Text by Bill Holland

Posted on April 7, 2021 on Dragzine.com

Axles 101: Everything You Should Know About Axles For Drag Racing

Drive axles are subjected to immense torque loads. Simply multiply the engine torque by the transmission ratio and the ring-and-pinion ratio (torque x low gear ratio x rearend ratio), and you see that forces multiply rapidly. And what’s on the receiving end of this force? The drive axles of a 1,300 horsepower drag car must collectively withstand upwards of 12,000 lb-ft of torque every time the driver drops the hammer. That’s why drag racing’s rule-makers mandated aftermarket axles for most classes more than 30 years ago. So while the performance requirements of street vehicles typically don’t require beefy racing axles, driveline swaps, power adders, and/or narrowing the existing axle housing most assuredly cry for sturdier replacements.

M-W manufactures three basic models (l to r) the “Ultimate Hi-Torque” made of 300M, the standard Hi-Torque, and the economical “MasterLine” series.

For some insight into axle technology past, present, and future we called upon industry veteran Mark Williams. While Mark didn’t invent the axle (that distinction likely goes to a Mesopotamian who developed the potter’s wheel about 5,500 years ago) in his lengthy career as a racer, car builder and driveline components manufacturer, Williams has accrued a wealth of knowledge on the subject of axles second to none.

Back in the ‘60s the primary method of making custom-length axles for narrowed rear ends Mark relied on junkyard cores. He’d chop off the shaft and bore a large hole in the center of the flange, weld in a new shaft of the proper length, and machine the splines in a mill. On the West Coast, Henry’s Machine in Bellflower, California, was probably the best-known source for these two-piece units. Sometimes, the stock shaft was thick enough and could simply be shortened and re-splined, but the cutting tools of the day weren’t up to machining the heat-treated stock, so it would typically require annealing the shaft to a softer state, then re-splining it.

The MasterLine axles are double induction-hardened in-house to assure precise, controlled heat-treating.

Speed equipment pioneer Ed Donovan developed the first one-piece forged axles, but their $300 price tag (equivalent to $2,520.72 today) fairly limited their use to Top Fuel. Williams saw the need for a popularly priced one-piece forged axle for other realms and set about developing his Hi-Torque line. Mark Williams Enterprises’ (M-W) first production axles went into Judy Lilly’s 1968 SS/AA Plymouth Barracuda. It was also drag racing’s first warranted axle.

As for current axles, Williams categorizes them in four basic types: carbon steel (commonly 1055 or 1552) as used by original manufacturers axles; Aftermarket induction-hardened carbon steel axles that employ 1552 steel; Aftermarket racing axles made from a through-hardened steel alloy like or similar to 4340 alloy; and Premium through-hardened 300M alloy steel.

Components are developed using contemporary CAD/CAM technology, as well as FEA (Finite Element Analysis).

In additiion to the material, a number of physical characteristics must by considered. These include heat treatment, spline count and shape, shaft profile, bearing size, flange design, and mass (a weight consideration important in racing only).

The most important (and the most-often overlooked) aspect of axle strength concerns the splined end. Viewed as a cross-section of the end, the outer tips of the splines define the major diameter. The bottom of the grooves between the spline defines the minor diameter, and it is this dimension that essentially determines the strength of the shaft. The pressure angle (or the basic included angle of the spline) is next in importance. Some are at 30-degrees (60-degree included angle), while the optimum for racing applications is 45-degrees (90-degree included angle) because the spline is shorter, allowing for a larger minor diameter of a given of axle.

All M-W axles are 100% magnafluxed to assure quality

Most modern axles splines are known as combination pitch splines. The 24/48-pitch spline is common to the GM, Ford and Chrysler axles that most high-performance street and drag racers use. If the shaft had a 1-inch circular pitch diameter (the mid-point between the major and minor diameters), the circumference of that diameter is: π times the diameter equals 3.141-inches. The shaft would have exactly 24 splines (or teeth). The distance between the centerline of adjacent splines remains constant, so as the diameter of the shaft increases, so does spline count. For example, a 35-spline axle has a major diameter of 1.500 inches; a 40-spline axle is 1.708-inches in diameter.

Another key aspect of the spline is its shape. All modern OE axles, differentials, and so on, have involute splines, which means that the faces of the splines are slightly curved to provide optimum contact and even pressure distribution during engagement. The most common way to achieve an involute form is by hobbing or spline rolling. Axles that have been re-splined or manufactured using a fly-cutting procedure, however, have straight-cut splines. Matching a flat axle spline with an involute spline differential concentrates pressure on a single point, both on the internal and external surfaces, creating excessive levels of stress. In terms of reliability, an involute spline beats a fly cut version hands-down.

Material and heat treatment are the next areas of concern.

