Although the engine and transmission usually get the most attention the driveshaft and rear axle assembly are two essential parts of the equation. In this chapter I address the necessary compromises to be made when vehicles are used in different operating modes, under changing conditions, and with differing budgets. I concentrate primarily on the driveshaft, rear axle, and their various components (yokes, U-joints, gear ratios, differentials, axles, housings, etc.).
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In many cases the driveshaft transfers power from the engine/ transmission to the rear axle/wheels and acts as a “fuse” because it can fail before other components when overloaded, the tires hook up, and the clutch doesn’t slip, etc. The driveshaft often becomes the weakest link in the system. Therefore, a primary consideration in choosing a driveshaft is that it is strong and safe enough to not fail. Weight (total and rotating) is a secondary concern. Because there’s little potential for parasitic/frictional loss the only other real considerations are reliability and serviceability.
For a vehicle primarily driven on the street that has only mild to moderate modification it’s hard to beat a steel driveshaft. The original driveshaft is likely fine as long as it’s not damaged and the U-joints are in good shape. Almost all firstgeneration Mustangs used a singlepiece, steel driveshaft with an integral soundproofing liner on the inside surface of the tube. Tube diameters ranged from 23 ⁄4 to 31⁄2 inches with various lengths and U-joint types. If your driveshaft is damaged or rusted just get a similar one from a salvage yard and put in new U-joints.
If a heavy-duty version (larger diameter and/or U-joints) was available for your car when it was fitted with a more powerful engine (yet it retained the same-style yokes on ends and is the correct length), you should be able to upgrade from a weaker standard OEM shaft to a stronger optional one. Just make sure the one you pick fits and is in good shape. Don’t worry about the condition of the U-joints because you’ll replace them.
The expense of a new steel driveshaft is rarely justified unless what you need isn’t available from a salvage yard. There are a few inexpensive (under $300) aluminum ones available from Ford Racing and others that make a good upgrade if they fit your car. They already come with new U-joints and a new slip yoke for the transmission. They may make sense if you need a new yoke and you want protection for future power increases. (Aluminum’s reduced weight and inertia don’t matter much in this case, unless you’re seeing more power and/or RPM.)
For the vast majority of situations an aluminum driveshaft is best in terms of strength per pound per dollar. Used steel driveshafts are not always easy to find in the size and type you need plus the cost adds up quickly if you also need to buy a new slip yoke and new U-joints. For mild to moderate power one of the lowerpriced, ready-made, direct-fit aluminum driveshafts cost little more than purchasing and refurbishing an OEM steel driveshaft.
A new steel driveshaft generally runs about $100 to $150 less than a better aluminum driveshaft. It is considerably heavier than aluminum and it may or may not be stronger. A steel shaft is likely more expensive than a basic, direct-fit aluminum shaft but it is less costly than an aluminum shaft designed for a highperformance application.
If you can’t use an OEM steel driveshaft with new U-joints, etc., the best solution is usually a directfit aluminum driveshaft. If this basic aluminum shaft doesn’t fit or lacks the necessary strength a stronger aluminum shaft with a larger diameter, thicker wall, larger U-joints, and/ or stronger slip yoke may be best. Aluminum driveshafts capable of handling 600 hp or more can be purchased off the shelf for about $500 to $600. You can also have a driveshaft custom-made to fit your particular situation for about $600 to $1,000.
For my 1965 fastback I was lucky enough to find that a ready-made aluminum driveshaft from Ford Racing (intended to fit 1979–1993 FoxBody Mustangs) was a perfect fit for the combination of small-block 302/5.0L with a T5 transmission and an 8-inch rear end. I will only be making about 450 hp so it can handle the power, and the slip yoke was a perfect fit because the T5 came in Fox-Body Mustangs. The 8-inch also had a compatible yoke and U-joints.
This Ford Racing aluminum shaft was the right length, saved a few pounds (and dollars), and still has some reserve strength if I decide to make more power. It also fit perfectly and worked just fine when I had an AOD in the car. The driveshaft in my 1968 coupe, however, had to be custom-made by Inland Empire Driveline Service (IEDS) because of all the parts I changed and the power I’m making. Changing out the stock C4 transmission for a TCI AODE resulted in the need to shorten the driveshaft slightly, as did the change from an 8-inch rear end to a much stronger 9-inch.
