Performance Parts Explained – Jeep 4×4 Systems

Alright, did you enjoy our last entry for “Performance Parts Explained” with a focus on Dodge vehicles? It was pretty fascinating stuff and all made a lot of sense in the end. This week, we have our second entry, with a focus on Jeep 4×4 Systems. We think it’s a fair warning, this is the kind of auto talk that can easily melt a common man’s brain in a single afternoon. Eat some granola or have a hard drink at the ready, because you’re gonna need a pick me up after firing up all those neurons. Let’s do this.

Components and Systems

These are the various components and mini-systems that make 4×4 systems possible.

Brake Lock Differential – In an open differential, any two wheels on an axle split the torque 50/50. This can be a problem if one wheel is on a low-traction surface, such as wet asphalt or ice, because if that wheel requires less torque than the other, then the other wheel will also get less traction, and the vehicle will ultimately stop. A brake lock differential (BLD) bypasses this problem by applying the brakes to the freely spinning wheel. By doing so, the now “braked” wheel requires more torque to be supplied to spin. Like the open differential, the torque is still split 50/50, but with a BLD, a freely spinning wheel is hindered so that both wheels get the same amount of torque, and the vehicle can keep moving.   

Electronic Limited Slip Differential – One could say a limited slip differential (LSD) works in conjunction with BLDs. This is because, like a BLD, a limited slip differential bypasses the rule of an open differential. A limited slip differential prevents the event of excessive power being delivered to one wheel and ensures that torque will not be limited in response to a wheel that requires less power to spin, such as the freely spinning wheel in the example above. An electronic LSD is simply an LSD controlled by a computer, such as the vehicle’s main computer, or another electronic control unit, such as a microcomputer.

Rear Axle Disconnect – The easiest way to explain this would be first to state that a rear axle disconnect is only installed into vehicles with a 4×4 drivetrain. This is because many Jeep 4×4 systems are part-time, or only direct power to all four wheels when required. Otherwise, even with a 4×4 drivetrain, the rear axle disconnect disengages the axle connected to the rear wheels so that power is only sent to the front wheels. In turn, with less power being exerted, fuel efficiency is improved and the vehicle defaults to front-wheel drive. In most cases, in the instance the front wheels start to slip, the 4×4 system will have a BLD to engage the 4×4 drivetrain and keep the vehicle moving.

Mechanical Rear Axle Lock – A mechanical rear axle lock is similar to an open differential, wherein torque is split evenly between two wheels. However, with a mechanical rear axle lock, a vehicle bypasses the rear axle disconnect and torque is sent to both axles; the output of torque to each axle is then split 50/50. For instance, with the Jeep 4×4 system “Active Drive Lock”, available on the Cherokee Trailhawk, the rear axle may be locked in an engaged position so that the vehicle is always using its 4×4 drivetrain and may freely drive off-road.

Mechanical “wet” clutch – A clutch is a mechanical device that engages and disengages the transfer of power from the driving shaft to the drivetrain. A wet clutch is simply a clutch immersed in a cooling lubricant to provide smoother performance and a longer lifespan.

Transfer Case – None of the above would be possible without a transfer case. Part of the drivetrain of four-wheel drive, the transfer case transfers power from the transmission to the front and rear axles via the drive shaft, the component responsible for transmitting torque and rotation to either axle.

Two-speed Transfer Case – Like the transfer case, responsible for transmitting power to each axle, a two-speed transfer case has two speed settings for each gear in an automobile. Think of a manual gear shift; it has multiple gears, typically 1 – 5, plus reverse and neutral. A two-speed transfer case can deliver a low or high speed in either gear if desired via the use of a clutch.

Electric Front Sway Bar Disconnect – We discussed anti-sway bars in the previous part of this series, with a focus on Dodge performance parts. In a Jeep, instead of connecting the front and rear axle, the sway bar actually stretches from the driver’s side of the axle to the passenger side, and can be split into two parts. This is because Jeeps are designed for off-road driving, and will at times require what it known as “flexing”, or having one wheel higher than the other. The sway bar in a Jeep still reduces the vehicle from rolling over, but when both parts are connected, flexing is near impossible. By disconnecting the two, each sway bar may move independently of the other, allowing for greater flexing. An electronic front sway bar disconnect is simply a device that allows the driver to disconnect the sway bars along the front axle with the push of a button.

Suspension Damping – We discussed shock absorbers in our section on Dodge, and suspension damping is related to this. Shock absorbers are designed to absorb and dampen the shock when traveling over rough terrain. Damping is the process of absorbing the energy of impact with the terrain, and suspension damping slows down the rate at which the suspension absorbs or releases that energy.

Quadra-Lift Air Suspension System – This is a system designed by Jeep that can raise the vehicle up 4.1 inches for a best-in-class maximum ride height of 10.7 inches for better off-road capability. In contrast, it can also lower the vehicle for a lower center of gravity for better handling and fuel efficiency. It can be controlled automatically by the driver, or set to operate automatically.

Common Phrases

The following are phrases used when describing some Jeep 4×4 systems, but are simple to understand.

Low-speed Traction – To understand low-speed traction, one must know of low-speed traction control systems. A low-speed traction control system helps to prevent front-wheel spin on slippery surfaces during acceleration up to 25 mph. Thus, one can assume that low-speed traction, when describing a Jeep 4×4 system, also indicates a vehicle’s speed is no more than 25 mph. Faster than that wouldn’t be very safe for going off road and rock-crawling anyways.

Flat Towing – The best way to explain flat towing would be an example. Have you ever seen someone driving down the highway in an RV with their primary vehicle being towed on it’s own wheels, meaning, not on a trailer? That’s flat towing. Some Jeep 4×4 systems allow for easy flat towing with the gearshift in neutral.

Throttle Response – This is simply a measure of how quickly a vehicle’s engine increases its power output in response to the driver applying pressure to the gas pedal for more acceleration. When describing a Jeep 4×4 system, some systems allow for greater throttle response at low speeds for better control.

Torque Multiplication – There’s a whole lot of information on torque multiplication, but the simplest way to put it? Torque multiplication is an increase in engine torque via a torque converter, a device that usually replaces the clutch in a vehicle with automatic transmission, and is responsible for separating the rotating power generated by the engine and the rotation transmitted via the driveshaft.

Steep Grades – This is just a fancy way of saying a steep slope or incline in either direction, up or down.

Crawl Ratio

Low-range and Crawl Ratio – The easiest way to explain what it means when a Jeep is in a “low-range” mode is this: a low-range vehicle moves slower but produces more torque.

It relates to crawl ratio, a term used to describe the highest gear ratio a vehicle is capable of. A gear ratio is how many times one gear has to spin to cover the same distance as another. A good example would be two gears, where the first is half the size of the other. The first gear has to rotate twice to cover the same distance as the second, causing a gear ratio of 2:1.

It’s confusing, but here’s the point. A low crawl ratio equates to a high gear ratio, and the higher the gear ratio, the higher the RPM of the engine, the slower the wheels spin, and the more torque the vehicle generates. Thus, a low crawl ratio will make traversing steep grades or crawling over large rocks easier.

So when you see a 56:1 gear ratio, that means the vehicle has insane off-road capability because it’s generating a huge amount of torque at a very slow speed.

Is your head hurting yet? This is probably the hardest part of the performance series so far, but we may not be done with it yet. Be sure to check out our inventory of Jeeps, now that you know what 4×4 systems to look for on your next adventure.