By Greg Miller

By using various combinations of transmissions, transfer cases, dual transfer cases, axle ratios, overdrives, and underdrives, there are a number of ways to achieve almost compromise free final results.

First, consider what kind of driving you would like to be able to do. Whether the vehicle needs to double as a daily driver. Whether you want to drive it much on the highway. What kind of wheelin’ you wanna do. What kind of motor you have or want. Then consider the physical length limitations of the vehicle. Now consider the various. Do you prefer to drive an automatic or stick? And finally, what is your budget?

Once you have these data as a starting point, proceed to research out all of the various combinations that will work in your application, work up the cost to make each happen, and compare each system’s functional capabilities to what your dream capabilities were. Balance this against your budget.  If you want, e-mail or call me with  the above data, and I can work with you on what options are available and at what costs.

The Pieces of the puzzle

In this article I will attempt and overview of what has been tried, what each achieved and what compromise was bought along with it.   

High Impact has available all of these components and more; Contact Us regarding the details of your application!

Optimal (Gearing) Enchantment:

First, I would like to point out that the gearing that works best for one type of driving will be unlikely to be best for all types of wheeling. Many things can be done, budget withstanding, but compromises may be in order. Highway driving, dunes, mud, rock crawling, forest service trails, all have their own specifics. What kind of driving do you do?  

(Link: discussion of what sort of gearing works best for what)
"Optimal Gearing Enchantment"

Axle Gearing

Wheelers used to install higher numerical ratio (axle ratios explanation) gears in their axles to gear down and get better off road performance. But this screws with your on-highway performance. 

Since a serious wheeler will be installing significantly larger-than-stock tires in most cases, some change in axle gearing to correct for this and maintain good drivability is usually in order.

It is not practical to lower just the axle gears to get low enough gearing for rock crawling, and if you did you would also mess up your gears for comfortable highway driving.  Like you are stuck in second or something. 

Conclusion: Set your axle gearing for a best compromise that includes highway driving, if you are not planning to trailer the thing everywhere.

"Axle Gearing" article

Transmissions

Automatic vs. Stick

There are die-hard proponents of both schools. Each has its benefits, and its drawbacks. It is an important decision.

Arguments for automatics:

Generally speaking, the  in the rocks the automatic boys claim they only have two feet (most do not have a third leg that will reach the pedals, boasting otherwise notwithstanding). In the dunes, mud or on a hill climb, you simply put your foot in it, and let the tranny figure out the rest.

Arguments for sticks:  

 

• Automatics provide less compression braking  on steep down hills. (Not true if you get the rest of the gearing equation right)

• Automatic transmissions make heat, cause overheating problems.

• Automatics are less reliable.

(Link: Automatic Transmission Page)

(Link: Stick Transmission page)

Granny Trannies

What I am calling "granny trannies" are 4 and 5 speed transmissions that are extremely wide ratio, and have a very low “granny gear” 1st gear. Popular granny 4 speed units are the SM420, SM465, NP435, T18 and the T19. Granny trannies can get your crawl ratio down into the 70 to one range still keeping sensible axle ratios.

The “granny” first gear is really too low to be generally useful on the street. Typically, you would drive them like a three speed using gears 2-3-4 around town, and the granny gear only when you needed that extra low gear.

NV4500 5spd is an extremely popular swap

Granny-Gear 5 speeds: The most popular is the NV4500, which can be adapted to a wide variety of applications. Others are available. A fifth gear is a wonderful bonus. Allowing a lower axle ratio with excellent highway gears.

(Link: Listing and expanded data on Granny Trannies)

Low Geared Transfer cases

Most transfer cases come from the factory with low range gearing between 1.96 to one and 2.72 to one. With a low geared transfer case, high range stays the same for the street, and you are geared lower for the trail. The main drawback is that while you get the lower gears, you lose the original low range gearing, which is usually best for sand and easy trails, and sometimes better for mud or hill climbing.

Replacement gears for factory Transfer Cases

A popular compromise is to replace the gears in the factory transfer case with lower gears .  Replacement gear sets are available for various T-cases with between 3.15 and 4.88 ratios.  4:1 replacement gear-sets are available for most Jeep transfer cases from 1980 on.  The nice thing about this modification is its relative low cost (around $750-1100, plus a bit more for installation) and that once you put the new gears in the stock case, it all bolts back together without any other changes.

