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Old 18th October 2010, 08:21   #76
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Slightly OT
Yesterday as part of recce for next weeks 4 Guna 4 event we drove through a slushy pond. I was driving a Grand Vitara with lockable central differential. However the moment one front and one rear wheel started spinning I was stranded. Had to be winched out.

While I was able to do this again without getting struck by keeping sufficient momentum. In contrast I was able to drive Forrestor through the same patch with much more ease. Now this car has Limited Slip Differential and so despite spinning wheels it kept on going.

Relevant picture:


On the other hand even with one rear wheel off the ground the GV was able to deliver traction at the front with locked central differential:
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Old 18th October 2010, 09:09   #77
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Slightly OT...
Absolutely not. Totally on topic.
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Grand Vitara with lockable central differential. ...one front and one rear wheel started spinning...
We talked about this earlier - the fact that any differential, whether fitted between left and right wheels to permit one to turn independent of the other, or between front and rear axles to allow each axle to rotate independent of the other - has this problem:
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...one wheel in the air leaves all the other wheels quite, quite useless in moving the vehicle.
[This would be the situation where all differentials (front, rear and centre) are "open" diffs]
...either we separate out left and right wheels, front and back axles through differentials, and lose movement as soon as [just] one wheel is spinning free, or we end up with transmission windup when we lock those diffs up.
We already know that all part-time 4WD vehicles have NO centre differential - which is the equivalent of the locked centre diff on the GV.

A part-time 4WD vehicle will perform better in the slush that you depicted, compared to a full-time 4WD or on-demand 4WD, where the centre diff is open - because, with the front and rear prop shafts getting 1:1 torque, the vehicle can be stuck when two wheels (one in front and one rear) spin free. Which did not happen in the situation below:
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On the other hand even with one rear wheel off the ground the GV was able to deliver traction at the front with locked central differential..
Both front wheels had full traction, so the GV continued to move.

But...
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Forrestor... has Limited Slip Differential...
...which means that with the centre diff locked, and an LSD intervening between left and right sides, we need to have three wheels stuck before the Forester completely loses movement (I am presuming it has LSD on the rear diff - if there are LSDs both front AND rear diffs, it'll keep going until all four wheels have lost traction 100%).
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Old 18th October 2010, 09:36   #78
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I could be totally mistaken here. But I remember reading this long time back. A torque sensitive LSD won't work if one side is spinning freely.

In open differential, if one side slips and needs only 20% of the available torque to turn, even the other side receives only 20% of the available torque. But in torque sensitive LSD, the other side gets, say 5X times the torque than the slipping side, which is 100%. However, the free spinning (say in air) wheel probably needs only .1% of the available torque to keep spinning. Then the other side gets .5% of the available torque, which is no good. This is why it is suggested to apply brakes along with accelerator to stop the free wheel from spinning so freely. The 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.
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Old 18th October 2010, 10:02   #79
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Quote:
Originally Posted by sudev View Post
Slightly OT
Yesterday as part of recce for next weeks 4 Guna 4 event we drove through a slushy pond. I was driving a Grand Vitara with lockable central differential. However the moment one front and one rear wheel started spinning I was stranded. Had to be winched out.

While I was able to do this again without getting struck by keeping sufficient momentum. In contrast I was able to drive Forrestor through the same patch with much more ease. Now this car has Limited Slip Differential and so despite spinning wheels it kept on going.

Relevant picture:

On the other hand even with one rear wheel off the ground the GV was able to deliver traction at the front with locked central differential:
Hi Sudev,

I think its some thing to do with Weight Distribution.

In the Flat muddy section, not all the weight is evenly or maximally distributed on the wheels.

But in the pic of the GV stuck in the rut, most of the vehicle weight is acting on the front axle thereby increasing traction.

Regards,

Arka
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Old 18th October 2010, 10:09   #80
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Originally Posted by Sutripta View Post
If it is not a CV joint, how did we use it even in our front engined, rear drive vehicles. As we are pottering along slowly, we don't see our cars hopping along, do we?
Hi Sutripta,

Wishing you a Very Belated Shubo Bijoya.

Could be the Traction Avant was using a Double Cardan "CV" Joint.

These Double Cardan Setup are preferred in 4WD Trucks. (Shaktiman & TMB)

http://2.imimg.com/data2/HN/OU/IMFCP...s4-250x250.jpg

http://www.ruf-inc.com/fm_features-double-cardan.jpg

Regards,

Arka
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Old 18th October 2010, 11:06   #81
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But in the pic of the GV stuck in the rut, most of the vehicle weight is acting on the front axle thereby increasing traction.
Arka, there is no front axle here.

Going from my GV experience, I think the GV front underbelly is sitting on the ground. That's usually when it gives up.
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Old 18th October 2010, 11:22   #82
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Arka, there is no front axle here.
Hi Sharath,

There has to be a Front Axle, otherwise its not a 4WD.

Regards,

Arka
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Old 18th October 2010, 11:27   #83
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Oh, that tiny thing. When you say axle, I can only think of solid axle.
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Old 18th October 2010, 11:51   #84
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Originally Posted by Samurai View Post
I remember... A torque sensitive LSD won't work if one side is spinning freely.

But in torque sensitive LSD, the other side gets, say 5X times the torque than the slipping side, which is 100%.

This is why it is suggested to apply brakes along with accelerator to stop the free wheel from spinning so freely.
Did you read the Hummer owner's manual by any chance?

