I’ve been meaning to write a pretty comprehensive guide to the various differential options for a while. Now I finally got around to it. But before I get into it, why should I be the one to write it? Or more to the point, why should anyone read what I write? I’m a mechanical engineer and I work for a company that designs and makes clutches. Since clutch-type limited slips, automatic and selectable lockers all use clutches (yes, they all do, just different kinds), they are really in my wheelhouse. And since I’m a fourwheeler who wanted a locker, I dug into how they all worked. So I understand how they work from the inside out. A lot of the time that’s more important than using them a lot and understanding them from that perspective.

However I have a fair amount of experience using several differentials as well. I’ve had automatic lockers in the rear of three vehicles (CJ5, ’95 F-150 SCSB and ’71 Bronco). All three were / are summer daily drivers, the CJ5 and the F-150 both were daily driven through Minnesota winters. And the CJ5 and Bronco were / are off-road driven. I also had an automatic locker in the front of the F-150 for about a year. I’ve had clutch-type limited slips in a ’93 Explorer and my ’97 F-250 CCSB. And I’ve had a bunch of different vehicles with open diffs.

In past threads discussing differentials I know that some people have thought me to be arrogant. I’m really not, but in the case of differentials I am very confident that I’m right, which often comes across as arrogant in internet discussions. So I’m sorry if I do come across that way here. I don’t mean to, it’s just that I am right!

I’ll have separate posts for each type of differential, in hopes of keeping this cleaner. I’ll try to keep my facts and opinions separate and clearly identified. And for the most part I’ll only be talking about using these diffs in rear axle applications. But I will have a separate post about front axles.

 

Open differentials
Let’s start with the basics. Open diffs always send the same torque to both tires. That’s exactly what you want when both tires have plenty of traction because it keeps everything balanced. But it’s not so hot when one tire has very little traction. In that case the tire with little traction supports little torque so the open diff gives the same little torque to the tire that has traction. At the limit, when you are crossed up with all the weight on one tire and the other in the air, the open diff sends zero torque to both tires and you go nowhere. The way they function, either tire can go slower than the ring gear as long as the other goes the same amount faster. So if one tire is stopped the other will go twice as fast as the ring gear. By the way, this is also why people say that open diffs send all of the power to the tire with no traction when they send the same amount of torque to both wheels. Power is torque times speed, so the tire at zero speed gets zero power.

By the way, people often say that an open diff only gives you one wheel drive. Technically that’s not true since it always drives both wheels with exactly the same torque. But in a practical way it does describe what happens when one tire doesn’t get traction and it won’t drive the vehicle forward.

Because an open diff makes it most likely that you’ll only spin one tire, it gives you the best stability. Once a tire is spinning or skidding it doesn’t care which way it slides, so the back end of the truck can slide sideways more easily. With one tire not spinning it wants to roll rather than slide, so it provides more resistance to sliding sideways.

My opinion:
Open diffs are obviously the low-cost leader. And they give the best street manners. Personally I wouldn’t have anything else in the front of a street-driven 4WD (selectable lockers are an exception, more on them later). And they would be my second choice for the rear of a primarily street-driven truck. I think an open diff with good tires and good weight distribution is a much better choice on snow covered roads than a clutch-type limited slip or an automatic locker, whether the truck is 2WD or 4WD.

 

Spool
Not that spools are almost as common as open diffs, but they’re simple, so we’ll go there next. Spools don’t allow any differentiating action, so they aren’t differentials, but I’ll be sloppy here and call them diffs sometimes anyway. They always turn both tires the same speed. Great when you are going in a straight line, less so when going around a corner. A spool will always need to scrub a tire when turning. But as long as one tire has traction it’ll push you ahead, and it’s dirt simple which is always a good thing in the strength vs. cost tradeoff. Welded spider gears (aka “Lincoln Locker”) turns an open (or limited slip) diff into a spool, functionally.

Because a spool doesn’t allow any differentiating action you always need to scrub when turning.

My opinion:
Spools have the worst street manners of any diff, and I’d never consider them in a primarily street-driven truck. But if you want a spool for a specific use, they aren’t nearly as unthinkable for street use as you might think, especially in a pickup with a light rear end. Yes tires will scrub, but back tires in an empty pickup don’t get enough traction to make that too bad.

Still, I wouldn’t recommend it for a truck that’ll tow or carry heavy loads (the scrub WILL be a problem then). And I wouldn’t recommend it for snow (scrubbing tires in every turn will make spinning out too hard to prevent).

