Lockers & Limited Slip Differentials
BusaDave9
Always Off-Roading Jeeper
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- Location
- Durango, Crawlarado
- Vehicle(s)
- 1979 Jeep CJ5 with 304 V8, T18 Transmission, Dana 20 Transfer case with TeraLow 3:1 gears, 4.88 axle gears, Detroit Locker up front and Ox Locker in back with 1 piece axle shafts, 36" SuperSwamper SX Tires, Shackle reversal, MileMarker Hydraulic winch, MSD 6A ignition.
Types of Traction Aids
Open Differential
The open differential is what is installed in the majority of all vehicles from the factory. It isn’t a traction aid. If one tire is on poor traction or starts to lift off the ground it will spin and the other tire won’t be able to apply any more torque to the ground either.
Advantage: excellent directional stability. If one tire spins or looses traction the other tire can keep the vehicle moving in the correct direction.
Disadvantage: You are most likely to get stuck with an open differential.
Limited Slips
A Limited slip differential will aid traction. If one tire starts to slip a Limited slip will apply more torque to the tire that has the best traction. The amount of torque that it applies to the tire with the best traction depends on the bias ratio of the Limited slip. The higher the bias ratio the more aggressive the Limited slip is and the more torque the Limited slip can deliver to the tire with the best traction. A Limited slip with a bias ratio of 4:1 (also called a 60% locking factor) can deliver 4 times more torque to one tire than the other.
There are three types of Limited slips:
Plate clutch Limited slip – these differentials have alternating friction plates. You can tune these Limited slip differentials by changing the order and number of friction plates to get a very high or very low bias ratio. Needs a special oil or oil additive for the clutch such as Amsoil Slip Lock.
Cone Clutch Limited slip – These differentials have a cone shaped clutch on each axle shaft that grips the differential to help send torque to the tire with the best traction. These Limited slip differentials are not tunable. In many cases they are not even rebuildable. This is because the cone grips the differential case itself. When the case is worn the whole differential must be replace. Also needs a special oil additive.
Gear type Limited slip – These Limited slips are a much different design from other differentials. The specially designed gears bind as one tire spins. This type of Limited slip does not require a special additive in the oil.
Automatic Lockers
A locking differential can deliver 100% of the torque to one or both tires even if one tire is off the ground. A locker can unlock to allow some differential action going around turns.
Lockers provide excellent traction to keep the vehicle from getting stuck but they also have the poor directional stability. If you give it a little too much gas going around a turn you can fishtail and loose traction. A locker can always keep you moving but you may not be moving the direction you want. This is not a big problem as long as you are careful when you drive on the street.
Selectable Lockers
A selectable locker (also called On-Demand locker) can be turned off or on. This is the best of both worlds. You can have an open differential for street use when you want control. Then when you are off-road and worried about getting stuck you can activate your locker and both tires on the axle will rotate in unison. Selectable lockers activate with different methods. Some use compressed air to lock the differential. Some are cable activated and the driver moves a small lever. Some selectable lockers are electrical and the driver presses a button to activate the locker.
Selectable lockers have two disadvantages:
1. They cost more
2. They are more involved to install. They may need an air compressor, air lines, cable or wiring.
Spool
A spool will rotate both axle shafts in unison at all times. They are not recommended for street use.
Spool
Credits: Information for this thread was gathered from several sources. Unless otherwise stated photographs came from the manufacturer's web sites.
The most important source of information was the book "Differentials: Identification, Restoration & Repair" by Randy Lyman and Jim Allen. An excellent book on the topic.
Open Differential
The open differential is what is installed in the majority of all vehicles from the factory. It isn’t a traction aid. If one tire is on poor traction or starts to lift off the ground it will spin and the other tire won’t be able to apply any more torque to the ground either.
Advantage: excellent directional stability. If one tire spins or looses traction the other tire can keep the vehicle moving in the correct direction.
Disadvantage: You are most likely to get stuck with an open differential.
Limited Slips
A Limited slip differential will aid traction. If one tire starts to slip a Limited slip will apply more torque to the tire that has the best traction. The amount of torque that it applies to the tire with the best traction depends on the bias ratio of the Limited slip. The higher the bias ratio the more aggressive the Limited slip is and the more torque the Limited slip can deliver to the tire with the best traction. A Limited slip with a bias ratio of 4:1 (also called a 60% locking factor) can deliver 4 times more torque to one tire than the other.
There are three types of Limited slips:
Plate clutch Limited slip – these differentials have alternating friction plates. You can tune these Limited slip differentials by changing the order and number of friction plates to get a very high or very low bias ratio. Needs a special oil or oil additive for the clutch such as Amsoil Slip Lock.
Cone Clutch Limited slip – These differentials have a cone shaped clutch on each axle shaft that grips the differential to help send torque to the tire with the best traction. These Limited slip differentials are not tunable. In many cases they are not even rebuildable. This is because the cone grips the differential case itself. When the case is worn the whole differential must be replace. Also needs a special oil additive.
Gear type Limited slip – These Limited slips are a much different design from other differentials. The specially designed gears bind as one tire spins. This type of Limited slip does not require a special additive in the oil.
Automatic Lockers
A locking differential can deliver 100% of the torque to one or both tires even if one tire is off the ground. A locker can unlock to allow some differential action going around turns.
Lockers provide excellent traction to keep the vehicle from getting stuck but they also have the poor directional stability. If you give it a little too much gas going around a turn you can fishtail and loose traction. A locker can always keep you moving but you may not be moving the direction you want. This is not a big problem as long as you are careful when you drive on the street.
Selectable Lockers
A selectable locker (also called On-Demand locker) can be turned off or on. This is the best of both worlds. You can have an open differential for street use when you want control. Then when you are off-road and worried about getting stuck you can activate your locker and both tires on the axle will rotate in unison. Selectable lockers activate with different methods. Some use compressed air to lock the differential. Some are cable activated and the driver moves a small lever. Some selectable lockers are electrical and the driver presses a button to activate the locker.
Selectable lockers have two disadvantages:
1. They cost more
2. They are more involved to install. They may need an air compressor, air lines, cable or wiring.
Spool
A spool will rotate both axle shafts in unison at all times. They are not recommended for street use.
Spool
The most important source of information was the book "Differentials: Identification, Restoration & Repair" by Randy Lyman and Jim Allen. An excellent book on the topic.
