LSD or Locking Differential?Please help!
MMNC (SS)(Ret)
Joined: Mar 2004
Posts: 11,598
Likes: 145
From: SE Georgia
2008 and older have a Limited Slip (LSD) available...not always installed though. FX4 has one guaranteed.
2009 + get an optional Limited Slip as well
2009 + FX4 gets a Locking Diff...only this model though
2009 + get an optional Limited Slip as well
2009 + FX4 gets a Locking Diff...only this model though
Elder User
Joined: Dec 2006
Posts: 559
Likes: 4
From: On a farm.
AFAIK, the FX4 has a Torsen, which is a gear type limited slip. It's got a much higher bias ratio than the wimpy Ford Traction-Lock, so it's more effective... but it ain't a locker. To qualify as that, it has to be able to deliver ALL the torque to either axle or both. A limited slip of any type can't do that. It can only deliver a percentage.
If a limited slip has a 2:1 bias ratio (many Traction Locks are around 1.8:1), it can deliver twice the torque to the high traction side as it has on the low traction side. In other words, if the low traction side of the axle has 100 lbs-ft of "resistance" (a combination of whatever tire traction there is, built in preload in the clutch packs, plus gear separation forces), then the diff can shuttle 200 lbs-ft to the high traction tire. That might be enough to pull you outta the mud, or not
Gear separation force is where the side and spider gears try to push away from each other under a torque load. The side gears, under which are the clutch packs, push outwards in this situation and add to the clamping forces being applied. Thing is, the less traction you have, the less gear separation force you have, so the effect is somewhat variable.
If you have zero tire traction on the low traction side (lifted tire), then you have lost all that, plus most of the gear separation force and all you are left with is the clutch preload. The clutch packs of limited slips are preloaded with various types of springs (coil, Belleville and S-springs are common). Traction-Locks have very little preload so they will operate smoothly on the street. A LS with high preload will bark tires and chatter in turns. A lightly preloaded LS will be easily overcome by side to side differences in traction. Bad manners get very notable on clutch type limited slips as you go past a 3:1 bias ratio, though less so with gear type limited slips.
Gear type limited slips use gear friction to apply the "resistance" on the low traction side. The easiest way to think about gear type LS is to use the worm gear principle as an example. With a worm gear, power can only be transmitted in one direction. Drive the worm and the unit turns freely. Try to drive the gear and there is lots of resistance. In the gear type LS, the axle is the "worm." When driving from the ring gear side, the unit holds and drive the vehicle forward. In turns, the outer wheel speeds up and because it's the "worm" it can do that while still transmitting power. It's more complicated that this in reality, because gear type limited slips aren't true worm gears, but the angles of the side and element gears come real close to being worm gears. The opposing angles of the gears, and their number, determine the bias ratio of the unit. The resistance of the gearing can be overcome by wide variances in traction side to side and gear types LS have no "preload."
The driving trick with LS is to augment "resistance" on the low traction side by using the brakes. If you apply the parking brake a little, for example, the diff sees that as more low traction side "resistance," so it will transfer more torque to the high traction side. You can use the footbrakes too, but tht applies all the brakes and increases the traction needed to push the vehicle forward. If you can lock the brakes individually and apply them on the low traction axle, even with an open diff, it will transfer all or most of the torque to the high traction side. That's how you can augment any limited slip. Google "brake pedal modulation" in connection with Humvees, which use gear type limited slips, and you'll see how the Army teaches troops to use the Torsen diffs in the Hummer to great advantage. The principle works to a degree on all open diffs and limited slips. In a nutshell, brake pedal modulation is either a light application of the brakes or an on-off tapping of the pedal.
A long post, but hopefully useful.
If a limited slip has a 2:1 bias ratio (many Traction Locks are around 1.8:1), it can deliver twice the torque to the high traction side as it has on the low traction side. In other words, if the low traction side of the axle has 100 lbs-ft of "resistance" (a combination of whatever tire traction there is, built in preload in the clutch packs, plus gear separation forces), then the diff can shuttle 200 lbs-ft to the high traction tire. That might be enough to pull you outta the mud, or not
Gear separation force is where the side and spider gears try to push away from each other under a torque load. The side gears, under which are the clutch packs, push outwards in this situation and add to the clamping forces being applied. Thing is, the less traction you have, the less gear separation force you have, so the effect is somewhat variable.
