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Off-Road Straight Axle - Straight Up

Posted in How To on January 1, 2001 Comment (0)
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Off-Road Straight Axle - Straight Up

As long as there have been trucks, the dominant drive axle system has been the Hotchkiss design, best known as the solid or straight axle. From the late-'40s through the mid-'80s, straight-axle assemblies were found at the front and rear of virtually every fullsize, built-in-the-U.S.A. 4x4. However, through the years, major American 4x4 manufacturers looked to unique front drive axle designs in order to keep their trucks' performance and image at the level consumers wanted, whether or not the public truly knew which mechanical systems were best suited to reliable, functional on- and off-road performance. Certainly, the motoring public knew what was trendy, and the popularity of independent front four-wheel-drive suspensions, combined with the top two American automaker's quest for improvement, caused solid front axles to fall out of favor with GM and Ford. Meanwhile, Dodge and Jeep stayed the course, continuing to offer solid front axles on their Ram and Jeep vehicles, although, in a bow to advancing technology, those straight axles were equipped with multiple locating links and coil springs.

However, while it's no longer the heyday of the straight front axle 4x4 - there's still a lot of life left in the venerable Hotchkiss design, as even the most casual off-road enthusiast can attest. Hundreds of thousands of solid axle-equipped trucks built during the '50s, '60s, '70s, and '80s remain on the roads and trails; certainly these trucks have stood the test of time and their continued popularity is well addressed by aftermarket drivetrain and suspension manufacturers.

What follows is a celebration of the straight axle: an overview of state-of-the-art solid-axle trends mixed with a strong dose of traditional straight-axle technology and augmented with a smattering of solid-axle performance goodies.

The Straight Axle: DefinedA solid, or straight, axle is all about simplicity and strength. Basically, a straight axle is a system of gears and axles contained within a metal housing, which transmits power to a truck's wheels. A solid axle is tremendously adaptable: It can be used at either the front or the rear of an off-road vehicle; it can be used with leaf springs, coil springs, coilover shocks, or even air springs, and it can be positioned under a vehicle's chassis with either locating links or spring packs. Most of all, a straight axle's strength is its simplicity: There aren't multiple CV joints to wear out or break; the differential's cover and gears are accessible, not buried between the truck's framerails; the axlehousings and the diff housing strengthen and support each other; and the exterior housing protects the inner workings of the moving parts.

Solid-Axle Suspension: From Fundamental to High-TechIn the past, straight-axle suspension systems were assuredly low-tech. That's not to say that leaf springs and solid axles aren't functional, but a straight drive axle system suspended by the antiquated design of the multileaf spring pack can only be a stellar performer when properly set up. Perhaps the biggest problem with a solid axle/leaf spring suspension is not with its performance, but rather its image - it's hardly high-tech in either appearance or image.

However, there's plenty of life left in the solid axle. These days, straight axles - especially straight front axles - are making a comeback, thanks in large part to Ford's Super Duty and Dodge's Ram, both of which use a straight drive axle to power the front wheels. There are differences, of course. Ford's straight axle is located by traditional leaf spring packs; the Ram's front axle is located by multiple links and suspended by coil springs.

Stepping further into the realm of advancing suspension trends, many enthusiasts are equipping their trucks with straight drive axles suspended by race-quality coilover shocks and directed through their travel with multiple - as many as five - locating links. Sure, these exotic straight-axle suspension setups are expensive, but their performance is unreal: smooth, well-damped suspension action and gobs upon gobs of wheel travel. Obviously, there's a lot of life left in the solid axle, although only Ford and DaimlerChrysler remain loyal to the Hotchkiss design on the front of the Ram and the Super Duty.

Dana AxlesDana Axles are perhaps the most recognizable name in axles and have been factory equipment at some point on almost every fullsize American solid-axle 4x4 over the past several decades. The most popular Dana axles, in both aftermarket and OE applications are the 30-, 44-, and 60-series, although the 70-series is also a popular axle for swapping into trucks with huge tires and powerful engines.

