train somewhere, often the transmission. The transmission will get very hot. Hot enough to carbonize the gear oil. Shortly thereafter bearings in the tranny melt and fuse. When you open such a transmission all the gear oil will be gone, vaporized and/or turned to tar and soot. The traction of tyres on a hard surface is enough to melt tranny bearings, and relatively quickly, too, say, 60K.

Offroad it is the tyres that slip on the surface thus saving the drivetrain (but, yes, sometimes you can still get windup offroad). In vehicles with a center differential, the difference in speed between front and rear slips here, hence no friction.

*******

My Invader is invincible off road because I have tow hooks front and rear and a good mobile with which I can call Rahoon and his intrepid M&M tractor .

[sudev]

@Sutripta: LOL

@SST: Thanks for starting this very interesting topic

In case of AWD's unless they also have Limited or Lock Differentials in front and Rear the "invincibility" suffers. Grand Vitara does not have them and while it does quite a bit of off-road work it suffers immediately if one front and one rear wheel is in air or slipping.

In the diagram below what needs to be appreciated that the radius differences shown A-B for front and C-d for rear get further multiplied by factor of 2*Pie when computing the circumference.


The example from Wikipedia needs to understood not only in terms of one axle stopping but also in terms of one axle facing more load vis the other. The differential sends more rotations to the axle which has low load. The outer axle in the picture above and this makes outer wheels travel more distance to keep parity with inner wheel.

The whole equation is further complicated by the fact that the wheels travel on undulating surfaces and hence the actual travel needed by each wheel varies continuously.

One question: Does Gypsy have a fixed central transfer case or with differential? I know most of the people using them in rallies drive for long distances in High or Low lock mode, albeit on loose surface, at pretty high speeds.