Let’s talk about axles...
If you’re into off-roading in a Jeep you might be pretty much up-to-speed when it comes to axles and which are the best to use - or are you?
For the uninformed, all wheeled vehicles have axles and they’re used for steering, driving, and braking, so they’re pretty important. The axles transfer power and torque from your engine to your wheels so must sustain the entire weight of the vehicle and everything inside. They must also withstand the accelerated forces of driving and braking.
There are two basic types of axle. The first is called the drive axle, which is connected to the driveshaft, which is a rod that extends up into the vehicle’s transmission and connects with the engine, which helps to turn the axle.
The second kind axle is the dead axle, which isn’t connected to the engine via the transmission and drive shaft and only turns when the vehicle is moving. Dead axles are present in your vehicle to help support the weight of the car.
But delve a little deeper and the science of the axle is a great deal more complex.
All vehicles use differential axles. This allows the wheels to differentiate rotational speed which is essential as the wheels will often rotate at different speeds, like when turning a corner, for instance.
Then there’s the open differential axle, which lets one wheel to rotate autonomously. This is particularly important for vehicles going off road. Usually, both tires have traction and the torque is split evenly but going off the road the vehicle may have to cope with mud or sand which can result in one of the tires losing traction, making the tire on the other side start to spin faster, leading to all of the power from the input shaft (the pinion gear) going to one wheel, whilst the other stops moving completely. In other words, the vehicle gets stuck.
Humankind has always looked to overcome problems and nowhere more than in the automotive industry. So let us introduce you to four other types of axles. These are the Limited Slip Differential, the Automatic Locker, the Selectable Locker and the Spool (not technically an axle but more on that later). Which is the best to employ for off-roading or daily driving depends on circumstance and what the vehicle needs to do. Let’s look at each type individually.
How Limited Slip Differentials Work
The Limited Slip Differential is designed to do what is says on the tin. Limit slip. The advantages of the LSD is that it’s always on and it inhibits differential speed, making it ideal for vehicles that are driven daily. The down sides are that they’re not usually as strong as the Selectable Locker, aren’t 100% locked and with clutch driven vehicles may need a rebuild.
The LSD transmits equal torque when driving straight but if one wheel loses traction the unit will automatically apply more traction to the wheel that has traction.
Furthermore, a clutch-type LSD has similar internals to open differentials but use cones or a clutch plate stack to add preventative force, which prevents the wheel spin. The greater the torque used, the greater the force will be applied on the clutches or cones and the more coupled the wheels will become. To greatly reduce uneven wheel spin friction and clamping force is used as the cone keep the axle shafts spinning in tandem.
A geared LSD is the considered choice for front wheel drive vehicles. It doesn’t have cones or clutches. It’s a torque-sensitive mechanical unit that uses helical worm gears to reduce wheel spin. This works through the spur pressing against the carrier, which creates friction, reducing spin on the faster moving wheel. This in turn sends more torque to the slower wheel. When unengaged, this torque-biasing unit stays open and not until it senses a difference in rotation speed will it apply pressure.
The LSD is recommended if it’s a vehicle that is being used daily and works well when meeting rain, mud, snow and ice. Sounds pretty good for a 4BT Jeep Swap guy out on a steep trail right?!
How Automatic Locker Differentials Work
As its name suggests the Automatic Locker is engaged as soon as power is applied and remains locked, except when cornering. Though this can be a little hit and miss with engagement sometimes only slightly engaging, which can cause a popping sound. It’s the most economic to use but wears out the tires quicker and can alter the handling of the vehicle. It’s also not great when encountering icy roads.
Not great for everyday driving and they’re more for off roaders and hot rodders. They’re relatively noisy and when getting on or off throttle they have a tendency to bang and clunk. Some manufacturers have come up with a design that reduces the noise but none yet have managed to eradicate it completely.
Another factor that makes it a bad choice for everyday driving is that they only drive on the inside wheel when cornering, particularly if making a right hand turn. There are two reasons for this. The torque on the rear end housing un-weights the right wheel and right hand turns are tighter, due to the fact that we drive on the right.
Other than that they have a reputation of being a reliable, no fuss option and is a popular choice amongst the off roading Jeep community.
How Selectable Locker Differentials Work
The Selectable Locker is the most expensive option and requires a lot of patience to get to grips with. For a start, it needs external connections outside the axle center section and makes the vehicle more harder to maneuver. It’s an option for those who are intending to do some serious off road journeys as it can handle some of the most severe terrains.
