michaelgroves
Explorer
I fixed Wikipedia's introduction to locking differentials, since it was just plain wrong:
http://en.wikipedia.org/wiki/Locking_differential
Understanding what a differential actually does (and of course a locking differential), helps you learn much faster what off-road driving lines are more likely to be successful. You can predict and understand what the vehicle does under what conditions, rather than solely by experience.
Most people imagine that an open diff allows all the torque to be supplied to the wheel with no traction (which then spins uselessly), and that a locked diff supplies equal torque to both wheels (giving you a better chance of getting out of the mire).
But in fact, it is precisely the other way around!
An open diff always puts equal torque on each half-shaft. A locked diff allows them to have unequal torques.
Here's the explanation: Torque is the twisting force on the halfshaft. (Think of it as the force that "tries" to break the shaft by twisting it). It is a product of the engine trying to turn the shaft, and the ground under that wheel resisting (this ground resistance is also called "traction"). Torque has nothing to do with how fast the wheel turns - you can spin a wheel really fast, and have nearly zero torque on that shaft, or you can have it entirely motionless, but have it under tremendous torque.
So... if you lift one wheel up into the air, the ground stops resisting, and torque drops to (approximately) zero, even though the wheel might be spinning. Conversely, if your truck is chained to a tree stump, and you're pulling at it hard, you may have a huge torque on both shafts, even though neither wheel is actually turning. (That's easier to imagine if the truck has automatic transmission).
With an open differential, if you lift a wheel into the air, so that its torque drops to zero, the open differential drops the torque on the other wheel to zero also. The wheel in the air spins because the differential is supplying enough power to overcome the (non-existent) traction. The opposite wheel - the one the ground - stops rotating, because the differential supplies too little torque to overcome the good traction.
The only time your vehicle ever moves is when there is sufficient total torque to move it. A lot of torque is needed to shift a heavy truck up a steep hill, much less torque is required to move a light truck along flat ground. Assuming your engine and transmission can supply whatever is needed, the limiting factor is how much traction the ground can provide. If the ground is slippery (or uneven, so that weight is taken off one of the driving wheels), then there may be insufficient resistance (traction) that your truck's engine can work against in order to generate the necessary torque.
By way of example - imagine you have a 2WD automatic (for simplicity) truck, with an open diff, going up a hill that requires a total of, say, 2000Nm of torque at the wheels to move it. Let's also say that your truck's transmission can generate way more than that, so traction is the only issue.
Now, to start with, let's look at the situation where you are on moderately solid ground, dirt capable of supporting, say 1300Nm of torque. If any wheel is pushed harder than that, it will start to slip on the dirt.
As you gradually press the accelerator, the torque will go up at both wheels equally, until it reaches 1000Nm at each half-shaft. At that point, the wheels will both start to turn, and the truck will move. No spin takes place, because neither wheel exceeds the 1300Nm limit of the ground.
Now imagine one wheel is on an muddy patch, which can only support 800Nm of traction. As you hit the pedal, the torque rises equally on both axles, until they each reach 800Nm. At that point, the muddy wheel reaches the friction limit of the mud, and starts to spin. The other wheel is also under 800Nm torque, but that's insufficient to make it rotate. In order for it to rotate, either it must spin, or the truck must go forwards. The total torque is only 1600Nm (800+800, which is below the 2000Nm required to shift the truck), and the "solid" wheel's torque is limited to 800Nm by the open diff, so instead, that wheel just sits there unmoving, invisibly pushing at 800Nm. Your truck is stuck with a shortfall of 400Nm.
So you try again with the axle diff locked. Now, each side is driven by the locked diff at whatever torque it can generate, individually. As you press the pedal, torque rises to 800Nm at each side.. but it doesn't stop there. The side with more traction continues to increase in torque, until it finally reaches 1200Nm. At that point, the total torque reaches the required 2000Nm (800+1200), so your truck start to move.
Had the muddy part been more slippery, supporting only 600Nm, say, then even the locked differential wouldn't have been enough. The slippery wheel would supply 600Nm, and the good side would need to provide 1400Nm. But at 1300Nm, the dirt under that wheel would start to give way too, and the truck would sit, both wheels spinning stubbornly, 100Nm short of what's needed (1300+600=1900).
Once you start to think in terms of differentials equalising the torques (and understanding that the torques are generated by traction), you can start thinking about your "lines" in terms of which lines will supply the best combination of torque at each wheel, and weigh those up against which lines will have the lowest total torque requirement. Most of it is doing what ntsqd described - picking lines based on experience and common sense. But it can also be refined by thinking about the effect of a locked or unlocked diff. With an unlocked diff, you never want the wheels to have very different tractions, because the wheel with better traction will always be reduced to the same level of torque as the wheel with worse traction. So, better to choose a line with moderate traction at both wheels, than a line with excellent traction at one wheel and lousy traction at the other.
Also, it's worth remembering that there are degrees of getting stuck - in the second example above, the traction was enough to supply a torque only 400Nm short of what was required. Yet an observer would probably just see that one wheel was slipping in the mud, and assume the stationary wheel was doing nothing at all. In fact, of course, it is supplying just the same amount of torque as the moving wheel, so perhaps all that is needed is a minor, rather than a major change of line. (All you need is another 200Nm of grip in the mud, and the other 200Nm comes "free of charge" from the other wheel!).
Anyway, I hope this rather long-winded description of differentials has helped dispel some of the misunderstanding of what diffs do, rather than generated more confusion!
