Brody
January 9th, 2011, 02:41 PM
LOCKING DIFFERENTIALS
Author:
With the attraction of harder trails and the addition of body armor, skid plates, etc, the very next thing you may be looking at are traction aiding devices of one kind or another for your rig. The first thing many people do is to have a locking differential installed. This article gives a brief description of lockers and other related devices.
When it comes to traction in off-road situations, differentials play a key role. In general, the differential in a vehicle's axle allows the outer wheel to rotate more quickly when the vehicle turns a corner. The outer wheel travels about 30 feet farther than the inner wheel in a 360-degree turn. A 4WD vehicle has one diff in each axle.
A full-time 4WD vehicle also has a center-diff in the transfer case, because the front wheels rotate more quickly than the rear ones when turning a corner and traveling forward.
A differential always:
distributes equal amounts of torque to both wheels
reacts to resistance/traction to allow a wheel with more resistance to rotate less and a wheel with less resistance to rotate more (The rpm difference created by the differential is always proportional. If the inside tire rotates 15 rpm less in a turn than going straight - then the outside tire will rotate 15 rpm more than going straight.)
Locking the differential is accomplished by doing one of two things:
1. Installing a locking center section which ALWAYS locks the differential
2. Installing a "lock-on-demand" unit like an ARB air locker
Open Differentials (what usually ships stock from the factory
Here's what happens if you're riding off-road and irregular terrain leaves one of your tires in the air or on a poor traction surface and you have open differentials:
With open differentials, power goes to the wheel with the least traction. So one tire on each axle may spin helplessly, despite having 4WD engaged. The differential action of open differentials is essential to stop tires scrubbing, to reduce transmission loads, and to reduce under steer during normal cornering. But if one wheel loses grip, due to slippery ground or through being in the air, the other wheel effectively loses all input torque and cannot propel the vehicle.
For example, a full-time 4WD, with an (unlocked) open center-diff loses traction if any one wheel does. A vehicle with 4WD engaged (or with a center-diff locked) and with open axle diffs loses traction if one wheel on each axle loses traction. such as getting crossed-up by being supported on diagonally opposite wheels, or if one side is on slippery ground.
Locking Differentials
Here's what happens if you're riding off-road and irregular terrain leaves one of your tires in the air or on a poor traction surface and you have locked differentials:
To get true 4WD, the differentials must be "locked".
Locking differentials (or diff locks) are the only cure for the shortcoming of differentials sometimes sending torque to the "wrong" wheel. Diff locks disable the differential's ability to distribute torque equally and its ability to allow the wheels to rotate at different speeds. In some ways a diff lock is the ultimate traction device. Since, all wheels rotate at the same speed when engaged, a vehicle with locks on all axles has traction if just one wheel does.
By disabling (or locking) the differential, steering becomes very difficult and the wheel with the most traction will get the most torque, as much as 100%. This guarantees that any wheel with traction will receive enough torque to move the car. In sum, diff locks will keep you going and keep you from getting stuck.
The drawbacks of diff locks:
Locking (disabling) the differential makes it impossible for wheels to roll at different speeds. Therefore, with lockers engaged, on high traction surfaces it becomes very difficult to make turns, and on low traction surfaces the turning radius gets very wide.
Unfortunately a diff' lock is on or off; there is no in-between. This can make the vehicle under steer (tend to go straight ahead in corners) or veer suddenly sideways if one side looses traction, like on ice.
These serious drawbacks of diff locks require an educated driver who knows exactly when to lock the diffs, usually only for a very brief moment, and when to unlock the diff to maintain steer
ability.
Limited Slip Differentials
Here's what happens if you're riding off-road and irregular terrain leaves one of your tires in the air or on a poor traction surface and you have limited slip differentials:
Some 4WD vehicle manufacturers offer a "limited slip differential" option. As the name implies, they limit wheel slip. These use clutch plates to lock the differential, and when turning on dry pavement they just slip.
Unfortunately, they also slip when there is a significant difference in traction between two wheels in an off-highway situation. Limited slip differentials do not prevent wheel slip; they only delay wheel slip. Therefore, traction is lost a little later than without limited slip, and you will be stuck a little later.
This article only information touches the surface of locking vs. open differentials.
