posi axle question
Lead Driver
Joined: Feb 2013
Posts: 5,006
Likes: 72
From: Roseville, MN
How differentials split torque and power always gets confusing. Open diffs send equal torque to both sides (as has already been said). But since power equals torque times speed, if that equal torque is spinning one tire and the other is sitting still, the tire that's not moving is getting no power (any amount of torque time zero speed = zero power). So it's accurate to say that an open diff sends all of the power to the tire with no traction.
Lockers and spools don't try to bias the torque, they just turn both tires the same speed. So if one tire is in the air and takes no torque to turn, it's getting no power because any speed times zero torque = zero power. So it's accurate to say that a locker sends all of the power and all of the torque to the tire that has traction.
As far as "true posi," that phrase doesn't mean much to me. No tire gets "positive" traction, they all rely on friction which is always a tenuous thing, not "positive" at all. Since all differentials are limited by the tire traction (although not in the same ways), none can give truly positive traction.
TrueTrac and Gleason Torsen are both pretty slick in that they allow full differential action (like an open diff) while giving limited slip operation as well. Clutch-type limited slips, to the extent that they limit slip, also limit the differential action. In the extreme, a really tight clutch-type limited slip will function exactly like a spool, with no differential action. A really aggressive TrueTrac will still freely allow the tire to go different speeds around a corner.
And a locker will only send torque to the inside tire in a corner (unless it spins to match the outside tire speed), while a TrueTrac will keep driving both tires, giving it's normal limited slip performance, even when going around a corner.
TureTracs (and Torsens) aren't perfect either. As EPNCSU2006 says, if one tire gets zero traction and will support zero torque, it will send 3 - 6 times zero (= 0) to the other tire as well. Either a clutch-type limited slip or a locker would work better in that situation.
Lockers and spools don't try to bias the torque, they just turn both tires the same speed. So if one tire is in the air and takes no torque to turn, it's getting no power because any speed times zero torque = zero power. So it's accurate to say that a locker sends all of the power and all of the torque to the tire that has traction.
As far as "true posi," that phrase doesn't mean much to me. No tire gets "positive" traction, they all rely on friction which is always a tenuous thing, not "positive" at all. Since all differentials are limited by the tire traction (although not in the same ways), none can give truly positive traction.
TrueTrac and Gleason Torsen are both pretty slick in that they allow full differential action (like an open diff) while giving limited slip operation as well. Clutch-type limited slips, to the extent that they limit slip, also limit the differential action. In the extreme, a really tight clutch-type limited slip will function exactly like a spool, with no differential action. A really aggressive TrueTrac will still freely allow the tire to go different speeds around a corner.
And a locker will only send torque to the inside tire in a corner (unless it spins to match the outside tire speed), while a TrueTrac will keep driving both tires, giving it's normal limited slip performance, even when going around a corner.
TureTracs (and Torsens) aren't perfect either. As EPNCSU2006 says, if one tire gets zero traction and will support zero torque, it will send 3 - 6 times zero (= 0) to the other tire as well. Either a clutch-type limited slip or a locker would work better in that situation.
Lead Driver
Joined: Feb 2013
Posts: 5,006
Likes: 72
From: Roseville, MN
For off-road I would definitely use an automatic locker in the rear. They are a little quirky on-road, but I put about 100k miles on an F-150 daily driver with one, and it's certainly livable. It's WAY more positive in low traction situations than any limited slip. And it's simpler to use and cheaper than a selectable.
I don't have much experience with traction-aiding diffs in the front (yet). I had an automatic locker in the front of an F-150 briefly. That was scary dangerous on a snow-covered freeway, so I went back to an open diff. In slow off-road it might be OK, but I'd never recommend an automatic locker for the front of anything that will be driven in 4WD at any speed.
I don't think I'd recommend a TrueTrac for serious off-road use either. When you get crossed up you get one tire on each axle that supports zero torque. Getting 3 - 6 times zero on the other tire is still zero.
So when I eventually put a traction aiding diff in the front of my Bronco I'm planning on putting a selectable locker in it. Open for higher speed driving so you don't have the stability problems of a non-open diff, but positive when you need it.
