By Dan Rich, Contributing Editor COPYRIGHT © 2004
This article will describe how a transfer case works and explain it’s basic operation. There are various kinds of transfer cases from different manufacturers, and there are exceptions to nearly every statement, but the basics described herein are common to most true 4-wheel drive transfer cases.
What is it?
A transfer case is unique to 4 wheel drive vehicles, and is what directs power to both ends (axles) of the vehicle through two separate driveshafts. 2 wheel drives don’t need a transfer case as the driveshaft comes directly out of the transmission to the drive wheels.
For the sake of discussion in this article, we’ll refer to a “typical” 4-wheel drive arrangement, which is a longitudinal drivetrain (engine and transmission in-line with the vehicle from the front to the rear – in other words, not normally a front wheel drive arrangement).
Where is it?
The transfer case is attached to the rear of the transmission and gets its input directly from the transmission. Power is directed from the engine through the transmission and then through the transfer case to either the rear wheels or both the front and rear wheels.
This is accomplished inside the transfer case by either chains or gears depending on the type of transfer case. In any case, a 4-wheel drive vehicle will have a selection of two gear ranges. This is called High range and Low range. This is one of the characteristics that distinguish “4-wheel drive” from “All-Wheel Drive”. (See definition: Two-speed transfer case).
What does it do?
As the transfer case accepts input from the transmission, and the Hi-Lo range selector is in the “Hi” position, the transfer case uses the highest gear ratio. This ratio is usually near or exactly a 1:1 ratio. (same input and output speed of the transfer case, i.e. no speed reduction or torque multiplication). If the driver chooses the “2WD Mode” of the transfer case, power is transferred to only the rear wheels, leaving the front driveshaft without any driving duties.
If the driver selects the 4-Hi mode, the front driveshaft is engaged inside the transfer case and power is now transferred to both the rear and the front driveshaft, allowing the front and rear wheels to pull the vehicle.
When the driver selects the 4-Lo mode, the second gear ratio is used and power is directed to both the rear and front axle. The lower gear ratio is used when a multiplication of torque is required for steeper hills or where more engine power must be used to pull the vehicle.
Gear ratio: Whenever gears are used (most automotive transmission of power typically uses gears of some type to do it), we have to understand the gear ratios. Ratios are expressed as the input rotation and the output rotation of a gearset. This is calculated by knowing the number of gear teeth on the input gear vs. the output gear. Simply put, if first gear in the transmission has a ratio of 3.5:1, it means that the input turns 3.5 times for every one time the output turns.
Said another way, the output gear has 3.5 times as many gear teeth. This allows a proportional multiplication of power (torque) through that gearset. Very effective in getting a 4,000 lb car underway from a stop. On the opposite end of the gear ratios is the “overdrive” gear. It’s called this because the input side now turns slower than the output side. This type of ratio is usually used as a top gear (ex: 5th gear in a 5-speed transmission). An example of an overdrive ratio would be 0:75:1. This allows the engine to run slower at cruising speed optimizing the fuel economy on that same 4,000 lb vehicle.
Two-speed transfer case: This describes a transfer case that has the choice of a high range and a low range. High range is usually a 1:1 ratio; low range is numerically higher such as 3.5:1. The alternative is a transfer case that only has one ratio and is typically used in an “All-Wheel Drive” system and is not considered a true “Off-Road” vehicle because it lacks the low-range required for maximum torque multiplication. These are common in today’s sport utility vehicles and are very useful in conditions where all-wheel traction is required, but the vehicle is typically not engineered for true off-road conditions such as steep hills and rocks. See the article “4-Wheel Drive vs. All-Wheel Drive” for a more thorough explanation.