Most Selectable Locking Differentials change between being open and completely locking the axles together. There are others that simply revert to an LSD when unlocked.
When locked, there is no allowance for a difference in rotational speed on either side of the axle.
Which brings us to the Spool.
How Spool Differentials Work
Not technically a differential, the spool is a chunk of metal that the ring gear rides on and it locks both axle shafts together, which means that there is no differentiation in wheel speed.
Though not suitable for everyday use they’re ideal for racing, off roading on the most challenging of terrains and hot rodders. They have three handyl applications. They lock both left and right axles together which negates power loss when one wheel rotates faster than the other, they support the ring gear better than differentials, and is lighter, which assists with rotating mass and unsprung weight which the suspension has to cope with.
If you’re looking for the ultimate in traction, weight, and durability with no other concerns, going straight to a spool might be your best choice. It’s the lightest, strongest and cheapest option even though it pushes the vehicle straight down the road, wears out the tires at a phenomenal rate and produces drivetrain bind. So, though cheap, not particularly cheerful. We know not everyone has 'Scrooge McDuck' type of money laying around but if you are spending some serious money doing a Cummins 4BT Swap in a Jeep already, why not get some axles that will really last out there on the trail.
Ultimately, there are dozens of different differentials on the market so now we’ve outlined the strengths and weakness of each type, let’s take a look at some of the most popular models.
Originally manufactured for GM vehicles, this is a strong and aggressive LSD. It has four preload springs and two steel preload plates. The latest model has an HD nodular iron case, forged side and pinion gears and for strength and durability, carbon fiber clutches. If you’re into customizing Eaton offer three levels of spring pressure which can be utilized to vary the clutch preload from 200 lbs to 800 lbs.
They’re now suitable for Ford applications and will soon be available for Dana Spicer, AMC and Chrysler applications.
Eaton differentials have been around forever. You can't go wrong with Eaton to support the 800 lbs beast of the 4BT in a Jeep Swap. Overall, this differentials would work best for everyday driving. If you aren't going off-roading a basic Eaton setup is fine.
Yukon Dura Grip
A four spring clutch driven limited slip that was used extensively in GM muscle vehicles during the 1960/70s. The internal gears are crafted from forged steel to add strength and the Raybestos composite clutches provide long lift, whilst giving mean lock up ability which provides plenty of traction. They come with 400 lbs of preload though with customization can be changed to 200 lb or 800 lb, depending on what level of handling is needed.
Yukon Bear Trac
A helical gear driven unit, the Bear Trac posi tractions feature an 8620 case and internals for added strength. They provide smooth operation and have no clutch surfaces to wear out. Suitable for front and rear, 2 wheel and 4 wheel drive vehicles, they’re designed for medium duty applications.
Yukon have eliminated the banging, clunking and clutch clatter and have engineered it to be a robust unit. The one negative is that it doesn’t give 100% lock up. Highly versatile when it comes to applications.
The Detroit Truetrac is a helical-gear style, limited-slip differential that maximizes wheel traction and enhances driving characteristics. Maintenance-free, it features smooth, automatic operation in forward and reverse. Its versatility makes Detroit Truetrac the ideal traction solution for almost any type of vehicle.
Inspired by worm gear engineering, the truetrac works like a clutch posi-traction. The torque comes from the light friction between the worm wheels and the case and is then multiplied by the worm wheel ratio. It can be adapted by the manufacturer to increase or decrease the torque biasing or lock up.
Highly durable, it works pretty well with tall tires, though not suitable for tires over 33 inches. Taking a look at the online forums, it works very well in mud and snow.
In more challenging terrains, where the vehicle will have one wheel off the ground, applying the brakes lightly helps the differential gain more transferring power to the wheel still on the ground.
A word of warning, Making the unit overly aggressive will lead to tire scrub during everyday driving causing the unit to quickly wear out. To summarize, if you got a Jeep YJ with 37" tires with a 6" inch suspension kit this isn't the axle for you. For your average Jeep Swap guy with a standard inline pump Cummins 4BT Engine who goes trail riding on occasion this is a good middle of the road axle that will get the job done.
Auburn Gear, Auburn Gear Pro and Auburn Race
Auburn Gear has been leading the way in limited slip differentials for over a half a century. Across its three models it caters for all vehicles and applications, from everyday driving to dragsters.