More can be found here:
http://www.frontrange4x4.com/forums/showthread.php?5026-Locker-Information-General
Author:
With the attraction of harder trails and the addition of body armor, skid plates, etc, the very next thing you may be looking at are traction aiding devices of one kind or another for your rig. The first thing many people do is to have a locking differential installed. This article gives a brief description of lockers and other related devices.
When it comes to traction in off-road situations, differentials play a key role. In general, the differential in a vehicle's axle allows the outer wheel to rotate more quickly when the vehicle turns a corner. The outer wheel travels about 30 feet farther than the inner wheel in a 360-degree turn. A 4WD vehicle has one diff in each axle.
A full-time 4WD vehicle also has a center-diff in the transfer case, because the front wheels rotate more quickly than the rear ones when turning a corner and traveling forward.
A differential always:
distributes equal amounts of torque to both wheels
reacts to resistance/traction to allow a wheel with more resistance to rotate less and a wheel with less resistance to rotate more (The rpm difference created by the differential is always proportional. If the inside tire rotates 15 rpm less in a turn than going straight - then the outside tire will rotate 15 rpm more than going straight.)
Locking the differential is accomplished by doing one of two things:
1. Installing a locking center section which ALWAYS locks the differential
2. Installing a "lock-on-demand" unit like an ARB air locker
Open Differentials (what usually ships stock from the factory
Here's what happens if you're riding off-road and irregular terrain leaves one of your tires in the air or on a poor traction surface and you have open differentials:
With open differentials, power goes to the wheel with the least traction. So one tire on each axle may spin helplessly, despite having 4WD engaged. The differential action of open differentials is essential to stop tires scrubbing, to reduce transmission loads, and to reduce under steer during normal cornering. But if one wheel loses grip, due to slippery ground or through being in the air, the other wheel effectively loses all input torque and cannot propel the vehicle.
For example, a full-time 4WD, with an (unlocked) open center-diff loses traction if any one wheel does. A vehicle with 4WD engaged (or with a center-diff locked) and with open axle diffs loses traction if one wheel on each axle loses traction. such as getting crossed-up by being supported on diagonally opposite wheels, or if one side is on slippery ground.
Locking Differentials
Here's what happens if you're riding off-road and irregular terrain leaves one of your tires in the air or on a poor traction surface and you have locked differentials:
To get true 4WD, the differentials must be "locked".
Locking differentials (or diff locks) are the only cure for the shortcoming of differentials sometimes sending torque to the "wrong" wheel. Diff locks disable the differential's ability to distribute torque equally and its ability to allow the wheels to rotate at different speeds. In some ways a diff lock is the ultimate traction device. Since, all wheels rotate at the same speed when engaged, a vehicle with locks on all axles has traction if just one wheel does.
By disabling (or locking) the differential, steering becomes very difficult and the wheel with the most traction will get the most torque, as much as 100%. This guarantees that any wheel with traction will receive enough torque to move the car. In sum, diff locks will keep you going and keep you from getting stuck.
The drawbacks of diff locks:
Locking (disabling) the differential makes it impossible for wheels to roll at different speeds. Therefore, with lockers engaged, on high traction surfaces it becomes very difficult to make turns, and on low traction surfaces the turning radius gets very wide.
Unfortunately a diff' lock is on or off; there is no in-between. This can make the vehicle under steer (tend to go straight ahead in corners) or veer suddenly sideways if one side looses traction, like on ice.
These serious drawbacks of diff locks require an educated driver who knows exactly when to lock the diffs, usually only for a very brief moment, and when to unlock the diff to maintain steer
ability.
Limited Slip Differentials
Here's what happens if you're riding off-road and irregular terrain leaves one of your tires in the air or on a poor traction surface and you have limited slip differentials:
Some 4WD vehicle manufacturers offer a "limited slip differential" option. As the name implies, they limit wheel slip. These use clutch plates to lock the differential, and when turning on dry pavement they just slip.
Unfortunately, they also slip when there is a significant difference in traction between two wheels in an off-highway situation. Limited slip differentials do not prevent wheel slip; they only delay wheel slip. Therefore, traction is lost a little later than without limited slip, and you will be stuck a little later.
This article only information touches the surface of locking vs. open differentials.
More can be found here:
http://www.frontrange4x4.com/forums/showthread.php?5026-Locker-Information-General