Far too often overlooked in the building of a street/strip project are the axles. Sure, the rear differential is always taken into consideration in terms of strength, width and whether or not it has a “posi,” but the axles themselves rarely get much love. In fact, most street/strip cars you see cruising Main Street may have upgraded differentials and gears, but the axles are bone stock. While stock axles are not necessarily all bad, there are some things to consider when putting together a strip-ready rear end. There are several considerations to be made when looking at axles—build material, spline count, axle thickness, retainer style and heat treating. We teamed up with Moser and Strange to get you educated on selecting the right axle for your street/strip project.

Axles often don’t get the attention they deserve. This the last link between the engine and pavement, and if you break one, you are done racing. These are Moser Custom Alloy units.

Bolt on a set of drag radials or slicks, and those stock axles may twist off on the first pass.

Street Tires Aren’t Race Tires

Before you go bolting on a set of stickies to your car, consider this—street tires are slippery; they don’t have the grip that drag radials or full slicks provide. Spinning the tires off the line means less stress on the axles, which means a quick car with street tires and stock axles may give you a false sense of security.

Bolt on a set of drag radials or slicks, and those stock axles may twist off on the first pass.  If you have plans to slip a set of stickies under your street ride, you need to consider some aftermarket axles. Jeff Anderson of Moser Engineering tells us, “For a street strip axle the standard high alloy thru hardened shafts are not options due to brittleness of the material. Streets here in the USA are far from perfect, especially in the northern half of the country where we deal with freezing and thawing of the roads. The road conditions that exist here can cause fractures on even a 40 spline thru hardened shaft pretty easily. We get calls all the time from people who chose the wrong material for their application.”

Alloys Used

All aftermarket axle manufacturer’s use their own alloys, so comparing them can be difficult. However, comparing those to OEM axles is simple enough. Most OEM axles are made from either 1039 steel (for flange-mount bearings) or 1050 steel (for C-clips). These alloys are used because they are cheap, strong enough for street use and easy to manufacture.

OEM axle splines are rolled into the metal, rather than cut. This is faster for mass production, but allows for more variance. Aftermarket axles use a wider range of alloys, depending on the manufacturer’s design. Strange Engineering uses Hy-Tuf alloy, which is proprietary steel. ““On the 35 spline S Series axle we recommend up to about 1000 HP before one might want to consider going to a race series axle,” recommends J.C. Cascio of Strange. “This low-carbon, high manganese, nickel and molybdenum steel was originally developed for landing gear in military aircraft, making it a logical choice for race axles.” Moser Engineering uses 1541H alloy steel for all of their axles, including stock replacement axles. The stronger the alloy, the stronger the axle.

Moser Uses: 1541H alloy steel, which is made from carbon, manganese, phosphorus, sulfur and silicon
Strange Uses:
 A proprietary steel called Hy-Tuf, which is a low carbon, high manganese, nickel and molybdenum.

Axles start out as forged blanks. These can be machined down to fit just about any application.

More Splines Doesn’t Always Mean More Strength

The alloy is only a part of the strength equation. Contrary to what you may think, less splines does not mean more strength, in fact, it is quite the opposite. Spline count is closely related to the overall diameter of the axle, the splines themselves are generally the same thickness. The more splines on the shaft, the larger the diameter of the axle. Cascio suggests, “…33- or more splined axles for any drag race application, with 35-splines being ideal for street/strip applications. That said, not all housings are capable of fitting 35 or 40-spline axles, the GM 8.5-inch 10-bolt for instance, is only large enough to handle 30-spline axles. The differential carrier has a lot to do with this as well, as the axle splines must match the carrier. The weak point of the 8.5” 10-bolt is the axles, the ring gear is less than ½” smaller in diameter (.375”) than the 12-bolt rear.” The 10-bolt is capable of handling 9-second Buick Grand Nationals, hanging the wheels well past the 60-foot mark, so take that into consideration. When in doubt, go with the largest spline count you can fit.

  • 33 or more splined axles for any drag race application
  • 35 splines for ideal street/strip use
  • Not all housings can accommodate large diameter axles

Cutting splines is performed on a lathe, using a special spline tool. This is a special cutting fluid that keeps the cutting tool cool and lubricates the process.