The Auburn Gear gives a smooth torque sensing operation with the correct bias ratios, outperforming many of its competitors. The friction is provided by a cone style clutch which wears directly against the carrier case. It works well on vehicles up to a ton and has good impact strength. The downside is that it’s not rebuildable and though it will perform well in high HP vehicles but will quickly wear if there is a lot of wheel spinning.
The Auburn Pro will boost your torque, giving you a higher bias ratio and quicker sensing reaction which will give a high level of controlled power transfer.
Again, cone style and not rebuildable, it provides a greater level of performance traction, but like the standard model, will still wear out fast when tested on extreme off road conditions and/or excessive wheelspin. A complaint is that it has a lot of chatter so avoid if you’re looking for a smooth drive. This is not the axle for the hard core rock crawling Jeep guys where accelerated wheel rotations are common.
The Auburn Race is a high Performance limited-slip differential specifically for road racing. The differential case is designed to allow the center pin to float along with the axle shafts. This aids cornering, so that when the driver is turning, the outside wheel axle shaft will apply a force to the center pin, hence, the pinion gears will firmly seat the inside cone/side gear assembly giving the racer an extremely “tight” rear axle. The greater the G-force the more bite you get.
Whether mild or extreme applications the Detroit gives the ultimate traction. Engineered to keep both wheels in drive mode – even if one wheel is lifted off the ground, it’s 100% locking and works in forward and reverse. It’s highly durable and pretty much maintenance free.
The new Sof-Locker design includes a damping device which reduces the banging and clunking, a problem in earlier models.
It automatically unlocks on turning so the outer wheel can turn faster than the ring gear inside the wheel. Having the outer wheel spin faster to obtain that low end torque is something built for the rock crawling community of Jeep enthusiasts.
Yukon Grizzly Locker
Highly popular amongst the four-wheeling community, the Yukon Grizzly is an automatic, speed sensing locker. It will lock up when there is a wheel speed difference side to-side. They are fitted with internal components that allow for differentiation in turns.
The forged steel internals gives it incredible strength. It comes in two versions, drop in and carrier replacement, depending on what application it’s being used for. It’s speed sensing design ensures 100% lock up. Banging and clunking is common but the latest units come with the same Sof Locker as the Detroit, reducing the noise levels. The Grizzly Locker is particularly popular with Jeep Rubicons that partake in some serious off roading.
Launched in 2009 the Spartan is one of the most economical and versatile lockers on the market. It’s compact and simple design is up there with more expensive lockers. Its features include 100% lock up for maximum traction, heat treated chromoly 9310 steel for added strength, comparably easier installation and removal, a redesigned pin shaft and it has eliminated the need for thrust washers.
Fitted with a center pin which slots in between two inner drivers, when under load the pin forces the drivers outwards which engages with the outer axle couplers, providing locked traction to both wheels. When the vehicle turns one of the drivers unloads so the teeth can ratchet across each other to create differential action. This reduces tire wear and gives easier handling when turning.
Highly versatile and very robust, it doesn’t require professional installation. This is great choice for a higher price range / heavy duty differential that will be dependable while doing some off roading or rock crawling. You see a lot of spartan lockers in Jeep Wranglers, JK, YJ and CJ Models.
Powertrax Lock-Right and Heavy Duty-Lock Right
Designed by Powertrax, the Lock - Right is the original locking differential that can be easily installed in existing case assemblies. It’s widely used in demanding off-road applications that require extreme traction output and high-strength.
It eliminates wheel spin-up which negates driveline trauma. It’s been used in some of the harshest conditions on the planet by hard-core off-roaders, used in towing applications and military vehicles that operate in the most challenging of terrains.
The Lock-Right is exceptionally strong, easy on the wallet and delivers as much traction as output.
It’s operation is fully automatic and designed to send the engine power to the wheels with the most traction. When traveling straight the Lock-Right will lock the axles together similar to a spool. However, when the wheels need to differentiate in a turn, it will allow the outside wheel to rotate faster to complete the turn as the internal gears overrun each other to allow this differentiation.
An ideal choice for off-roading and its extreme traction makes it an attractive option for racing, hot-rods and muscle cars. You will see the a lock right axle in just about anything from old M-37 and WWII Willis Jeeps to F-150s and Land Cruisers. This axle will easily hold the robust weight of 4bt engine will giving the vehicle the versatility to make all kinds of tight turns or steep inclines.
The Heavy Duty model drives the same as the standard unit but are made from heavier springs and sturdier materials. With no damping mechanism it will bang and clunk when driven on the street. Ironically, the standard Lock-Right comes with a 2 year warranty but with the Heavy Duty version you only get a 1 year.
