Axle Upgrades

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PHASE 2

In my goal to achieve more ground clearance, a better ride, and a more trail-worthy Jeep, I had finally settled on a three-phased plan. Phase 2 of my plan includes one-piece axles, new gearing, and lockers. Because I wanted to go with the ARB air lockers from the start, I needed to install the new taller springs first (see Phase 1 project, the Rubicon Express Extreme Duty Lift), so the proper air line lengths could be set when the lockers were installed. Since this phase of the project is really not a do-it-yourself project, this page will only provide some of the details of this important step before adding bigger tires to the Jeep (which is Phase 3).

ONE-PIECE AXLES

If you're going to increase the traction potential of your CJ by adding lockers (of any brand or type), you'll need to make sure your axles can handle the extra load. Even if you're not adding lockers, the stock AMC 20 rear axle has a weak link looming inside it. This first part of Phase 2 can be done at home by yourself with the right tools. However, since the locker and gear installation requires the removal of the axle shafts, the labor to install the one-piece axles are absorbed in the locker install process.

	One of the most notable problems with the AMC 20 rear axles has been their use of a two-piece axle design. The flange (the part with the wheel studs fitted through it) is pressed onto the axle shaft. Note the splines on the axle shaft just above the large nut.
	The large nut is used to bind the flange onto the axle splines. If the nut comes lose, the flange can wallow-out, which will cause the splines to be stripped. Once that occurs, the axle shaft will begin to spin freely causing total loss of power to the rear wheels if you don't have a locker.
	With a one-piece axle shaft, the flange and shaft are one solid unit. No longer is there a need for the large nut to hold the flange against the splines. The weakest link in the system has now been removed.

There are now several makers of one-piece axle shafts for the AMC 20 rear axles. The average price hovers around $300 to $320 for the shafts and bearing kits. If your Jeep sees a lot of off roading in four-wheel drive, this investment will give you a good level of comfort.

NEW GEARING

Do you know the gearing for your axles? The CJ-7s came with a variety of gear ratios depending on the transmission and motor size combination in the option. For both the AMC 20 wide track and narrow track axles, the factory gearing ranged from 2.73 (highest), 3.31, 3.54, 3.73, to 4.10 (lowest).

So why the fuss? Well, the stock 258 motor is designed to run on the freeway at an RPM range between 2,000 and 2,500 (give or take). The CJ-7s typically came with a 235/75R15 tire size that translates to just under 29 inches in diameter. There's a basic formula that factors your axle gearing, vehicle speed, and transmission ratio by the tire size to give you the final RPM reading. If you plan on significantly changing the size of your tires beyond the stock setting, you can use the formula to see what the impact will be on the RPM reading. Too low of an RPM and your power will be lost and the motor will be lugging (a common complaint). Change a number in the formula (like axle gearing) and the RPMs come back into the desired envelope. Watch what happens to the RPMs when the tire size changes...

Axle ratio	x	MPH	x	Constant	x	Final Transmission Ratio*	/	Tire Size	=	RPM
3.31		60		336		0.86		29		1979
3.31		60		336		0.86		31		1851
3.31		60		336		0.86		33		1739
4.10		60		336		0.86		33		2154

* T-5 Transmission, fifth gear (OD) x Dana 300 Transfer Case High Range (1:1) = 0.86

Since many people opt for even lower gears to help with the off-road performance, the 4.56 ratio becomes even more attractive! Using the above formula with 4.56 gears and 33 inch tires, the RPMs are 2396 on the highway, but still within the acceptable range. Keep in mind, if you change your axle gearing your speedometer/odometer readings will be affected and the speedometer gear may need to be changed. Check out this link to see how to do a Speedometer Gear Change.

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ARB AIR LOCKERS

By design, an open differential will allow only one wheel on the axle to receive the power (which ever one it can turn the easiest). This helps to aid in turning the vehicle. It also means there is less wear and tear on the drive train as both wheels on the axle can turn at separate rates (but one side is coasting). Off-road, this becomes more of a problem when traction is needed. With the open differential, once a wheel is lifted off the ground, there is no more power available to drive the wheels on that axle.

The job of a locker is to make both wheels turn at the same rate, receiving equal amounts of power. This greatly aids in traction when on uneven surfaces or slick/loose surfaces. However, a problem comes when the same axle with a locker is taken on the road and cornering makes the locker "think" it's losing traction and engages. Loud noises result and, over time, more wear and tear is expressed through the drive train circuit. Sometimes, on slippery roads, a suddenly engaged locker can cause odd driving manners that could adversely affect the control of the vehicle.

For years there have been attempts to reach some sort of compromise. "Limited slip" or "soft lockers" are examples of locking differentials that use a variety of means (usually clutches) to soften the impact on the drive train. Sometimes, these lockers require quite a bit of wheel spin before they will engage.

Then along came the ARB air locker. This unit uses air pressure to manually activate the locker. When it's on, the locker will not drift in or out, it stays on. When it's off, the axle functions just as if it has a normal open differential. Thus, it's the best of both worlds!

	Normally, the major drawback of the ARB locker system is with its cost. Part of that cost is the need for an on-board air compressor. However, most any air compressor will do as the requirements for air pressure and volume are not that great! For me, the ARB system was tied into my existing on-board air compressor and I was able to save that expense. An air manifold was added up near the compressor unit (mounted to the dual battery tray) to allow the additional air line connections.
	Each unit (front and rear locker) has its own solenoid that is electronically controlled from within the cab. This allows you to either select the rear locker or the rear and front locker together. (As a safety feature, the front locker cannot be activated by itself.)
	From each solenoid on the manifold, an air line runs to the differential. (The air line is in blue. The black line is the normal breather hose.) Inside, the air pressure activates a piston mechanism that expands and almost instantly locks the two axle shafts together. When the air pressure is released (via switching off the the button in the cab), springs contract the mechanism and the locker is instantly shut off.
	The routing of the air lines is important! The blue plastic lines are vulnerable to damage by sticks and debris along the trail. Some companies offer stainless steel air lines. But if those lines are snagged by a branch, something else may give! The blue lines are fairly easy to repair in the field and are a weaker link by design.
	Each switch is labeled and lit for ease of use in the day or night. The photo at the top of this page shows the rear switch being lit, indicating that it's activated. It's that easy!

Putting it all together results in a very thin pocket book! However, the performance gains (off-road) are enormous! The Jeep will now walk up hills it struggled with before thanks to the gearing. The axles and lockers give the ability to get through (or over) obstacles that normally require the winch. Now I was ready for Phase 3!

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Axle Upgrades