4 wheel drive question

[wvpolekat]

Not meaning to be difficult here, just wondering if you get that they are receiving equal torque and unequal speed with an open differential and unequal torque but equal speed, with a locked diff.

What it also means is that it is incapable of sending enough torque to the wheel(s) with usable traction and therefore, ability to DRIVE the vehicle. Rendering them no longer drive wheels.

Equal torque to all wheels is only useful when enough of it can be applied to the wheels with the traction to convert it to movement. Meaning, that under this system, in all conditions, all wheels are not capable of being drive wheels.

Use of the word "drive" in this case is semantics, and I suspect you know that.

 

[TripleR]

I love threads like these. I Jeep. I hardcore Jeep. I have a complete understanding of differentials, lockers, limit slips, etc.

I find that those who get caught up in "word perfect" terminology tend to have the least amount of practical application under their belt.

When you are stuck in 4wd and open diffs and are walking for help because you are spinning only two tires and not moving, you can think of plenty of things to "call" it.

Sometimes, but not always, some professions insist on absolute correctness in terminology. Some professionals also use only correct terminology when speaking with their peers and maybe not so much when speaking with others. I dealt with legal matters mostly and you darned well better use the exact precise terminology as it was going to wind up in court or some government office to be read by many and open to misinterpretation and challenge.

I saw a recent discussion of the use of "gun" by Soldiers/Marines/Sailors. I would never refer to a S&W revolver as being a pistol to some of my friends as a pistol, it is a "revolver", our Glocks are "pistols". We won't get into the whole military thing.

As to my understanding of differentials, until recently I thought maybe there were hamsters on wheels in there or maybe elves. Actually, I have helped my brothers and knew there were gears in there, but it beat the heck out of me as to how they worked; interesting thread.

 

[Egon]

When you are stuck in 4wd and open diffs and are walking for help because you are spinning only two tires and not moving, you can think of plenty of things to "call" it.

In this situation is it still "Four Wheel Drive"?:laughing::laughing::laughing:

 

[Raspy]

The basic problem (aside from not enough traction and getting stuck) is the notion that seeing a wheel spin, somehow equates to "it is the only one getting power" misconception. You can't see torque, but you can see spinning. In other words, if I can't see it, it doesn't exist. "That non-spinning wheel is just sitting there and not being driven at all. Must be, because I can't see anything happening. It's one wheel drive". The quick human analysis, even with one wheel up off the ground and spinning helplessly, is that that wheel is the only one getting power.

Next comes the argument. "Yes it is one wheel drive and I can prove it." Then the solution from someone else: "well, my Daddy used to just step on the brake when he spun a wheel and just drive right out".

Man, I love all the stories and ways to describe how four wheel drive is really two, etc. These days, machines are so user friendly that we don't have to know anything about how they work, then we push them out of their envelope and ............uh, oh. But instead of trying to figure it out, it's time to argue about "one wheel drive", or get out the dictionary and argue definitions.

So, let the stories continue. :thumbsup:

 

[Gary Fowler]

I just have 1 more thing to say on this with regard to tractors, If it is too icy or muddy for your tractor to move in "3 WD" then perhaps you should be home by the fireside or kicked back with a cold one rather than out on the tractor to start with. I have had my tractor stuck once in "bottomless" soil when I tried to mow across fresh landfill in a slough area and again in a dried up pond we were digging out and when the frame is under the dirt, it really doesnt matter if you have 1 wheel or 8 wheel drive, you aint going nowhere. When all the wheels are spinning and the FEL cant help push you out, it's time for another piece of equipment.
I rarely get into situations where "3 Wd" and FEL wont get me out with differential locked on the rear. I think you would be asking for disaster to the front drive system to have a front lock on a tractor should you engage it and then try to turn. It would be like trying to turn a dozer by hitting the brakes without disengaging one of the drive clutches.

 

[JoelD]

Did you take one wheel off? Did you disconnect one wheel from the power train? Which "three" does it "run".

Not meaning to be difficult here, just wondering if you get that they are receiving equal torque and unequal speed with an open differential and unequal torque but equal speed, with a locked diff.

How is it unequal torque with lockers?