Having mentioned the stability problems in the front I'll expand on that a bit. When a front tire is driving, it's trying to pivot around the ball joint. That would lead to torque steer, except that the other front tire is doing the same thing in the opposite direction. And since open diffs send the same torque to both tires, it always balances out. But put a locker in there and what do you think happens when one tire gets no traction and all of the torque goes to the other? You get a LOT of torque steer. If it were to happen when you were hypothetically changing lanes on a snowy freeway, as the right front tire hit the ridge of snow between lanes it could lose traction. Then the left front tire would get all the power and the truck would pull to the right. Hypothetically. The first time that happened to me the truck had moved 10 feet to the right before I could catch it. The second time I was ready for it and it only jumped over about 5 feet. That was an automatic locker, which is probably the harshest. But even a TrueTrac or a clutch-type limited slip would have the same effect, just probably not quite to the same extent. That's why I'll never recommend anything other than an open diff, or a selectable that can be driven open, for the front axle of a truck that will be driven in 4WD at any speed.
Elder User
Joined: Jan 2014
Posts: 513
Likes: 1
From: smock pa
Personally I'd never choose a clutch-type limited slip for anything (edit: love how every once in a while something adds a link to a post, in this case to a limited slip gear oil additive). If they're set up "loose" they're no better than an open diff, and if they're set up "tight" they lean toward a spool. If I wanted an open diff I'd just have one. And I don't want a spool. So for me the choices fall between an automatic locker (like a Detroit), a selectable locker (like an ARB), and a clutch-type limited slip (like a TrueTrac)
For off-road I would definitely use an automatic locker in the rear. They are a little quirky on-road, but I put about 100k miles on an F-150 daily driver with one, and it's certainly livable. It's WAY more positive in low traction situations than any limited slip. And it's simpler to use and cheaper than a selectable.
I don't have much experience with traction-aiding diffs in the front (yet). I had an automatic locker in the front of an F-150 briefly. That was scary dangerous on a snow-covered freeway, so I went back to an open diff. In slow off-road it might be OK, but I'd never recommend an automatic locker for the front of anything that will be driven in 4WD at any speed.
I don't think I'd recommend a TrueTrac for serious off-road use either. When you get crossed up you get one tire on each axle that supports zero torque. Getting 3 - 6 times zero on the other tire is still zero.
So when I eventually put a traction aiding diff in the front of my Bronco I'm planning on putting a selectable locker in it. Open for higher speed driving so you don't have the stability problems of a non-open diff, but positive when you need it.
Having mentioned the stability problems in the front I'll expand on that a bit. When a front tire is driving, it's trying to pivot around the ball joint. That would lead to torque steer, except that the other front tire is doing the same thing in the opposite direction. And since open diffs send the same torque to both tires, it always balances out. But put a locker in there and what do you think happens when one tire gets no traction and all of the torque goes to the other? You get a LOT of torque steer. If it were to happen when you were hypothetically changing lanes on a snowy freeway, as the right front tire hit the ridge of snow between lanes it could lose traction. Then the left front tire would get all the power ant the truck would pull to the right. Hypothetically. The first time that happened to me the truck had moved 10 feet to the right before I could catch it. The second time I was ready for it and it only jumped over about 5 feet. That was an automatic locker, which is probably the harshest. But even a TrueTrac or a clutch-type limited slip would have the same effect, just probably not quite to the same extent. That's why I'll never recommend anything other than an open diff, or a selectable that can be driven open, for the front axle of a truck that will be driven in 4WD at any speed.
For off-road I would definitely use an automatic locker in the rear. They are a little quirky on-road, but I put about 100k miles on an F-150 daily driver with one, and it's certainly livable. It's WAY more positive in low traction situations than any limited slip. And it's simpler to use and cheaper than a selectable.
I don't have much experience with traction-aiding diffs in the front (yet). I had an automatic locker in the front of an F-150 briefly. That was scary dangerous on a snow-covered freeway, so I went back to an open diff. In slow off-road it might be OK, but I'd never recommend an automatic locker for the front of anything that will be driven in 4WD at any speed.
I don't think I'd recommend a TrueTrac for serious off-road use either. When you get crossed up you get one tire on each axle that supports zero torque. Getting 3 - 6 times zero on the other tire is still zero.