A rear spool would be my first choice for a mud truck, and my second for a bracket racer. A spool or a welded diff would be my third choice (or my low-buck first choice) for a rock crawler. Personally I couldn’t see putting a spool in the front except in a mega-power mud truck (for the strength and simplicity), but a welded front diff could be a low cost option for mud or rock crawling.

 

Clutch-type limited slip
Ford calls it Traction-Lock (Eaton calls it Positraction, but as far as I know only GM used Eaton Positraction from the factory, so “posi” is more-or-less a Chevy term). There are aftermarket brands as well. Like open diffs, clutch-type limited slips will allow one tire to go slower than the ring gear as long as the other goes the same amount faster. But rather than allowing this to happen freely, limited slips put friction between the two sides. That way, when one tire supports less torque than the other, the diff will send the same torque plus the clutch friction to the other side. That could leave you spinning one tire if the clutch isn’t set up “tight” enough to push you ahead, but it will help at least some. And if the clutch is set up “tight” enough it can help a lot. But the downside is that whatever that torque is also needs to be overcome when turning, so a “tight” clutch-type limited slip is more likely to scrub tires in corners.

My opinion:
OK, here’s where I know people are going to disagree with me. But remember, this part is my opinion, not something I’m saying is fact. Your mileage may vary. A lot of people like clutch-type limited slips. I don’t. To me they seem to be the worst of both worlds: not positive enough to really help when you need it, not free enough to avoid handling problems on snow. The only vehicle that’s ever spun me into a ditch was a ’93 Explorer with a factory limited slip. It was always a handful to keep the back from sliding out in turns on slippery roads, and it was harder to keep from spinning both tires on that than it was to keep just the one driven tire from spinning in my automatic-locker-equipped F-150 (the F-150 had a fiberglass topper which helped the weight distribution and it had better snow tires).

The only place I’d choose to use a clutch-type limited slip is in a street racer where I was almost willing to use a spool but wanted a little better manners. Then I’d want it set up tight. Otherwise any time I had a clutch-type limited slip I’d hope it was like the one in my ’97 F-250, completely worn out so it works just like an open diff which I see as not enough worse traction and enough better handling in snow to be far superior.

 

Automatic locker
Detroit is the most famous of these, Yukon Grizzly is another. “Lunchbox lockers” (like Lock Right, Spartan or Aussie Locker) are automatic lockers you can install in your open diff without needing to set up your gears. There may be other brands as well. They all (except the G80, also known as Gov Loc, which I’ll cover later) work basically the same way: dog clutches will allow one tire to over-run the ring gear but will never allow either tire to turn slower than the ring gear. This means that both tires will always be positively driven (like a spool), but when going around a corner the outside tire can over-run to prevent tire scrub. When this happens the automatic locker is only driving the inside tire (yes, an automatic locker is the only diff that actually gives you one wheel drive in some circumstances).

I’ve heard some people say that an automatic locking diff should really be called an automatic UNlocking diff because they are normally locked and will unlock automatically when over-driven. That’s actually a helpful way to think about them, to avoid the mistake a lot of people (including some off-road magazine writers) make when they say that they automatically lock under power. That’s not how they work. They unlock when one tire tries to go faster, allowing it to overrun. Then they re-lock when the driven tire catches up to the overrunning tire. This catch up will happen when you straighten out and the outside tire slows down to the same speed as the inside tire. It will also happen if the inside tire starts spinning and catches up in speed to the outside tire.

By the way, I’ve also heard some people use the term “true posi”, I believe in reference to full lockers. As described above “Positraction” is Eaton’s brand name for their clutch-type limited slip, so that is “true posi(traction)”. At the same time, a full locker (or a spool) will positively drive both tires, so that is “true posi(tive drive)”. “True posi” isn’t really a recognized term, and since it’s not completely clear what it means I encourage people to use terms that are more descriptive.

My opinion:
Automatic lockers do have some quirks when driven on road, but in my opinion those quirks are overplayed and pretty easy to get used to. I would not recommend an automatic locker in a vehicle that will be driven by people who won’t want to (or be able to) compensate for the quirks (for instance, my wife, or my sons when they were teenagers). But for anyone who is willing to adapt to an automatic locker because they value its benefits, the quirks aren’t hard to live with.

