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The drive axle bears huge torque loads. Simply multiply the engine torque by the gear ratio and ring gear ratio (torque x low-speed gear ratio x rear-end gear ratio), and you will see that these forces multiply rapidly. What is the receiving end of this force? Each time the driver puts down the hammer, the drive shafts of the 1,300-horsepower tractor must collectively bear up to 12,000 lb-ft of torque. This is why the rule makers of drag racing mandated most levels of aftermarket axles more than 30 years ago. Therefore, although the performance requirements of street vehicles generally do not require a sturdy racing axle, powertrain replacements, power adders, and/or downsizing existing axle housings most certainly require more sturdy replacements.

MW produces three basic models (l to r): "Ultimate Hi-Torque" made of 300M, standard H-Torque and economical "MasterLine" series.

In order to gain insight into the past, present and future of axle technology, we visited industry veteran Mark Williams. Although Mark did not invent the axle during his long career as a race car driver, car manufacturer, and manufacturer of transmission components (this distinction is probably due to the Mesopotamians, who developed it about 5,500 years ago) Tao Wheel), but Williams has accumulated a wealth of knowledge on the subject second to none.

As early as the 1960s, the main method of manufacturing custom-length axles for narrow rear ends, Mark relied on the junkyard core. He would chop off the shaft and drill a large hole in the center of the flange, weld a new shaft of suitable length, and then machine the spline on the milling machine. On the West Coast, Henry's Machine in Bellflower, California is probably the most famous source of these two-piece installations. Sometimes, the blank shaft is thick enough to be simply shortened and resplined, but the cutting tools at that time cannot process the heat-treated blank, so it is usually necessary to anneal the shaft to a softer state and then respline.

MasterLine axles are double induction hardened internally to ensure precise and controlled heat treatment.

Speed ​​equipment pioneer Ed Donovan developed the first one-piece forged axles, but its $300 price tag (equivalent to today's $2,520.72) considerably restricted their use in Top Fuel. Williams saw the demand for one-piece forged axles at a common price in other fields and set about developing his Hi-Torque series. The first production axles of Mark Williams Enterprises (MW) were used in Judy Lilly's 1968 SS/AA Plymouth Barracuda. This is also the first warranty axle for drag racing.

For current axles, Williams divides them into four basic types: carbon steel used by the original manufacturer’s axles (usually 1055 or 1552); after-sales induction hardened carbon steel axles made of 1552 steel; and made of similar or similar 4340 alloys After-sales racing axles made of fully hardened steel alloy; and high-quality through-hard 300M alloy steel.

The components are developed using modern CAD/CAM technology and FEA (Finite Element Analysis).

In addition to materials, many physical properties must also be considered. These include heat treatment, number and shape of splines, shaft profile, bearing size, flange design and quality (important weight considerations only in racing cars).

The most important (and most often overlooked) aspect of shaft strength is related to the spline end. Viewed from the cross section of the end, the outer tip of the spline defines the large diameter. The bottom of the groove between the splines defines the small diameter, and it is this size that determines the strength of the shaft. The second is the pressure angle (or the basic angle of the spline). Some are 30 degrees (60 degrees included), while the best angle for racing applications is 45 degrees (90 degrees included) because the splines are shorter, allowing the small diameter of a given shaft to be larger.

All MW axles have 100% magnetic flux to ensure quality

Most modern axle splines are called composite pitch splines. The 24/48 pitch spline is a common feature of GM, Ford and Chrysler axles, and most high-performance street and drag racing riders use these axles. If the circular pitch of the shaft has a diameter of 1 inch (the midpoint between the major and minor diameters), then the circumference of that diameter is: π times the diameter equals 3.141 inches. The shaft will have exactly 24 splines (or teeth). The distance between the centerlines of adjacent splines remains constant, so as the diameter of the shaft increases, the number of splines also increases. For example, the major diameter of a 35 spline shaft is 1.500 inches; the diameter of a 40 spline shaft is 1.708 inches.

Another key aspect of the spline is its shape. All modern OE axles, differentials, etc. have involute splines, which means that the surfaces of the splines are slightly curved to provide optimal contact and uniform pressure distribution during joining. The most common method to obtain the involute shape is gear hobbing or spline rolling. However, axles that are resplined or manufactured using the fly-cutting program have straight-cut splines. Matching a flat shaft spline with an involute spline differential will concentrate the pressure on a single point, whether it is the inner or outer surface, thereby creating excessive stress. In terms of reliability, the involute spline is better than the flying cut version.

(Left) Axle flange drilled and tapped in a CNC machining center. (Right) The spline adopts real involute design and processing to achieve the best contact efficiency.

Materials and heat treatment are the next areas of focus.

OE carbon steel shafts (usually SAE 1055 or 1541) only induction harden the bearing surface, making the flange much softer than the rest of the shaft-so people can hit curbs or potholes without breaking the flange, The story is like this. Induction hardening (the shaft passes through an electromagnetic coil to excite a strong current in the shaft to heat it) penetrates approximately 0.150 to 0.300 inches, so the shaft core remains relatively soft. Generally, the surface hardness of the shaft is 55-58 Rockwell and is very brittle-this is great for curbs, but it certainly cannot handle the impact loads associated with huge torques.