OE carbon steel axles (typically SAE 1055 or 1541) are induction-hardened only up to the bearing surface, leaving the flange much softer than the rest of the shaft — that’s so people can run into curbs or potholes and not snap the flange off, or so the story goes. Induction hardening (shaft passes through an electromagnetic coil that excites a powerful current within the shaft, thereby heating it) penetrates about 0.150- to 0.300-inch, so the axle core remains relatively soft. Typically, the shaft surface hardness is 55-58 Rockwell and is very brittle — this is great for curb-bangers, but certainly not capable of handling the shock loads associated with massive torque.

On the flip side, MW’s Hi-Torque axles are made of high-strength chromium-molybdenum-nickel alloy and are subsequently heat-treated in a process called Austempering. In this phase, the machined axle is submerged in a special solution at 1,550 degrees Fahrenheit in a vertical furnace for about an hour. This treatment provides an ultimate tensile strength in the range of 235,000-263,000 psi, along with exceptional ductility (the ability to change shape or form without breaking). The shaft surface hardness is in the 48-52 RC range, far less brittle than an induction-hardened carbon steel axle. When these axles are subjected to thousands of pound-feet torque they twist and rebound like a torsion bar instead of snapping like a wishbone.

All M-W axles are custom made to order and spline length optimized for application. Axles shown with a companion spool.

The profile of the axle shaft strongly influences ductility. MW’s Hi-Torque unit tapers from the axle bearing shoulder (1.774-inches) down to the minor diameter of the spline for part of the length. This “triangulation” section gives the axle more resistance to bowing (more powerful cars that can actually create a toe-in situation and impede performance). Moreover, a significant portion of the axle remains at or below the minor diameter to allow torsion bar-like twisting and prevents the axle from permanently deforming.

The diameter of the axle bearing is another area that requires improvement. Standard 12-bolt Chevy axles use a 1.400-inch I.D. bearing, while the one for the small Ford is 1.378-inches. Most mid-range performance axles typically utilize a 1.562-inch I.D. bearing. MW’s Hi-Torque Racing axles can, employ a 1.774-inch bearing.

Overview of some of the many CNC machining centers at M-W’s state-of-the art facility in Louisville, Colorado.

Another area to consider is flange design and method of wheel attachment. The flange on OE axles is soft and receives press-in studs with a serrated shank. Aftermarket Mark Williams street axles have thicker flanges and normally use ½-20 thread inch diameter cap screws. An option for street and racing axles is to use a drive stud with 5/8”-18 thread. The studs have 11/16-inch shoulder that locates the wheels. This is the method that M-W designed in 1974 and allows an extremely accurate bolt pattern location, plus it prevents wheel stud failure that can be catastrophic. This setup has come to be adopted for most high-horsepower applications.

If you’re a racer, the final consideration is mass.

Most competitors who seek ultimate performance (as opposed to category and bracket racers primarily interested in consistency) desire to reduce rotational weight. For such applications, M-W offers its Super Light line that features pocket-milled flanges, a tulip end, and other flange lightning options. They’re gun-drilled 11/16-, 7/8- or 1.0-inch depending on the length, spline and applications. Compared to conventional designs, they’re about 12 pounds lighter per pair. M-W also offers its MasterLine series, a budget-priced performance axle (designed for the street and bracket racing markets) that is made to exact length and features hobbed, involute splines that can match a stock differential, posi-traction, or aftermarket spools. They’re offered in all popular bolt patterns, with 1/2- or 5/8-inch studs.

Hi-Torque axles are heat-treated using the highly regarded Austempering process where they’re heated in a special solution using vertical racking to optimize grain flow and ductility.

What does Williams recommend? Any car capable of a 9.99 or better 1/4-mile elapsed time should be equipped with 35-spline axles at a minimum. Heavier cars, or those with an anticipated load of 9,000 lb-ft torque should have nothing less than 40 splines. For bracket and street use, avoid OE 28-spline axles; in fact, even 31-spline axles are borderline. A late-model Camaro with a 7.62-inch ring gear and pencil-thin 26-spline axles are at the absolute bottom of the reliability scale. Further, says Williams, pairing a spool with 28- or 31-spline axles is a surefire driveline grenade waiting to pop. Another waste of money is using a 33 spline spool for the 9-inch stock bore third-member cases, or 10- and 12-Bolt GM rears. M-W can supply a 35-spline spool for all three applications that bolts right in. The 35-spline spool axle set up is the minimum size spline that should be considered for any drag race application.

The final take-away is there is no substitute for size. Material strength and heat treatment notwithstanding, size is the most important consideration. Williams also cautions the use of straight-cut splines, especially in conjunction with posi-traction and spools that accept involute (curved) splines.

Consider the pressure angle; you shouldn’t mix 30-degree and 45-degree components together. Even if they happen to engage, the contact points will be focused on a very concentrated area. Also, keep in mind that a 35-spline axle with the 45-degree pressure angle spline is about 5-percent stronger than the same axle with a 30-degree pressure angle spline. All manufacturers of 40 spline axles use a 45-degree pressure angle spline.

Williams recommends that you have a clear understanding of the physical properties of all components and of the load potential involved when you choose an axle to fit you needs.

Find the complete and original article here on www.dragzine.com