As a young adult I was more interested in going fast than going off-road, so my thoughts on modifications were always focused on the one question: “How can I get more power?”. It didn’t take me long to realize that, off-road, power didn’t matter a hill of beans without traction. We’ve all been there, stuck, trying to get up a hill, watching as one wheel spins away furiously and the opposite one sits motionless. You may have wondered the same thing I did with that first experience: “Why can’t both wheels be turning?”. Today I’d like to help answer that, along with the accompanying question, “What can I do to fix it?”.
Notice the drivers side tire spinning while the passenger sits still.
The problem arises from one of the basic needs of any vehicle, the need to turn. If a vehicle only needed to go straight you could just lock both axles together, But in a turn, the inside wheels have a shorter distance to travel and must turn slower, while the outside wheels have a longer distance to travel and must turn at a much faster rate. The solution to this quandary was the invention of the differential. Rather than go into the technical details the video below (made in the 1930s) does the best job of explaining that I’ve seen to date. (Forward to 1:55 to skip intro)
The solution to this problem is to modify the differential so that the axles can both turn at different speeds to corner while driving on the road and have some sort of mechanism that can send torque to both wheels while off-road. These differentials can be broken into two main categories, limited slip and locking.
Limited slip differentials (LSD) have been in use since the 1930s, and basically function as an open differential until one wheel’s speed exceeds what is normal in every-day driving. When this happens a clutch or geared mechanism transfers torque from the wheel that is spinning to the one that is not. The main advantage of this system is streetability and smooth operation. The LSD is completely invisible in its operation; while on the street you will never know it is there. It is also the perfect solution for slippery or icy roads where you just need that extra little bit of traction to get through. LSDs can be limiting, however, in very low traction situations such as deep mud or rock crawling. The problem arises because it doesn’t kick in until there’s already excessive wheel spin, and most do not lock up fully. This means that you can lose precious momentum before the wheel with better traction receives any torque, and in rock crawling, the necessary power is not always transferred to the wheel with the best traction. LSDs are a popular option for the front axle where smooth operation and the ability to turn are more important, or for any 4X4 that will not see extreme use.
The clutch plates in the LDS below shift torque to the wheel with less traction.
Fully-locking differentials (lockers) function just the way they sound. Both wheels are fully locked together so that no matter what the traction situation they turn at the exact same speed. Lockers used to be rare and only used in dedicated trail rigs, but modern technology has brought them into the mainstream, and they even come on many factory vehicles. The Jeep Wrangler Rubicon is a good example of this and has front and rear lockers, so when fully engaged, all four tires turn together equally. Lockers can be broken into two basic categories, automatic and selectable.
Automatic lockers function nearly opposite the LSD; both wheels are locked together until the locker senses the need to disengage in order to allow the vehicle to make a turn, then will reengage upon the application of torque. The advantage of the automatic locker is its simple design and always engaged functionality. Simply, when you are off-road it just always works. The disadvantage is more on the street, as they require that you modify your driving style because they can lock up in corners and accelerate tire wear. Automatic lockers also come in two main designs, full carrier and drop-in (lunchbox). The difference between the two are in strength and cost, but both function the same. The lunchbox style is the least expensive route to add traction, but can be clunky and may not have the strength needed for larger (35”+) tires. For strength and reliability where street manners are of less concern, the automatic is clearly the way to go.
Below is a typical lunchbox style locker, note the simple design and few moving parts.
Selectable lockers have become, by far, the most popular way to add traction to vehicles that are driven on the street and off-road as they combine the best of both worlds. The selectable locker functions as an open differential until engaged by the driver, then it fully locks both axles until it is disengaged. Engagement methods vary by brand but include electrical, pneumatic, and cable/lever operated. The advantages of this design are obvious as it is seamless on the street and can be fully engaged off-road. The disadvantages are the high cost of parts and installation, and the complexity of the systems that can be more failure prone than an automatic locker.
Below is a promotional video made by ARB Australia, demonstrating the functionality of their selectable locking differential system. (p.s. love the dry humor of the blokes down under)
If you are new to wheeling, be assured that open differential 4WD systems are more than enough to take on the majority of easy to moderate trails, but sooner or later most want to tackle more difficult challenges where upgraded differentials are mandatory. The balance of cost, reliability, streetability, and need is unique to each individual. I am personally a fan of hybrid systems that use a different option on the rear axle from the front. Because my Jeep sees limited commuting usage, I like the Detroit style automatic locker for the rear because its strength and reliability. I have seen too many selectable lockers have a failure and not engage properly, therefore I don’t want both axles to rely on air or electric to engage. I do, however, prefer a selectable locker for the front axle, as a locked front can be dangerous in snow and harder to make tight maneuvers in highly technical rock crawling.
Whichever route you choose, know there are no absolute right or wrong answers, just a set of trade-offs. Upgrading differentials is one of the more costly modifications in building your Jeep, so choose carefully based on experience, needs, driving style, and overall build plan.