High-Performance Differentials, Axles, and Drivelines. Joseph Palazzolo
This Ford stock-style banjo center section has been damaged from way too much torque. A stronger center section may have prevented this failure, but this goes to show you that the stock Ford 9-inch is not bullet proof and actually every axle has a torque or impact value where it will fail. Notice that one bearing cap is missing a section in the middle while the other is in two pieces. This failure also cracked the pinion straddle-mount bearing support and even the front of the housing cracked. There is nothing salvageable from this center section.
On this rack of axle housings, the Salisbury-style are the second one from the top and the bottom one. The other six housings (some are stacked behind one another) are all banjo-style. (Randall Shafer)
The banjo-style axle has a removable center section. This cast center section is removed from the front of the axle and supports all of the gears and bearings. Many small steel pieces are welded together to form the banjo-style axle housing. Toyota still uses this style axle housing on its truck platforms today. The most well known banjo axle is the Ford 9-inch, which will be covered later. The banjo axle, with its studs sticking out, resembles its namesake musical instrument, with the drum and tuning keys sticking out.
Probably, the most significant weakness and most common failure point of the banjo axle is at the differential bearing supports, which are cantilevered out from the cast structure. The design has two arms reaching from the main casting, like diving boards, that are trying to support the hypoid-ring-gear separating loads. Under heavy torque loads, these bearing supports can, and frequently do, break in stock form. If you are going to send tremendous amounts of torque through a banjo-style axle, it is crucial that you use a readily available aftermarket center section, or a factory nodular-iron unit that is stronger than the common, stock-gray-iron center section found on most passenger car applications.
Independent carrier axles (ICAs) utilize halfshafts instead of axle shafts to transfer torque to the wheel-end hubs. ICAs, therefore, do not react to suspension loads. The axle-carrier movement is typically independent of suspension movement. These style axles are more commonly found on luxury vehicles and ultra-high-performance cars, such as Ferraris. Certain domestic performance vehicles have also utilized ICAs. The purpose-built, limited-production 1999 Cobra R Mustangs debuted with an ICA, and all Cobra Mustangs from 1999 to 2004 were factory-built with ICAs.
The latest Dodge Vipers and Corvettes use ICA rear axles, and the reinvented 2010 Camaro has an ICA. Many folks have grafted ICAs from other applications into their muscle cars. I need to caution you on performing such a modification, as the ICA load-bearing frame members, mounts, and dampers are originally tuned for the original chassis stiffness and compliance.
When you rip that subframe out of a Jaguar or Cadillac CTS and make it fit under your Nova, chances are it will not perform the same as the original donor car because the entire rear subframe module is tuned for a specific chassis arrangement, wheelbase, and weight transfer characteristic. Keep in mind that there are companies that specialize in conversion kits for such swaps. These are expensive swaps and do not always garner the expected benefits. A custom-fabricated ICA can be a great talking point while at a car show. In some racing circles, an independent suspension may yield significant advantages, especially when there are abrupt changes in the track surface.
A complete ICA from a Jaguar XK8 connects directly to the rear suspension. The black halfshafts connect the axle outputs to the wheel ends. This ICA also uses the jiggle-style vent cap on the top of the axle cover discussed in Chapter 1. (Randall Shafer)
The current Corvette axle is unique in the fact that the pinion is located behind the ring gear when the ICA is installed in the vehicle. The pinion shaft is actually hollow and driven from an internal spline in the head of the pinion. Conventional pinions are driven from an external spline at the tail end of the pinion. The Corvette pinion is also very short with a small distance between the pinion bearings. Using large pinion bearings makes this arrangement possible and provides adequate support for the pinion gear.
The front surface of a Corvette ICA bolts to the rear mounted transmission. The outputs to the halfshafts are on the right and left. Note the extensive use of cooling fins and the side covers on both sides. There is even a rear cover on this axle. It tends to resemble a square box more than a traditional axle.
Rear-Cover and Side-Cover Housings
There is a subset of ICAs defined by the cover style or split line of the axle housing. Most ICAs have the axle cover split line like the Viper axle, which is parallel to and just behind the output shafts. There are some that utilize a side-cover design like the Corvette. There are many schools of thought regarding which design is better. But one issue that all agree on is that most side-cover designs today do not easily lend themselves to visual pattern checks and traditional gear backlash inspection. Since the side cover is required to properly support the differential bearings and subsequently the hypoid ring gear, you cannot see the pattern or check backlash unless the cover is removed. If a large access port were available, a pattern and backlash check could be performed. The other method is to assemble the unit and rotate the pinion to disburse the pattern compound, and then remove the cover to check the pattern.
An internal spline in the head drives the Corvette hollow-style pinion. The traditional-style pinion is next to it for comparison. That pinion head bearing is almost as large as the pinion head itself.
The venomous Dodge Viper ICA, easily spotted by the snake on the rear cover, is based on a combination of Dana 44 and Dana 60 internal components. This ICA uses a four-point mount system, two on the rear cover and two more on either side of the pinion. This is also a more traditional rear-cover design than the Corvette side-cover.
ICAs have been manufactured in both cast iron and cast aluminum. The cast-aluminum units have a thermal-expansion and gear-alignment issue that needs to be taken into account during the design process. Aluminum expands and shrinks at a faster rate than the steel gears inside the axle. Depending on the design layout of the gears and bearings, when the aluminum axle housing heats up from normal operation, the gears may be shifted out of their ideal mesh point. Also, the pinion bearings typically increase in preload, while the differential bearings lose preload. This can turn into gear noise at elevated temperatures in the axle.
If the axle receives a cool stream of air while driving to help maintain the temperature, this heating of the axle housing is minimized and these problems do not exist. The typical ICA is tucked up under the vehicle, and does not usually receive adequate airflow. This is important if you are going to stick one of these axles under your muscle car; the transmission will have poor airflow, but also increased torque well beyond the factory design limit. Many manufacturers are now going back to cast iron for their ICAs in order to eliminate the concerns that come with an aluminum structure.
The Cadillac CTS axle housing is cast aluminum and not only has a side cover but also a pinion-cartridge arrangement. The side-cover arrangement and combination of the pinion cartridge make this a unique axle arrangement to service, as most axles are not assembled in this fashion.