On the other hand, MW's Hi-Torque axles are made of high-strength chromium-molybdenum-nickel alloy, which is subsequently heat treated in a process called austempering. At this stage, the processed axle is immersed in a special solution at 1,550 degrees Fahrenheit in a vertical furnace for about an hour. This treatment provides ultimate tensile strength in the range of 235,000-263,000 psi, as well as excellent ductility (the ability to change shape or shape without breaking). The shaft surface hardness is in the range of 48-52 RC, which is far less brittle than induction hardened carbon steel shafts. When these axles are subjected to thousands of pound-feet of torque, they twist and bounce like a torsion bar, rather than breaking like a wishbone.

All MW shafts are customized to order, and the spline length is optimized for the application. The axle shown comes with matching spools.

The contour of the axle shaft has a great influence on ductility. MW's Hi-Torque device tapers from the shaft bearing shoulder (1.774 inches) to the small diameter of the spline for part of the length. This "triangulation" part makes the axle more resistant to bending (more powerful cars can actually cause toe conditions and hinder performance). In addition, a large part of the shaft is kept at or below the small diameter to allow torsion bar-like distortion and prevent permanent deformation of the shaft.

The diameter of the bearing is another area that needs improvement. Standard 12-bolt Chevrolet axles use 1.400-inch inner diameter bearings, while small Ford axles use 1.378-inch inner diameter bearings. Most mid-range performance axles usually use 1.562 inch bore bearings. MW's Hi-Torque Racing axle can use 1.774 inch bearings.

An overview of some of the CNC machining centers at MW's state-of-the-art facility in Louisville, Colorado.

Another aspect to consider is the flange design and wheel connection method. The flanges on the OE axle are soft and can accommodate press-in studs with serrated shanks. Aftermarket Mark Williams street axles have thicker flanges and usually use cap screws with a diameter of ½-20 inch thread. One option for street and racing axles is to use drive studs with 5/8”-18 threads. The bolts have 11/16” shoulders for positioning the wheels. This is a method designed by MW in 1974 to achieve Extremely accurate bolt pattern positioning, in addition to preventing possible disastrous consequences of wheel bolt failure. Most high-horsepower applications have adopted this setting.

If you are a race car driver, the final thing to consider is quality.

Most contestants looking for ultimate performance (as opposed to categories and bracket racing that are primarily interested in consistency) want to reduce spin weight. For such applications, MW offers its Super Light series, which has pocket-milled flanges, tulip ends, and other flange lightning options. Depending on length, spline and application, they are gun drills of 11/16 inch, 7/8 inch or 1.0 inch. Compared with traditional designs, each pair is about 12 pounds lighter. MW also offers its MasterLine series, which is an affordable high-performance axle (designed for the street and bracket racing market) with precise length and hobbed involute splines, which can be used with stock differentials and positive traction Or match the aftermarket spools. They offer all popular bolt styles with 1/2 or 5/8 inch studs.

Hi-Torque axles are heat treated using the highly regarded austempering process. In this special solution, vertical shelves are used to heat them to optimize grain flow and ductility.

What does Williams recommend? Any car that can achieve a 1/4 mile elapsed time of 9.99 or better should be equipped with at least 35 spline shafts. Heavier cars, or those with an expected load of 9,000 lb-ft of torque, should have no less than 40 splines. For support and street use, avoid the use of OE 28 spline shafts; in fact, even 31 spline shafts are in a critical state. The new Camaro with a 7.62-inch gear ring and pencil thin 26 spline shaft is at the absolute bottom of the reliability rating. In addition, Williams said that pairing a spool with a 28 or 31 spline shaft is a foolproof transmission grenade, waiting to be ejected. Another waste of money is the use of 33 spline spools for the 9-inch butt hole of the third member box, or 10 and 12 bolt GM rear wheels. MW can provide a 35-spline spool for all three applications, which can be bolted directly. 35 The spline spool setting is the smallest size spline that should be considered for any drag race application.

The final conclusion is that the size is irreplaceable. Despite material strength and heat treatment, size is the most important consideration. Williams also warns of the use of straight cut splines, especially when combined with positional traction and spools that accept involute (curved) splines.

Consider the pressure angle; you should not mix 30-degree and 45-degree components together. Even if they happen to touch, the contact points will be concentrated in a very concentrated area. Also, remember that a 35-spline shaft with a 45-degree pressure angle spline is about 5% stronger than the same shaft with a 30-degree pressure angle spline. All manufacturers of 40 spline shafts use 45 degree pressure angle splines.

Williams recommends that you have a clear understanding of the physical characteristics of all components and the potential loads involved when choosing the axle that suits your needs.

Comparison of the strength of common OE car and racing splines

Comparison basis: GM 12 bolts

GM Buick & Pontiac '64-'70 axle

Dana 60 Strange & Moser spool

GM 10 bolt 7-1/2" '82 and higher

General 10 bolts 8.5" & 8.2" '65-'81 & Ford 9" & 8.8"

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