Axle Retainers

How the axles are kept in the housing makes a difference too. C-clips are by far the most common method of axle retainers, but they are also not legal for drag racing below 10.99 ET. That doesn’t mean that you will always get tossed from the track if you have a stock-axle GM 12-bolt with C-Clips, but you could. “The problem is that C-clips are located past the splines on the axle, in the center of the differential, there is nothing keeping the axle inside tube at the end of the housing,” explained Anderson “When the axle shaft breaks, the wheel can come out of the car, creating a potentially deadly situation.

Anderson continued, “For a 10.99 or faster car to legally run on a dragstrip, it must use either a non-C-clip rear or C-clip eliminators. Eliminators retain the axle at the end of the axle tube flange, so if the axle brakes, the wheel does not come out of the car. C-clip eliminators are not suggested for consistent street use, mainly due to the fact that most kits are made of aluminum, which can bend and warp over time from cornering, leaving you with a leaky rear end.”

The other retainer is the flange-style, most famously used in the Ford 9” and a few others.  The flange retainer uses 4 bolts and a plate that keeps the outer wheel bearing in the housing. These are simple and pretty much fool-proof. Much like a C-clip eliminator, when an axle breaks, the wheel continues to roll, as the remainder of the axle is held in place by the flange. This is one of the biggest assets the Ford 9-inch has going for it. Certain GM rear ends use this design as well, specifically the Buick and Pontiac (Oldsmobile always used the C-clip) 8.5” 10-bolt in the A-body cars. These rear ends are hard to find, they can only distinguished by taking them apart. Not all Buick and Pontiac A-bodies used the non-C-clip rear either, finding one is really just a big crapshoot.

Flange style retainers, such as this axle (from a Buick 8.5 10-bolt), feature a larger outer bearing with a built-in seal. IF the axle breaks, the wheel stays on the car.

Anderson suggests converting a C-clip style rear end to a flange-style retainer. “By cutting and welding new ends on the housing and then ordering 9” style press on bearing axles, it will eliminate the need to run eliminators in a street application. The eliminators, regardless of who manufactured them are subject to flexing against the housing end that they bolt up against. These ends are not welded to the tube, only bolted. No matter how thick or what material they are made of, flex is going to occur; when this happens the bearings wear out the seals. Sometimes you get lucky and they will last a few years, but usually, they only make it a year with regular street use.”

  • C-clips are not NHRA legal on vehicles running 10.99 or faster
  • C-clip eliminator kits can help fix this problem, but are not recommended on any vehicle that might dual purpose on an autocross or road race
  • Flange-style end bearing retainers are the choice for drag racers on all popular rear end combinations

Spline count dictates the overall diameter of the axle shaft, the larger the axle, the stronger it is.

Heat Treating

Heat treating is an important aspect of an axle, without it, the axle would twist like a red rope licorice

Heat treating is an important aspect of an axle, without it, the axle would twist like a red rope licorice (which when combined with Mr. Pibb, crazy delicious; the candy, not the axle). There are two main types of hardening; induction and thru hardening. Stock axles are induction hardened, which hardens the outer layer of the axle, leaving a soft core. This is achieved by heating up the axle quickly, typically by running it through a heating coil, then quenching it in oil or water. Since the outside of the metal heats up much faster than the interior, when it is quenched, only the outer shell of the metal is hardened. Stock axles are typically hardened to 1/16”, whereas aftermarket axles are usually hardened to 3/16” or more.

Induction hardened (also referred to as case hardened) axles have very high torsional and bending strength, as the soft core allows more flex while the hardened outer shell keeps it all together. Moser Engineering uses induction hardening for all of their Custom Alloy axles, only the 4140 alloy axles get thru hardened, and that is only in specific applications. “We do all of heat treating here in our facility using the induction process,” explained Anderson. “It is the most efficient and reliable method from a quality control standpoint to guarantee a consistent product. The depth of the heat treat actually varies depending on the diameter of the shaft and the spline count. Our Custom Alloy Street Axles are never thru hardened to avoid the common breakage that occurs with thru hardened shafts on the street.”

