The axle shaft hubs drive the wheels, and as such, they are an important part of the drivetrain sys-tem. Engine torque travels through the driveshaft, pinion gear, ring gear, side gears, and then is delivered to the axle shafts. Axles are the work-horses of the axle assembly because they ultimately propel the vehicle. If the axles break, the car loses its drive, and it might not even roll.
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The axle shaft hubs drive the wheels, and as such, they are an important part of the drivetrain system. Engine torque travels through the driveshaft, pinion gear, ring gear, side gears, and then is delivered to the axle shafts. Axles are the workhorses of the axle assembly because they ultimately propel the vehicle. If the axles break, the car loses its drive, and it might not even roll.
Axles are simple components that have a long shaft with splines on one end and a flange on the other, which is drilled for the wheels. The axles see the most brutal application of torque for the entire vehicle. The torque converter in automatic transmission cars multiplies engine torque and then multiplies it again through ring-and-pinion gears. Thousands of foot-pounds hit the axles every time you stomp on the gas pedal. Dump the clutch for a manual, and the shock is even bigger. Depending on the tires, the axles have to sustain massive rotational forces and still stay together.
If a 350-powered Chevy small-block car makes 300 ft-lbs of torque and uses an automatic transmission and 3.55:1 gears, each axle receives more than 2,300 ft-lbs of torque. If the tires spin from heavy acceleration, this number decreases. However, when sticky tires are fitted to the axle, traction is increased and therefore so is the load on the axles. Too much torque load for the axles results in a rotational fracture and axle failure.
Other loads are at work on the axles as well. Both GM 10- and 12-bolt differentials are a semi-float design, which means that the axles carry the weight of the vehicle on one bearing at the end of the axle shaft. The axle bearings, or wheel bearings, must support the entire weight of the vehicle. Therefore, the bearings need to be in good condition and the axle surface they ride on needs to be perfectly round and smooth.
A typical street axle must cope with enormous side loads. Every corner means that the one axle is pushing in on the wheel bearings and crossshaft pin in the differential, while the opposing axle is pulling on the C-clip. Most high-performance cars run sticky tires, so these forces dramatically increase, putting several hundred tons of force on the bearings and the 1/4-inch-thick area that retains the C-clip.
Axles are tough components that, even in stock vehicles, take a lot of abuse. And if you add more power and harder driving conditions, they simply must be capable of taking a beating. In the final analysis, the axles used in your housing are just as important as the gears and differential.
Stock axles are suitable for normal street use, but they can be used for mild-performance builds too, depending on the spline count, strength, and shape.
The spline count is the number of splines on the axle shaft. Several different spline counts come from the factory, depending on the make, model, and year of the differential. For custom-built rears, you have your choice, but how do you choose the appropriate spline count for your application?
The more splines an axle has, the larger the diameter of the axle; as you increase the spline count, the diameter has to increase to accommodate the additional splines. Each spline is the same size whether the axle has 28 or 35 splines. A typical GM 8.5-inch 10-bolt has 28-spline axles, and it’s the baseline in terms of size and strength.
Most 8.5-inch 10-bolts installed in trucks received 30-spline axles. The actual outside diameter (OD) of a 28-spline axle is 1.205 inches while the 30-spline axle is larger at 1.310 inches. Therefore, the 30-spline axle is 35 percent stronger than the 28-spline.
Anytime you increase the spline count, the differential carrier must be upgraded to match the axle diameter of the particular shafts. So arbitrarily upgrading to a larger count costs you money that may be better spent elsewhere. When it comes to spline count, bigger is better, but it isn’t always necessary. Manufacturers offer guidance for a particular vehicle, engine, transmission, and other factors, so you can select the correct axle shafts for your combination.
When it comes to spline count, there isn’t a magic formula or number for determining axle strength because other variables are at play, such as tires, transmission, intended use, etc. For example, a 700-hp full-race drag car with slicks and a transmission brake would put more stress on the driveline than a street-tired foot-brake car with 1,000 hp.
The way the splines are created makes a difference too. Four factors determine spline strength: count, pressure angle, major diameter, and minor diameter.
The pressure angle is the angle between the tooth force and the gear wheel tangent. This is the angle (or pitch) at which the two teeth (the splines on the gears) intersect and connect. Most axles are either 45 or 30 degrees.
The major diameter is the OD of the spline circle at the top land. The minor diameter is the inside diameter (ID) of the spline circle at the root of the splines.