The quirks all come because when an automatic locker is unlocked it only drives one tire. That can cause some torque steer, like pushing on one end of the handle of a grocery cart. That will be more pronounced in shorter wheelbase vehicles (it was somewhat annoying in my CJ5, noticeable in my Bronco, and almost unnoticeable in my F-150). It is also more noticeable with a manual transmission because the softer coupling of an automatic gives you more time to compensate with the steering.

Probably the biggest handling quirk comes from the fact that with only one wheel being driven it’s easy to spin that tire when you apply power. This results in a lot of tire spin until/unless you learn to drive with a lighter foot in corners. And in some conditions it can be impossible to start (or keep) moving without spinning the inside tire. Pulling a trailer is one thing that emphasizes this. It adds load that the drive wheel(s) need to get moving, but not much more weight to the drive wheel(s), so it can be impossible to start up without spinning a tire. For this reason I choose not to put an automatic locker in my F-250.

Winter driving is another place this quirk can be worse. Starting or accelerating on a slippery surface in a turn can be impossible with only one wheel drive, so you end up spinning the inside tire. If you aren’t careful, when that tire catches up with the outside tire, the abrupt reengagement of the locker can break the outside tire loose too, causing you to spin out. But it really isn’t hard to get used to this (especially in a longer wheelbase vehicle, but even in my CJ5 it wasn’t too hard to live with). And you do have both tires driving then (even though one is spinning).

Getting back to my dislike for clutch-type limited slips, in my opinion an automatic locker has better manners in snow (when you get used to it) than a clutch-type limited slip. Clutch-type limited slips try to scuff the tires when going around corners (even when coasting), so they are more likely to cause one or both tires to skid. In contrast, an automatic locker never tries to scrub a tire. An automatic locker is more likely to spin one tire when powering through a corner, but as long as you are a little careful it’s less likely to break the other one loose. So in my opinion and experience, a clutch-type limited slip hurts stability more than an automatic locker. Combine that with the fact that it doesn’t help traction as much, I would never choose or suggest a clutch-type limited slip for winter driving.

Again, I wouldn’t recommend an automatic locker for heavy towing, or for a vehicle that will see a lot of less experienced drivers. And even though I’ve minimized the handling quirks here, I’d still choose an open diff with good tires over a locker for just street driving. But if I wanted the locker for any other reason I wouldn’t hesitate to have one in my winter daily driver (again).

 

Selectable locker
ARB air lockers are the best known, Yukon Zip Locker is another air operated one, OX locker is a cable actuated selectable, and Eaton’s ELocker is electrically actuated. There may be other brands as well. When unlocked these work like open diffs (although I think there might be some that act like clutch-type limited slips when unlocked). But then when locked they work like a spool.

My opinion:
These are usually a little more expensive than automatic lockers. Also when locked they have the handling quirks of a spool (worse than an automatic locker in my opinion). For a rock crawler they give the bad traction of an open diff when unlocked and the bad handling of a spool when locked, so I’d choose an automatic locker over a selectable in the rear axle to avoid needing to think about when to switch the locker on or off. But in most other situations you don’t go back and forth between needing a locker or not so often, so a selectable gives you the benefits of a locker when you need it with the stability of an open diff when you don’t. In a daily driver you’d never lock it except when needed to get unstuck. In a mud truck you’d never unlock it until you’re back on the street (or harder surface trail). Kind of the best of both worlds. And this would be my only choice (other than an open diff) in a front axle (more on that later).

 

Gear-type limited slip
There are two of these available as far as I know. Gleason Torsen was the first of this type and is still around, but Eaton’s Truetrac is more common in the aftermarket. I have to admit that I don’t know as much about either of these as I do the diffs I’ve discussed above. These don’t use clutches, but rather use the pressure angle of gears to control slip. The manufacturer could get very different performance, ranging from a full locker to a full open diff, depending on what pressure angles they choose, and I don’t know what choices either of these manufacturers has made.

Below is a link to a You Tube video that shows how a Torsen diff works. Based on how they describe it, Torsen uses pressure angles that give full locking performance, but I can’t confirm whether that’s accurate. I’ve heard that the Truetrac uses pressure angles that give a 3.5x torque bias, but I can’t confirm that either. However I’ve also heard enough times about a drawback that the Truetrac has which would indicate that the 3.5x bias could be right that I believe it is, so that’s what I’m going with here. But again, this isn’t as much in my wheelhouse as the different clutch-type diffs.

One thing I do know it true is that both Torsen and Truetrac allow free differentiating action in corners, allowing one tire to go slower than the ring gear as long as the other goes faster, just like an open diff, with no tendency to scuff tires like a clutch-type limited slip. Another thing I know is both Torsen and Truetrac will send more torque to the tire with more traction, making it less likely that you’ll get stuck, just spinning one tire.

