Guide to 4x4, open and locked diffs, and Traction Control
05-11-2018, 01:59 PM
#1
Senior Member
Thread Starter
Guide to 4x4, open and locked diffs, and Traction Control
Hey all,
I wanted to create a little write-up on how 4x4 works, how open diffs and locked diffs work, and what Traction Control does.
I've seen a lot of questions and threads from people new to the truck community on this topic, so I thought it might be good to have some info all in one spot.
Everything below is related to regular late-model F-150's (not Raptors) and the 4x4 info is about standard 4x4 (4Hi and 4Lo), not the 4A found in some of the higher trims.
Open Differentials
This is one area where I see some of the most confusion. One of the most common things you might hear is "Open Differentials send power to the wheel with the least traction" or something to that effect. That's actually not correct. An Open Diff sends power to both wheels equally all of the time. The split is 50 / 50 left right, 100% of the time. That's not to say that one wheel can't be spinning faster than the other. That's how you get stuck on ice or in mud with one wheel spinning and the truck going nowhere. So let's backtrack and see how it works.
Open Diffs are necessary because when you go around a corner, the left and right tires have to turn at different speeds from each other because they travel different distances. The tires on the outside of the turn travel farther and have to spin faster. If both tires spun at the same rate, when you went around a corner, it would cause problems. That said, torque is applied to the left and right wheels 50 / 50.
Now I know many of you are thinking of a truck doing one-wheel peels, watching one wheel spin on ice or in the mud while the other wheel is stationary, etc etc. All of that can happen. We already know that the left and right wheels can turn at different rates. But it's not because the diff is "transferring power" to one wheel or another. It's because of the way the diff is designed. As a "side effect" of the design, and this next part is important: The total torque at the wheels at any given time (which is equal at both wheels) is the max amount of torque that can be applied until a wheel loses traction. So the amount of torque (or power, or force, or however you want to call it) to the wheels actually varies depending on traction. In most situations, when you have good traction, the torque that is split to each wheel is the full amount that is put out by the truck (after going through the transmission gears and diff gears). So on dry pavement, just pulling away from a stop light normally, you're getting all the torque available to you, split 50 / 50 left and right. Where things change is when traction is poor.
Let's use the classic example of one wheel on ice, the other on pavement. The max torque that will go to the wheels is the amount that causes a tire to lose traction. Just for discussion, let's say it only requires 10% of the total torque to spin the tire on ice. That means the total torque to each wheel is 10% of what would otherwise be available. That 10% is enough to spin the tire ice, but it is not enough power to turn the wheel that is on pavement. To an observer, you see the wheel on ice spinning wildly, while the wheel on pavement just sits there doing nothing. That's exactly what it looks like. But it's not because all the power is "going to the wheel on ice". It's because a very small amount of power is going to both wheels equally, and that power is enough to spin the one on ice, but not enough to turn the wheel on pavement. So any time you see a single wheel spinning in an open diff, you'll know that the torque to the wheels has been reduced to the amount that is required for that spinning wheel to continue spinning. Now you might say "well my truck always spins this wheel every time" or "in this situation my truck behaves like this". Nobody would dispute what you're saying is happening in your truck. But what's happening is due to the way the open diff is engineered, and your truck is behaving according to the principles above. If you floor your truck on dry pavement and leave one strip of rubber, it's because one wheel has broken loose of its traction, and the torque to the wheels is just enough to keep that tire spinning.
For more info on the 50 / 50 split see the following articles and a video at the bottom of this post:
https://auto.howstuffworks.com/differential3.htm
https://oppositelock.kinja.com/your-...rks-1661277563
Locked Differential
A locked differential is pretty easy to understand. Imagine a solid axle with both wheels attached such that both wheels must always turn at the same rate since they're both connected to the same axle. That's what "locking your diff" does. When you activate the locker, it essentially connects the left and right sides together. The left and right tires can no longer spin at different speeds like they can in an open diff. Why would you want this? Because it eliminates the "torque reduction" problem you have with an open diff. As discussed above, an open diff only transmits as much torque as can be handled by the tire with the least traction. A locked diff doesn't care what one individual tire is doing. Both tires are going to turn at the same rate no matter what. So in the one wheel on ice example, if your diff is locked, even though there is only a small amount of traction for the tire on ice, there is plenty of traction for the tire on pavement and the truck moves forward. The downside? The benefits of the open diff are gone. The open diff was designed so that when you go around corners, the wheels can spin at different speeds. A locked diff eliminates the ability of the wheels to spin at different speeds. So you really only want to use it in situations where there is poor traction. Mud is a great place to use a locked diff. I'll explain more when we talk about Traction Control.
