Axleshaft Theory 101
More often than not it’s the parts that you don’t see that make the biggest difference off-road. Sure, slapping on taller tires and a lift provides instant visual gratification, but those giant knobbies won’t take you far if you are twisting-off axleshafts each time you hit the dirt. The good news is that the aftermarket is full of heavy-duty axleshaft upgrades to strengthen your factory axle set. Figuring out which axleshafts are best for your ride often comes down to price, material type, and availability.
Look, we’re all about going overkill if it means that our rig won’t break down on the trail, but there is no sense in throwing out big coin if you don’t actually need that extra bit of strength. Before you can decide on the right aftermarket axle set, it’s best to understand what you are actually paying for. Since we know not everyone is a metallurgist or axleshaft expert, we’ve categorized some of the most common axle types and configurations to help you determine what’s best for your 4x4.
A key to understanding axleshafts is to know what the numbers tell you. One of the most common numbers you will see promoted in the aftermarket is 4340. Anytime you see a 4xxx it indicates that it is part of the nickel-chromium-molybdenum group (chromoly). The numbers following are indicative of the shafts strength, properties, and grade (as in aviation grade). Numbers beginning 1xxx are part of the carbon group, which is what your OE axles and middle-of-the-road aftermarket axleshafts are comprised of.
You’ll often see axleshaft strengths placed into two categories—yield and tensile.
Yield is the point at which the steel has permanently deformed and lost all of its strength. Tensile represents the number just before the material breaks.
Your rig’s factory axleshafts are most likely a 1040 carbon-steel. Carbon-steel is easier to manufacture over harder alloys and gives the OEs a balance between strength and elasticity. Some OEs use a higher 1050 grade, which is about 38 percent stronger than 1040.
One of the more affordable aftermarket alternatives is 1541H. The tensile strength of 1541H is 181,000 psi, whereas the standard carbon-steel 1040 is 120,000 psi. The silicon and heat-treatment give the 1541H substantial strength gains, but not as much as a chromoly group.
The chromoly group is what the majority of the heavy-duty aftermarket axleshafts are comprised of. As we previously mentioned, chromoly falls under the nickel-chromium-molybdenum group. Each element plays a vital role in making chromoly the go-to level for the aftermarket. Chromium, for example, increases corrosion resistance, while molybdenum and nickel improve the hardness. In terms of strength, 4340 generally has a yield of 210,000 psi and a tensile 228,000 psi. Imported chromoly axles tend to differ from those made in the U.S. Much of it has to do with heat treating and material composition. You may pay a little more for U.S. made chromoly axles, but it’s worth it in more ways than one. Aftermarket axle manufacturers such as Yukon Axle & Gear (www.yukongear.com) offer 4340 replacement axles for a variety of axles.
300M, which is commonly known as 4340M, gets a helping of vanadium, plus additional silicon, carbon, and manganese over standard 4340 chromoly. After being thoroughly hardened, 300M gains around to 2.33 times the tensile strength over 1040. The M rating indicates the steel has been modified or enhanced to improve or change it in some way. Companies such as Ouverson Machine and Equipment (www.ouversonusa.com) offer direct-replacement 300M shafts for the 14-bolt rear axle.
Axleshafts splines are generally rolled or hobbed. Rolling the splines forges the steel at the splines and is widely considered to be the strongest method. Hobbed splines are machined, which cuts into the heat-treating and creates what’s known as a involute (curved-tooth profile) spline.
The strengthening process known as hardening adjust the metals crystal-line arrangement. The most common type of axleshaft hardening is induction hardening. This process involves heating and cooling the metal quickly. In this method an electromagnetic field is used, along with a current that can be adjusted based on the material’s size and ideal depth of the hardened layer. This process is not cheap, and unfortunately some companies skimp out on it, which will result in a weaker shaft.
Something we used to see more of, but not as much nowadays, is cryogenic axleshaft freezing. This process uses nitrogen gas to cool the shaft nearly 300 degrees F below zero. This process is intended to battle metal fatigue more than gain strength. The idea is that by chilling the metal it will tighten the metal’s grain. For hard-to-find and classic drivetrain parts, it can be that extra step that makes them last longer.
We understand more about the chemistry of steel now than we ever have. This knowledge has led companies like RCV Performance (www.rcvperformance.com) to break out of the norm and develop its extremely heavy-duty CV axleshaft set. Material types, mixtures, source, and manufacturing tolerances all play a part in creating the best end product. When purchasing aftermarket axles from any company, be sure to ask about the different variations available and don’t be afraid to ask if it’s made in the U.S.A.