What I’ve heard often enough that I believe it to be true is that, unlike a locker or a spool, a Truetrac can leave you stuck spinning one tire. This is consistent with the idea of a 3.5x torque bias. That says that the Truetrac will send 3.5 times as much torque to the tire with more traction as it does to the tire with less traction. But if one tire gets zero traction and supports zero torque, the Truetrac will send 3.5 x 0 = 0 torque to the other tire and your stuck just like with an open diff. I’m pretty sure that this is how the Truetrac works. But even there it’s still a lot better than an open diff, because rarely (except in rock crawling) does either tire have zero traction, so in most situations it effectively sends more torque to the tire with traction. And even when one tire does have zero traction you can trick a Truetrac into driving the other tire by lightly applying the parking brake. That will put some drag on the tire in the air (or on ice) so the Truetrac will send 3.5 times that much torque to the other tire. Yes, the parking brake also puts drag on this tire which must be overcome, but you still have 2.5 times the parking brake torque going into driving the vehicle forward (just don’t forget to take the brake off when you’re unstuck!). And if the video is accurate, the Torsen doesn’t have this limitation because it uses different pressure angles that go give full locker performance when one tire is in the air.

I’ve heard that the Torsen and Truetrac both drive both tires when driving through a curve (like an open diff, but unlike an automatic locker). I haven’t worked through the physics of how that would work well enough to be able to confirm it from an engineering perspective, so I’ll present that as hearsay that is likely true.

My opinion:
I wouldn’t choose a Truetrac for a rockcrawler or a mud truck due to the less than positive performance. And even if a Torsen does give positive locking I’d still choose a locker (automatic or selectable) with dog clutches for those applications due to simplicity and strength. But for primarily street use this (and the G80) are the only traction aiding differentials that don’t give much worse handling than an open diff. It’s only drawback in handling over an open diff is that it is more likely to spin both tires than an open diff, so it would be less stable in that narrow zone where an open diff would spin one tire and a Truetrac or Torsen would spin both. But I think that would be very tolerable, especially given the traction improvement. Unless towing was a factor I’d probably choose an automatic locker over a Truetrac or Torsen for my own vehicle, based on simplicity and strength. But for a ¾ ton or bigger truck that was expected to tow heavy trailers, or for a vehicle that would be driven by my wife or teenage sons I’d definitely choose a Truetrac. And, budget permitting, it’s what I’ll eventually have in my F-250.

Edit (added in Jan 2020): In the fall of 2018 I did put a Truetrac in the rear axle of my F-250. I report on that farther ahead in this thread, but I thought I'd add an update here for people coming on this thread later. In practice I'm not as big a fan of the Truetrac as I'd hoped I'd be. I definitely like it better than a clutch-type limited slip because it doesn't try to scrub tires in corners. But while it's better than an open diff, too frequently for me it's not enough better. The main issue is when I've got the right rear tire on ice and the left rear on bare pavement as I'm starting from a stop sign (a pretty common occurrence on residential streets in a Minnesota winter). When that happens I end up spinning the right tire more than I'd hoped. It does still send more torque to the left tire than an open diff would, so usually I can get going, but I was hoping for a little more.

And while it's definitely smoother in operation than an automatic locker (it's completely invisible in use), it still tends to kick the back end out when starting around a corner on snow. It does this almost as much as an automatic locker, and much more than an open diff. It's easy to predict and get used to, but it offers less help than I'd hoped for, and has at least as much handling down-side as I'd feared. So it's just not quite as good as I'd hoped.

It does work well on the wet-leaves-on-dirt hill at my folks cabin. But if I had it to do it over again I'd either save the money and leave it open, knowing that I can just use 4WD when I need more than the open diff offers, or I'd go with an automatic locker if I wasn't planning to tow a lot with it. (Of course, I didn't have an open diff, I had a worn out limited slip. I do think this was a good upgrade from that.)

 

G80 (aka Gov-Loc aka Eaton MLocker)
(edit: after a little more digging I found that G80 is the GM order code for this diff, Gov-Loc is the informal name they are often called by, and MLocker is what the manufacturer, Eaton, calls it.)

