What’s the difference between a locker, limited slip, and spool?
Which one is best for you?
What are some of the pros, and cons of each?
Find out in this comparison of a
Locker vs Spool vs Limited Slip Differential
To help you have a better understanding about the basic function.
And the difference of these devices.
Let’s discuss the
Differential Carrier Components
First you have the…
The carrier houses the spider-gears , side gears, and holds the ring gear.
Are on the inside of the carrier you have one on each side.
They’re a straight bevel gear.
Which the side gears are what the axles spline/go into.
Also connecting/meshing with the side gears are
Sometimes also referred to as planet-gears or pinions.
So, you will typically have 2. Sometimes up to 4 spider/planet-gears.
Which are also a straight bevel gear.
The benefit of having more spider gears is that it adds more strength.
Dividing the torque across more meshing gear teeth. So you have less tooth strain.
From the spider to side gears.
Spider gears are attached to the diff carrier on a cross-pin.
The spider gears can rotate as a solid unit. With the ring-gear/carrier-case.
When axle torque is equal.
As well as spin on their own axis (cross-pin). Rotating around on the side gears.
Which this action (movement). Allows for a differentiation in torque, and speed between both axles.
Or, what is referred to as an open rear.
Just hold that thought for a minute. More talk on that a little further below.
The ring gear gets bolted to a flange. On the outside of the carrier.
It’ a spiral-bevel-gear
The ring gear is the big gear.
Now, the ring gear is driven (meshes) with the…
Which the pinion gear is also a spiral-bevel gear.
The pinion gear is not attached to the diff carrier.
But, the shaft of the pinion-gear goes in/through the diff housing (or 3rd member) itself.
Which then has a yoke. That attaches on the end. Connecting the U-joint, to the driveshaft.
Or also called the diff housing for short. Is the exterior housing you see
When you look under your vehicle.
And, what the axles go in, and holds all the components mentioned above.
Lets discuss the
Because, most vehicles come factory with open differentials.
Typically, to allow for the best control/handling of the vehicle.
An open diff like mentioned above.
Will allow for differentiation of outer, and inner-wheel speed when turning.
Might be hard to visualize
How an Open Differential Works
Is when you turn, inside an open-differential. The side-gears spin opposite of each other.
The outside-axle/side-gear. Starts going forward and faster
And the inside-axle/side-gear is going backward and slower.
Relative, to the carrier-case and ring-gear itself.
So, for example,
With equal rolling traction.
If the outside wheel, when you’re turning a corner, is going 4 RPM faster.
Than the ring-gear/carrier.
The inside wheel, will be going 4 RPM slower. Than the ring-gear/carrier
Because, of the spider-gear functionality.
With an open diff,
Most of the torque always goes to the tire with the least amount of resistance.
The wheel that loses traction.
An open diff has a 1 : 1 TBR (torque-bias-ratio). And what is called a 50/50 torque-split.
So, going straight on a high-traction-surface tires having equal-traction,
Both tires, will receive the same amount of torque.
The carrier is locked, as a unit,
So there’s no differentiation going on.
And 100%, of the torque to the diff carrier /ring-gear. Is being split 50/50, between both wheels.
But, let’s say it takes zero-torque, to spin a wheel.
Because of no load or traction.
The high-traction wheel will receive the same amount, zero.
And, your vehicle goes no where.
Open-diffs work well on high traction surfaces, Allowing for great steering and control.
But, when you add wet, slick, slimy, surfaces. Or, a wheel in the air.
Like, when you’re in, uneven terrain, off-road, (off-camber) situation,
And chances are, you’re going to get stuck, a lot easier.
That’s where the importance of some sort of traction-aiding-differential.
Starts, coming into play.
So first up is the
Now, keep in mind a limited slip differential.
Will help “limit slip” between the axles. But remember they do not “lock”
Now, there are different types, of limited slips
But common aftermarket ones you will see are
- Clutch Type
- Cone Type
- Helical Gear Type
And one of the popular clutch type you will see is called the
Positraction Limited-Slip Diff
And, sometimes also referred to as a Posi or Trac Loc.
They’re a flat-plate clutch-style LSD.
How a Posi Limited-Slip Works
Is on the same basic concept, and components as an open diff.