This is a great thread

Thanks
Joel

 

[wvpolekat]

The basic problem (aside from not enough traction and getting stuck) is the notion that seeing a wheel spin, somehow equates to "it is the only one getting power" misconception. You can't see torque, but you can see spinning. In other words, if I can't see it, it doesn't exist. "That non-spinning wheel is just sitting there and not being driven at all. Must be, because I can't see anything happening. It's one wheel drive". The quick human analysis, even with one wheel up off the ground and spinning helplessly, is that that wheel is the only one getting power.

The problem is that you are trying to apply a textbook answer to a real world question. ON PAPER, there is power to both wheels. In reality, the wheel with enough traction to produce work is not getting enough power to allow it to do so. Technically, you are right, in reality, it doesn't matter because they are still stuck with one wheel on dry pavement and the other on ice.

Next comes the argument. "Yes it is one wheel drive and I can prove it." Then the solution from someone else: "well, my Daddy used to just step on the brake when he spun a wheel and just drive right out".

And this is a 100% valid way of getting a open diff to work. It causes it to send enough torque to both wheels to give the one with traction enough to move the vehicle. They may not understand why it works, but they have learned to manipulate the system to do what they want, to overcome it's biggest weakness.

Man, I love all the stories and ways to describe how four wheel drive is really two, etc. These days, machines are so user friendly that we don't have to know anything about how they work, then we push them out of their envelope and ............uh, oh. But instead of trying to figure it out, it's time to argue about "one wheel drive", or get out the dictionary and argue definitions.

Except you are arguing for the sake of arguing. The "proof" is right there in plain sight. Regardless of what is technically happening, the reality is that the wheel without traction is spinning and the one with traction is sitting there like a bump. The why of this doesn't matter except to you. To the user, the system has failed them by not delivering sufficient power to the wheel with traction to allow the vehicle to move.

I don't think anyone here has said you are wrong about the operation of an open diff. What I think is you are fighting a lot of windmills because people say something that is technically wrong, but true in reality. Nobody is going to say that they got stuck because the amount of torque required to loose traction on the right wheel was not enough to produce enough work from the one with traction to move forward.

 

[QRTRHRS]

Sometimes, but not always, some professions insist on absolute correctness in terminology. Some professionals also use only correct terminology when speaking with their peers and maybe not so much when speaking with others. I dealt with legal matters mostly and you darned well better use the exact precise terminology as it was going to wind up in court or some government office to be read by many and open to misinterpretation and challenge.

I saw a recent discussion of the use of "gun" by Soldiers/Marines/Sailors. I would never refer to a S&W revolver as being a pistol to some of my friends as a pistol, it is a "revolver", our Glocks are "pistols". We won't get into the whole military thing.

As to my understanding of differentials, until recently I thought maybe there were hamsters on wheels in there or maybe elves. Actually, I have helped my brothers and knew there were gears in there, but it beat the heck out of me as to how they worked; interesting thread.

This is my weapon, this is my gun. This is for fighting, this is for fun.

 

[slowzuki]

Diff Locker = equal speed at each tire, not equal torque
Open diff = equal torque at each tire limited by the tire with the least traction

Differentials not as common in tractors:
Torque biasing = think of as a differential without a 1 to 1 ratio between the opposing tires but rather say 10 to 1. The torque on the spinning side is multiplied by the gears to the other side but not locked. Hummers have this type of diff.
Limited slip = clutches that have a minimum torque to overcome before letting the two sides turn at different speeds.


There are lots more too but not found at all in tractors.

 

[Egon]

The horses have come to the trough. They are not drinking as the water is not to their knowledge or understanding. :thumbsup:

Surprisingly; many of the Horse's, understand the equal torque bit on an open differential. They can also understand why there "MAY" be unequal torque on the axles of a locked differential. Some of them have gone as far as blocking one rear wheel, jacking up the other wheel an open differential and using it for a prime mover of various different tool's they use in the course of their work.:thumbsup:

Slowzuki, thanks for a real simple expalation.:thumbsup:

 

[Raspy]

How is it unequal torque with lockers?

This is a great thread

Thanks
Joel

With a "locker" locked, there is no differential action. So both wheels must turn at the same speed all the time.

Now imagine driving up onto a rock with one rear wheel, in a two wheel drive pickup. A rock big enough that the other rear wheel is off the ground.

With an open diff you can't move because the wheel off the ground just spins in the air. Equal torque, unequal speed.

With a locked diff, both wheels must turn at the same speed. So you drive off with unequal torque to the two wheels. The wheel on the rock does all the work while the other one hangs in the air.

