Every vehicle has a differential, but 4×4 owners and off-roaders tend to care a little bit more about what’s going on with theirs — the type makes a world of difference on the trails.
There are various types of differentials, including:
- Limited Slip/Posi
Here’s what you need to know about the benefits and drawbacks of each for off-roaders.
Purpose of the Differential
The differential is cased in the center of the front and rear axles on a 4×4. It consists of a set of gears that allow the wheels to rotate. These gears convert rotating motion from the driveshaft into power that goes to each of the driving axle shafts.
A differential functions in three roles:
- It directs power from the engine to the wheels
- It acts as the final gear reduction in your vehicle
- The differential transmits power to different wheels while allowing them to rotate at different speeds
The differential’s main purpose is to convert power from the driveshaft to power that can turn the vehicle’s wheels, while sometimes controlling the rotational speed of each axle for each wheel as well.
What’s an Open Differential?
An “open” differential is a differential that doesn’t try to control the rotational speed of each wheel. As a result, open differentials don’t work very well for off-road use, as they typically send all the power from the driveshaft to the wheel with the least amount of traction.
Still, open differentials can be found on many vehicles as they’re generally “good enough” for most on-road applications, and because they’re very affordable. But any sort of 4WD vehicle will have either a limited slip or locking differential, as these types of differentials are far better for off-road use.
Open Diff Pros:
- Low cost
- Adequate for most on-road situations
Open Diff Cons:
- All power is sent to the wheel with the least amount of traction. If one of your wheels is on a sheet of ice, and the other is on pavement, odds are good you won’t move forward with a purely open differential (at least not unless you’ve got tires that can “grab” a sheet of ice).
NOTE: Many (not all) locking differentials are actually open differentials when they’re unlocked.
What About Limited Slip or “Positive Traction” Differentials?
As the name implies, limited slip differentials are made to limit the tendency of an open differential to send power to a wheel that’s losing traction. Limited slip and positive traction (aka “posi-trac”) differentials send power to both wheels at the same rate when traveling straight. If one wheel loses traction and begins faster, the differential will automatically direct torque to the other wheel. Essentially, limited slip or posi-traction differentials send power from the wheel that’s slipping to the wheel that isn’t.
Limited-slip style differentials can function using any number of mechanisms: clutches, gears, viscous coupling, etc. There’s even such a thing as a limited slip differential that limits wheel spin by electronically controlling the brakes on the wheel that’s spinning. Each mechanism has its’ own pros and cons, but most off-road limited-slip differentials utilize a clutch system to control the speed of the axle.
Limited slip/positive traction differentials are good for daily driven vehicles as well as 4×4 vehicles. While not quite perfect for aggressive off-road use, they’re a great compromise between off-road ability and daily drivability in an affordable package.
Limited Slip Diff (LSD) Pros:
- Better off-road traction than a simple open differential, as LSDs direct power to wheels with traction
- Solid performance and traction on paved surfaces
- Minimal tire wear and stress on axle shafts, as axles can rotate at different speeds during turns
- Traction is unpredictable on mud, sand, and loose rocks…the LSD sends power to the wheel with less grip, but the power isn’t supplied continuously. It is re-routed as the gripping wheel begins to slip. This can cause the vehicle to pull to one side when traction is reduced, then the other, etc.
- LSDs usually aren’t capable of sending 100% of the power to the wheel with grip. A small percentage of power is always going to the wheel that slips.
- Not all LSDs are the same…there are different levels of wheel control, different durability levels, etc.
Finally, Let’s Cover Locking Differentials
A locking differential (aka “locker”) locks the rotational speed of the axle shafts, forcing the left and right wheels to turn at the same rate. This happens regardless of which axle has traction or is losing traction. While this isn’t quite as helpful as an LSD that directs power to the wheel with grip, it is far more predictable.
In the 4×4 world, locking differentials are available as either “automatic” locking diffs OR as “selectable” lockers. The key difference between selectable lockers and automatic lockers is drivability. Automatic lockers are less expensive and do not generally require the driver to do anything – the differential locks when there’s a substantial torque on the differential (as there would be when you’re stuck or launching), and then unlocks when the torque load is light). This locking and unlocking isn’t exactly seamless, however, and it can make for a miserable pavement driving experience (depending on the vehicle and situation). The locking and unlocking is also often quite loud.
Selectable lockers are probably a better option than automatic lockers, but they can be quite expensive. The selection is just a switch that controls a electrical or hydraulic system that locks the differential. When you flip the switch, the differential is locked and ideal for off-road challenges. When you disable the locker, the differential is open and the vehicle is easy to drive on the street.
Most top-end off-road packages that are available on popular off-road vehicles like the Wrangler and Tacoma feature an selectable locking differential.
Locking Diff Pros:
- Consistent performance/acceleration, which is especially helpful off-road
- When unlocked, behaves just like an open differential and is therefore easy to drive on the street
Locking Diff Cons:
- Automatic lockers can be loud and bothersome on the street, as they often take some getting used to. Until you become accustomed, you can “trick” your automatic differential into locking just by driving down the road.
- Lockers can place a great deal of stress on axles shafts. By forcing both wheels to turn at the same speed, a locker can sometimes create a situation that snaps an axle shaft.
Which Differential is Best?
If we’re just talking about off-roading – and money is no object – a selectable locking differential is the best option. While locked, the differential is the perfect tool for serious off-roading. But on pavement, the differential is unlocked so that wheels can turn at different rates (as needed during turns).
Since most of us have to live inside the constraints of a budget, automatic lockers and/or limited slip differentials are a good option. While they’re not perfect for off-roading, many 4WD enthusiasts rely on limited slip systems to get them over obstacles every day. Considering that LSDs are standard equipment on many 4WD vehicles, and considering that most vehicles aren’t driven aggressively off-road, LSDs are a reasonable choice for occasional off-road use.
Finally, open differentials should be avoided for off-road use. If your 4X4 came with an open differential, an upgrade is probably in your future. Open differentials are great on pavement, but they come up short even in mild off-road situations.