With straight bevel spiders, and side gears
But, with the addition of clutches. And some preload spring tension.
Depending on the brand,
They will either have a disc (bevel) spring washers, in each clutch pack. For clutch preload tension.
They will have a coil spring pack. Or what is called an S spring in the center.
That pushes out to the side-gears.
Putting tension outward, to both sides of the carrier/case.
And, sandwiched between the side gears, and carrier/case are clutch-packs.
Now, the spring preload on the clutches creates “artificial traction”
To keep you moving. When a tire has very little, or no grip (traction).
But, additionally when there’s enough grip (torque/load) on the tires/axles.
Slight “gear-separation-forces” can occur as well,
Between the spider, and side-gears
Adding slightly more clamping/braking force to the clutch-packs.
Which helps to transfer more torque over to the high traction wheel.
By trying keep both side gears (axles) spinning at the same speed as the carrier itself,
To maintain that 50/50 torque split to both wheels.
Or, let’s say at least as much as it can.
But, keep in mind. This transfer of torque, over to the other wheel.
Will only be as strong, as what the clutch packs (friction plates) will hold.
Depending on how “aggressive” or “light” the diff is tuned.
Also, referred to as a Higher, or Lower torque bias ratio (TBR)
Now, most aftermarket clutch-type limited-slip diffs are rebuildable. And tunable, for different driving characteristics, and conditions.
By changing shims, spring preload/tension,
And or friction modifier additive. To adjust clutch plate friction.
Now the more aggressive (tighter/stiffer) the tension is. The more the axles stay synced.
And stay spinning at the same speed.
If it’s too tight.
The tire grip may not be able to overcome. The “breakaway-force” of a heavy preloaded clutch pack.
Causing the tires to bark/chirp. While also, creating understeer,
Forcing the vehicle, to go straight. When trying to turn.
The lighter the tension is. The easier it allows the vehicle to turn.
Without chirping/barking the tires. For better safer handling, and control.
Now there’s also a
Cone Type Limited Slip Differential
And it works, on the same principle. As the posi clutch type.
But, it has no clutches.
Instead, the back of the side gears, are coned shaped. Which fits into a matching, conical recess in the carrier.
With springs, pushing out, on side gears for preload
But, they still require maintenance, and they are not rebuildable.
Of a cone, or clutch-style limited slip diff.
Is that they can be helpful, With the occasional need for extra traction.
Of the Cone and Clutch-Type LSD. For one is that every time, you make a turn.
You’re forcing the wheels, to go a different rate of speed.
Which in return, is dragging the cones, or clutches causing wear.
It’s not really a big deal though,
Because, all that happens.
Is the limited slip will just turn into an open rear diff, once it wears out.
So, it usually wont hurt anything.
But, you will lose the traction benefits. That you wanted to begin with.
They’re typically not strong enough. If, you get a lot of wheel spin,
And or, a wheel in the air all the time, in uneven terrain.
Like when, you’re in extreme off-roading, rock crawling, type conditions.
Because, the spring pre-load on the cone or clutch-packs
Can’t typically hold the forces, put on them continually. And they can slip
So, you do all that work to install it. Just for it to possibly not last very long.
Then, you would have to go back in. And repair or replace, and, or set it all back up again.
So, for a more reliable maintenance free LSD option.
Leads us to the
Helical-Gear Type Limited-Slip Diff
Or also known as
An automatic torque biasing (ATB) limited-slip-differential
They work based on the principle, of friction, and gear separating forces.
And that a worm gear, can turn a worm wheel. But a worm wheel, can’t turn, a worm gear.
Although, they actually don’t use worm gears, or worm wheels.
They have, what’s called helical-gears.
Keep in mind there’s different designs of these units. But they work on the same basic principle.
A popular option you will find.
When is comes to the helical gear type LSDs is the
Now, the general concept
How an Eaton TrueTrac Limited Slip Works
Is just like an open diff. When driving straight down the road. On a high traction surface.
And the wheels have equal traction.
The carrier unit is locked. With a 50/50 torque split, to both tires.
When you have a loss in traction, on one of the tires.
The helical gears will start to spin.
And now, more torque, will try going to the low traction wheel.
But, as soon as there’s a difference in torque (traction). From one wheel to the other.
