YJ1 is being built as a rock crawling machine that can still be driven back and forth to the trail on highways where 75 mph is slow.

In my opinion there are 3 very critical factors when it comes to rock-crawling: Ground Clearance, Traction & Control. To improve ground clearance we use lift kits and larger tires, traction is supplied by those larger tires with some some help from lockers or other traction aids and by flexibility in our suspension so we can keep all of the tires on the ground. Control is a function of the driver and how well the vehicle can be controlled going over obstacles.

In Rock Crawling one of the most important modifications that we can make to our vehicles to enhance control is to improve the "crawl ratio" - how slowly we can get and keep our vehicle moving. For example, a stock jeep with 3.73:1 gears in the transmission, 2.72:1 gears in the transfer case and 3.07 gears in the axles has a pretty abysmal crawl ration of 3.73 x 2.72 x 3.07 = 31.1:1.

At 31:1 way too much clutch slip is needed as you attempt to get over obstacles. Inevitably you will hear a lot of crashing as the under carriage of your Jeep meets the rocks. You cringe inside as you hear each of the hard cracks, grinds and crunches you are making while you attempt to look graceful and hope nobody is watching (no really - I meant to do that).

Lack of control turns what could have been smooth moves through the obstacles on the trail into a series of barely controlled crashes. This is tough on you and your vehicle, and is one of the number one sources of trail breakdowns (right up there with a lack of basic vehicle maintenance).

When we went to larger tires on YJ-1 we switched to 4.10:1 Ring and Pinion gears in the axles. This was done to allow us to run the Jeep with close to stock performance with these larger tires. Gear swapping is a popular way to increase you crawl ratio. With 4.10:1 gears our crawl ratio technically went up to about 41:1. BUT the larger tires negated this increase in crawl ratio - so in essence we were left with an effective crawl ratio that was only 7% better than stock. Big Whoop. We could have gone lower than a 4.10:1 ratio, for example to 4.88:1 ratio. This would have lowered the crawl ratio to 49:1 a number approaching something reasonable, but highway cruising would have been tougher with those tall gears and the stock transmission. Ring and Pinion swaps are not cheap either - swapping both ends can cost easily between $1000 & $2000 in some places (including parts & installation).

A transmission swap would help as well. A variety of low or granny geared 4 and 5 speed transmissions would improve crawl ratio by lowering the first gear ratio. Gearing from 5:1 down to 7:1 is possible. We did in fact switch over the transmission to a HD NV4500 on YJ1 (see http://www.off-road.com/jeep/reviews/yj1-nv45001.htm) with it's 5.61:1 first gear we increased the crawl ratio to 60:1, a 50% improvement on our original crawl ratio. But these wide ratio granny geared trannies do affect street performance as well, especially on a vehicle like the 1990 YJ with a fairly narrow torque band.

Comparing the Atlas side by side with the NP231 that came out of YJ1 you can see the increased beef of the Atlas and it's overall shorter length.

Well, from there the simplest solution to improving crawl ratio on the trail without impacting highway performance is to lower the low ratio in your transfer case. This leaves street performance untouched but lets us improve our control on the trail - a win:win situation. Several methods will allow you to do this. There are dual transfer case setups, in-line planetary reduction units, replacement gears for your existing transfer case or even complete replacement transfer cases designed from the ground up to meet the needs of the rockcrawler for a bullet proof heavy duty unit with some extra oomph in the low range. For project YJ1 we choose to go all out and swap in one of these purpose built hardcore cases - the Atlas Transfer Case from Advance Adapters.

Comparing the Atlas gears side by side with a set of Dana 300 gears.

The Atlas is based on the same basic design of the well respected Dana 300 case but it has been refined and improved quite a bit. Like the Dana 300, and unlike the chain driven NP231 that has been standard on Jeep Wranglers since 1987, the Atlas is a gear driven transfer case. It uses helical cut hardened steel gears riding on needle bearings (like the legendary NP205) inside a one piece aluminum alloy case with access covers (instead of having a split case). All of the gears are fully synchronized for on-the-fly shifting and the output is a virtually unbreakable 1.375" 32 spline shaft. An added benefit is that the Atlas transfer case is 2.5" shorter than the NP231, even with a tailcone eliminator installed, so the rear driveline problems that short wheelbase Jeeps are plagued with are also helped.