If you have zero tire traction on the low traction side (lifted tire), then you have lost all that, plus most of the gear separation force and all you are left with is the clutch preload. The clutch packs of limited slips are preloaded with various types of springs (coil, Belleville and S-springs are common). Traction-Locks have very little preload so they will operate smoothly on the street. A LS with high preload will bark tires and chatter in turns. A lightly preloaded LS will be easily overcome by side to side differences in traction. Bad manners get very notable on clutch type limited slips as you go past a 3:1 bias ratio, though less so with gear type limited slips.
Gear type limited slips use gear friction to apply the "resistance" on the low traction side. The easiest way to think about gear type LS is to use the worm gear principle as an example. With a worm gear, power can only be transmitted in one direction. Drive the worm and the unit turns freely. Try to drive the gear and there is lots of resistance. In the gear type LS, the axle is the "worm." When driving from the ring gear side, the unit holds and drive the vehicle forward. In turns, the outer wheel speeds up and because it's the "worm" it can do that while still transmitting power. It's more complicated that this in reality, because gear type limited slips aren't true worm gears, but the angles of the side and element gears come real close to being worm gears. The opposing angles of the gears, and their number, determine the bias ratio of the unit. The resistance of the gearing can be overcome by wide variances in traction side to side and gear types LS have no "preload."
The driving trick with LS is to augment "resistance" on the low traction side by using the brakes. If you apply the parking brake a little, for example, the diff sees that as more low traction side "resistance," so it will transfer more torque to the high traction side. You can use the footbrakes too, but tht applies all the brakes and increases the traction needed to push the vehicle forward. If you can lock the brakes individually and apply them on the low traction axle, even with an open diff, it will transfer all or most of the torque to the high traction side. That's how you can augment any limited slip. Google "brake pedal modulation" in connection with Humvees, which use gear type limited slips, and you'll see how the Army teaches troops to use the Torsen diffs in the Hummer to great advantage. The principle works to a degree on all open diffs and limited slips. In a nutshell, brake pedal modulation is either a light application of the brakes or an on-off tapping of the pedal.
A long post, but hopefully useful.
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Senior User
Joined: Jan 2008
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Likes: 0
AFAIK, the FX4 has a Torsen, which is a gear type limited slip. It's got a much higher bias ratio than the wimpy Ford Traction-Lock, so it's more effective... but it ain't a locker. To qualify as that, it has to be able to deliver ALL the torque to either axle or both. A limited slip of any type can't do that. It can only deliver a percentage.
If a limited slip has a 2:1 bias ratio (many Traction Locks are around 1.8:1), it can deliver twice the torque to the high traction side as it has on the low traction side. In other words, if the low traction side of the axle has 100 lbs-ft of "resistance" (a combination of whatever tire traction there is, built in preload in the clutch packs, plus gear separation forces), then the diff can shuttle 200 lbs-ft to the high traction tire. That might be enough to pull you outta the mud, or not
Gear separation force is where the side and spider gears try to push away from each other under a torque load. The side gears, under which are the clutch packs, push outwards in this situation and add to the clamping forces being applied. Thing is, the less traction you have, the less gear separation force you have, so the effect is somewhat variable.
If you have zero tire traction on the low traction side (lifted tire), then you have lost all that, plus most of the gear separation force and all you are left with is the clutch preload. The clutch packs of limited slips are preloaded with various types of springs (coil, Belleville and S-springs are common). Traction-Locks have very little preload so they will operate smoothly on the street. A LS with high preload will bark tires and chatter in turns. A lightly preloaded LS will be easily overcome by side to side differences in traction. Bad manners get very notable on clutch type limited slips as you go past a 3:1 bias ratio, though less so with gear type limited slips.
