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Differential Differences

Posted in How To: Transmission Drivetrain on August 14, 2017
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Photographers: Courtesy of Manufacturers

How much do you know about your axles? Assuming you’re at least mildy off-road savvy (hey you’re reading this magazine!), it should be no news to you that a locking differential or a limited slip differential will greatly enhance traction and help you go further into the great outdoors.

Any modern drive axle (one that transmits engine power to your wheels and tires) has a differential that allows the wheels to differentiate rotational speed. A differential is necessary on the drive axle(s) under any street-driven vehicle because the inner wheels on a vehicle rotate at a slower speed than the outer wheels when turning.

A differential that can allow one wheel to spin completely freely of the other is called an open differential. In normal driving situations, where both wheels (tires) have traction, torque is split almost equally to both wheels. But when one tire has less friction force on it, because of oh say, sand, it’s more prone to break traction. This less resistant side will start spinning faster and faster, until all power from the input shaft (the pinion gear) is going to one wheel and the other wheel stops moving completely. Not what you want off-road.

For this reason, traction-aiding differentials are extremely beneficial. The most common type of traction-aiding device in stock vehicles is a limited slip differential (LSD). A limited slip differential uses either clutches, cones, or helical gears to greatly inhibit a differentiation in wheel speed, without prohibiting it. This enhancement in traction (while still allowing one wheel to rotate faster when necessary) makes an LSD a popular aftermarket traction aid to add to trucks and cars, too.

For more radical terrain, a locking differential might be what you need. A locking differential can either be on full-time, or it can be selectable. A full-time locker, or automatic locking differential, has no external control and engages either when power is applied (via driveshaft) or when a difference in wheel speed is sensed, depending on what type of auto locker. A selectable locker uses a cable, electricity, or air pressure to actuate from an open to a locked position.

There are also a few selectable locking differentials out there like the factory Rubicon Wrangler diffs that toggle between being a limited slip and fully locked. Powering both wheels at the same speed, “locked” together, both tires would have to spin at the same speed at all times.

There is a third option for extreme traction—a spool—that isn’t really a differential, but instead goes in place of one. A spool is basically a solid chunk of metal that carries the ring gear and is broached to accept two splined axle shafts, forever locking both wheels to the same wheel speed. Most race-only trucks and cars use spools, and while locking both axle shafts together is ideal for race situations where extreme traction is required, the associated poor handling characteristics and advanced tire wear makes it so a spool is never recommended for street driving.

There is a variety of traction-aiding differentials out there, but let this be clear: Lockers and spools absolutely lock both axle shafts together with no difference in rotational speed. Anything less than 100-percent locked is either a limited slip diff, an open diff, or not properly working.

Open Differentials

A typical open automotive differential is one that uses gears inside a carrier to allow a difference in wheel speed for everyday street driving, while delivering almost the same torque to both wheels as long the resisting force on both sides is congruent. Open differentials make it possible to turn smoothly in your vehicle, where the inside tire is spinning at a slower rotational speed than the outside tire. A limited slip differential could be considered a type of open differential, as well.

Limited Slip Diff—Clutch or Cone Type

Limited slip differentials do just that: They limit slip. A clutch-type LSD makes use of cones or a clutch plate stack. Friction and clamping force are used to keep uneven wheel spin (side to side) to a minimum, as the clutches (or cone) try to keep the axle shafts spinning together. These clutch-type LSDs have similar internals to open differentials, but with the added preventative force of the cones or clutches causing friction, ready to combat wheel spin. As more torque is applied, more force will be applied on the clutches or cones, and the more coupled the wheels will become. These differentials occasionally need rebuilding and replacement of the internals that can wear.

Limited Slip Diff—Geared

A geared LSD is a torque-sensitive mechanical unit that does not have cones or clutches, and instead uses helical worm gears to inhibit wheel spin. The helical worm (or spur) gears press against the carrier to create friction and inhibit wheel spin on the faster moving side, directing more torque to the more slowly-moving wheel. When unengaged, this torque-biasing unit remains otherwise open, until it senses a difference in rotational speed and applies pressure. These geared LSDs are considered the best type to use in front drive applications.

Locking Differential—Selectable

Selectable locking differentials toggle between being open and completely locking both axles together. There are a couple exceptions out there that act as LSDs when unlocked, but the large majority of selectable lockers go from fully locked to fully open. Depending on what locker, engagement and disengagement can be done using a mechanical cable, electricity, or air pressure. The OX Locker (pictured) can actually accept four different locking and unlocking methods—cable, electric, air actuation, and an emergency trail lock.

When a selectable locker is locked, there is no allowance for a difference in rotational speed on either side of the axle. No matter what, both tires will turn together. In a front application, it will be very difficult to turn the steering wheel with the locker engaged if both tires have traction. Many enthusiasts find a front selectable locker very desirable—one that they can turn on and off as necessary so turning the front tires is not a problem.

Locking Differential—Automatic

Full-time lockers, or automatic lockers are locking differentials that engage when power is applied to them or when a variation in wheel speeds on the same drive axle are detected. Theoretically, an auto locker unlocks when turning corners or when not under throttle. But some enthusiasts find that the automatic engagement and disengagement are not always spot-on, and sometimes an automatic locker will suddenly pop or slightly engage in a corner. This pop can sometimes sound like you’ve just broken an axle shaft and will definitely get your attention when it happens. Generally, auto lockers have been found to be a reliable, heavy-duty, no-fuss option that many off-roaders prefer.

Locking Differential—Drop-in (Automatic)

A more cost-effective, but usually weaker locking differential option is a “drop-in” locker. Also known as a lunchbox locker, it retrofits your existing open differential carrier in place of the spider gears to make it an automatic locking differential. These are usually the cheapest type of traction aid you can purchase (aside from a spool) and cheapest to install, but they are not as strong as a complete replacement locking differential.

Spools

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. A spool locks both axle shafts together and allows for no differentiation in wheel speed. Because a spool is a solid chunk of metal that the ring gear rides on, there are no moving parts inside of a spool to heat up or wear out. A spool is lighter, cheaper, stronger, and will run lower axle temperatures than any other option you can put in your axle. It will also push your car straight down the road creating bad understeer issues, wear out your tires in hundreds (not thousands) of miles, and create unwanted drivetrain bind. Spools should not be used for streetable applications and never in front axles.

Do You Need a Locker or a Limited Slip?

If you’re going to be off-road, then any type of traction-aiding differential that inhibits differentiation in wheel speed is going to propel you further in the dirt. But contrary to what most people’s egos might tell them, not everyone needs a locking differential. You may find that a limited slip differential is actually better suited for the way you drive and/or the conditions you drive in. If you need to completely lock the two rear tires together for the ultimate traction, then a locker is going to be what you need. But you’ll have to choose between a selectable locker that stays “open” a majority of your driving time, and an automatic locker that can sometimes unexpectedly engage on the street, making for quite a different driving dynamic. A limited slip differential is on all the time, always discouraging a variation in wheel speed and always helping with superior traction over an open differential, but never locking both wheels together.
And if you’re always in the dirt and don’t care about tire wear, you might want to look into the cheapest, lightest, and strongest option for your axle: A spool.

Sources

Eaton
Cleveland, OH 44114
800-386-1911
http://www.eaton.com
South Bay Truck and 4x4
310-219-0727
http://www.southbay4x4.com
Richmond Gear
Chicago, IL
864-843-9275
RichmondGear.com
ARB USA
Renton, WA 98057
425-264-1391
http://www.arbusa.com
Yukon Gear and Axle
888-905-5044
www.yukongear.com
OX USA
727-230-7803
www.ox-usa.com

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