You need to pay attention to the overall length of the transmission you use and to whether or not the transmission output shaft sticks out past the shaft seal (as in the AOD) or is internal (as in the C4). This affects the location and length of the splines in the slip yoke, which must be correct to avoid failure of the slip yoke (too short) or having it hit or bind inside the transmission (too long).
Because I’m making around 600 hp with the 427 stroked Windsor I also wanted to go with bigger U-joints and a beefed-up slip yoke. I provided IEDS with the necessary measurements and they were able to make a beautiful custom piece in about a week; the cost was quite reasonable ($600 to $700, depending on options).
Companies that specialize in driveline components are definitely the way to go if you can’t use an offthe-shelf item. They can pretty much make whatever you need based on your parts and power level. In many cases they also have components on hand for some of the more common situations such as swapping an AOD or a T5 into a first-generation Mustang. They may also stock many of the small, oddball parts you might need such as special conversion U-joints, hard-to-find slip yokes, and similar items related to the driveshaft/ driveline for most vehicles.
If your engine’s power level is extremely high (over 1,000 hp or so) you may find that even an aluminum driveshaft is not up to the task. Some companies offer aluminum driveshafts that have been partially wrapped with carbon-fiber material to reinforce them. This certainly does gain you a bit more strength and safety versus aluminum alone. However, if you need maximum strength the answer is a composite driveshaft with a carbon-fiber (or similar) tube and metal (usually aluminum) end pieces.
A composite shaft can handle up to about 4,000 ft-lbs of torque and 14,000 rpm. They’re much lighter than aluminum or steel, have a longer fatigue life, and are better at dampening (and withstanding) vibrations. Should a composite shaft fail it shreds itself rather than whipping around and beating up the vehicle (and/or driver). Composite shafts from IEDS further benefit from a wet-filament winding process, which contains no metal substrate so you have the lightest and strongest possible construction.
Composite shafts are almost always custom-made to spec so they normally cost upward of $1,000. If they’re the only thing that can do the job for you they’re well worth it.
Yokes and U-Joints
The benefits of having an upgraded driveshaft cannot be realized unless the connections at each end of it are also correct. Because the yokes are determined by the transmission and the rear axle assembly this becomes primarily a function of matching the U-joint size and type. The slip yoke at the front of the driveshaft must match the splines of the transmission output shaft and also be of the correct length. If you go to a more robust transmission you’ll likely end up with a larger-diameter slip yoke. You also want to ensure you chose U-joints sufficiently strong for your power level.
Ford doesn’t have many variations when it comes to slip yokes. Common ones are the 25-, 28-, and 31-spline versions. The smallest was generally found on the 6-cylinder, and the less-powerful small-block V-8s were usually equipped with a 3-speed manual or small automatic transmission.
The 28-spline versions were commonly used on the more powerful small-blocks with 4-speed manual or C4 automatics as well as on some of the later transmissions such as the AOD and T5.
The largest yokes were reserved mainly for big-blocks, which came with a 4-speed Toploader or an FMX/ C6 auto. The 31-spline yoke also fits some later transmissions such as the T45 and the Tremec TKO and T-56. The point is that you should choose your transmission not only based on its capacity but also the ability to use a sufficiently strong slip yoke. You can also go with slip yokes that are forged/steel instead of the usual cast iron if you must use smaller transmission components.
At the other end of the driveshaft things are considerably simpler with the pinion yoke. This yoke size is basically determined by the diameter of the driveshaft tube and the size of the U-joints. You want to match the U-joint sizes at both ends of the driveshaft. This can be difficult sometimes because the rear axle may need something different. There are conversion U-joints to resolve this but they’re not the most desirable option with higher power levels. In general, you should try to have the same-size U-joints all along the way. If it means having a different slip yoke or pinion yoke on the rear axle, it’s preferable in terms of strength, reliability, and serviceability.