All modern axles are designed with a 24-degree angle for the splines. Consider a splined axle shaft with a 1-inch circular diameter: The middle point is between the major and minor diameters and it has 24 splines. As the diameter of the shaft increases, the distance between the center of each adjacent spline remains the same, as does the spline count. For example, a 35-spline axle has a major diameter of 1.50 inches; a 30-spline axle is 1.31 inches in diameter.
Another key characteristic of the spline is its shape. All OEM axles and differentials use involute splines. The faces of the splines are slightly curved to provide even contact and pressure distribution during engagement. Rolling the splines into the axles does several things. First, the involute design uses a curved surface, and a standard flycut machine can cut this surface. Second, the metallurgy of a rolled spline determines its strength and elasticity, which is an important factor.
Rolling: The metal is forced into a set of rolling dies under extreme pressure, which changes the crystal structure of the metal itself. The molecules of the metal align, becoming denser, and thus the metal is stronger. After the splines are rolled, the axle is then heat-treated.
Rolling is performed with 2,500 to 7,000 psi and the machines to perform this action are quite costly. Therefore, most custom axles are made with a different manufacturing method.
Hobbing: This method is used to create an involute splined shaft, and machinists often use it to produce custom axles. A spline hob is a specialized machine that cuts all the splines at once. The hobbing process yields an accurate involute spline, and when done before the shaft is hardened is just about as accurate as a rolled spline.
Flycutting: Most machine shops can flycut axle shafts. Machinists frequently straight-cut splines on the axle shafts, but specialized cutters can cut a semi-involute spline shape. The key to a properly flycut spline is keeping the depth above the hardening line, or having the axle re-hardened once cut.
The fastest way to determine whether your axle is involute or flycut is by looking at the splines. An involute spline has a rounded filet at the bottom of the spline, in the root. Fly-cut splines show a boxed corner angle.
There are a couple of potential strength issues with cut splines. Cutting means ripping away metal. Regardless of how fine the cut is, it still rips the material apart. This results in potential stress risers that could cause a failure. The other potential problem is that splines have to mesh with an involute spline in the differential side gears.
Installing machine-cut splines on an involute side gear is possible, but there will be some potential strength loss, as much as 30 percent. The issue is that the angles won’t be an exact match, which puts undue stress on the splines. Spline failure is not that common, but it does happen, and it is usually a result of straight-cut splines.
Stock versus Aftermarket Axles
Only a few stock axles are offered for the Chevy 10- and 12-bolt. The only stock axles available for the GM truck 10- and 12-bolt units are 28-spline, and this axle shaft fits only the 8.5-inch 10-bolt. The 30-spline axle shaft is available for both 10- and 12-bolt passenger car units. The 7.5- and 8.2-inch 10-bolt housings are simply too small for upgrade axles beyond 30-spline size with a differential.
Aftermarket axles provide some upgrade potential, but size is not one of them. The truck 12-bolt had two factory axle sizes. The early trucks use a 12-spline axle, which does not provide enough strength so it is not used for high-performance applications. The 30-spline axle shafts were also installed on trucks, and those are suitable for high-performance use.
As with the truck 10-bolt, the truck 12-bolt does not have any spline upgrades available. That said, 30-spline axles are not weak. The 8.5-inch 10-bolt is capable of handling large amounts of torque. For example, a Buick Grand National 3.8 turbo has the reputation for hanging the front wheels well past the 60-foot mark on the dragstrip with the stock 8.5 10-bolt axle assembly.
The aftermarket offers the passenger car 12-bolt with larger 33- and 35-spline axles. However, you need to install a larger aftermarket housing or center section to accommodate those larger-diameter axle shafts. The 33-spline axles measure 1.41 inches, with a whopping 1.50-inch diameter in the 35-spline units.
Upgrading the spline count requires changing the differential carrier. The 33-spline option requires an Eaton Truetrac gear-driven LSD. Switching to the massive 35-spline carrier means upgrading to a Detroit Locker.
One other option for all three differentials is available: a spool. By switching to a spool from a differential, you have more room for larger-diameter axles. Moreover, 33- and 35-spline axles are available for the 8.5-inch 10-bolt and the truck 12-bolt. Keep in mind, however, that a spool is not street friendly. In fact, they can be dangerous on the street.
Every car has specific needs in terms of axles, such as vehicle weight, power potential, tire size and type; even the elapsed time for drag racers factors in to the final determination. Anytime you are unsure of your application’s specific needs, 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.