The same is true while driving down the road with a diff locked. As you turn, the wheels try to rotate at different speeds and one begins to slip. If one has less traction, like on ice, the other will keep putting power to the road. This is why we have "Limited Slip" differentials. So the wheels can rotate at different speeds, without severe binding on dry pavement, but still apply some power to the wheel with the most traction when needed. A LS diff might not get you off a rock as in the example above, but it will definitely help in mud and snow conditions. On ice it's questionable. Sometimes it might be better to spin one wheel and not both. Spinning one wheel, sometimes, allows you to stay on the road and not just slide sideways into the ditch. In those conditions, even a LS can lock up and cause a spinout. If the LS is "aggressive" with its locking, it will also lead to faster tire wear. But the benefits of LS greatly outweigh the drawbacks. There are many types of LS diffs that range from very little torque biasing to practically locked all the time. Some act like ratchets, and will allow one wheel to rotate faster on the outside of a curve, but neither one can rotate slower under power. My favorite is the Gleason Torson. Through sort of a reverse worm gear mechanism, it will differentiate, but not rev up the wheel with least traction. Some simply have clutches behind the spider gears that resist differentiation under load. One simple design simply replaces the spider gears with dog clutches that engage whenever power is applied, but release when coasting (a terrible design for driveability, but very simple and easy to retrofit). The list goes on and on.

 

[Raspy]

The horses have come to the trough. They are not drinking as the water is not to their knowledge or understanding. :thumbsup:

Some of them have gone as far as blocking one rear wheel, jacking up the other wheel an open differential and using it for a prime mover of various different tool's they use in the course of their work.:thumbsup:





That is how hay was loaded onto trucks in my area back in the '70s. A diff mounted on a rig with an engine turning the input shaft. One bare wheel with no tire on one side and a rope around it acting like a variable brake. The other bare wheel had a rope around it that went up over a pulley on a boom and down to a hook. By braking more, the boom rope would lift more, by slacking the brake the hook would come down to be attached to the next bale. It was fast, simple and could be built from parts around a ranch.

I was exploring an old mine once and found a car frame, engine, trans and differential that were adapted to be used as a winch to bring up ore in carts that were pulled up a slope. The old car frame was staked to the ground at the top. With one wheel permanently locked, the other could be used as a rotating winch drum controlled by the car engine and clutch.

A pulley or extra wheel can also be mounted outside a truck wheel and, with a rope wrapped around it, can act as a winch to get you unstuck.

 

[PILOON]

Step one: don't go down a steep gravel over pavement hill with a heavily loaded bucket in a low gear in 2 wheel drive. What happens is the rear tires will start to slip and they will not regain traction until you get to the bottom. Do not ask how I know this. In my case, both tires lost traction at the same time, I suppose if one tire had good traction and one slipped, the tractor would try to skew sideways a bit, but the front tires would probably prevent this until the other tire lost traction also. If you are in 4 wheel drive the front tires will provide some engine braking action through the driveline connection. I can tell this to you as a fact. Some here discount this notion, but I have personal experience.:laughing:

James K0UA

LOL, been there, did that.
That is why I always keep a hand on the bucket controls, one quick flip of the lever and you have 'bucket skid brakes'.
Down hill in 4WD allows the engine compression to control the speed.

 

[diesel466]

With a "locker" locked, there is no differential action. So both wheels must turn at the same speed all the time.

Now imagine driving up onto a rock with one rear wheel, in a two wheel drive pickup. A rock big enough that the other rear wheel is off the ground.

With an open diff you can't move because the wheel off the ground just spins in the air. Equal torque, unequal speed.

With a locked diff, both wheels must turn at the same speed. So you drive off with unequal torque to the two wheels. The wheel on the rock does all the work while the other one hangs in the air.