U-joints don’t fail often if they’re properly lubricated and maintained but you have an easier time finding a replacement when all the bearing caps are the same size. It’s best to determine what the largest U-joint size is in the system and change all components to that size. Because there are only three main U-joint sizes (1310, 1330, and 1350) and the slip yoke usually determines what options you have, it’s relatively simple.
The smallest U-joints (1310) are sufficient for street use with a mildly modified vehicle but not much more. After a few modifications, or if you take the car to the track, you’re pushing it. In general it’s better to have at least 1330 U-joints throughout the system. They are good for moderately to heavily modified vehicles when autocrossing or on a road course, and at least a moderately modified vehicle at the strip. You can get away with more when you’re not at the drag strip because the shock loading of a road course or an autocross is much less. Similarly, an automatic transmission is less abusive than a manual transmission.
A 1330 U-joint is fine for the vast majority of cars still driven on the street and run mostly on street tires. A possible exception is the heavier 1971–1973 cars but these tend to not be modified as often.
Every car is different and you need to protect for what you have and what you’re planning. Going from a 1310 to a 1330 U-joint shouldn’t be very difficult or cost very much plus it provides more protection should you modify the vehicle further. The 1350 joints are normally only needed for heavily modified vehicles that spend a lot of time at the track. However, a powerful and/or heavier street car with grippy R-compound tires or drag radials may be enough reason to go with 1350s, just in case.
Bigger doesn’t generally cost much more as long as you can make it work. I went with 1350 joints on my 1968 because I planned to use drag radials with 3.70:1 gears and a torquey 427W. It was no problem making them work with the AOD transmission and 9-inch rear.
Be sure to buy permanently lubricated U-joints because they’re stronger (a grease fitting creates a weak point) and use better materials (such as silicone or polyurethane) for the seals. Use a high-grade iron or steel for the main body/trunnion. A lifetime heavy-duty U-joint doesn’t cost much more than a basic version but it gives you more strength and protection. Keep the insides of the bearing caps clean and properly lubricated with an appropriate grease. If you don’t check before final assembly, it’s too late.
Early Mustangs came with three types of rear axle assemblies: 7.5-, 8-, and 9-inch. The 8.8-inch axles found in later Mustangs and other vehicles can also be used but it requires some fabrication to retrofit them. For a performance vehicle the 7.5-inch unit isn’t up to the task and is not discussed here. The 8-inch, contrary to some opinions, can prove to be quite acceptable for most street-driven vehicles and even many vehicles taken to the track (but not the drag strip). The 9-inch is the choice of many racers and highperformance vehicle owners and is the best option when performance levels rise significantly beyond stock. Even the 9-inch requires upgrades for extreme power.
There are several pinion yoke options for each type of rear axle assembly. Different yokes are needed for the various U-joint types (1310, 1330, and 1350). All except the 1350 come in long and short versions, depending on the original vehicle. The original factory parts were usually made from cast iron. Aftermarket examples available from rear axle specialists (such as Currie Enterprises) can be made from aluminum (for less weight and rotational inertia), nodular iron (stronger than OEM), or forged steel (strongest). It’s generally preferable to use the short style because it’s lighter and stronger. This is easily accommodated if you’re having a custom driveshaft made; just be sure you specify the correct dimensions. It may also be possible if you use a pre-made shaft.
The 8- and 9-inch have removable center sections but the 8.8-inch does not; it simply has a rear cover to allow access to the internals. The 9-inch is the high-performance rear axle of choice.
There can be significant differences in the basic strength of the housing. Some have swedged (narrowed) axle tubes, which considerably reduces the strength of the housing. Most do not have any reinforcement from the center section to the tubes; some do.
The size of the axle tubes can vary; bigger is generally better. Likewise, the size of the bearings at the end of the tube can vary from early small bearings to the later, large Torino size. The latter is preferable in most cases.
The heart of any rear axle assembly is the center section. This is where the most critical components (gear set and differential) are located. Other components such as axles, bearings, etc., are also important in the respect that they must be able to perform without failure but they don’t directly affect performance. The choice of gear ratio and the type of differential clearly do.