Speedometer gear assembly transfers from the old case to the new Atlas.

Off-Road.Com was one of the first to receive the new production Atlas (Serial number 0003) and there have been some improvements since we received our unit - we will note differences between our unit and the newer model (Atlas II). Since installation we have put more than 10,000 miles on this new design, both on and off-road, and we now feel comfortable giving you our impressions of the unit.

Installation was a snap. Unbolt the old transfer case, pull it off swap the vacuum actuator and speedo gear assembly from the old transfer case to the new Atlas case, and bolt up the new Atlas transfer case.

The Vacuum switch for the front axle disconnect is transfered from the NP231 to the Atlas Case

It really only took and hour or so to do. In our particular installation (with the added length of the NV4500) we did have to trim the lip of the skid plate to clear the transfer case. With the stock transmission or with the skid plate dropped, this would not have been necessary. The new Atlas II has a "clocking" feature that lets you rotate the mounting position of the case. With the new unit we could have rotated the case and we would not have had to trim the skid plate. The only other serious modification was that the rear driveshaft had to be lengthened.

We filled the unit up with gear oil and we were ready to rumble!

The Atlas II now ships with twin stick shifters and offers clocking holes to allow you to customize the rotation of the case.

On the road the case runs a bit quieter than the NP231 did. The helical gears do not have the hum and occasional snap that you would hear from the NP231's chain and planetary gears. Our unit had a single stick shifter (all current unit have dual shifters). The single shifter has an odd 4HI 2HI N 4LO pattern and is somewhat vague. With the shifter laying right against my leg it was easy to pop the unit in or out of 4HI. We will be replacing this with the dual shifter (now the only way the Atlas II can be ordered). The twin stick shifter will allow independent shifting in and out of high and low range as well as shifting into and out of 4 wheel drive. This will also allow us to run 2 wheel drive low ranges.

On the trail is where this unit really shines. Our Atlas I has a 3.5:1 low range. This is a vast improvement over the 2.72 low range in the NP231. Newer Atlas II units can be had with gear ranges from 3.8:1 to 4.3:1 and they can be had in left hand or right hand drop.

The Atlas II twin stick shifter looks great installed in this Jeep TJ.

There is nothing that drops the difficulty rating of a trail better than the increased control you get with a truly low crawl ratio. With the stock transmission the crawl ratio in YJ1 would have improved from 41:1 to a good 53:1, with the 5.61 geared NV4500 the crawl ratio improved to a very good 80:1. The holy grail of crawl ratios is above 100:1 and we are fast closing in on that and improving the strength and reliability of the drivetrain to boot.

We ran the Rubicon Trail with the Jeeper's Jamboree and Sierra Trek and the unit really performed great on these tough trails. The low gears allowed us to idle over rocks and obstacles we would have crashed over before. The difference truly is incredible.

One thing we noticed after being on the trail for a while is obvious in retrospect: When you lower you crawl ratio you will use up quite a bit more fuel on the trail. You can use more of the gear box to help out, shifting from first into second or third gear, since you will not need those ultra slow crawly gears over all of the trail. If you do have a tough trail that will keep you in 1st gear low range for most of the way, you better plan on it taking up to twice the gas that it used to.

With a brand new design like the Atlas, I was worried that some sort of problem might crop up after some miles and hard trails where under the belt - that the promise of the Atlas was just too good to be true.

Now, with 10,000 tough miles on the unit, I can say that the extensive research and design work that Advanced Adapters put into this unit has paid off. After this long term introductory testing I can report that I are very pleased with the Atlas transfer case and I confidently recommend it to anybody who is serious about rockcrawling.