It will instantly cause “thrust force”. Which creates instant “friction”
And then in return, from the friction. It pushes the helical pinions apart from each other, and the side gears
Causing the “gear separation” to happen.
Which then puts binding pressure out, against the carrier/case itself.
And this binding pressure, causes the so called “lock up”
Turning the unit basically into a spool.
To help maximize traction of both tires
Up to the point where both tires would lose traction.
From either too much torque. And both tires cannot maintain traction, to the surface they’re on.
Which among other things. Can be dependent upon momentum, and or throttle control.
If you have a scenario. Where there’s little to no traction/resistance on a tire. (exceeding the TBR torque bias ratio)
Like when you loft a wheel up in the air, on uneven terrain.
Which causes, a lack of differentiating forces.
So, the gears, can’t create enough friction, to bind up.
Now, the wheel will spin like an open diff would and you go no where.
But, a little
All you typically have to do, in this case. Is either apply some parking brake.
For a rear helical gear LSD.
But of course, you need to make sure. That the brakes are properly & evenly adjusted.
And, this will only work. If the parking brakes, are on the axles.
As, some vehicles have parking brakes on the driveshaft.
Or, you can use the main foot brake. And drive with two feet. (brake and throttle)
Lightly, applying the brakes. With throttle modulation, helps to create, some brake-load/resistance on the axle.
Because, the gears need, some form of negative torque/load. To bind up the gears.
The more load applied accelerating. With enough traction/load. The more friction can be created to bind the gears.
A helical gear Limited slip diff. Can also act just like an open diff as well.
When making a turn in low load conditions. As, the helical gears, can differentiate freely.
From the lack of frictional forces on them. Coasting around turns, turning in parking lot etc.
Allowing the outer, and inner axle. To turn in opposite directions in relation to the carrier case.
Of a Helical Gear LSD is that they’re a good, reliable, upgrade/replacement,
To a factory type open differential. Or, positraction limited-slip diff.
And there’s typically, little to no noticeable resistance in the steering wheel.
When using it in front diffs. They have a smooth operation, with no chatter,
Like what can commonly happen. With, clutch style limited slip diffs.
They don’t wear out prematurely.
You don’t need friction-modifier-additive. Because there’s no clutches.
As they’re all gear construction.
And there’s no need for any spring-preload, or clearance adjustments.
So, if you want a limited-slip diff. That’s virtually maintenance free,
Only needing to perform your regularly required maintenance gear oil changes.
These are great for a daily driver, towing. And it’s capable in a wide variety of conditions on and off road.
But some of the
And a few things to keep in mind, with helical gear limited slip diffs.
Is for one they’re directional specific.
So, they have different units for a front, and rear diffs. The rotation of the gears are different.
And they’re not rebuildable
They also recommend, the use of quality conventional gear oil
Because, synthetic is a lot slicker (reducing-friction).
And the problem is with that.
Now, it could take a little more effort. To create friction to bind up the gear set (reducing TBR).
But, you hear of people using it with no problems
The helical-gear-type limited-slips, are a nice, simple, reliable upgrade, for most.
If you can keep your wheels on the ground.
If you find yourself. Off the pavement four wheeling a lot.
And or, tend to tackle a lot of tough uneven terrain. Lifting a wheel, in the air all the time.
To help save you from wheel spin. Mixed with a lot of pedal pumping,
A bunch of fancy-foot-work. When you’re in, tricky situations.
Then even with doing all that. Sometimes they still may not lock up when needed.
You’re best bet is a…
Now you have a different types of lockers
- Selectable Lockers
- Automatic Lockers
Or, also sometimes referred to as on-command-lockers
There are different types of selectable lockers
- Electric lockers
- Manual Lockers
- Air Lockers
Now, when it comes to selectable lockers. If they’re not engaged.
They will operate just like an open-differential would. Allowing for great on-road-handling.
And, then when you’re off road, and need more traction at times.
Depending on the type of selectable-locker you choose.
You would either
Pull a manual lever. Or flip a switch to engage it.
Which once engaged you form a solid spool. Locking both axles together.
And, then once you unstick yourself. You can disengage it.
So it will give max even torque. To both wheels all the time when locked.
And then when disengaged. It also gives you the best control and handling of an open diff
So the best of both worlds
If you have the budget. A selectable locker is most of the time the way to go.