So when I eventually put a traction aiding diff in the front of my Bronco I'm planning on putting a selectable locker in it. Open for higher speed driving so you don't have the stability problems of a non-open diff, but positive when you need it.
Having mentioned the stability problems in the front I'll expand on that a bit. When a front tire is driving, it's trying to pivot around the ball joint. That would lead to torque steer, except that the other front tire is doing the same thing in the opposite direction. And since open diffs send the same torque to both tires, it always balances out. But put a locker in there and what do you think happens when one tire gets no traction and all of the torque goes to the other? You get a LOT of torque steer. If it were to happen when you were hypothetically changing lanes on a snowy freeway, as the right front tire hit the ridge of snow between lanes it could lose traction. Then the left front tire would get all the power ant the truck would pull to the right. Hypothetically. The first time that happened to me the truck had moved 10 feet to the right before I could catch it. The second time I was ready for it and it only jumped over about 5 feet. That was an automatic locker, which is probably the harshest. But even a TrueTrac or a clutch-type limited slip would have the same effect, just probably not quite to the same extent. That's why I'll never recommend anything other than an open diff, or a selectable that can be driven open, for the front axle of a truck that will be driven in 4WD at any speed.
Lead Driver
Joined: Feb 2013
Posts: 5,006
Likes: 72
From: Roseville, MN
There are also a variety of selectable lockers, actuated by air, cable, or electric. I don't have any experience with any of them. So do your research there.
edit: I should add that an open rear diff has advantages at times, both on and off road. It's more stable (less likely to slide sideways) and a rear locker can make it harder to turn on slippery surfaces. So there are advantages to a selectable locker in the rear. But for simplicity and cost I lean toward an automatic locker in the rear.
Freshman User
Joined: Nov 2014
Posts: 35
Likes: 0
From: In the Great White North.
So much mis-information going on in this thread I don't know where to start. I'll have to sleep on it and get back to you all because it's going to take a lot of typing to correct this mess.
As for factory offered limited slip diff.'s and Ford the two I can think of right now where called the Trac-lok (what I like to call an un-limited slip because they are a very poor LSD) and the better and earlier model the Power-Lok a stronger and more capable LSD. Anyways it's late and that's all I got for now...
Cheers D
As for factory offered limited slip diff.'s and Ford the two I can think of right now where called the Trac-lok (what I like to call an un-limited slip because they are a very poor LSD) and the better and earlier model the Power-Lok a stronger and more capable LSD. Anyways it's late and that's all I got for now...
Cheers D
Lead Driver
Joined: Feb 2013
Posts: 5,006
Likes: 72
From: Roseville, MN
I'm really looking forward to you setting us all straight.
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I put a Spartan brand locker in my red & grey truck, a 2wd truck. I've posted this before but figured it worth repeating.
I have a lot of fun on grass, dirt, or gravel pitching the truck sideways and drifting, its effortless and very manageable with practice.
I put a Spartan brand locker in my red & grey truck, a 2wd truck. I've posted this before but figured it worth repeating.
I have a lot of fun on grass, dirt, or gravel pitching the truck sideways and drifting, its effortless and very manageable with practice.
I put a Spartan locker in my '87 F150. It's a 2wd truck and I was having traction issues in wet grass if you know what I mean. This locker costs $300 and its pretty simple to install. You don't have to take the gears out of the pumpkin. The directions weren't clear on how to get the C-clips back in but I found a video on YouTube that explained it perfectly. Now that I've done one the next one will be a breeze.
With this type of locker the wheels are always locked in a straight line. When you turn and the outer wheel wants to go faster it is allowed to do so because the locker can unlock when driven by the wheel. It just will not unlock when driven by the gear. It's like a one-way thing. You here a noticeable CLICK - CLICK - CLICK when this happens. I think it sounds cool, doesn't bother me at all.
The unlock function of this design is 100% dependent on traction. When I turn sharp in my gravel driveway there's not enough traction to hold the tire. So the tire cannot transmit enough force to make the locker unlock and that wheel spins in the gravel. Of course this only happens when you are turning. If I goose the throttle it will pitch the truck sideways. If I am easy on the gas, that wheel just spins and it's no big deal. It's actually a lot of fun on dirt & gravel roads and I imagine it would be fun to play in the snow when acceptable!
