A popular option for the Rock Bouncer crowd is a spool with a minimum of 35-spline axles. 40-spline versions are also now available for the ultimate in strength.
Author: Ian Johnson Photos: Ian Johnson
Dirt Sports Blueprint Series
The hunt for the “Holy Grail” of axle assemblies has been going on ever since off-roading began. With growing race series like Ultra4, The King of the Hammers and the Southern Rock Racing Series, there is an expanding number of off-roaders who are trying to eliminate any and all weak links in their rigs.
In some parts of a buggy it is easy to eliminate possible failure points just by adding a few secondary or back up items to the vehicle. Two fuel pumps with ball valves can be switched if one should fail, or individual circuits with circuit breakers can be used to help eliminate the possibility of an electrical failure. These types of solutions work well for small components, but inside larger components like axle assemblies it requires the entire assembly to be examined as a whole to remove all the weak links.
When it comes to an axle assembly that is being used in extreme off-road conditions, it is important to choose the right axle to start with. I have heard an axle described as a 27-piece screwdriver, with many moving parts, and just as may failure points inside. This makes eliminating the weak links difficult, but not impossible. By understanding the possible failure points, the axle can be upgraded to help eliminate weak links and will explain why two axles that are regularly found under the most hardcore rigs are often a Ford 9-inch or a Corporate 14 Bolt.
Factory joints often destroy themselves when the needle bearings inside the caps fail. The extra “slop” in the joint when the needles fail creates a domino effect and the cross fails next.
Freewheeling Hubs: The First Item to Fail A common failure area for hardcore off-roaders is the free-wheeling hubs that are the final connection between the wheel and the outer axle shafts. This connection was never originally meant for serious off-road abuse. Rather they were designed to allow the tire to “free-wheel” and disconnect the drive axles from the tires and wheels. The problem is that when they fail, no power is transmitted to the drive wheels. The solution? Drive flanges or “slugs” are solid pieces of chromoly that are used to provide a solid connection between the outer shaft, and the tire and wheel, eliminating this weak link. Unfortunately, this upgrade does not make the axle “bulletproof,” it simply moves the weak link deeper into the axle assembly, making factory axle joints the next possible item to break.
14 Bolt Support:
The pinion support found on the 14 Bolt is removable from the axle housing, and once removed the machined surface engages the nose bearing.
Axle Joint Upgrades The axle joint in a steering axle is a common weak point in the axle shaft assembly. When the axle joint fails, it often causes more damage to the axle shaft “ears.” To prevent this failure from happening, upgraded axle joints are used in the axle assembly. The upgraded joints use bushings, instead of the needle bearings found in stock style joints, to strengthen the friction surface inside the joint. Also, upgrading the “cross” or trunnion of the joint to a chromoly steel billet material helps to eliminate the failure point. A recent development in off-roading has been the use of the constant velocity joint. This joint provides six surfaces of contact during cornering instead of the four points found inside typical axle joints. By adding additional load surfaces inside the constant velocity joint, they provide more strength when the tires are turned and under a heavy load, making these the ideal choice for serious off-roaders.
Axle Shafts: Larger Shafts, with More Splines With drive flanges installed and upgraded axle joints, the next area of failure that needs to be addressed is the axle shafts. The solution is simply to upgrade the shafts to a better material like chromoly or 300M, and upgrade the spline count to a larger number of splines. By building the axle with shafts made with 35, 40 or 47 splines, the load on the shaft is distributed across a larger surface area. High-spline-count axles also have an added benefit that the shaft is physically larger in diameter, than the lesser count shafts, making them stronger.
Splined shafts machined with a 35-spline count for extra strength. Custom axle blanks are available in many different lengths.
Last Line of Defense: The Ring and Pinion The last possible failure point inside the axle after the shafts, joints and hubs are upgraded will be the ring and pinion assembly. The purpose of a ring and pinion is to change the rotational direction 90 degrees inside the axle, taking the power from the driveshaft and providing it to the axles, tires and wheels by using a “hypoid” (low pinion) or “ampoid” (high pinion) gearset. The issue that arises with this type of gear set is the rotational load causes the gears to separate when under load. In a factory setting this is not an issue, but with the additional load of low-range gears in a transfer case, larger wheels and tires, and high horsepower applications, the gears can deflect to a point where the tooth contact between the ring and pinion is diminished. When the gears deflect too much, the tooth contact reaches a failure point and the teeth are shattered, causing a massive failure inside the axle.
What makes the 9-inch and 14 Bolt axles different? To prevent a failure due to deflection from happening, the 9-inch and 14 Bolt have a three bearing pinion support assembly. The third bearing is located on the head of the pinion in addition to the two bearings on the pinion shaft that all axle assemblies have. It helps to prevent the pinion from deflecting away from the ring gear, leading to a catastrophic failure. This is why the Ford 9-inch and the American Axle Manufacturer 10.5 (often called the Corporate 14 Bolt) have become the axle of choice for many Ultra4 and rock racers -- along with hard-core trail use.
One Step Further! For severe-duty applications like off-road racing, aftermarket 9-inch third members are equipped with a load bolt assembly to prevent deflection on the ring gear. The combination of the load bolt and the pinion nose bearing support makes it virtually impossible for the ring gears to deflect under even the most severe load to the gear set. Larger aftermarket fabricated axle housings also have clearance for a larger 10-inch ring gear, which is popular in racing applications as well.
In order to get your rig to handle the type of abuse that Rock Bouncers see on a regular basis, the axles need to be strong.
Once you understand the potential weak links in an axle assembly, it becomes very clear why the Ford 9-inch and the Corporate 14 Bolt with 40-spline spools, CV joint axles and chromoly drive slugs have become popular axles of choice for hardcore use on the rocks.
Slugs are available in different spline counts to upgrade the outer axle shafts the same way inner shafts are upgraded by increasing the pressure area.
RCV Performance has changed a lot of people’s opinions of a CV joint, proving that in severe-duty applications they can handle serious abuse.
The Third Bearing:
In this Ford 9-inch cutaway, the location of the two bearings behind the head of the pinion, as well as the third bearing located on the head of the pinion is visible.