Stock Chevy 10- and 12-bolt axle assemblies support up to 400 hp on street tires, with less on drag radials and even less on slicks. If your car is producing more than 400 hp and/or fitted with drag radials or slicks, stock axles cannot support this so they will most likely fail. Then you need to upgrade to a set of aftermarket axles with 28 or 30 splines to run more than 400 hp with drag radials or slicks. There are limits to the stock spline counts, and as you approach 750-plus hp, you may need a complete aftermarket axle assembly from Moser or Strange.
This means that you can bolt a set of drag slicks to an otherwise stock 8.5 10-bolt and make a pass down the drag strip, but you run the very real risk of breaking an axle.
All aftermarket axle manufacturers 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 inexpensive, strong enough for street use, and easy to manufacture.
Aftermarket axles also offer greater overall thickness for increased strength. Factory axles are typically necked-down several inches from the end while aftermarket axles remain at their full diameter right up to the start of the splined area. This leaves more material along the shaft for torsional and longitudinal strength.
Aftermarket axles use a wider range of alloys, depending on the manufacturer’s design. Strange Engineering uses Hy-Tuf alloy, which is proprietary steel. This low-carbon, high-manganese, high-nickel, and high-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. Most of the leading brands use 1541H alloy for 10- and 12-bolt axles.
The method of axle shaft retention is an important consideration. C-clips are by far the most common axle retainers, but they are also not legal for drag racing below 10.99 ET. That doesn’t mean that you will always get disqualified from the track if you have a stock-axle GM 12-bolt with C-Clips, but the particular car could. It all depends on the application.
C-clips are located past the splines on the axle in the center of the differential, so there is nothing keeping the axle inside the tube at the end of the housing. When the axle shaft breaks, the wheel can come out of the car, creating a potentially dangerous situation for everyone around it and being particularly destructive for the car. For a 10.99 or faster car to legally run on a dragstrip, it must use 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 breaks, the wheel does not come out of the car. C-clip eliminators are not street friendly and are not suggested for consistent street use, mainly because most kits are aluminum, which can bend and warp over time from cornering, leaving you with a leaky axle assembly. These kits are available for the 10- and 12-bolt housings from Moser Engineering, Yukon Gear, and Strange Engineering. (See Chapter 2 for more detail on C-clip eliminators.)
The other most common axle shaft retention method is a flange style. The Ford 9-inch and a few differentials use flange axle tubes. With the flange retainer, four bolts and a plate keep the outer wheel bearing in the housing, and this retains the axle shaft inside the tube. Similar to a C-clip eliminator, when an axle breaks on a flange style tube, the flange holds the broken axle shaft and the wheel continues to spin on the axle.
This is one of the greatest assets of the Ford 9-inch. Certain GM axle assemblies use this design as well, specifically the Buick, Oldsmobile, and the occasional Pontiac 8.5-inch 10-bolt in A-Body cars. These axles are hard to find, and they can only be distinguished by taking them apart. Not all Buick and Pontiac A-Bodies used the non-C-clip rear; finding one is really just a big crapshoot.
Heat treating is an important process that increases the strength of the axle shafts. Without heat treating, the axle would twist like a red rope licorice, and that’s a big problem. Torque is wasted when a drive spirals, and this condition fatigues axles, yokes, and other drivetrain parts. So, failures can occur.
Axles must be hardened in order to live under the stress of heavy torque. How deep the axle is hardened is key to its function. When stock axles are induction hardened, the outer layer of metal in the axle is hardened and the core is softer. This is achieved by heating up the axle quickly, typically by running it through a heating coil then quenching it in oil or water. The outside of the metal heats up much faster than the inside. When it is quenched, only the outer shell of the metal is hardened.
Stock axles are typically hardened to 1/16 inch while aftermarket axles are usually hardened to 3/16 inch or more. Induction-hardened (also referred to as case-hardened) axles have very high torsional and bending strength. 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 Street Axles (only 4140 alloy axles receive through hardening, and that is only for specific applications.) Moser performs all heat treating in its own facility using the induction process. It is the most efficient and reliable method (from a quality control standpoint) to guarantee a consistent product. The depth of the heat treat varies depending on the diameter of the shaft and the spline count.
Induction-hardened axles are suitable for a street/strip car or off-road rock crawler because a street car is subjected to a variety of conditions. These vehicles cope with 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.
Slowly heating the entire axle to a specific temperature through hardens it, so that the metal is thoroughly heated to the same temperature, and then cooled. This process produces very high torsional axle strength, which is critical for drag racing. Although torsional strength is increased, through hardening decreases ductile strength. Ductile strength is the metal’s ability to spring back from bending.