Dana's model 30 axlehousing was used extensively as a front unit in the '70s-and-earlier Broncos, in Jeeps built from '72-'86, in late-model Grand Cherokees, and in '87-and-later Wranglers. The 30 is a full floating design, meaning that the axleshafts don't support the vehicle's weight; the axles only transmit power.

The 44 is perhaps the most popular and adaptable axle in the Dana series, and it has been used by Dodge, Jeep, Ford, and GM in front applications. It features a nodular iron centersection. There are two basic models of the Dana 44: 1/2-ton and 3/4-ton. The 3/4-ton 44 is the way to go if you're seeking a replacement front drive axle for your truck because the 3/4-ton model has a larger U-joint between the outer and inner axles. One down side of the Dana 44 is its width. The 44 was designed for fullsize 4x4s, so the axles and axle tubes will need to be narrowed if you want to run a 44 on the front of a Jeep, a Toyota, or other compact 'wheeler. Another aspect of the 44 that can sometimes cause a problem is its weakness when paired with large (36-inch-diameter and larger) tires and the wide wheels such tires require. The Dana 44 can be used successfully at the rear of a Jeep as long as its not expected to contain the power of a V-8. Don't bother using a 44 at the rear of a fullsize hauler because it will not prove to be reliable.

The Dana 60 is the next step up from the 44, and the front model is somewhat of a rare find in wrecking yards since the front 60 was primarily used in 1-ton 4x4s, although a few 3/4-ton trucks were factory-equipped with the 60. The Dana 60 has proven so durable and adaptable that DynaTrac manufactures 100 percent new models - no more scrounging around at the wrecking yard. The OE Dana 60, as well as the DynaTrac replica, uses a center diff housing made from nodular iron, which is notably stronger than the cast-iron centersection used on many other makes and models of axlehousings. As for placement, the 60 is an excellent choice for the front or rear of fullsize American iron, and like the 44, it will work on a Jeep or a similarly sized 4x4 if the axle tubes and axles are narrowed.

Dana 70 axlehousings are large and rugged; 44-inch-tall tires are no problem for the big Dana's 10-1/2-inch-diameter ring gear and its 35-spline axleshafts. Like the 60, the 70's centersection is formed from nodular iron. If you're looking for a used Dana 70, try checking under the rear of '94-'96 3/4-ton Dodge Rams because the 70 was OE in both two-wheel-drive and 4x4 versions of those trucks.

General Motors' Corporate AxlesThe General's own brand of drive axles are the 10-, the 12-, and the 14-bolt, with each number referring to the number of bolts, which hold the differential cover plate in place. The 10-bolt solid axle is the smallest in regard to ring gear and axle size; the 14-bolt is the largest. By the way, if you're checking the frontend of a fullsize GM truck built during the '60s or '70s, don't be surprised to find a Dana 44 front axle assembly; Dana supplied GM with front axles before the General took up building its own front 10-bolt drive axles in '79.

GM's 10-bolt drive axles are manufactured in two sizes: The version with the 7-1/2-inch-diameter ring gear is used at the rear of GM's S-series trucks; the larger 10-bolt with its 8-1/2-inch ring gear was used at the front of fullsize GM trucks until '88 and is still used at the rear. Although many 10-bolt drive axles survive the stress of 36-inch-diameter tires (and larger), a 10-bolt isn't really up to the task of wearing large meats because the 10-bolt's axles are retained by C-clips. If one of these clips or the axle breaks, the remaining section of the axle and the wheel and tire attached to it will slide out of the axlehousing and separate itself from the vehicle.

The 12-bolt drive axle is a better choice than the 10-bolt, although it too is equipped with C-clip axles. As for 12-bolt availability, there are plenty of good used units to be found in wrecking yards, but since GM has ceased production of the 12-bolt, the supply is sure to dry up sooner or later. One aspect of the 12-bolt (and the 14-bolt for that matter) is the fact that swapping out a 10-bolt in favor of 12-bolt is straightforward. Basically, it's a bolt-in, although you will have to change the wheels from six-lug to eight-lug if the12-bolt is so equipped.