"Transfer Case Replacement Gear sets available"

Atlas Transfer Case

Atlas Transfer Case on rear of NV4500 5speed

The Atlas II Transfer Case is an extremely important contribution to this category. Developed and manufactured by Advance Adapters, the Atlas is a complete, purpose built fully gear driven transfer case available as a bolt in replacement for a wide variety of applications. It is available ina conventional 2spd case in 2.0, 3.0, 3.8, 4.3, 5.0 and 6.0 low range ratios  It is designed as a fully synchronized unit: This helps make up for the fact of losing the in-between range because you can go between high and low range while driving without stopping. This unit is also a length improvement over the chain drive cases used in most 4x4s since the late 80s. Drawbacks are relatively high cost (Over $2k for the unit), plus you will most likely need to modify at least one of the driveshafts.

April 2006 - the New Atlas 4 speed begins to ship. This unit qualifies as an "Ultimate" system as described below

Atlas Transfer Case Page

"Ultimate" Gearing systems

Extreme Low Gearing:

"Extreme Underdrives" and  "Dual Transfer Cases"

This type of system has by far the greatest flexibility and versatility in gearing options. Properly done, it is the Cadillac option: Awesome. In this manner you can accomplish enough gearing flexibility to always find the right gear from highway all the way to extreme crawl ratios. Cons: Tends to make a longer drivetrain, and is a more complicated installation.

An "Extreme Underdrive" is an underdrive gearbox that bolts in place between the transmission and transfer case. We call it "extreme," because the underdrive multiplies your existing gear reduction by up to 4 times, rather than a usual underdrive of 1.2 times, or so. It is intended for extreme four-wheel-drive applications, rather than towing or gear splitting in a big truck. It works equally well with both manual and automatic transmissions.  Pros: Because the unit can be purchased with a 4 to one ratio, when used with a conventional transfer case typically between about 2 to one and 2.7 to one) up to four different transfer case ranges can be selected: Both boxes in “High”, you get a 1:1 highway range. Put the Tcase in “low”, leaving the underdrive in “High”, you get a 2 to 2.7 to one transfer case range. Put the Underdrive in “low, leave the Tcase in “high”, you now have a 4:1 low range. Put them both in “Low” you get an extreme crawl range, equivalent to transfer case gears of between 8 to 12 to one.

Link: Klune-V Extreme Underdrive brochure

Link: Install Article: Klune-V into Jeep Wrangler

(Dealing with the short wrangler driveshaft issue)

Link: For 87+up Jeeps: Klune-V/Bronco Dana 20 Combo

(Direct Replacement for Wrangler NP231 Tcase, Has 3" shorter overall length than stock NP231 Tcase)

A Dual Transfer Case is a method of doing a similar thing. A transfer case is essentially two parts: A two speed gear box (High and Low ranges) and the a system of gears or a chain to send power to both the front axle and the rear axle. What is done in a dual transfer case is the reduction aspect is used, discarding the front-axle drive portion. A systems of adapters, shafts and custom shifters bolt the second transfer case’s reduction portion between the main transfer case and the transmission. This system accomplishes much the same thing as the Extreme Underdrive, and is sometimes a less costly alternative due to the lower cost of the recycled parts from the “donor” transfer case. Drawbacks to this design is that they are frequently heavier or clunkier than the purpose designed “Extreme Underdrive”, and  won't usually give four gear ranges discussed above. 

The "Doubler" consists of the gear reduction portion of an NP203 Tcase coupled to the main transfer case. In the example shown the Doubler is mounted to an NP205 transfer case, allowing possible effective transfer case ratios of approximately 1:1, 2:1 and 4:1.

Link: "Dual Transfer Cases"

(Link: Additional Applications for Extreme Low Gearing")

 

Atlas 4 speed

This unit is functionally a Klune-V Goliath bolted to either a 3.8 or 2.0 Atlas. (Klune-V / Atlas Combo - Article) There is an advantage to this Advance Adapters version:  By purpose-engineering the combo, Advance Adapters has done a nice job of integrating the assembly, shortening the overall combo by 3.5". Another benefit of this system is that the Atlas 4-Speed as a unit costs less than buying a Klune and buying an Atlas 2 speed and combining them

 

Summary

As you can plainly see, there are a vast number of options to consider. Read through these pages. Then give us a call, we are happy to work with you to sort out the optimum gearing solution for your rig. 

Tech Consultation

Then, if useful, we can research out all of the details to make your system a reality, and provide you with a complete kit.

Complete Conversion Kits

Thank you!

Greg Miller