Rgds,
Michael...
http://en.wikipedia.org/wiki/Locking_differential
Understanding what a differential actually does (and of course a locking differential), helps you learn much faster what off-road driving lines are more likely to be successful. You can predict and understand what the vehicle does under what conditions, rather than solely by experience.
Most people imagine that an open diff allows all the torque to be supplied to the wheel with no traction (which then spins uselessly), and that a locked diff supplies equal torque to both wheels (giving you a better chance of getting out of the mire).
But in fact, it is precisely the other way around!
An open diff always puts equal torque on each half-shaft. A locked diff allows them to have unequal torques.
Here's the explanation: Torque is the twisting force on the halfshaft. (Think of it as the force that "tries" to break the shaft by twisting it). It is a product of the engine trying to turn the shaft, and the ground under that wheel resisting (this ground resistance is also called "traction"). Torque has nothing to do with how fast the wheel turns - you can spin a wheel really fast, and have nearly zero torque on that shaft, or you can have it entirely motionless, but have it under tremendous torque.
So... if you lift one wheel up into the air, the ground stops resisting, and torque drops to (approximately) zero, even though the wheel might be spinning. Conversely, if your truck is chained to a tree stump, and you're pulling at it hard, you may have a huge torque on both shafts, even though neither wheel is actually turning. (That's easier to imagine if the truck has automatic transmission).
With an open differential, if you lift a wheel into the air, so that its torque drops to zero, the open differential drops the torque on the other wheel to zero also. The wheel in the air spins because the differential is supplying enough power to overcome the (non-existent) traction. The opposite wheel - the one the ground - stops rotating, because the differential supplies too little torque to overcome the good traction.
The only time your vehicle ever moves is when there is sufficient total torque to move it. A lot of torque is needed to shift a heavy truck up a steep hill, much less torque is required to move a light truck along flat ground. Assuming your engine and transmission can supply whatever is needed, the limiting factor is how much traction the ground can provide. If the ground is slippery (or uneven, so that weight is taken off one of the driving wheels), then there may be insufficient resistance (traction) that your truck's engine can work against in order to generate the necessary torque.
By way of example - imagine you have a 2WD automatic (for simplicity) truck, with an open diff, going up a hill that requires a total of, say, 2000Nm of torque at the wheels to move it. Let's also say that your truck's transmission can generate way more than that, so traction is the only issue.
Now, to start with, let's look at the situation where you are on moderately solid ground, dirt capable of supporting, say 1300Nm of torque. If any wheel is pushed harder than that, it will start to slip on the dirt.
As you gradually press the accelerator, the torque will go up at both wheels equally, until it reaches 1000Nm at each half-shaft. At that point, the wheels will both start to turn, and the truck will move. No spin takes place, because neither wheel exceeds the 1300Nm limit of the ground.
Now imagine one wheel is on an muddy patch, which can only support 800Nm of traction. As you hit the pedal, the torque rises equally on both axles, until they each reach 800Nm. At that point, the muddy wheel reaches the friction limit of the mud, and starts to spin. The other wheel is also under 800Nm torque, but that's insufficient to make it rotate. In order for it to rotate, either it must spin, or the truck must go forwards. The total torque is only 1600Nm (800+800, which is below the 2000Nm required to shift the truck), and the "solid" wheel's torque is limited to 800Nm by the open diff, so instead, that wheel just sits there unmoving, invisibly pushing at 800Nm. Your truck is stuck with a shortfall of 400Nm.
So you try again with the axle diff locked. Now, each side is driven by the locked diff at whatever torque it can generate, individually. As you press the pedal, torque rises to 800Nm at each side.. but it doesn't stop there. The side with more traction continues to increase in torque, until it finally reaches 1200Nm. At that point, the total torque reaches the required 2000Nm (800+1200), so your truck start to move.
Had the muddy part been more slippery, supporting only 600Nm, say, then even the locked differential wouldn't have been enough. The slippery wheel would supply 600Nm, and the good side would need to provide 1400Nm. But at 1300Nm, the dirt under that wheel would start to give way too, and the truck would sit, both wheels spinning stubbornly, 100Nm short of what's needed (1300+600=1900).
Once you start to think in terms of differentials equalising the torques (and understanding that the torques are generated by traction), you can start thinking about your "lines" in terms of which lines will supply the best combination of torque at each wheel, and weigh those up against which lines will have the lowest total torque requirement. Most of it is doing what ntsqd described - picking lines based on experience and common sense. But it can also be refined by thinking about the effect of a locked or unlocked diff. With an unlocked diff, you never want the wheels to have very different tractions, because the wheel with better traction will always be reduced to the same level of torque as the wheel with worse traction. So, better to choose a line with moderate traction at both wheels, than a line with excellent traction at one wheel and lousy traction at the other.
Also, it's worth remembering that there are degrees of getting stuck - in the second example above, the traction was enough to supply a torque only 400Nm short of what was required. Yet an observer would probably just see that one wheel was slipping in the mud, and assume the stationary wheel was doing nothing at all. In fact, of course, it is supplying just the same amount of torque as the moving wheel, so perhaps all that is needed is a minor, rather than a major change of line. (All you need is another 200Nm of grip in the mud, and the other 200Nm comes "free of charge" from the other wheel!).
Anyway, I hope this rather long-winded description of differentials has helped dispel some of the misunderstanding of what diffs do, rather than generated more confusion!
Rgds,
Michael...