Once you understand the limitation of the locker it's perfectly safe. But you will want to be careful until you get it figured out. The truck will go sideways on you when you goose the throttle in a turn. This is because the outer wheel is turning faster than the inner wheel, so the outer wheel is unlocked. At this point the only wheel being driven is the inner wheel. If this wheel loses traction & spins it allows the ring gear to go faster. Once the ring gear catches up to the unlocked outside tire it will LOCK and now you are driving both tires again. Hopefully traction is regained, but if you were goosing the throttle probably not - and now both rear tires will break and the rear of the truck will go sideways. Not a problem as long as you don't play happy feet with the throttle.
For me with a 2wd truck this type of locker was perfect and a lot cheaper than an Off/On style of locker. Plus there is nothing to wear out, it should last the life of the differential.
Update 2/14/2014: Tire squeal on pavement depends heavily on what tires you are running. I had 30x9.50 Cooper ATV/3 off-road tires on the truck when I installed the locker. These tires slipped enough when turning on pavement that I didn't notice any tire squeal. I swapped these wheels and am now running 30x9.50 Michelin LTX M/S tires. These tires squeal much more on pavement, in turns when you give it a little throttle. It can be hard to pull out into traffic when turning without chirping the tires. It helps to gas the truck in a straight line then let off the gas while turning hard, then ease back into the gas as the truck straightens out. On a gravel road this is not an issue because a tire can slip.
With this type of locker the wheels are always locked in a straight line. When you turn and the outer wheel wants to go faster it is allowed to do so because the locker can unlock when driven by the wheel. It just will not unlock when driven by the gear. It's like a one-way thing. You here a noticeable CLICK - CLICK - CLICK when this happens. I think it sounds cool, doesn't bother me at all.
The unlock function of this design is 100% dependent on traction. When I turn sharp in my gravel driveway there's not enough traction to hold the tire. So the tire cannot transmit enough force to make the locker unlock and that wheel spins in the gravel. Of course this only happens when you are turning. If I goose the throttle it will pitch the truck sideways. If I am easy on the gas, that wheel just spins and it's no big deal. It's actually a lot of fun on dirt & gravel roads and I imagine it would be fun to play in the snow when acceptable!
Once you understand the limitation of the locker it's perfectly safe. But you will want to be careful until you get it figured out. The truck will go sideways on you when you goose the throttle in a turn. This is because the outer wheel is turning faster than the inner wheel, so the outer wheel is unlocked. At this point the only wheel being driven is the inner wheel. If this wheel loses traction & spins it allows the ring gear to go faster. Once the ring gear catches up to the unlocked outside tire it will LOCK and now you are driving both tires again. Hopefully traction is regained, but if you were goosing the throttle probably not - and now both rear tires will break and the rear of the truck will go sideways. Not a problem as long as you don't play happy feet with the throttle.
For me with a 2wd truck this type of locker was perfect and a lot cheaper than an Off/On style of locker. Plus there is nothing to wear out, it should last the life of the differential.
Update 2/14/2014: Tire squeal on pavement depends heavily on what tires you are running. I had 30x9.50 Cooper ATV/3 off-road tires on the truck when I installed the locker. These tires slipped enough when turning on pavement that I didn't notice any tire squeal. I swapped these wheels and am now running 30x9.50 Michelin LTX M/S tires. These tires squeal much more on pavement, in turns when you give it a little throttle. It can be hard to pull out into traffic when turning without chirping the tires. It helps to gas the truck in a straight line then let off the gas while turning hard, then ease back into the gas as the truck straightens out. On a gravel road this is not an issue because a tire can slip.
Elder User
Joined: Jan 2014
Posts: 513
Likes: 1
From: smock pa
Subscribed to this thread.
I put a Spartan brand locker in my red & grey truck, a 2wd truck. I've posted this before but figured it worth repeating.
I have a lot of fun on grass, dirt, or gravel pitching the truck sideways and drifting, its effortless and very manageable with practice.
I put a Spartan brand locker in my red & grey truck, a 2wd truck. I've posted this before but figured it worth repeating.
I have a lot of fun on grass, dirt, or gravel pitching the truck sideways and drifting, its effortless and very manageable with practice.