Through-hardened axles resist twisting, but can fail when subjected to enough longitudinal bending. Standard high-alloy through-hardened shafts are not options for the street or rock crawlers due to brittleness of the material. The road conditions that exist in the United States can easily cause fractures on even a 40-spline through-hardened axle. Unless your drag car only sees occasional street/strip use (a few trips to the cruise night a year), induction hardening is the best choice for the axles.
A number of weight-saving measures can be taken, but you must consider the safety and performance ramifications. An easy and common way to increase dragstrip ETs is to shave a little weight off the axle. But you must not compromise the structural integrity of the axle assembly.
Gun drilling, also referred to as rifle drilling, 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 to 26 percent. Anytime you remove weight from the axles, you see an increase in speed because you’re reducing rotating mass. However, the drawback is reduced strength of the axle. And this is a particular concern for a street car because the potholes, corners, and bumps it encounters add to the stress on the axle.
As a result, I do not recommend gun-drilled axles for a car that sees regular street use because one significant impact, such as hitting a bad spot in the pavement or pothole, increases the possibility of breakage. An airplane’s fuselage can flex ever so slightly and not fracture, and you want some of the same characteristics for a street axle. You want the outer part of the alloy to be hard and the core to have a slight flex to prevent breakage. There are weight limits for drilled axles, so check with your manufacturer to ensure that your decision is a safe one.
You can also remove weight from the wheel hub to lighten the axle. Most aftermarket axle manufacturers offer lightened axles. But you don’t have to buy custom axles for every car and application.
Lightweight cars, such as street rods and fiberglass-bodied dragsters, don’t put as much stress on the axles as a full-size 1970 Buick GS, which tips the scales at 4,000 pounds.
Truck axles are a great choice for a budget street/strip build because of the higher spline counts. With thicker flanges, this means that they can 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 the axle tubes, and that can be difficult. Most aftermarket axle manufacturers offer custom cutting and splining services, but that means shipping them back and forth, which, of course, takes additional time and money.
Another option is to find a local machine shop that can cut splines. If you can find one, your options greatly increase. You need to order a set of uncut axles with no bolt pattern or splines and have your local guy do the finish work. That way, you get what you want to fit your housing. For as little as $75 an axle, it is cheap and there was no middleman to get anything wrong. Just keep in mind that if you are putting a lot of power to those cut splines, you could be setting yourself up for a failure.
Project: Replacing Wheel Studs
Wheel studs fail and it’s not uncommon when you are running a muscle car. Let me paint a picture for you: Sweat is beading up on your forehead, the midday sun is blistering the skin on the back of your neck, and a blast of 80-mph wind just about knocks you over from the semi-truck screaming down the highway.
You had a blow out, and now you are stuck on the shoulder of a busy highway swapping tires. Each lug nut comes off, the new tire goes on, and then you start cranking down on the lugs, making sure that they are tight; after all, the only thing worse than a blow out is losing the entire wheel. Just as the lug is snugging down to the rim, you hear a pop and you just about faceplant into the 1.5-million-degree asphalt. Yeah, you really did it this time; you busted a wheel stud.
Don’t stress too much; it happens. Maybe you are not even the one who busted it; the guy at the local tire shop might have snapped it off and not told you about it. (It happens more often that you think.) But the situation is the same. The results are the same; a busted wheel stud and you have to drive around with one less lug nut on your car. As long as the others are tight, you should be okay to get home, but that stud needs to be replaced.
Fortunately, there is an easy way to fix a busted wheel stud without taking half of your vehicle’s suspension apart.
If you do not have an impact wrench, you can use a socket wrench, but you will need a way to lock the axle hub from spinning. The following process works on front hubs and rear axles. The entire process typically takes about 20 minutes to complete.
Remove the lug nut and washers. Check the fitment of the stud. The head of the stud should be flush with the backside of the axle hub.
Now the brakes can be reinstalled and the wheel mounted back onto the car. Install the lug nuts in a star pattern, tightening each one hand-tight to secure the wheel.
Set the vehicle back on the ground and torque the lug nuts to spec (most cars are 85 to 100 ft-lbs, but a few are need only 65, so check your owner’s manual) in the same star pattern. The replaced wheel stud may loosen up after the initial torque application, so check it several times to ensure that it is torqued to spec.
Written by Jeferson Bryant and Posted with Permission of CarTechBooks