I don’t know a lot about G80 diffs either, so take this for what it’s worth. I know they are made by Eaton, and as far as I know were only available in GM vehicles from the factory and are only available for GM axles. They work like an open diff most of the time, freely allowing one tire to go slower and the other to go faster so you can corner with no tendency to scuff tires. But when the speed differential between the two sides gets too high a centrifugal governor engages some type of clutch to lock the two sides together.

This could be the best of all worlds, but I’ve heard that they are prone to breaking. That, along with my understanding that they’re only available for GM axles, make me less interested in them. So sorry that I don’t have more info on them. And again, I only know what I’ve read about them, so take this for what it’s worth.

 

Front differentials
As I said at the start, most of what I’ve written so far applies to rear axles. You can also put limited slips or lockers in a front axle. But should you?

I had an automatic locker in the front axle of a daily driver ’95 F-150 for one winter. The first time I drove in in the snow, I went to change lanes on the freeway. As the left front tire hit the ridge of snow between lanes the truck suddenly pulled so hard to the left that the truck had jumped over 10 feet before I could catch it! I thought I was more ready for it when I went to change lanes again, and that time it only jumped over about 6 feet. I shifted into 2WD and never used 4WD on the road again until I removed the locker and went back to an open diff.

I’d experienced the torque steer of an automatic locker in a rear axle before, so why was this so much worse? Physics. Torque steer from a locker in a rear axle is due to pushing the vehicle from one side, like pushing a grocery cart from one end of the handle. But it’s really not even that bad because, unlike a grocery cart, a truck has front tires that are trying to keep you going straight rather than a grocery carts casters that don’t care which direction they go.

But in a front axle, not only does driving only one tire give you torque steer from pushing on one side of the truck, it also tries to turn the steering wheel. It’s like if you jack up both front tires and unlock the steering. If you grab a front tire and try to turn it to the right or left you can, and the steering wheel will spin as you do. Normally with both front tires driving the right tire is trying to turn the steering wheel to the left. But at the same time the left front tire is trying to turn the steering wheel to the right and the two perfectly balance out.

With an automatic locker, when I hit snow with one front tire that balance was suddenly upset, and the steering wheel turned in my hands before I could react to it.

My opinion:
I’ve heard others say that clutch-type limited slips or Truetracs are fine in a front axle. They might be right. But the physics tells me that any differential that can put more torque to one tire than the other can give this unbalanced steering force. Yes an automatic locker will do that the most abruptly, going from balanced to zero on one side instantly. But for me, in any vehicle that might be driven in 4WD on snowy roads at higher speeds, it’s not worth it, and I’d never have anything but an open diff in that situation.

I’d make the same call for a vehicle that would be driven at higher speeds on less slippery surfaces, like in the desert. The risks of the changes in steering balance at higher speeds aren’t worth it to me, although I could maybe see using a Truetrac in that situation.

For a rockcrawler or mud truck I could see using an automatic locker. In my one year experience with an automatic locker in the front of my F-150 I found the low speed handling quirks to be pretty bad, but livable. So in rocks or mud, where positive drive of both tires is important, I could see it being worth the quirks.

Likewise in a mud truck I could see using a spool for the strength and traction in spite of the handling issues. I wouldn’t do that in a rock crawler that has to turn more often than a mud truck. But I could see using a welded diff in the front of a low-buck rock crawler.

The exception to me “open front diff only” rule would be a selectable locker. It IS an open diff when unlocked, which it would be all of the time unless it was needed. And I’d only use it at slow speeds where there’s time to react to changing steering balance. I would readily put a selectable locker in the front of a vehicle for any use except for one thing: cost. Still, eventually I hope to have them in both my rockcrawling Bronco and my daily driver pickup.

 

Loving it, keep it coming, great job! Thank you.

 

You want more? Really? I figured what I wrote there was enough to put just about anyone to sleep already!

Seriously, I 'm glad someone appreciates it, but I wasn't planning on adding anything more. However if there are any questions I'm sure I'll respond.

 

Oh ok, I thought that there might be more. I am good with it then and yes thank you for taking the time to put it all on here, I appreciate it very much.

 

what happened Bob, the dog keep you up all night?

 

Actually I wrote all of this over about a year. Every so often I'd end up writing part of it (again) in some thread and I thought I'd like to have a single place to point people so I didn't have to write the same thing over and over. Obviously it wasn't very high on my priority list, I'd write a little one day and then not get back to it for a month or more. But eventually I got through it, then posted it all at once.

 

I'll admit I skimmed most of it but it's a good read. I didn't see any mention of of a shotgun sound when ones engaged lol pucker effect goes up real quick