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Old 11-08-2013, 12:55 AM   #21
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Originally Posted by gbaglo View Post
I remember my 1987 Subaru wagon. If you put it in four wheel drive and tried to turn a corner on pavement, the engine would die. Didn't have the oomph to turn the wheels. Then you had to straighten the wheels and back up several yards to unlock the 4X4.
Yep, that was the front tires fighting the rear tires: the fronts had to move faster than the rears, just as the outside tires have to move faster than the inside). It didn't have the power to spin the rears to match the fronts, and in trying it jammed the front-to-rear coupling.

This was a front-or-four system with no centre differential, just as a traditional 4x4 truck is a rear-or-four system. I knew a guy who had a similar vintage 4WD Toyota Tercel wagon, and convinced him to come to a performance driving event on an ice track (plowed out of the snow on a frozen lake); by the end of the day as his skill improved he found that he could lap the track faster in 2WD, because in 4WD it resisted turning so much that it slowed his cornering down more than enough to negate the acceleration advantage. We're talking obsolete designs here (for cars - trucks still come this way)... a modern AWD car is certainly fastest and most stable with drive to all four tires working .
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Old 11-08-2013, 01:11 AM   #22
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People go too fast in all conditions AFAIAC. Sunny dry day, snowy day, no difference.

A front-wheel-drive vehicle in snow may not be able to make it up an incline, even a slight incline, maybe even with some sand down. A 4X4 will go up easily and will also go up with heavy snow and no sand.
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Old 11-08-2013, 02:18 AM   #23
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Brian,

No dispute about the torque being equal. I was talking about usable power, horse power to the driving wheels to be specific which definitely isn't equal at times. One wheel does have to turn faster/freewheel relative to the other on a corner. If one wheel isn't turning then technically there is no usable power/horsepower at that wheel. From that perspective for all intents and purposes the vehicle becomes one wheel drive. Technically I guess it could vary from 100% one wheel drive to 50%.

I did forget about the steering fighting you on posi front ends and I agree that having a winch mount on the rear is sometimes a very good idea.


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Old 11-08-2013, 06:03 PM   #24
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No dispute about the torque being equal. I was talking about usable power, horse power to the driving wheels to be specific which definitely isn't equal at times.
Absolutely agree. Power and torque are not the same thing, despite very confusing and sloppy descriptions by auto manufacturer's marketing people and others around the industry. Power (for a rotating shaft) is rotational speed multiplied by torque.

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One wheel does have to turn faster/freewheel relative to the other on a corner.
Yes the outside is faster, but the one going faster doesn't freewheel (which means to turn without any torque applied or power transmitted). Both are turning, both are being driven, both are transmitting power, nothing is freewheeling, neither is just along for the ride. Well, those non-driven wheels at the other end of the car are...

The point of the cool old GM differential models is that regardless of the speed that each side gear (of the diff) is turning, the spider gear between them is always pushing both side gears around, each with the same force on the gear teeth.

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If one wheel isn't turning then technically there is no usable power/horsepower at that wheel. From that perspective for all intents and purposes the vehicle becomes one wheel drive. Technically I guess it could vary from 100% one wheel drive to 50%.
Sure, but in a corner neither is stopped. At full steering lock my typical minivan runs around a 5.7 metre (18.7 foot) radius circle at the outside front tires, and a 4 metre (13.1 foot) radius circle at the inside front tires. With the same torque applied to each, and the tire speed proportional to the radius, that's a 59% / 41% power split between outside and inside tires... hardly one-wheel-drive, and that's the most extreme case.

If one tire has no traction and is just spinning (on ice, for instance), while its mate is just stopped, there is no power being transmitted to either drive wheel, so that's still not one-wheel-drive... it's two equally ineffective wheels, both trying to drive. Because the drive torque is split evenly between two tires, the same degree of driving the vehicle forward is accomplished with half as much drive force at each tire as a one-wheel-drive vehicle would have, so this whole spinning thing is much less likely to occur. This is the logic of 4WD... it's not how many tires you can spin, but how much force your can drive the vehicle forward with before any tire slips - more tires means more force before limited traction matters.

In between the ideal no slippage case and the pointlessly spinning frustration, such as slogging through the mud shown in the video that started this discussion, the faster-spinning (less traction) tire is transmitting some power, and the other one is also transmitting some power, because both are driving with the same torque... still not one-wheel-drive, still two-wheel-drive with one of them being limited by the other.

An analogy: If you're walking somewhere and you can't go as fast as you want because you are with some child/elderly person/whatever, your speed is limited by the slower person... but there are still two people walking.
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Old 11-08-2013, 06:09 PM   #25
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A front-wheel-drive vehicle in snow may not be able to make it up an incline, even a slight incline, maybe even with some sand down. A 4X4 will go up easily and will also go up with heavy snow and no sand.
That's an extreme comparison. Most of the time - even in snow and ice - the front-wheel-drive vehicle has enough traction; if it didn't, I would miss work for half of the year. On the other hand, a 4X4 will often not go up a slope through heavy snow (or on ice); if it were always easy, towing companies would make a lot less money around here.

Yes, 4WD is fundamentally more capable than 2WD, but 4WD is not magic; in the best case, it means about twice as much drive traction. Sometimes twice as much is still not enough...
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Old 11-08-2013, 10:33 PM   #26
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Brian,

Under very slippery conditions when one wheel on a conventional axle is stationary and the other is spinning there is most definitely power (HP) to the spinning wheel and some tractive force (in this case 100% of the power is going to one wheel). We can quibble about the shift in power as the vehicle just begins to move but essentially it is one wheel drive at this point. Again both are correct depending upon circumstances.

I possibly could have used a better term than "freewheel" but you keep missing the "relative to" part. Since it was combined with "turn faster" I think it was clear what I meant.

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Old 11-08-2013, 11:08 PM   #27
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I think we're stuck.

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Old 11-08-2013, 11:37 PM   #28
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I possibly could have used a better term than "freewheel" but you keep missing the "relative to" part. Since it was combined with "turn faster" I think it was clear what I meant.
No, I noticed the "relative" part and understood that. No Dave, it wasn't clear what you meant, especially since a freewheel can be used for this purpose, but that's completely different from how a differential works. I now understand that you didn't mean freewheel at all.

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I think we're stuck.

Are we unstuck now?
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Old 11-09-2013, 11:31 AM   #29
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So in conclusion.....
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Old 11-09-2013, 12:19 PM   #30
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A 4-wheel drive is superior. Thank you for listening....
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