4x4
Now that we know what's going on in the diffs, we can talk about 4x4. This is also pretty straightforward. When you engage 4Hi or 4Lo, the transfer case engages the front differential. The truck's power is now going to the rear diff and the front diff equally. In a situation where all four tires have traction, the power is evenly split and all four wheels drive the truck forward. The diffs still act the way they do when not in 4x4, You can have a variety of combinations of how traction can affect your ability to move. For example, in 4x4, you can still get stuck. If one rear tire and one front tire are on ice, you're stuck. Even though you have 4x4. The rear diff is only transmitting enough force to the wheels for the tire on ice to spin. The front diff is doing the exact same thing. You're stuck. If you had a locking rear diff, you could engage it and the one rear tire on pavement would help get you out. In a different example, if both rear tires were on ice, and both front tires were on pavement, you'd easily move along as both front tires would pull you out.
Traction Control
This has a variety of names, but I'm just going to call it Traction Control. Traction Control in late-model F-150's is a way of getting around the problems with an open diff, and it can work very well in certain situations. One of its main functions is to solve the "torque reduction" problem that occurs with an open diff. It does this by automatically applying the brakes to just the spinning wheel, in a situation where one tire is spinning because it has no traction. Let's say we're back at our example with one rear tire on ice, the other on pavement. Torque is reduced to 10% to both wheels. You're stuck. Traction Control kicks in. It applies the brakes to just the wheel on ice. That wheel slows down. Traction Control is "simulating" traction. There's not really any new traction, but to the open diff, it acts as if there is because the brakes are applied to that wheel. The torque available to both wheels goes up as a result, and the tire on pavement (which is not being braked by the Traction Control system) starts to roll and you move along. This system works very well in a lot of situations. There are some situations though, where you'd want to turn it off. For example, in mud or sand where several of the tires all have poor traction. In this case, the Traction Control system will constantly be detecting wheel slip and applying the brakes to many of the wheels at various times. In situations like mud where you want to maintain forward momentum, having Traction Control applying the brakes to your wheels is not what you want. In this case, it's better to turn it off. Earlier I said mud is a great place to use a locked diff. If you have a locking diff, you can lock it and turn off Traction Control. With the locker, you get the benefit of having both rear tires turning, no matter what the other tire is doing, and because of that traction advantage, you can turn off Traction Control and not worry about it applying the brakes while you're trying to maintain forward momentum.
Limited Slip Differentials
I'm not going to talk too much about Limited Slip Differentials, since late-model F-150's don't have them (apart from the Raptor). Just know that a Limited Slip is a mechanical addition to the diff that helps overcome the disadvantages of an open diff. I've heard some people refer to the Traction Control system as an "Electronic Limited Slip". That's probably not the best way to describe it. Traction Control and a Limited Slip aim to provide the same goal, but Traction Control applies the brakes and the diff is still open, while a Limited Slip is mechanical and achieves its goal in a different way. It's best not to confuse the two.
Update: Description of 4A here:
https://www.f150forum.com/f118/guide...3/#post5883772
Well, I hope this helps those of you new to the world of 4x4, and maybe sheds some light on how things work, even for those who have been wheelin' for a while.
I wanted to create a little write-up on how 4x4 works, how open diffs and locked diffs work, and what Traction Control does.
I've seen a lot of questions and threads from people new to the truck community on this topic, so I thought it might be good to have some info all in one spot.
Everything below is related to regular late-model F-150's (not Raptors) and the 4x4 info is about standard 4x4 (4Hi and 4Lo), not the 4A found in some of the higher trims.
Open Differentials
This is one area where I see some of the most confusion. One of the most common things you might hear is "Open Differentials send power to the wheel with the least traction" or something to that effect. That's actually not correct. An Open Diff sends power to both wheels equally all of the time. The split is 50 / 50 left right, 100% of the time. That's not to say that one wheel can't be spinning faster than the other. That's how you get stuck on ice or in mud with one wheel spinning and the truck going nowhere. So let's backtrack and see how it works.
Open Diffs are necessary because when you go around a corner, the left and right tires have to turn at different speeds from each other because they travel different distances. The tires on the outside of the turn travel farther and have to spin faster. If both tires spun at the same rate, when you went around a corner, it would cause problems. That said, torque is applied to the left and right wheels 50 / 50.