The main function of the center section casting is to hold these components precisely in place and not deflect under load. Different options do this to varying degrees. The design features of the casting (or billet/forging) such as extra ribs, the use of larger and stronger bearings and gears, and other associated components all vary. This is one of the main advantages of the 9-inch over the 8-inch; everything is just bigger and stronger. The 9-inch axle is the most widely used for performance purposes but many concepts I discuss also apply to the 8-inch, and some even to the 8.8-inch.
I went to the driveline experts at Currie Enterprises for some specific details on these axles. Some of the obvious visual differences include strengthening the ribs, machining for weight reduction, using different pinion bearing options, and using a different casting material. These differences vary depending on the specific center section type and the manufacturer.
The first decision to make regarding your gear set is what ratio to use. The ratio you choose depends on your priorities. What’s available varies by the rear axle type but it’s safe to say the 9-inch has far more options than the others. In general, ratios of 3.00:1 or less are not used for performance purposes, except perhaps where a higher top speed or better fuel mileage is the priority. Most street-driven vehicles intended for performance use have a ratio in the range of 3.25:1 to 4.10:1 with the higher numbers being best suited for drag racing.
The transmission also affects the choice of gear set because the use of an OD transmission such as an AOD or a T5 allows the use of a numerically higher gear without the penalty of higher engine RPM on the highway. A higher numerical ratio generally provides better acceleration while the lower numbers, theoretically at least, offer the potential of a higher top speed and/or better fuel mileage. You can be sure the latter reduces engine RPM at any given vehicle speed.
Ratios over 6.00:1 are available for the 9-inch axle though such extremes are only appropriate for drag racing and other specialized uses; they’re not suitable for vehicles driven on the street.
An OD transmission and/or one with more ratios (5- or 6-speed versus 4-speed) and a wider ratio spread provides more flexibility. There’s not much difference in the cost of the gears in terms of the ratio chosen, except perhaps for the really high (over 5.00:1) ratios. The cost of the gears is more dependent on the material and any special finishing operations (lightening, polishing, linecoating, heat treating, cryogenics, etc.).
For low to moderate performance levels and ratios, new factory/OEM gears are commonly thought to have the best wear and noise characteristics. Even used OEM gears can be better than some aftermarket brands if they’re not too worn or otherwise damaged (pitting, chipping, scoring, heat checking, etc.). You should never use any set of used gears that shows any sign of damage or wear. The possibility of a catastrophic failure does not justify the risk.
The gear ratio determines how well you match the engine’s RPM and torque characteristics to the way the vehicle is used. The carrier/ differential determines how the torque gets transmitted to the tires. You need to have a powerful engine with the proper transmission and the correct rear-end ratios in a limitedslip differential (LSD).
The most common type of LSD on early Mustangs with an 8- or 9-inch axle is the Traction-Lok type, which uses the same concept as the Eaton Positraction (Posi) and others. This design is far superior to the Ford LSD found in most smaller rear ends, including the 8-inch, which were neither strong enough nor durable enough for serious performance use.
This rear design uses a series of steel plates with corresponding friction plates sandwiched between them to keep the wheels spinning. It relies on a preloaded spring setup to keep the plates together when going straight yet the springs allow for differential action when turning.
Older units were prone to wear though newer versions of the Eaton Posi feature carbon-friction technology along with precision-forged gears for greater durability and strength. These units are rebuildable, if necessary, and relatively inexpensive. They operate automatically so they are a good all-around unit for a mildly to moderately modified vehicle. Their main disadvantage is they’re not always smooth and may cause tire squeal or gear chatter when going around tight corners. They also change their characteristics as they wear. Still, they’re very robust and economical.
The 9-inch axle type is able to accommodate much larger, stronger axles with up to 40 splines, whereas smaller axles types may be limited to 31- or 33-spline axles, which may not be strong enough. You must match the splines of the carrier/differential to those of the axles, which further affects your number of options.
It’s best to do all rear axle upgrades at the same time, if possible, because there are many interdependencies among parts and you avoid disassembling the rear axle more than once.