Having the most versatility. When it come to on, and off road use.
Let’s start with the
E-Lockers (Electric Lockers)
An electric locker engages by 12v current
So you will have to install a switch on the dash.
Which is powered by 12v
Now, when the switch is switched on it will send power to an electro magnet inside the locker carrier
Which will engage the unit.
If you turn the switch back off and it will disengage it.
Pretty straight, forward, right?
Also E lockers are fairly easy to install. Just run some wire and add a switch
But you will have to. drill a hole in the diff housing for wires to connect power to the locker.
And you will also have to replace the whole carrier to install them
One type is the
A little more about
How an Eaton E Locker Works
Ok so inside the locker carrier you have a magnet
And, this magnet has pins on it
To, keep it in place so it doesn’t spin around, and rip the wire out.
Now, inside the E-Lockers diff carrier there are wedge/plates .
Which have three ball-bearings between them
And, these wedges are what you call ball-ramp-cams,
So when you apply power to the electromagnet, it then pushes them sandwiching it all tight together.
And, then when the diff starts to turn.
It will cause those three ball bearings. To ride up the ball-ramp-cams.
Pushing outward more and more and wedging them apart.
Which, now these wedged apart ball-ramp-cams
Will then, in return push on a plate. That has 6 heavy-duty pins.
That will go into matching grooves/recesses in the back of the side-gear.
The E-Locker is a pretty simple, robust and reliable setup overall.
With the E locker comes in with those ball-ramp-cams.
As, they don’t start to push apart with out some axle travel.
So, you have to typically turn them about a quarter turn.
Before they ride up the ball-ramp enough. To push the pins into the side-gear locking-it.
So with roughly a quarter turn of play when going forward before it actually engages.
And on top of that if you would need to reverse out of something.
You would have roughly an additional quarter turn of rotation going back.
From the cam resting position.
On top of the quarter rotation you went forward
So, with this additional quarter turn.
Now, you have roughly one-half of an axle turn of rotation. Before the diff-locks back-in.
So, it’s not really the best solution. For a serious off-road use rig.
Probably better suited for a street driven vehicle (daily-driver)
That needs a bit of extra traction every once in a while.
But, now, the new Eaton E-Locker4
For the Dana 60 and 70 axles. They have a Direct-Acting-Locking-Plate.
Giving you an instant-lock
Instead of that delayed engagement. With the ball-ramp-cam setup.
So let’s move on to the
Now, when it comes to air lockers. You will have to have an air system.
Which would be more products to purchase separately and install.
For example you need an:
Air compressor, or a CO2 tank
Solenoid valve controlled by a switch. To put in the dash to turn the locker on and off.
These replace the whole carrier unit as well.
Also you would have to wire in, and plumb in air lines.
To an air compressor mounted somewhere on the vehicle.
Making this a little more difficult. To install than the manual ox locker or electric locker.
A popular air locker is the
ARB Air Locker
The basic concept of
How an ARB Air Locker Works
Is the air is supplied through the diff housing to the carrier.
The air goes to a collar/seal-housing, that sits next to a carrier bearing (in the diff).
With O-rings on both sides.
Then the air is fed from that seal housing. To a pin-hole/journal, into the inner part of the carrier, to a piston
That will then push an engagement collar.
Locking the side-gear, to the carrier/case, creating a solid spool.
Of an Air Locker, is that they are most of the time very reliable.
But some of the
Are that they will have more components. With the added compressor, airlines, O-rings.
So, you need to make sure air-lines, stay away from heat sources, and the lines don’t get pinched/kinked.
You also need to make sure. That the electrical connection are in good shape.
And the wires are ran properly. So they don’t get chaffed, or damaged.
Because, a pinched air-line, air leak, or bad electrical connection. Can cause you problems, with the switch, and or solenoid.
Also, in below freezing temps.
Moisture in the air lines, could freeze, creating a problem for you, by causing the locker to fail.
When installing an air locker it’s real important to make sure. That there’s no dirt on the O-rings. in that seal housing.
Which can cause air leak failure inside the diff.
While, also becoming a very complicated fix out on the trail.
Also, when in comes to selectable lockers, in general,
It might take a bit to learn. As to when to engage them before, and after an obstacle,
So, the driveline isn’t in a bind. Possibly hindering engagement/disengagement.
