The difference in strength between the 29-spline (left) and 27-spline Mopar 8.25 axles is on the order of 25 percent. It's a bolt-in swap to boot. This is an easy way to gain strength to compensate for larger tires and/or lower gears.
Shaft strength can be reasonably accurately calculated on paper. Since the axleshaft is the most common first failure, it provides a useful yardstick by which to judge overall strength. There are a lot of ways to look at axleshafts, including diameter, spline count and so on, but the most useful is minimum diameter. The smallest part of the shaft is the weakest part, wherever it is. Most often, but not always, it's at the minor spline diameter (MSD). This is the diameter at the bottom of the splines, with more teeth having a larger MSD and vice versa. You will also see different spline pressure angles, i.e., the angularity of the teeth (45 degrees versus 30 degrees), and these equate to a slight difference in MSD, but it's not a hugely important strength issue. The main thing is that the types don't interchange.
A good acronym to use in evaluating axles is DMD, or Diameter, Material and Design. Diameter (minimum diameter) is the most important. Bigger is better. An increase of 0.100-inch in the diameter of a 1-inch shaft (a 10-percent increase in size) equals a 33-percent improvement in strength. Material is almost as important. There are many steel grades, but only a few used in axles. A nearby sidebar rounds them up. There can be a strength difference of 100 to 150 percent between the high-performance 4340 alloys and the ordinary 1040 carbon steel often used for OEM applications. There are options in between these as well, so there are plenty of choices.
Finally, we come to design. The shape and physical characteristics of the axle can contribute to strength. A "fluted" or "waisted" axle has the main section of the shaft at about the same diameter as the MSD. The transition point at the spline roots, where mainshaft diameter dips into the splines on a common axle, is a high-stress area and where non-fluted axles often break. Fluted axles eliminate these stress risers. Unfortunately, fluted axles are rare.
The smoother the surface of the shaft, the less chance there will be of a stress riser being formed, hence the value of polished or micropolished shafts. Cryogenic treatments, during which the metal is subject to temperatures as low as 300 degrees below Fahrenheit, add slightly to strength and greatly to fatigue resistance.
The way splines are formed can add or detract from strength. At the top of the list are rolled splines, which are rolled into the steel under very high pressure. This forges the steel at the splines and increases strength in that area. Close behind are hobbed splines. In this case, the curved-tooth profile (called an involute spline) is machined, rather than rolled. There are cut splines, which have a flat profile and are not compatible with parts designed for an involute profile. Though nitpicked in many circles, the difference between properly done rolled and hobbed splines is very small.
There is a way to calculate the strength of an axleshaft of a given material and diameter (see the "Useful Formulae" sidebar). It's subject to some variable factors but can get you in the 10-percent ballpark, or better. Use it when you have no other strength yardsticks or if you've upgraded an axle with aftermarket shafts. Remember that a stronger shaft may expose weak links elsewhere in the axle.
1) Weigh the vehicle to get the weight on the front and rear axles individually. You can do this at truck scales, waste-disposal sites, scrapyards, grain elevators and so on. Load your rig as you would for the trail, using the actual gear or simulated weight in the same relative position in the vehicle as it will be on the trail. Drive halfway on, getting the weight on the front tires and halfway off and getting the rear weight. The front and rear weights should equal the total.
2) Calculate traction torque using the axle weight and the radius of the tires you run, or will run. Use a CoF based on your style and type of 'wheeling. We'd recommend a worst case, especially if you rockcrawl in hard-core terrain. Use at least 0.7 and if you are wild and crazy, use 1.0. You can also add weight to account for weight transfer if you need to generate more worst-case scenarios. Add 25 percent more weight to the rear axle to be safe.
3) Compare traction torque to MOT or to the calculated axleshaft strength. Traction torque should be lower than MOT or axle strength. If not, it's upgrade time. If it's within 10 percent, you are probably better off upgrading in some way. In front-axle applications, compare traction torque to U-joint strength (see sidebar). Judge this against the terrain in which you generally run. If you don't 'wheel hard, you can get by at or near the limits, but remember those limitations. Moderating tire diameter gives you a lot of flexibility in retaining drivetrain strength without a lot of axle mods.