The necessities of ordering axles

Accurate measurements are a must when ordering new axles. You must check with the engineers to find out what measurement they want. We have found that axle measurements tend to vary, some include the C-clip, some don’t. These identical axles measure the same, but each manufacturer used different end points; spline end and actual shaft end.

“Thru hardening is applied by slowly heating the entire axle to a specific temperature, so that the metal is thoroughly heated to the same temperature, and then cooled,” said Casico “This provides very high torsional strength, which is critical for drag racing. Where thru-hardening loses favor is in ductile strength. Ductile strength is the metal’s ability to spring back from bending. Thru-hardened axles resist twisting, but they do not respond well to longitudinal bending.”

For a street/strip car, induction hardened axles are superior, because a street car is subjected to cornering forces, jarring longitudinal forces and changing weight loads, often all at the same time. A street car needs an axle that can handle more than just torsional strength. “For a street/strip axle, the standard high alloy thru hardened shafts are not options due to brittleness of the material,” Anderson told us. ”The road conditions that exist here in the US can easily cause fractures on even a 40 spline thru hardened axle. We get calls all the time from people who chose the wrong material for their application.” Unless your drag car only sees occasional (a few trips to the cruise night a year), induction hardening is the best choice for street/strip axles.

  • Induction hardening –  Hardens the outer layer of the axle, leaving a soft core. This is achieved by heating up the axle quickly, typically by running it through a heating coil, then quenching it in oil or water. This is ideal for a street/strip axle
  • Thru hardening – Applied by slowly heating the entire axle to a specific temperature, so that the metal is thoroughly heated to the same temperature, and then cooled. This does not allow the axle to flex and thus not recommended for anything outside of a purpose-built race car

This is the hub end of a gun drilled axle from Strange. The entire hub is machined to reduce weight.

Lightening Your Axles

Stripping weight from a car is always an easy way to increase ETs. Shaving a little weight off the axle is a great idea right? Kind of. Gun drilling, also referred to as rifle-drilled, is the process of removing weight from the center of the axle shaft by drilling it out. This can reduce the weight of an axle by as much as 17-26 percent. Anytime you cut weight in the axles, you will see an increase in speed. The drawback of a drilled axle is that they are not as strong, particularly in a street car, where potholes, corners and bumps add to the stress on the axle. For a car that sees regular street use, gun drilling is not the best option.

“The likely hood of hitting a bad spot in the pavement or some other debris or pothole just opens up that possibility of breakage, explains Anderson. “Just like how an airplane can flex ever so slightly to keep from fracturing you want some of the same characteristics for a street axle. You want the outer part of the alloy hard and the core to have an ever so slight bit of flex to prevent breakage.” There are weight limits for drilled axles, so check with your manufacturer to ensure your decision is a safe one.

With all that said, you don’t have to buy custom axles in every situation. Lightweight cars, such as street rods and early-body dragsters don’t put as much stress on the axles as a full-size 1970 Buick GS, tipping the scales at 4,000 pounds.

Truck axles are a great choice for a budget street/strip build. Truck axles get the higher spline counts, with thicker flanges; this means that they will handle more torsional and longitudinal stress, especially in a light car, such as a T-bucket dragster. Swapping truck axles into car housings or a narrowed housing means cutting, and that can be tricky.

Most aftermarket axle manufacturers offer custom cutting and splining services, but that means shipping, which by the time you get done, you could order custom axles. The next option is to find a local machine shop that can cut splines. There are still a few local shops that will cut and spline axles, you just have to look. We had ordered a set of uncut axles (no bolt pattern, no splines) and had our local guy do the finish work to match the custom housing he built for us. At $75 an axle, it was cheap and there was no middle man to get anything wrong.

Every car has specific needs in terms of axles; vehicle weight, power potential, tire size and type, and ¼-mile ET all factor in to the final determination. Anytime you are unsure of your needs, or just want to consult a professional, give one of the manufacturers a call. Their tech department will gladly answer any questions you have to ensure you get the right axles for your application.