All of the horsepower in the world won’t be very enjoyable if you cannot put it to the ground. Part of that equation is the suspension (see Chapter 1), but the other part is the rear axle. A proper axle assembly for a high-performance Chevelle has the right components to deliver nearly equal power to both rear tires and is strong enough to do so time and time again without a mechanical failure.
Both of these are tall orders, especially as the ability to make gobs of horsepower and torque has become easier and easier. The idea of making 700 hp with a naturally aspirated engine is completely reasonable. Factor nitrous or a supercharger into the equation, and you can easily knock on the 1,000-hp door. There are quite a few items that further complicate the rear-axle solution: more traction, greater lateral forces, and increased braking power.
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A modernized high-performance Chevelle often has considerably more engine power than it had originally. That’s a lot more twisting force being applied to the rear axle. Today’s tire width (and contact patch) can be triple that of the original tires, and performance rubber compounds offer significantly more traction. The traction coefficient of the compound combined with the increased contact patch can be as much as 100 times greater than when these Chevelles sat on new-car dealer lots. Aftermarket suspension systems have also greatly increased the traction, planting the tires on the ground rather than allowing wheel hop or uncontrollable tire spin.
It’s also possible to take your Chevelle around corners better than engineers of the 1960s ever thought possible. That places an entirely new type of stress on the rear axle: lateral loads. When you turn hard into a corner, in addition to a twisting force driven by the engine, the weight of the vehicle is applied laterally against the wheels and tires. This translates into force trying to bend the axle shafts and axle housings, stressing all of the attachment points of the axle assembly. This force is exponentially larger than what the original axle assemblies were designed to handle.
Finally, there is significantly more braking power. Most of these cars had four-wheel drum brakes, which provide only a percentage of the stopping power of today’s 12-, 13-, and 14-inch rotor disc brake systems. Every time you nail the brakes, the axle has to manage rotational force that is, again, exponentially greater than what the original design could generate.
Upgrades to Your Original Axle
With only a handful of factory Chevelles manufactured with a 12-bolt rear axle, most came with a 10-bolt rear end. These 10-bolts had an 8.2-inch ring gear and 28-spline axle shafts. And the newest one is more than 40 years old. That’s the definition of weak and worn. However, you can greatly improve the strength and durability with either a 10- or 12-bolt.
I don’t recommend building a 10-bolt for a very high-performance Chevelle, but there are several upgrades you can make for use in a driver that you’re not going to abuse. In fact, I had a reliable 10-bolt under my Chevelle behind a 410-hp engine and Muncie 4-speed for 15 years, but the car was basically a daily driver and made a few trips to the racetrack.
These upgrades also apply to building a 12-bolt to handle moderate power and abuse. If you have a 12-bolt, though, it probably has value to a restorer as an original Chevelle high-performance rear axle. If you really want to build your car to perform to today’s standards, you may want to sell your original axle and purchase an aftermarket 12-bolt or 9-inch, both of which are stronger than the original 12-bolt.
The key components in a 10- or 12-bolt that should be upgraded for strength and performance are the differential and the axle shafts.
The best differential offered from the factory was a limited-slip Posi- Traction. This is a clutch-type differential that delivers torque to both rear tires up to a point when the clutches slip. This design allows for smooth driving around corners when you need that slippage, and moderate performance. You can install a brand-new Posi, which is still made by Eaton.
There are several alternatives that have a greater bias for performance, though, and the most popular are helical-gear types, such as the Detroit Truetrac, which is also made by Eaton. Instead of clutches, this differential uses helical gears to send power to both tires, but still provide differentiation around corners. This type is preferred with cars that are expected to perform well on an autocross course and drag strip, but not create any negative driving characteristics on the street. It delivers the power smoothly, without ratcheting or harsh catch-and-release mechanisms.
The latest helical-gear differential for muscle cars is the Wavetrac. It is similar to a Truetrac in that it uses the helical gears to provide smooth differentiation. It’s different because it has a wave-shaped design in the center of the differential applying force against the two side gears when one tire approaches zero traction. This causes the Wavetrac to send nearly equal power to both tires, even if one of them is completely off the ground during hard cornering. The Wavetrac uses 9310 steel gears in a case-hardened billet steel body.
There are also several automatic and selectable lockers. However, these operate as open differentials until they are locked. When locked, there is zero differentiation. These are very good for drag racing, but not well-suited for all-around performance Chevelles.
There are two factors to consider when choosing new axle shafts. The first is size. You hear people talk about spline count, and I’ve already referred to axle shafts by this measure. The spline count tells you the diameter of the shaft. A 30-spline shaft is larger than a 28, and a 33 is larger than a 30. Assuming the material is the same, the larger the diameter, the stronger the axle shaft.
The axle shaft spline count needs to match the number of splines on the side gears inside the differential. If you have a 12-bolt with 33-spline axles, you can’t just step up to 35-spline axle shafts. You have to change the axle shafts and the differential. If you’re doing a complete axle rebuild and will be using a new differential and new axle shafts, go with the largest spline count you can get, which is typically 35.
