Overland Tech: Transfer Case basics

What separates a capable off road rig from a street machine is usually the inclusion of a transfer case that splits power to both the front and rear wheel sets.  There are many different kinds of transfer cases ranging from tried-and-true gear driven cases, chain driven classics like the venerable 231 common in Jeeps, to modern day viscous and clutch-pack cases providing variable power to all four wheels.  So, what does that mean for the overlander and when does 4×4 not really mean 4×4?

First things first, let’s talk terminology:  
What do things like 2wd, 4wd, 4×4, RWD, FWD, and AWD mean, how do they relate, and are any of them interchangeable?
  • 2wd – “Two Wheel Drive” – Most cars on the road are two-wheel-drive.  This means out of the four tires on the car only one pair of them receives power from the engine.
  • RWD – “Rear Wheel Drive” – A 2wd vehicle in which only the rear tire set receives power.
  • FWD – “Front Wheel Drive” – A 2wd vehicle in which only the front tire set receives power.
  • 4WD – “Four Wheel Drive” – A vehicle in which both tire sets receive power through a center differential case. This includes both 4×4 and AWD systems.
  • AWD – “All Wheel Drive” – A full-time differential system in which both tire sets receive power. Commonly used to designate on-road 4wd systems with a variable front to rear torque split that are high range only and do not offer selectable gear ratios.
  • 4×4 – “Four-by-four” – A selectable 4wd system typically with a fixed equal 50/50 front to rear torque spilt usually reserved for off-road capable 4wd systems.
  • 4-High – “Four-High” – Also known as “high range.” The highest range gear ratio of a selectable 4×4 system. Usually a 1:1 gear ratio.
  • 4-Low – “Four-Low” – Also known as “low range.” The lowest range gear ratio of a selectable 4×4 system.  OEM gear ratios typically range from 2:0:1 to sometimes as low as 4.1:1. Aftermarket manufacturers offer lower range gear sets or even complete aftermarket housings
  • Part Time – A 4×4 system that locks the front and rear outputs together. Should only be used in conditions with limited traction such as dirt, gravel, mud, and/or snow.  Not recommended for dry solid surfaces due to the lack of differential action front to rear which may cause driveline vibration or binding.  Part Time settings can either be 4-High or 4-Low.
  • Full Time – A variable 4×4 system letting the front and rear outputs move at different speeds. Can be used on a variety of off or on road conditions when traction is compromised. Typically provides some differential action front to rear.  Not to be confused with an AWD system which are high range only.  Most 4×4 systems with a Full Time 4-High setting are usually paired with both Part Time 4-High and Part Time 4-Low settings.
So let’s look at two Jeeps: 

Stock TJ/LJ Shifter with the 231 “Command Trac” 4wd System
2H, 4H, N, 4L.
Note: Both 4WD settings are “Part-Time”
  • My 2004 LJ has the the New Venture Gear 231 series transfer case.  This is a part-time 4×4 system with a selectable 2wd, Neutral, 1:1 4-High, and 2.72:1 4-Low options.  The 231 is the most common Jeep transfer case and is found in Wranglers, Cherokees, and Grand Cherokees.  It’s chain drive and mechanically engaged through a manual level.  Variants of the 231 appear in other 4×4 makes and models and in some cases can be engaged through an electronic solenoid.
Stock ZJ shifter with the 242 “Selec-Trac” 4WD System
2H, 4H-PT, 4H-FT, N, 4L-PT
Note:This system features both a full-time and part-time 4H options
  • My old 1997 ZJ had the New Venture Gear 242 series transfer case.  In addition to the gear drive options of the 231 the 242 also provided a Full-Time 4-High option.  This allowed for the 242 to engage a 4wd system useful on the road in wet conditions such as heavy rain or wintery mix.  It also allowed for 4wd to be used on terrain where sections were a mix of hard and soft surfaces.  While not as strong as the 231, the 242 proved to be a nice option for the dual-purpose light duty 4×4.
So what about a Subaru, how do they compare?

The basics of the Subaru AWD systems per-2012.

Subaru’s are considered AWD.  Their symmetrical system distributes power not only side to side but also front to rear.  Basically it’s a three-way limited slip.  Power can move side to side in both wheel sets through two limited slips, but can also move power front to rear as needed, through a center mounted limited slip.  This could mean equal power to all four wheels or power to only one wheel.  In some models this is electronically controlled sending a majority of the power to the front wheel set until wheel slippage is detected.  To get the same effect in a Jeep it would need limited slips front and rear as well as a transfer-case capable of shifting power front to rear.  This was the type of system in my 1996 Grand Cherokee which had a full-time viscous transfer case as well as limited slips in both axles.  The AWD system in a Subaru is only 1:1 where as my Jeep was able to shift into a low-range setting with a fixed Full-Time Low-Range 2.72:1 gear reduction.

What other types of cases are out there?

I could spend all day giving you a history of various 4wd systems that are out there.  This post is just to give you a primer on the terminology and the basics.  Hopefully later I will get into a more detailed write up into the differences between chain, gear, and viscous internals, over/under gear boxes,  aftermarket transfer cases, as well as multi-gear transfer cases and doublers.  We’ll save that for another day because to be honest that’s a lot more technical than overlanders really need to worry about.

What 4wd system is the “best “for overlanding?

Simply put, the one that works reliably.  The advantages and disadvantages of one system compared to another have more to do with the rigs themselves than the systems.  Subarus with AWD are great on the road year round and on semi-groomed unpaved roads.  Their major limitation isn’t the AWD system but usually ground clearance, the independent suspension front and rear, and the lack of a low-range gear for more technical terrain.  Jeeps on the other hand have great 4×4 control with low range in technical terrain but lack any 4wd traction unless locked in 4×4 which limits fuel milage and isn’t appropriate on paved surfaces.  Since they are RWD by default, this makes them somewhat squirrelly in the rain and loose terrain when in 2wd.  Other smaller SUV’s and Crossovers tend to be FWD when in 2wd or have a FWD bias.  So to circle back, both AWD and 4×4 will work for general overlanding.  It just depends on how technical a terrain you plan on and if you need to modify the factory suspension and driveline appropriately.

Data plate for a Bantam Model 18 4wd system.

Bonus! – But wait, there’s more.
Ever here the term “Crawl Ratio” and wonder what it means?  Crawl Ratio is calculated by multiplying the gear ratios of the transmission, transfer-case, and axle.  This is typically the lowest possible combination using first gear and low-range.  Example: My 2004 Wrangler has a stock Crawl Ratio of 28.4:1 (2.8 first gear, 2.72 transfer case, 3.73 axle).  In contrast, a Rubicon from the same year with a manual transmission would have a crawl ration of 64:1 (4.0 first gear, 4.1 transfer case, and 4.1 axle).  At first glance you might think the Crawl Ration of my automatic is terrible compared to the manual Rubicon, but there is some torque multiplication through an automatics torque converter.  This  ranges between 1.5 to as high as 2.5.  I’ve never been able to find out exactly what it is for the LJ’s 42LRE transmission, but I suspect the lower-end of the range is probably a safe bet.  That makes comparing the two Crawl Ratios a little less frightening at 64:1 for the manual to somewhere in the high-40’s to low-50’s.  For a more accurate number we’ll need to study hydrodynamics and that’s well outside the realm of this blog.  That’s also why I will always refer to automatic transmissions as “black boxes of voodoo magic”… and that’s a post for another day.