Now I know many of you are thinking of a truck doing one-wheel peels, watching one wheel spin on ice or in the mud while the other wheel is stationary, etc etc. All of that can happen. We already know that the left and right wheels can turn at different rates. But it's not because the diff is "transferring power" to one wheel or another. It's because of the way the diff is designed. As a "side effect" of the design, and this next part is important: The total torque at the wheels at any given time (which is equal at both wheels) is the max amount of torque that can be applied until a wheel loses traction. So the amount of torque (or power, or force, or however you want to call it) to the wheels actually varies depending on traction. In most situations, when you have good traction, the torque that is split to each wheel is the full amount that is put out by the truck (after going through the transmission gears and diff gears). So on dry pavement, just pulling away from a stop light normally, you're getting all the torque available to you, split 50 / 50 left and right. Where things change is when traction is poor.
Let's use the classic example of one wheel on ice, the other on pavement. The max torque that will go to the wheels is the amount that causes a tire to lose traction. Just for discussion, let's say it only requires 10% of the total torque to spin the tire on ice. That means the total torque to each wheel is 10% of what would otherwise be available. That 10% is enough to spin the tire ice, but it is not enough power to turn the wheel that is on pavement. To an observer, you see the wheel on ice spinning wildly, while the wheel on pavement just sits there doing nothing. That's exactly what it looks like. But it's not because all the power is "going to the wheel on ice". It's because a very small amount of power is going to both wheels equally, and that power is enough to spin the one on ice, but not enough to turn the wheel on pavement. So any time you see a single wheel spinning in an open diff, you'll know that the torque to the wheels has been reduced to the amount that is required for that spinning wheel to continue spinning. Now you might say "well my truck always spins this wheel every time" or "in this situation my truck behaves like this". Nobody would dispute what you're saying is happening in your truck. But what's happening is due to the way the open diff is engineered, and your truck is behaving according to the principles above. If you floor your truck on dry pavement and leave one strip of rubber, it's because one wheel has broken loose of its traction, and the torque to the wheels is just enough to keep that tire spinning.
For more info on the 50 / 50 split see the following articles and a video at the bottom of this post:
https://auto.howstuffworks.com/differential3.htm
https://oppositelock.kinja.com/your-...rks-1661277563
Locked Differential
A locked differential is pretty easy to understand. Imagine a solid axle with both wheels attached such that both wheels must always turn at the same rate since they're both connected to the same axle. That's what "locking your diff" does. When you activate the locker, it essentially connects the left and right sides together. The left and right tires can no longer spin at different speeds like they can in an open diff. Why would you want this? Because it eliminates the "torque reduction" problem you have with an open diff. As discussed above, an open diff only transmits as much torque as can be handled by the tire with the least traction. A locked diff doesn't care what one individual tire is doing. Both tires are going to turn at the same rate no matter what. So in the one wheel on ice example, if your diff is locked, even though there is only a small amount of traction for the tire on ice, there is plenty of traction for the tire on pavement and the truck moves forward. The downside? The benefits of the open diff are gone. The open diff was designed so that when you go around corners, the wheels can spin at different speeds. A locked diff eliminates the ability of the wheels to spin at different speeds. So you really only want to use it in situations where there is poor traction. Mud is a great place to use a locked diff. I'll explain more when we talk about Traction Control.
4x4
Now that we know what's going on in the diffs, we can talk about 4x4. This is also pretty straightforward. When you engage 4Hi or 4Lo, the transfer case engages the front differential. The truck's power is now going to the rear diff and the front diff equally. In a situation where all four tires have traction, the power is evenly split and all four wheels drive the truck forward. The diffs still act the way they do when not in 4x4, You can have a variety of combinations of how traction can affect your ability to move. For example, in 4x4, you can still get stuck. If one rear tire and one front tire are on ice, you're stuck. Even though you have 4x4. The rear diff is only transmitting enough force to the wheels for the tire on ice to spin. The front diff is doing the exact same thing. You're stuck. If you had a locking rear diff, you could engage it and the one rear tire on pavement would help get you out. In a different example, if both rear tires were on ice, and both front tires were on pavement, you'd easily move along as both front tires would pull you out.