Crawl ratio: This is a term used describe the lowest gear ratio you can possibly achieve. Crawl ratio describes how slow you’ll be going and is usually only calculated using the lowest ratios of the transmission and the transfer case. You would rarely need to know the crawl ratio of 2nd gear. The lower the gear ratio, the higher your crawl ratio. This is calculated by multiplying all the ratios through the entire drivetrain. For example, if first gear in the transmission is 3.65:1 and the low ratio in the transfer case is 1.82:1, and the differential gear ratio is 5.13:1, the crawl ratio is about 34:1. The lower your crawl ratio (numerically higher), the slower you’ll go. This is very useful when slow speeds are required when crawling over rocks or going up a steep hill when a wide range of engine rpm is required for maximum control.
Tire size and crawl ratio: Your tire size is the final determining factor for so many things, not the least of which is your crawl ratio. Shorter tires will have a lower “final” gear ratio than taller tires. Let’s compare two common tire sizes. A 28-inch diameter tire is a common size found on some production vehicles. A popular upgrade for the off-road folks might be a 35-inch tire. Using these two sizes, we can see the difference they make for the final crawl ratio. For example, at a walking pace of 3 mph, and a crawl ratio of 34:1, the engine speed of the vehicle with the 28-inch tire would be about 1,225 rpm. For the 35-inch tire on the same vehicle, the engine speed would be about 980 rpm.
Your engine probably makes more power at 1,225 rpm than it does at 980 rpm so you’d better like those tall tires. To compensate for the loss of power due to taller tires, you could change one of your gear-sets in the crawl ratio equation. The easiest one to change is probably the differential ratio. In our example above, our diff ratio is already quite high at 5.13:1, but if we wanted identical performance from our 35-inch tire as our 28-inch tire, we would have to change our diff ratio to a fictional 6.40:1.
Looking at it another way, lets say your engine performs it’s best at around 2,000 rpm. How fast will you be going using our two tire sizes as a case study? With the 28” tire, your speed will be about 4.9 mph. With the 35”, it will be about 6.1 mph. At 3,000 rpm, the speeds are 7.4 mph for the 28” and 9.2 mph for the 35”. Remember, slower is better when crawling!
Transfer mode: This describes the method the manufacturer of your vehicle has chosen for you to be able to select how the transfer case shifts into its various modes. Some systems use a “stick” or lever located next to the transmission shift lever on the floor. (There were earlier systems that are different but are out of the scope of this article.) Somewhere in your vehicle is a diagram to show you what position the lever should be in to select 2-Hi, 4-Hi and 4-Lo. Some manufacturers use buttons or dials located on the instrument panel, some manufacturers a combination of a button/dial and a lever. If a button or dial is used, it signals a motor of some kind to physically change the gear position in the transfer case. Where a combination button and lever are used, it is simply a combination of electric motor and mechanical lever to select the transfer mode.
Drive / range selection: This describes how to choose which wheels will get power from the engine and what the gear ratio will be. Drive selection indicates either 2 or 4 wheel drive. Range selection indicates either Hi-range or Lo-range. The usual choices of drive/range selection are: 2-Hi, 4-Hi and 4-Lo. Sometimes a Neutral position is available as well. This will disengage everything at the transfer case. Also, with some systems or with some modifications, a transfer case can be configured to be able to select 2-Lo. This is a specialized modification usually reserved for hard-core ‘wheelers that have a lot of extra money and time!
Double transfer cases: When an extremely low crawl ratio is desired, two transfer cases can be attached in series (one after the other) and the choice of gear ratio is again multiplied. This is popular in today’s rock crawling competition vehicles where ultimately low vehicle speed is required to crawl up incredibly steep rocks. Super low crawl ratios have been known to be in the neighborhood of 400:1. Attaching a transfer case with the same ratios on the back side of the example above (see gear ratios), with a 3.65:1 gear in the transmission, two 1.82:1 reductions in our double transfer case arrangement, and then through the differential of 5.13:1, we would have a high crawl ratio of 62:1! Use the formulas below to calculate the difference in engine speed while trying to keep the same vehicle speed.
COPYRIGHT© Dan Rich, 2004.
Material in this article may not be reproduced in any fashion without the express written consent of the author.