For the uninitiated a transfer case sounds like something you might take on vacation. But it is actually a critical part of all-wheel and four-wheel drive vehicles. So if you have ever asked yourself what is a transfer case you will be pleased to know that we’re going to tell you.
What Exactly Is A Transfer Case?
A transfer case is an essential part of the drivetrain of all-wheel drive and four-wheel drive vehicles. You will also find them in multiple powered axle vehicles.
The job of a transfer case is to transfer power from the transmission to the rear and front axles or to the rear axle only via the driveshaft.
Some vehicles allow the driver to select either with the option for 2-wheel drive or 4-wheel drive through a shifter which can look like a manual transmission selector, or it may operate electronically via a switch.
Other vehicles have transfer cases that are locked into all-wheel drive permanently with no option to select any other drive.
The transfer case also allows the synchronization of the difference between the rotation of the rear and front wheels. Some transfer cases will have one or more sets of low range gears that can be employed for off road use and to handle more demanding terrain.
Functions Of A Transfer Case
The transfer case divides the engine torque coming from the transmission and sends it to output shafts. These are in turn connected to driveshafts which lead to the front and rear axles.
For off road vehicles most transfer cases can mechanically lock the rear and front driveshafts together. This allows each end of the vehicle to receive equal amounts of engine torque.
In this sense the transfer case is the center of the drivetrain system for four-wheel and all-wheel drive vehicles. Torque may be delivered to the front output shaft by a chain or via a set of gears depending on the age of the transfer case.
Normal road use vehicles with transfer cases synchronize the variation in rotation of the rear and front wheels, in the same way that the differential behaves on an axle. This is important as the rear and front wheels never turn at the same speed.
This is because there are often different gear ratios in the rear and front differentials to help with control. If there was no transfer case the two different rates of rotation would cause binding and damage the transfer case.
Transfer cases that are not designed for on-road use can cause issues with driveline windup if they are driven on dry roads.
Types Of Transfer Case
Modern transfer cases tend to have chains as they are quieter and lighter than gears. However, gears have more tolerance under extreme stress and are capable of withstanding larger amounts of torque.
Gear Driven
A gear driven transfer case will use a set of gears to drive the front or both the rear and front driveshafts. These are typically heavy and strong units often used in large trucks. This is a more practical use for them as they are often too heavy for smaller vehicles.
However, some four-wheel drive vehicle owners will swap out their chain driven transfer case for a gear driven one to gain the extra strength they give. The downside is the extra weight and noise.
Chain Driven
More often than not a chain driven transfer case is used to drive one axle but can be used to drive both. These are quieter and lighter than gear driven transfer cases and are used in SUVs, Jeeps, compact trucks and full-sized trucks.
While they have a lot of advantages, chain driven transfer cases may be susceptible to stretch if put under extreme stress.
Single Speed & Two Speed Units
The majority of transfer cases in all-wheel and four-wheel drive vehicles are two-speed units. This effectively gives you an auxiliary set of gears that can be engaged to lower the ratio of the gears.
So when you engage low-range the torque going to the wheels is multiplied. For pulling and climbing power it is best to have the lowest low-range ration that you can get.
In factory produced transfer cases, the low-range ratios are between 2.50:1 and 2.70:1. However it is possible to have even lower ratios in specialized applications.
Single speed transfer cases found in some all-wheel drive cars and lighter SUVs are designed to send power to the front and rear of the vehicle. They do this without a direct mechanical link between either end of the vehicle.
A single speed transfer case also has no provision for low-range gear reduction. These cars work well on wet pavement or graded dirt roads but are not capable of tackling any kind of heavy terrain.
Part Time, Full Time Or Hybrid?
There was a time when the only options for transfer cases in all-wheel or four-wheel drive vehicles were either part time or full time. However, there has been a lot of development in this area and there are now many different types of hybrid systems as well.
Part time transfer cases allow the driver to shift between two-wheel and four-wheel drive according to the road conditions.
In a part time system the wear on the front-drive elements is reduced as they are not constantly engaged. This also allows for better fuel economy.
Full time transfer cases are permanently in four-wheel drive and no input by the driver is needed to engage the system.
Hybrid systems have been developed over the years to give drivers a lot more choice for selecting the drive system.
The options include two-wheel drive, full time four-wheel drive, and part time four-wheel drive. These systems are sometimes called active four-wheel drive.
Final Thoughts
Getting to know your four-wheel or all-wheel drive vehicle is always helpful for preventative maintenance and good performance.
We hope this insight into what a transfer case is and what its function is has been helpful to you.