Traction Control
This has a variety of names, but I'm just going to call it Traction Control. Traction Control in late-model F-150's is a way of getting around the problems with an open diff, and it can work very well in certain situations. One of its main functions is to solve the "torque reduction" problem that occurs with an open diff. It does this by automatically applying the brakes to just the spinning wheel, in a situation where one tire is spinning because it has no traction. Let's say we're back at our example with one rear tire on ice, the other on pavement. Torque is reduced to 10% to both wheels. You're stuck. Traction Control kicks in. It applies the brakes to just the wheel on ice. That wheel slows down. Traction Control is "simulating" traction. There's not really any new traction, but to the open diff, it acts as if there is because the brakes are applied to that wheel. The torque available to both wheels goes up as a result, and the tire on pavement (which is not being braked by the Traction Control system) starts to roll and you move along. This system works very well in a lot of situations. There are some situations though, where you'd want to turn it off. For example, in mud or sand where several of the tires all have poor traction. In this case, the Traction Control system will constantly be detecting wheel slip and applying the brakes to many of the wheels at various times. In situations like mud where you want to maintain forward momentum, having Traction Control applying the brakes to your wheels is not what you want. In this case, it's better to turn it off. Earlier I said mud is a great place to use a locked diff. If you have a locking diff, you can lock it and turn off Traction Control. With the locker, you get the benefit of having both rear tires turning, no matter what the other tire is doing, and because of that traction advantage, you can turn off Traction Control and not worry about it applying the brakes while you're trying to maintain forward momentum.
Limited Slip Differentials
I'm not going to talk too much about Limited Slip Differentials, since late-model F-150's don't have them (apart from the Raptor). Just know that a Limited Slip is a mechanical addition to the diff that helps overcome the disadvantages of an open diff. I've heard some people refer to the Traction Control system as an "Electronic Limited Slip". That's probably not the best way to describe it. Traction Control and a Limited Slip aim to provide the same goal, but Traction Control applies the brakes and the diff is still open, while a Limited Slip is mechanical and achieves its goal in a different way. It's best not to confuse the two.
Update: Description of 4A here:
https://www.f150forum.com/f118/guide...3/#post5883772
Well, I hope this helps those of you new to the world of 4x4, and maybe sheds some light on how things work, even for those who have been wheelin' for a while.
Last edited by Florida_F150; 08-17-2018 at 12:54 PM.
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05-11-2018, 11:41 PM
#2
Member
Great writeup. Clarifies a few things so that I better know what to do with 4x4 and slicker. Any differences i should note about 4a or what's the difference between 4h and 4l?
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Florida_F150 (05-12-2018)
05-12-2018, 12:08 AM
#3
2018 XL Sport 4x4
Great thread, thanks!
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Florida_F150 (05-12-2018)
05-12-2018, 06:35 AM
#4
Senior Member
Thread Starter
Thanks all, I'm glad you found it helpful.
I'll put some info on 4Lo and 4A below:
4Lo
When you engage 4Lo, you're using different gearing than you use in 4Hi. The 4Lo gearing does two primary things at the same time:
1) It multiples the torque that goes to the wheels so you have more pulling power. This may be useful if you're going up a steep hill with a heavy load, for example
2) It causes the truck to travel less distance when you press the gas pedal. This might be useful if you're rock crawling for example. You don't want the gas pedal to be too "touchy" where a tiny press of it would make you launch off the rock you're perched on. 4Lo allows you to "inch along" very carefully.
While 4Lo can be useful in certain specific circumstances, 4Hi is the more "general use" 4WD mode.
4A
Before we talk about 4A, let's recap how the "standard" 4WD modes (4Hi / 4Lo) work. When you put your truck in 4Hi or 4Lo, the transfer case essentially locks the front of the truck to the back of the truck. Similarly to how a locked differential locks the left and right wheels together, 4Hi / 4Lo essentially locks the front driveshaft to the rear driveshaft so they turn as one. They will always turn at the same speed, together. This is fine if you're traveling straight and are in slippery conditions. But now imagine if you're on dry pavement and you try to make a U-turn. When you make a U-turn, the front of your truck swings around more than the rear, and so the front tires travel a greater distance. If your front driveshaft is connected to your rear driveshaft and you try to make a U-turn, it's not good. The front driveshaft wants to spin at a faster rate than the rear, but they're connected. That's why you should never use 4Hi / 4Lo on pavement.
4A, on trucks that have it, uses a different method to engage the front wheels. Instead of a "locked" 50 / 50 split front and rear like when you're in 4Hi / 4Lo, 4A allows the front to act independently of the rear. This allows you to go around turns and use 4A in a wider variety of situations. In slick conditions, you can leave 4A on, even on pavement, and it will automatically engage the front wheels as necessary.
Hope that helps.
BTW, I meant to post the video about open diffs in my original post. Here it is:
I'll put some info on 4Lo and 4A below:
4Lo
When you engage 4Lo, you're using different gearing than you use in 4Hi. The 4Lo gearing does two primary things at the same time:
1) It multiples the torque that goes to the wheels so you have more pulling power. This may be useful if you're going up a steep hill with a heavy load, for example
2) It causes the truck to travel less distance when you press the gas pedal. This might be useful if you're rock crawling for example. You don't want the gas pedal to be too "touchy" where a tiny press of it would make you launch off the rock you're perched on. 4Lo allows you to "inch along" very carefully.