Gear type limited slips use gear friction to apply the "resistance" on the low traction side. The easiest way to think about gear type LS is to use the worm gear principle as an example. With a worm gear, power can only be transmitted in one direction. Drive the worm and the unit turns freely. Try to drive the gear and there is lots of resistance. In the gear type LS, the axle is the "worm." When driving from the ring gear side, the unit holds and drive the vehicle forward. In turns, the outer wheel speeds up and because it's the "worm" it can do that while still transmitting power. It's more complicated that this in reality, because gear type limited slips aren't true worm gears, but the angles of the side and element gears come real close to being worm gears. The opposing angles of the gears, and their number, determine the bias ratio of the unit. The resistance of the gearing can be overcome by wide variances in traction side to side and gear types LS have no "preload."
The driving trick with LS is to augment "resistance" on the low traction side by using the brakes. If you apply the parking brake a little, for example, the diff sees that as more low traction side "resistance," so it will transfer more torque to the high traction side. You can use the footbrakes too, but tht applies all the brakes and increases the traction needed to push the vehicle forward. If you can lock the brakes individually and apply them on the low traction axle, even with an open diff, it will transfer all or most of the torque to the high traction side. That's how you can augment any limited slip. Google "brake pedal modulation" in connection with Humvees, which use gear type limited slips, and you'll see how the Army teaches troops to use the Torsen diffs in the Hummer to great advantage. The principle works to a degree on all open diffs and limited slips. In a nutshell, brake pedal modulation is either a light application of the brakes or an on-off tapping of the pedal.
A long post, but hopefully useful.
If a limited slip has a 2:1 bias ratio (many Traction Locks are around 1.8:1), it can deliver twice the torque to the high traction side as it has on the low traction side. In other words, if the low traction side of the axle has 100 lbs-ft of "resistance" (a combination of whatever tire traction there is, built in preload in the clutch packs, plus gear separation forces), then the diff can shuttle 200 lbs-ft to the high traction tire. That might be enough to pull you outta the mud, or not
Gear separation force is where the side and spider gears try to push away from each other under a torque load. The side gears, under which are the clutch packs, push outwards in this situation and add to the clamping forces being applied. Thing is, the less traction you have, the less gear separation force you have, so the effect is somewhat variable.
If you have zero tire traction on the low traction side (lifted tire), then you have lost all that, plus most of the gear separation force and all you are left with is the clutch preload. The clutch packs of limited slips are preloaded with various types of springs (coil, Belleville and S-springs are common). Traction-Locks have very little preload so they will operate smoothly on the street. A LS with high preload will bark tires and chatter in turns. A lightly preloaded LS will be easily overcome by side to side differences in traction. Bad manners get very notable on clutch type limited slips as you go past a 3:1 bias ratio, though less so with gear type limited slips.
Gear type limited slips use gear friction to apply the "resistance" on the low traction side. The easiest way to think about gear type LS is to use the worm gear principle as an example. With a worm gear, power can only be transmitted in one direction. Drive the worm and the unit turns freely. Try to drive the gear and there is lots of resistance. In the gear type LS, the axle is the "worm." When driving from the ring gear side, the unit holds and drive the vehicle forward. In turns, the outer wheel speeds up and because it's the "worm" it can do that while still transmitting power. It's more complicated that this in reality, because gear type limited slips aren't true worm gears, but the angles of the side and element gears come real close to being worm gears. The opposing angles of the gears, and their number, determine the bias ratio of the unit. The resistance of the gearing can be overcome by wide variances in traction side to side and gear types LS have no "preload."
The driving trick with LS is to augment "resistance" on the low traction side by using the brakes. If you apply the parking brake a little, for example, the diff sees that as more low traction side "resistance," so it will transfer more torque to the high traction side. You can use the footbrakes too, but tht applies all the brakes and increases the traction needed to push the vehicle forward. If you can lock the brakes individually and apply them on the low traction axle, even with an open diff, it will transfer all or most of the torque to the high traction side. That's how you can augment any limited slip. Google "brake pedal modulation" in connection with Humvees, which use gear type limited slips, and you'll see how the Army teaches troops to use the Torsen diffs in the Hummer to great advantage. The principle works to a degree on all open diffs and limited slips. In a nutshell, brake pedal modulation is either a light application of the brakes or an on-off tapping of the pedal.
A long post, but hopefully useful.
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