What you are describing is true for the Torsen-type LSD - where the design incorporates a limited transfer of torque to the non-slipping wheel - say, 5x or 7x the torque being transmitted to the slipping wheel. The advantage here is that torque starts to be transferred well in time, before free spinning of one wheel can occur.

But LSDs are of different types:
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Originally Posted by SS-Traveller View Post
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.
Speed-sensitive LSDs will allow transfer of almost 100% torque from a totally free-spinning wheel to a completely stalled wheel. For example - with the viscous coupling (type 2-A above) - the faster the free wheel spins, the more sticky and thick the fluid becomes - and transfers torque to the other side. A totally free spinning wheel is the best thing that can happen to make the viscous coupling work to its full potential.

Disadvantage with the viscous coupling is that one wheel has to start spinning much faster than the other wheel before the other wheel gets to receive its share of torque and keep moving the car.

Similarly, the 1-B type LSD with clutch plates - IIRC the Safari LSD is of this design. Faster the non-gripping wheel spins, the harder the clutch plates are pressed together. The limit of torque transfer in this case would be the limit of grip between the clutch plates (the plates wear out with use, similar to a car's regular clutch, in which case torque transfer becomes progressively less).

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Originally Posted by Samurai View Post
Oh, that tiny thing.

Last edited by SS-Traveller : 18th October 2010 at 11:53.
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Old 18th October 2010, 12:00   #85
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Did you read the Hummer owner's manual by any chance?
No, in 2007 when I first found out that GV has torsen LSD, I googled around and found that info.
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Old 18th October 2010, 12:04   #86
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The Hummer owner's manual has the brake+accelerate technique mentioned in it, which you described earlier.
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Old 18th October 2010, 14:06   #87
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The safari has clutch type LSD, and if you get royally stuck, one of the rear wheel starts spinning free.
However, if you are not so stuck, then LSD definitely works.
The Forrester getting free early could have been due to lower weight also, not just LSD.
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Old 18th October 2010, 15:24   #88
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Originally Posted by Sutripta View Post
The trick is in this. If the input and output shafts are in the same plane, then if we introduce a shaft in between these, and if we maintain the deflection angle at each end to be the same, then with proper phasing of the UJs at each end, we can exactly compensate for and eliminate the variations. The output shaft will now accurately follow the input. Viola! we have our propeller (cardan) shaft. (An interesting point to be noted is that though the input and output shafts are following each other properly, the propeller shaft itself is not. It is speeding up and slowing down over each cycle. Luxury car makers nowadays object to this.)
Thought of attaching a pic for clearer understanding. Driving all four wheels: how is it done?-1.jpg

It needs to be mentioned that the major drawback of a UJ when compared to a CV joint is the "range of operation". The angle between two intersecting shafts in an Independent suspension may sometimes range up-to 40 degrees, for smaller degrees even a UJ will suffice, but when it comes to 40 degrees the anomalies multiply.

(novice) Spike
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Old 18th October 2010, 18:07   #89
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Originally Posted by Sutripta View Post
How does a cycle rickshaw work?
Totally , but here's one that works with a Team-BHP LTD sticker on its back.

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Originally Posted by ceebeezee View Post
I dunno if this has been uploaded in the past. location is the French quarters (aka White Town), pondicherry.
Driving all four wheels: how is it done?-img00165201010081711.jpg
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Old 18th October 2010, 20:40   #90
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Originally Posted by SPIKE ARRESTOR View Post
Thought of attaching a pic for clearer understanding. Attachment 440341

It needs to be mentioned that the major drawback of a UJ when compared to a CV joint is the "range of operation". The angle between two intersecting shafts in an Independent suspension may sometimes range up-to 40 degrees, for smaller degrees even a UJ will suffice, but when it comes to 40 degrees the anomalies multiply.

(novice) Spike
Thanks. If the diagram had been rotated slightly so that the input and output shafts were horizontal, I think people would more easily equate it to the gearbox, and differential, and the connecting shaft to the propeller shaft.

The diagram also answers the age old question of every schoolboy:- why dont the manufacturers tilt the differential so that the tail is inline with the prop shaft?

Thanks for highlighting the UJs limited range of operations. I did mention in an offhand way
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Originally Posted by Sutripta View Post
Another thing I forgot to mention was that in a front engine rear drive config, the deflection angles are small.
An important point to note is that we can keep on shortening the intermediate shaft without any change in the equations governing its motion. In the extreme case we get two back to back joints with no intermediate shaft:- the double cardan joint. Which brings us to

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Originally Posted by ex670c View Post
Could be the Traction Avant was using a Double Cardan "CV" Joint.
I think you are right. From very vague recollection, there was a UJ at the diff end, and a double cardan at the wheel end. I also think there was a separate slip joint.

Some pages can be seen at
Universal joints and driveshafts: analysis, design, applications By Hans Christoph Seherr-Thoss (Graf von), Friedrich Schmelz, Erich Aucktor

Even amongst the fan sites, they talk of notchy (and heavy) steering, 'snatchiness' in the transmission, and very large turning radius. These are related. So all was not rosy in the land of universally jointed Citroen.

@SS. I think a section on different hubs should also be included.
Re: the discussion on the Grand Vitara getting stuck, I think its way too early to discuss it. However, it does clear up

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Originally Posted by Sutripta View Post
An interesting discussion would be on why a Torsen needs a lock.
Regards
Sutripta
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