For higher-output heavier vehicles and/or more severe use, the Eaton Detroit Locker is an excellent solution, especially on the drag strip. This design was intended mainly for racing use only, though it could be bought over-the-counter at a Ford dealer (for the 9-inch). It operates on the same basic principle as the Posi but replaces the plate arrangement with a couple of very strong ratcheting “dog ring” gears that are constantly meshed together when going straight. There’s no potential for slippage as there is with the Posi.
When turning, the gears separate by sliding along their splines and the teeth on their sides ratchet (jump) over one another for differentiation. This results in a very audible clunking sound and a harsh action, which is really not suitable for street use. This style is available for the 8.8-inch axles with C-clips and even the 7.5-inch axles so it may be a viable solution in cases where the ultimate in strength and durability is needed.
An interesting variation on the Detroit Locker is the Eaton ELocker, which is most commonly used with 8.8-inch axles. This differential allows the driver to lock or unlock it at will via a pushbutton connected to built-in electromagnets. The Detroit Locker is ultimately stronger but the ELocker comes close and spares you the harshness. The ELocker is maintenance free though it’s serviceable if necessary.
Versions using air instead of electricity are called Air Lockers. They essentially give you an open differential for the street (smoother, no chatter/clunking) but allow you to have a no-slip LSD when you get to the track/strip. While originally intended for all-wheel-drive and off-road vehicles they have been adopted by performance-oriented drivers because they provide smooth operation on the street with tremendous strength in competition.
The Eaton Detroit TrueTrac (TT) helical gear-style differential is an excellent all-around choice for a vehicle driven on the street; it is also aggressively used on the track/strip. This design is by far the simplest and most effective for the street. It’s also more expensive due to the precision needed during its manufacture. They are torque-sensing rather than speed-sensing differentials.
Their biasing and differentiation functions are constantly working, regardless of speed, in a fully automatic manner. The transfer of torque from one axle to another is totally consistent and seamless. There is no locking/unlocking, which can destabilize the vehicle and break traction in a turn. Helical differentials are the smoothest, quietest, and most progressive design, which makes them the best choice for street, road course, and autocross use.
TTs work great on the drag strip and are a better choice than any of the other differentials unless you’re dealing with extremepower, slicks, and a heavy car with a manual transmission. In other words, they are for anything but the “worst case” scenario and/or where the budget is limited. These units are extremely strong and are suitable for all but the most extreme power levels when used on the drag strip. In almost any other use there virtually is no power limit because something else is likely to break first.
The original helical gear differential was the Torsen. It adds to the features of the TrueTrac by being available in several different styles (T-1, T-2, T-2R, etc.), which have different bias ratios ranging from about 1.5:1 to over 3.0:1 (up to 5.0:1 in the T-1) to better suit different situations. Their design is also better able to handle the highest torque levels with minimal chance of failure. Torsen differentials were available as an option on the 2013 and newer Boss 302 Mustang (standard on the Laguna Seca model) and were also included in Ford’s factory road race cars such as the FR500S with winning results.
The T-2R race version incorporates serviceable friction plates to supplement the helical gears to “tune” performance and provide a higher bias ratio under the most severe conditions. It also makes the unit compatible with electronic traction control.
Rear Axle Assembly
Always use the appropriate factory manual and/or manufacturer’s instructions when assembling a rear axle. It’s also extremely important to keep everything as clean as possible and ensure your torque wrench is properly calibrated. Failure to properly torque the fasteners for the bearing caps and other items will almost surely cause a problem of some kind if not outright epic failure.
The following photos are highlights of installing a rear axle at Currie Enterprises, where they have the benefit of specialized fixtures and tools to make the process easier and quicker. This is not a step-by-step guide because there are too many differences among cars.
The final steps in the assembly process involve the installation of the center section into the axle housing, and then the installation of the axles with their bearings and seals. Other than choosing the correct axles the main thing to consider is what type of axle bearing to use (ball, tapered, etc.) as determined by the size of the bearing pocket in the axle housing and the intended use .
Once the axle assembly is complete you may still have to address the installation of brake lines, suspension brackets, brake hardware, and so forth before you install the whole assembly in the car. Before the axle is finally installed you must also fill it up with the appropriate amount and type of gear oil.
Written by Frank Bohanan and Posted with Permission of CarTechBooks
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