While GM's Corporate 10- and 12-bolt drive axles are adequate performers, the big 14-bolt monster is extremely durable and is able to tolerate the effects of massive tires and a torque-y V-8 engine. With its 10-1/2-inch ring gear, the 14-bolt is as capable as a Dana 70 and a lot more plentiful. There is no front steering 14-bolt, and the 14-bolt only comes equipped with eight-lug hubs, so if you are using this axle for a swap, you will likely need to change the bolt pattern of the front wheels so they'll match the front. As with a 12-bolt swap, upgrading from a 10- or 12-bolt rear to a 14-bolt rear is basically a bolt-on. The best 14-bolt axle to get is the full floating version, which boasts 32-spline axles and 1-1/2-inch-diameter axles.

Ford Corporate AxlesAs previously stated, Dana 44 drive axles were used at the front of straight axle-equipped fullsize Ford 4x4s, while Ford's 8.8- and 9-inch drive axles were common at the rear of these trucks. Ford's smallest solid drive axle, the 7-1/2-inch model, was used at the rear of non-4.0L V-6-equipped Rangers and Bronco IIs.

Ford's 8.8-inch drive axle is a semi-floating design and is currently used in the rear of Rangers, Explorers, and certain 1/2-ton fullsize trucks. It's also used under the big Super Duty 4x4. The next step up in drive axles is the best known and perhaps one of the best units on the market: Ford's venerable 9-inch. The 9-inch is impressive because its pinion is supported by three bearings. Used at the rear on trucks until '86, the 28-spline 9-inch is heavily supported by the aftermarket.

For example, the normal cast-iron construction of the 9-inch isn't known for its strength, but Currie Enterprisesmarkets a nodular iron center section that's stronger. In fact, Currie manufactures complete, 100 percent new reproduction 9-inch Ford rearends set up in any manner you want.

Ford's Super Duty 4x4 rides atop a pair of Dana drive axles; the front axle is a Dana 50, the rear is a Dana 70. Both drive axles are robust; the front Dana is rated to carry 5,200 pounds. The rear Dana axle is capable of supporting 6,084 pounds when installed under an F-250. The Super Duty F-350 model is equipped with a Dana 60 at the front and a 70 drive axle at the rear that's capable of holding 6,830 pounds.

DANA AXLE SPECIFICATIONS
AXLE RING GEAR
DIAMETER
AXLE SPLINES AXLE DIAMETER
AT SPLINES
Dana 30 7.2 inches 27 1.16 inches
Dana 35 7.56 inches 27 1.16 inches
Dana 44 8.5 inches 30 1.31 inches
Dana 60 9.75 inches 30 1.31 inches
Dana 60 9.75 inches 35 1.50 inches
Dana 70 10.5 inches 32 1.37 inches
Dana 70 10.5 inches 35 1.50 inches
GM CORPORATE AXLE SPECIFICATIONS
AXLE RING GEAR
DIAMETER
AXLE
SPLINES
AXLE DIAMETER
AT SPLINES
10-bolt 8.5 inches 28 1.285 inches
10-bolt 8.5 inches 30 1.285 inches
12-bolt 8.875 inches 30 1.285 inches
14-bolt (semi-floating) 9.5 inches 30 1.285 inches
14-bolt (full-floating) 10.5 inches 30 1.50 inches
FORD CORPORATE AXLE SPECIFICATIONS
Axle Ring Gear
Diameter
Axle Splines Axle Diameter
at Splines
8.8-inch 8.8 inches 30 1.31 inches
9-inch 9 inches 28 1.19 inches
9-inch 9 inches 31 1.33 inches
Dana 50 9 inches 30 1.30 inches
Dana 60 9.75 inches 30 1.30 inches
Dana 70 10.50 inches 35 1.50 inches

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Sources

Currie Enterprises
Corona, CA 92880
714-528-6957
http://www.currieenterprises.com
Dynatrac Products Co.
Huntington Beach, CA 92647

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