Detroit Gearless Locker
The Gearless is a speed sensitive automatic locking differential that, says it’s manufacturer “dramatically changes both the fun and function of any ATV (All Terrain Vehicle). It powers both wheels yet automatically lets the differential operation to compensate for wheel speed differences when cornering or overcoming obstacles, eliminating individual wheel spin-out.
It’s also suitable for Light Utility Vehicles as the Locker increases traction on all surfaces, including slippery mud, ice, sand and extremely wet conditions. A plus is that it is almost noise free as it doesn’t use internal gear teeth. A negative is that there is strange handling in short wheel-base vehicles when making sharp turns, as well as spinning of the inside tire when turning from a motionless position, on a slippery road.
The clutches lock simultaneously and there is no slippage, and they instantly release when any overriding forces are made when cornering. If you’re a fair to middling mechanic, installation is pretty straight forward and it works equally well on the front and back wheels.
Basically the same as the standard version but with a dampening mechanism to mostly eliminate bang and clunk. The mechanism doubles up as a holdout ring meaning the teeth last longer and cornering is smoother and quieter.
We've put this axle in the Locker section as it comes most commonly with a locker differential, though some come with the ARB Airlocker. The Dana 44 was first introduced just after WWII and was found in a wide variety of Jeeps, as well as International Harvester, Dodge, Studebaker and Ford trucks. This is standard go to axle for just about anyone with a Jeep and 4BT Engine.
There are many versions of the 44 so but we've picked the Ultimate Dana 44 to talk about.
Made from nickel chromoly steel it has thicker tubes and brackets along with Spicer gearing and is manufactured to combine strength, optimal performance and little noise pollution. Its newly improved caster angle reduces bump steer.
Another feature are the Spicer Life Series 1350 u-joints with triple lip seals and full-circle clips, designed for ultimate protection and retention.
They’re not cheap, so, as mentioned, serious off roading is what you need this for, not the school run.
If the 44 is not enough for the serious off roading your vehicle endures, then the 60 will be as it is built for the most extreme demands and terrains known to man.
With massive Spicer 70 universal joints it gives optimal performance to lifted vehicles with larger tires. It has a higher torque load and new ‘rock solid’ seals give the ultimate in protection from environmental contaminants. You not only get a higher torque load, but with new “rock solid” seals you get peak protection from environmental contaminants. Cast from Spicer 35 spline SAE-4340 nickel chromoly steel its strength is unmatched.
Dana have made many improvements and the Ultimate version includes high-strength, heavy gauge brackets, selectable electronic locking differentials, a nodular iron differential cover with protective ribbing and upgraded heavy-duty brakes. However brutal the terrain is, the 60 will handle it.
Again, not cheap and should only be considered if your vehicle is going to be challenged to the extreme. When asked what type of axles you put in that Jeep CJ with a 4BT behind it you will most certainly hear Dana 60s as being in the top tier.
If unsure whether you should chose the 44 or 60 we've written a comparison article HERE.
The Yukon Zip Locker is the latest in air locker technology. It switches the differential from open to locked on demand. Contrary to a limited slip, the Yukon Gear & Axle Zip Locker gives the driver ultimate control when needed. Each unit is manufactured with 4320 Chrome Moly internals for added strength and durability, it’s ideal for midsize daily driving use.
Offering the option of 100% locked or fully open the advantage is that when open it gives a tighter turning radius and when locked gives maximum traction, making it an excellent option for off road use and driving on snow and ice. It comes with a self-installation kit which has everything needed, with the exception of a compressor.
ARB Air Locker
Another air locker, the ARB gives 100% traction on demand, without sacrificing driving performance. Employing a 12 volt air compressor it will activate and deactivate the locking mechanism inside the differential. This is done by simply flicking a switch located close to the driver’s seat.
If the ARB air pump and ARB differential are bought together, all of the parts and wiring are included. The ARB pump though, is not a compulsory purchase as any decent pump supply 95 - 115 lbs of pressure will do.
The downside to this bit of kit is that the after-sales service is rumored to be poor.
Well, we've reached the end of the review about the best axles for a 4BT Jeep Swap. Hopefully, we've given you enough information to start the discussion of which axle you want to investigate further. There are tons of mods you can do to your Jeep and engine but it is always important to give full attention to your axles (the backbone of your vehicle). There are countless other differentials out there and it’s worth doing the research before purchasing one as they all have strengths and weaknesses, depending on the application that you require.