The other consideration when choosing axle shafts is material. There are many mixtures of steel alloys used to make aftermarket axle shafts. Some of the strongest materials manage twisting loads exceptionally well but don’t tolerate impacts, such as potholes, without the risk of breaking. The best advice is to select an alloy that fits your intended use of the vehicle. Typically, a street/strip axle shaft is made from a good, all-around alloy that holds up to the shock impacts of street use as well as the twisting forces of moderate racing.
Complete Aftermarket Housings
For the strongest possible rear axle assembly, use a complete after-market housing. This gives you a brand-new housing, and the axle tubes and center section are typically thicker and stronger than the origi-nals. Also, with a new housing you know it hasn’t been bent at some time during its 40 years of use. There are quite a few companies offering 12-bolt and Ford 9-inch housings that have the suspension mounts, spring pads, and shock mounts in the proper location, so you can bolt the housing right into your 1964–1972 Chevelle without any modifications. Whether a 12-bolt or a 9-inch is best for you depends on your intended use. Generally, the 9-inch is considered stronger because of its larger-diameter ring gear, pinion gear mount, and the engagement between the ring and pinion gears. The biggest difference between the two axle housings is in the basic design. The 9-inch uses a drop-out center section to mount the differen-tial and gear set. This makes it easy to change if you want a different setup for various racetracks, or if you find you don’t like the specific differential or gear ratio that you started with. Some also argue that the mounting of the differential in this third mem-ber is a stronger design. The 12-bolt is a Salisbury design, and the cast center section holding the differential and gears is an inte-gral part of the housing. To change the gear ratio or differential, you have to set up the entire axle hous-ing. With either style of aftermarket rear axle, you have a significantly stronger housing than the original.
Project 1: Setting Up a Ford 9-inch Differential
Step-1: Press Bearings onto Differential
The team at Moser showed how they set up a Ford 9-inch differential. The first step in assembling the differential is to press the carrier bearings onto the differential. Use a hydraulic press to press the bearing into the differential case. Moser does as much or as little of the assembly work as the customer wants.
Step-2: Install Ring Gear
Heating the ring gear on a hot plate makes it easier to slide onto the differential. The heat expands the ring gear slightly. Install the ring-gear bolts (available in a ring-gear installation kit from Moser) with Loctite Red threadlocker. Torque the ring-seam bolts in a star pattern to 85 ft-lbs.
Step-3: Remove 12-Point Bolts
With all of the ring-gear bolts in place, you can remove these two small, 12-point bolts. These hold the two halves of the Wavetrac differential together during shipping, but serve no purpose once you torque the ring-gear bolts in place.
Step-4: Set Pinion Depth
The pinion depth on a Ford 9-inch is adjusted with shims behind the bearing support. This makes it a bit easier to adjust the pinion depth compared to a Salisbury axle design, which uses a crush sleeve to set resistance, and shims under the nose of the pinion for pinion depth.
Step-5: Note Pinion Gear Support
On a 9-inch differential, the rear of the pinion gear is held in the third member. The pinion tries to climb the ring gear during acceleration. The design of a 9-inch has a significant strength advantage, because the pinion is supported on both ends.
Step-6: Install Differential in Housing
The differential bolts into the housing with two large caps. In a Salisbury axle assembly, the gear mesh is adjusted with washers on either side of the differential. A 9-inch has spanners on either side that allow you to adjust the position of the carrier without removing it. Tighten the main bearing cap fasteners to 85 ft-lbs.
Step-7: Set Backlash
With the pinion gear and the carrier installed, there are two critical measurements to check: backlash and mesh pattern. Backlash is how much the gears rock back and forth between engagement. The ring and pinion gear manufacturer determines acceptable backlash, which is printed in the setup instructions.
Step-8: Check Mesh Pattern
The other thing to check is the mesh pattern. Apply a thin layer of grease with a brush on the ring gear teeth and rotate the pinion as if the car was driving forward. A pattern like the one shown is ideal. The teeth are meshing in the middle part of the ring gear without being too close to any edge. There are adjusters on the side of the differential to affect the gear mesh.
The complete third-member assembly is ready to bolt into the 9-inch housing. At the pinion flange, you have several choices. The original U-joint in your Chevelle was most likely a 1310. A 1330 has a wider cross-bar construction and larger caps, which is considered a good performance option. There is also a 1350, which has the same cross-bar width as the 1330 and slightly larger caps, making it an extremely strong U-joint.
If you’re building an all-new axle, go for the biggest axle shafts that fit with your differential. In this case, the Wavetrac for the 9-inch is available in 35-spline. Moser forges these 35-spline axle shafts, which are induction heat-treated to optimize torsional strength, and Magnafluxed. These axles are specifically designed to endure the stress of racing and street use.
Written by Cole Quinnell and Posted with Permission of CarTechBooks