Doesn’t need any air supply. Or 12v electric going to it.
You can leave it locked in as long as you want. And it won’t matter if the vehicle is on or not.
Or you could just leave it disengaged as a open diff as well.
A brand you will see for a manual locker is the
How an OX Locker Works
Now, with an Ox Locker has a manual has a lever,
That’s connected to a cable, running to the diff cover.
Which moves a shift fork. That will then move an engagement collar (locking ring).
So, when the locking ring is engaged, it will lock the side gear, to the carrier/case
And when it’s disengaged it’s an open diff.
One thing nice about this type of locker is that you can feel, and know whether or not, it’s engaged
Because, of the more direct feel of the manual lever.
The shift lever has a lifting lock collar on it. To keep it from being moved by accident.
And the knob it self screws down. Locking the shift lever for additional security.
Which can keep it from accidentally getting knocked into gear, riding down the road..
Also it comes with its own diff cover, with the shift fork. That engages and disengages the locking collar for the diff.
And they also offer what is called the drive-away-lock. That you can install in the diff cover to lock the diff.
Why do you need a Drive-Away-Lock?
Well if something happens to the cable.
You can put the Drive Away Lock in the place where the cable goes in the diff cover.
The Drive Away Lock. Will then push the shift-fork over, locking the diff.
What nice about these units is there is no hoses (air lines) to leak, or wiring to get damaged
Making this a really good all around simple solid locker for many purposes.
The down side and
To this one. Is that it can sometimes be a little bit of trouble.
To run, and route the cable.
Just make sure to route the cable nicely. And don’t kink the cable around too tight of a bend.
Use nice wide turns, so the cable operates smoothly and doesn’t bind.
And make sure it’s adjusted properly.
Also make sure to keep it away from heat sources like the exhaust.
And you will also have to measure. To get the right length of cable you need for your application.
So, next up is the…
Automatic-lockers work automatically, by themselves. With no driver intervention.
They also, can be referred to, as a gear-less differential.
But, in reality they’re more like a spool. With a ratchet release functionality.
Rather than working like a actual differential so to speak.
There are two main types of automatic lockers
- Full case
- Lunch Box (Drop-in locker)
So, one of the most popular brands of full case type is the
So, a full case Detroit locker. You will have to replace the whole carrier.
How a Detroit Locker Works
Is that they have three steel plate assembly sections (ratchet gears)
A spider assembly in the center.
The spider-assembly (center section) is attached to carrier/case via the trunnions
With two clutch assemblies meshing on each side of it.
The clutch assembly plates stay engaged or “locked” in position all the time in the spider assembly.
By a set of heavy, springs. Pushing between the clutch assemblies, and the side-gears.
On, the inside of the carrier/case.
Both clutch-assemblies, are attached to the axles via splines to the side-gears.
Also, on the inner part of the spider, and clutch, assemblies.
They have tapered-cut-teeth. (cam-ramps).
Which creates the ratchet mechanism.
So, when both wheels have high traction, and you start to turn.
Those inner tapered teeth, begin to push the clutch assembly plates out (against the spring tension), and away from the spider
So, now the outer teeth, of the clutch assemblies. Can clear the spider assembly teeth on each tooth click.
Allowing differentiation, to happen. Between the outer and inner wheel.
But, then as soon as you’re going straight again. It will snap back in place.
And, become fully-locked in the outer grooves of the clutch, and spider-assembly.
Are that it allows the axles to move independently from each other. And gives you maximum traction for your off road rig.
Detroit Lockers have been used. From ultra4, desert-racing. To drag-racing, and extreme-off-road use.
Like rock crawling, or even mudding applications.
But, keep in mind
And though these unit are great for off-road applications.
These units do have draw backs. And reason why they’re not suitable for daily driving on the street.
The teeth are cut with a little bit of extra slop.
So, when you let on and of the throttle.
You will typically, have some play forward, and back. When taking off the line from a stop.
Or, even when, letting on, and off, the throttle, cruising.
They also, make a loud clicking noise. When you turn, because of the ratchet style teeth mechanism.
And the locker mechanism itself, isn’t a very smooth transition.