The same is true while driving down the road with a diff locked. As you turn, the wheels try to rotate at different speeds and one begins to slip. If one has less traction, like on ice, the other will keep putting power to the road. This is why we have "Limited Slip" differentials. So the wheels can rotate at different speeds, without severe binding on dry pavement, but still apply some power to the wheel with the most traction when needed. A LS diff might not get you off a rock as in the example above, but it will definitely help in mud and snow conditions. On ice it's questionable. Sometimes it might be better to spin one wheel and not both. Spinning one wheel, sometimes, allows you to stay on the road and not just slide sideways into the ditch. In those conditions, even a LS can lock up and cause a spinout. If the LS is "aggressive" with its locking, it will also lead to faster tire wear. But the benefits of LS greatly outweigh the drawbacks. There are many types of LS diffs that range from very little torque biasing to practically locked all the time. Some act like ratchets, and will allow one wheel to rotate faster on the outside of a curve, but neither one can rotate slower under power. My favorite is the Gleason Torson. Through sort of a reverse worm gear mechanism, it will differentiate, but not rev up the wheel with least traction. Some simply have clutches behind the spider gears that resist differentiation under load. One simple design simply replaces the spider gears with dog clutches that engage whenever power is applied, but release when coasting (a terrible design for driveability, but very simple and easy to retrofit). The list goes on and on.

To me torque is rotating force. In a open diff with one wheel off the ground , the wheel on the ground is not getting any rotating force because the spider gears just walk around the side gear on that side. To me that is not equal torque.

 

[slowzuki]

But it is equal torque. When you accelerate, the torque to accelerate the free spinning tire is also applied to the other side through the diff and generates the same torque on both sides, which is partly why tramping it and rocking can work in an open diff vehicle to get it free.

If you had split brakes like a tractor, and feathered the brake on the free spinning tire, you can generate equal torque on the planted tire and braking force/torque applied to the spinning tire.

To me torque is rotating force. In a open diff with one wheel off the ground , the wheel on the ground is not getting any rotating force because the spider gears just walk around the side gear on that side. To me that is not equal torque.

 

[diesel466]

But it is equal torque. When you accelerate, the torque to accelerate the free spinning tire is also applied to the other side through the diff and generates the same torque on both sides, which is partly why tramping it and rocking can work in an open diff vehicle to get it free.

If you had split brakes like a tractor, and feathered the brake on the free spinning tire, you can generate equal torque on the planted tire and braking force/torque applied to the spinning tire.

If force is being aplied to the wheel on the ground it would move, but it does not because the spider gears just rotate around the side gear and the axle dosent turn.

 

[284 International]

Slowzuki has this covered, but the wheel that is stationary still sees the torque applied. It doesn't magically see no force. Think of power-braking your car: When you rev the engine and stand on the brakes, the axles still are transmitting torque, it just happens not to move the car until the torque of the axles overcomes the resisting force of the brakes.

In the example, Diesel466, the force to move the vehicle is less than the torque to spin the free wheel, so it doesn't move, but the stationary wheel still sees SOME torque, just not enough to move, since it has the same torque applied to it as it takes to spin the free wheel. Applying a brake to the lifted wheel will add to the torque that the stationary wheel sees. They will be equal forces. At some point, as braking is added to the spinning wheel, either the vehicle must move, both tires will spin, or neither will turn if the engine's torque is insufficient. Slowzuki is right.

 

[diesel466]

I understand what you guys are saying and You are right, I guess My point is that in a open diff it depends on resistance from the other side to provide full torque to both axles.

 

[rjkobbeman]

If force is being aplied to the wheel on the ground it would move, but it does not because the spider gears just rotate around the side gear and the axle dosent turn.

Force IS being applied to the wheel on the ground, just not enough. If you suddenly jacked the wheel that is on the ground up off of the ground, I assure you it would start spinning. This is because there is torque being applied to it.

Those spyder gears rotating around the end gear are TRYING to turn the end gear, however there is too much resistance on that end gear (connected to the wheel that is not spinning) in relation to the other end gear (connected to the wheel that is spinning). There is torque being applied to both. There is a direct connection between both wheels and the drive shaft. If only one wheel was driven, the other wheel would not be connected in any way.

Just because you are spinning one wheel doesn't mean the other wheel has been completely disconnected from the drive system and is receiving no torque.

In summary, just because an open differential may get you stuck does not mean it is a "one wheel drive" (or in the case of a 4wd "two wheel drive). All you have to do is jack up both wheels off of the ground and turn the drive shaft. I assure you, both wheels will turn unless a brake is dragging and preventing one wheel from turning. Both wheels are driven.

 

[slowzuki]

Nope, the force has to be greater than the forces holding the vehicle at rest.

If you attach a chain to the back of your tractor and the other end to a giant stump, on a great traction surface, you can generate enough torque to maybe even break an axle shaft without ever turning a tire. You don't need movement to have force.

If force is being aplied to the wheel on the ground it would move, but it does not because the spider gears just rotate around the side gear and the axle dosent turn.