What they did before 4 wheel drive was available….

[Brian B-P]

Originally Posted by gbaglo

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 .

[float5]

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.

[Dave D.]

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.


Dave

[Brian B-P]

Originally Posted by Dave D.

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.

Originally Posted by Dave D.

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.

Originally Posted by Dave D.

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.

[Brian B-P]

Originally Posted by Floating Cloud

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...

[Dave D.]

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.

Dave

[gbaglo]

I think we're stuck.

baglo

[Brian B-P]

Originally Posted by Dave D.

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.

Originally Posted by gbaglo

I think we're stuck.

Are we unstuck now?

[J Mac]

So in conclusion.....

[float5]

A 4-wheel drive is superior. Thank you for listening....

[Dave D.]

So in conclusion:

1. Under the worst circumstances and when you need it the most a standard differential results in your vehicle having only one driving wheel. Traction differentials (Posi-trac, Track- loc etc.) and computer controlled traction systems were developed to supply power (hopefully) to both drive wheels under these circumstances.
2. Under regular conditions it is 2 wheel drive with a varying ratio of power going to each wheel depending upon conditions.
3. Both answers are correct depending upon your perspective or parameters.
4. All things being equal 4-wheel drive is superior to conventional all wheel drive systems.
5. All things being equal 4-wheel drive pick ups are really light in the rear and are likely inferior to conventional all wheel drive.
6. Variables like having the proper tires, driving skill, horsepower, clearance, terrain and traction systems does not make all things equal and will change the outcome.
7. A pickup, jeep or similar 4-wheel drive has a low range position in the transfer case which gives it a large advantage in deep mud or loose sand and going down a slippery slope in control.

Clear as mud?

Dave

[Brian B-P]

Nice summary, Dave.

My version, shamelessly copied from Dave's:

• A conventional axle with an "open" differential is always 2 wheel drive with a varying ratio of power going to each wheel depending only upon wheel speeds.
• Under the worst circumstances and when you need it the most a standard differential results in your vehicle being limited by the driving wheel with the least traction. Variations of differential design (Posi-trac, Track- loc etc.) and computer controlled traction systems were developed to transfer power to the slower-moving drive wheel (with more traction) under these circumstances. Some "torque vectoring" differential designs can even apply more torque to the faster-turning wheel if that is desired for conditions.
• Modern 4WD or AWD systems distribute power between front and rear axles in many of the same ways as the differential of an axle does between left and right wheels.
• Variables like having the proper tires, driving skill, horsepower, clearance, terrain and traction systems are of comparable or greater importance to the basic features such as 2WD versus 4WD.
• Some 4-wheel drive systems - most often found on pickup trucks, some Jeeps, and more traditional SUVs - have a low range position in the transfer case. Such very low gearing (in the transfer case or otherwise) provides a large advantage in deep mud or loose sand and going down a slippery slope in control.

That performance in mud of that car in the antique video that started the discussion probably had a lot more to do with ground clearance than anything else. Hopefully you won't be that deep in mud when towing your Escape!

[Dave D.]

Brian,

There are no absolutes in this e.g. "is always 2 wheel drive". There is only an absolute if you consider only torque and not power. It is power (HP) that is propelling the vehicle forward, not torque alone. If a wheel isn't turning there is no power going to it. 100% of the power can and will go to only one wheel under extreme conditions. Hence the conclusion by people that a conventional axle is or can be one wheel drive. They arrive at this conclusion from the practical but not pleasant experience of being good and truly stuck with one wheel merrily spinning. Again both views are correct.

If you had said is mainly, usually or mostly 2 wheel drive I would agree with you. If I can give you an example where one wheel can not rotate yet the vehicle is able to move with a conventional differential would you agree that sometimes a standard differential is one wheel drive?

In addition to greater clearance the big advantage antique cars had were skinny tires which often sunk down to firmer ground and didn't require nearly as much power.

Dave

[J Mac]

Is this still the Escape forum ?