So, when it engages or disengages. (pops in-and-out)
It can go, from locked, to unlocked, when taking a turn,
Which can cause a loss of control, irregular/squirrelly unsafe driving characteristics.
And that can be, especially troublesome. During rain/snow/ice type weather conditions.
Now, on longer wheelbase heavier commercial type trucks. They can possibly work a little better.
Because there’s typically enough weight/down-pressure on the tires.
So when the plates pop in an out, it’s not as noticeable.
The lunch box locker works on the same principle as the Detroit locker.
Though different in design. and
How a Lunchbox Locker Works
Attached to the axles via splines is a left and right side (gear) couplers.
That then mesh on each side of the center drivers (left and right cam gears)
Now between the two center drivers is alignment pins and springs. To keep some preload out to the side (gears) couplers
Now the center drivers have a slight oval shape. Where the cross pin goes through.
So when under power the cross pin will ramp up, and push out more on the center drivers.
Locking them to the side couplers.
So when going straight the unit is locked in.
Then without power when coasting around a turn.
Now one side will be able to unlock side (gear) coupler, from the center driver. As it can push in on the spring preload clicking over the teeth.
A lunch-box locker will replace the spider-gears. Inside the carrier of your factory open-diff.
Allowing it to be easier to install, than the full case Detroit locker.
Because you don’t need to remove carrier, and reset ring and pinion gears.
You just remove the cross-pin and spider-gears. And replace it with the lunch box locker.
Which is nice because if it doesn’t work out. You can always put the spiders back in.
Allows movement between two axles
And some, lunch-box lockers, have tighter tolerances.
And typically, don’t have as much slop, in the teeth. Compared to Full-Case automatic-lockers.
Which helps, to eliminate some of the play. When you’re letting on, and of the gas.
Lunch Box lockers are most of the time a more affordable locker to put in your off road rig.
And easier to install.
However keep in mind
They are designed for off road use. and not recommended for a daily driving use.
As you could experience irregular driving characteristic at times.
And they can have the same clicking noise, as the full case automatic locker does, as well.
Now a spool isn’t a differential by definition.
Because, it doesn’t have any gears, and it doesn’t differentiate anything.
Spools, are for Off Road Use Only
And mainly used for drag-racing, trophy truck, desert racing, or competition rock crawling type applications.
Also, when it comes to spools there is two types
- Full Spool
- Mini Spool
Replaces the whole differential carrier.
And creates a solid connection from the ring gear to the axles.
So, it will send equal Torque/speed to each axle all the time.
They are lighter weight.
Reducing some of the rotating mass, and or unsprung weight, vs a standard diff carrier. Which can help with performance.
And since there is no moving parts spider to side gears. They can handle more torque than a standard differential
Keep in mind as well. That some spools are made to be used with larger diameter axles.
Remember a spool is for Off Road Use Only
If you use your vehicle for on-road use at all. You Do Not want one of these
Because if you’re going straight they may work fine
But if you try to make a turn. Now things are about to get wild, and whirly (severe understeer)
From lack of control to chirping/chewing, tires up on the pavement.
As one wheel tries to go faster than the other around a turn, but it can’t.
It begins twisting/cranking, and yanking on stuff.
Something could eventually bust. And it’s probably an axle.
(Thats why it’s probably best. To go with a Detroit Locker in some cases. If you can over a spool to eliminate the binding)
Something to keep in mind, is that you should, only put a spool in the rear diff.
Never, in a front diff.
Because, it can, cause too much binding.
Making it too hard or nearly impossible to make tight turns on tight trails
Might be a good option. If you need a spool that’s a little more budget friendly and easier to install. (or swap in-and-out).
Because, you don’t need to remove the carrier.
A mini spool just replaces the spider gears in the diff carrier.
Which then will lock both axles together like a full spool would.
More economical than a full spool
Easier to take in or out
Because, a mini spool isn’t typically as strong.
And, since with the mini spool you’re using the same stock diff carrier. It doesn’t support the ring gear like a full spool would do.
If you really find you need a spool. You’re best going with the full carrier spool.
One last thing to
Wrap it Up
Remember some of these units. You’ll have to replace the whole diff carrier
Which will require you to reset the ring/pinion gears back up.
So, you might want to consider a new gear-set/gear-ratio change at the same time.
If you would need to do that.