While 4Lo can be useful in certain specific circumstances, 4Hi is the more "general use" 4WD mode.
4A
Before we talk about 4A, let's recap how the "standard" 4WD modes (4Hi / 4Lo) work. When you put your truck in 4Hi or 4Lo, the transfer case essentially locks the front of the truck to the back of the truck. Similarly to how a locked differential locks the left and right wheels together, 4Hi / 4Lo essentially locks the front driveshaft to the rear driveshaft so they turn as one. They will always turn at the same speed, together. This is fine if you're traveling straight and are in slippery conditions. But now imagine if you're on dry pavement and you try to make a U-turn. When you make a U-turn, the front of your truck swings around more than the rear, and so the front tires travel a greater distance. If your front driveshaft is connected to your rear driveshaft and you try to make a U-turn, it's not good. The front driveshaft wants to spin at a faster rate than the rear, but they're connected. That's why you should never use 4Hi / 4Lo on pavement.
4A, on trucks that have it, uses a different method to engage the front wheels. Instead of a "locked" 50 / 50 split front and rear like when you're in 4Hi / 4Lo, 4A allows the front to act independently of the rear. This allows you to go around turns and use 4A in a wider variety of situations. In slick conditions, you can leave 4A on, even on pavement, and it will automatically engage the front wheels as necessary.
Hope that helps.
BTW, I meant to post the video about open diffs in my original post. Here it is:
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ColoFatherOf3 (08-17-2018)
05-12-2018, 07:29 AM
#5
Member
Thanks for the additional info!
So does 4a affect mpg much if there is no loss of traction in the front? I imagine there is some loss due to spinning what I assume is some sort of clutch.
Trying to integrate it all... sounds like highway travel during snow/ice conditions would be best handled in 4a with locker? Will the locked diff affect steering capability enough where you wouldn't want to use it with twisty roads or obstacles (i.e. mustangs and challengers with summer tires)?
So does 4a affect mpg much if there is no loss of traction in the front? I imagine there is some loss due to spinning what I assume is some sort of clutch.
Trying to integrate it all... sounds like highway travel during snow/ice conditions would be best handled in 4a with locker? Will the locked diff affect steering capability enough where you wouldn't want to use it with twisty roads or obstacles (i.e. mustangs and challengers with summer tires)?
05-12-2018, 11:00 AM
#7
Senior Member
Thread Starter
Thanks for the additional info!
So does 4a affect mpg much if there is no loss of traction in the front? I imagine there is some loss due to spinning what I assume is some sort of clutch.
Trying to integrate it all... sounds like highway travel during snow/ice conditions would be best handled in 4a with locker? Will the locked diff affect steering capability enough where you wouldn't want to use it with twisty roads or obstacles (i.e. mustangs and challengers with summer tires)?
So does 4a affect mpg much if there is no loss of traction in the front? I imagine there is some loss due to spinning what I assume is some sort of clutch.
Trying to integrate it all... sounds like highway travel during snow/ice conditions would be best handled in 4a with locker? Will the locked diff affect steering capability enough where you wouldn't want to use it with twisty roads or obstacles (i.e. mustangs and challengers with summer tires)?
Highway during snow / ice, 4A would be great to use. I wouldn't use the locker for highway as it might cause fishtailing or other unwanted steering issues. You could certainly use it if you were stuck in a snow bank and needed to get out, but I wouldn't drive around with it on. I think of the locker as a brief temporary helper to get me out of or through some difficult terrain. If I'm about to go through a big mud pit, or over some uneven terrain where a tire might come off the ground, things like that. I engage it, get through the obstacle, and turn it off.
Last edited by Florida_F150; 05-12-2018 at 11:15 AM.
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05-12-2018, 11:05 AM
#8
Senior Member
Thread Starter
That's a great combo. 2wd, locked diff with traction control off will get you through a lot of stuff. Pair that with a decent All Terrain tire (or "Hybrid Terrain" -- somewhere between an All Terrain and a Mud Terrain) and you'd have a great setup for a daily driver that also sees some general trail riding, hunting trails, etc.
Last edited by Florida_F150; 05-12-2018 at 11:08 AM.
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Florida_F150 (05-12-2018)
05-12-2018, 11:20 AM
#10
Old Fart
Great, just what we need another sticky at the top of the forum! Seriously, if this doesn't qualify nothing does.
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Florida_F150 (05-12-2018)