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On a scale of 1 to 10, I think an expert on this thread is someone who's at least achieved 4 or 5 in terms of basic knowledge.
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Let us close this by saying 'amongst the most knowledgeable regular participant on this forum.'
We set out to talk about a fairly simple but extremely misunderstood topic, i.e. how all the 4 wheels of a car are driven together by multiple methods. At the heart of such methods is one of the oldest designs of machinery (perhaps second only to the wheel) that runs a car. And it is the arrangement of differentials in multiple patterns that distinguishes the type of drive - RWD, FWD, 4WD (part-time, full-time, on-demand) - and understanding the functioning of a differential is so essential to understanding these acronyms that keeps flying around to everyone's confusion.
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You've got to thank Sutripta for thinking this up!
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Thanks SS, but suggestions/ suggesters are a dime a dozen. Doing matters, and you are the doer. Remember when you said "then why don't you put up a thread on this", My response was "Me! No. Too much hard work".
(To others wondering whats going on, an explanation. Call started with exchange of Puja greetings, then veered round to the hot topic of the day - the Aria. And then to its drivetrain (of which we still don't know much), and its perceived positioning. It was in that context that I said that there was too much of misconception about 4WD, which needs to be addressed, and also going back a step, about RWD, FWD, etc. )
Since I started the cycle rickshaw question, let me close it. The basic question was - if the cycle rickshaw does not have a differential, why does everything else?
Let me start by saying that our cycle rickshaw does not have a differential not because it does not need one, but because it makes do without one. Just like it makes do without proper brakes, or proper gearing. How does it make do without these. Because at the slightest sign of adverse conditions, the 'engine' jumps off the rickshaw, and manhandles the vehicle.
Not having a differential does not mean it will not benefit from one. It is a cost benefit analysis done by both buyer and seller. This same analysis says that for a powered vehicle, a diff is a must.
How about the other option for the cycle rickshaw (leaving aside lateral thinking alternatives like front wheel drive, as in a kids tricycle) is an axle with wheels firmly attached at both ends (ie a spool). It is really not an option.
When taking a turn, total tyre slippage has to be equal to the difference in the arc lengths transcribed by the two wheels (a slight amount can be accomodated by deformation of the tread area of the tyres). And this slippage will be against the traction afforded by the tyre. You have to give enough torque to break traction. Anything less, and you will stall. And this force (and energy) is totally wasted. Components like chain, freewheel, and axle flanges/ keys are going to be stressed, but before any significant damage, the 'engine' is going to give up. As an experiment to gauge the effort necessary, (and going to waste) may I suggest getting on a pedalcycle, locking the front wheel by whatever means, and trying to pedal forward a few metres.
So the only thing left is the 1WD system used. Let us see where this leads us.
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I am putting forward two more explanations of why automobiles need differentials:
1. The final mover for the car is those little palm-sized patches of tyre on the road. And two of them are better than one. If the car was a one-wheel drive like the cycle-rickshaw, the possibility of wheelspin and wasted effort in moving the car would have been much higher. In a straight line, when the car is set in motion, BOTH tyres get equal turning force - and because 2 tyres grip better than one, we get better acceleration.
2. If there was one-wheel drive, say for the left wheel - and if one was driving through multiple S-bends - the left wheel would have to suddenly slow down on a left-hand curve, and speed up rapidly on a right-hand curve, to maintain a steady speed through these S-bends. Obviously, this would have unnecessarily strained the engine and transmission. The differential helps to average out this speeding up and slowing down of the wheels.
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I would put more emphasis on 2 rather than 1. Motorcycles transmit quite a bit of power through one wheel.
The other way of looking at 2 would be for uniform rpm of the power source, speed of the vehicle (as seen from its geometric centre, or CoM) would be a function of its direction of travel.
Also we would expect (even demand) that the vehicle behave identically for a left turn as a right turn. For a 1WD, that is certainly not going to be the case.
Powering 1 wheel of a pair is not an acceptable solution. (Proof:- It is not used on the Tata Nano
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Regards
Sutripta