Expert Tips for Selecting & Assembling Driveshafts & Rearends

Nov 16, 2011

The driveshaft and rearend usually aren’t the first parts a hot rodder boasts about or shows off to admiring friends (well, maybe the rearend, if it’s trick like a quick-change or has been nicely polished). But the proper selection of these parts and, perhaps even more importantly, the precision and accuracy of their installation, can make the difference between OEM-grade refinement and riding-mower roughness. Choose poorly and you’ll find yourself mopping up a catastrophic failure.

“Engines produce power, transmissions and rearends modify power, and driveshafts deliver power,” said Greg Frick, president of Inland Empire Driveline Service (IEDS) in Ontario, California. “The devil is in the details of their placement and relation to each other.”

IEDS provided us with a wealth of information regarding driveshaft selection and installation, much of it taken from company publications. We’ll take a look at that later.

For now, however, let’s begin at the posterior of the hot rod driveline and work our way forward.

Do You Need a 9-Inch?

The 9-inch is arguably the  industry’s most-admired rearend.

“The 9-inch Ford has become such a buzzword that everybody with a hot rod thinks they have to have one, or they’re cheating themselves,” said Brian Shephard of Currie Enterprises in Anaheim, California.

Although the 9-inch may be all the rage, in reality an 8-inch Ford is a better choice of rearend for a lot of traditional street rods.

“It’s smaller, lighter and costs less but still features that same ‘old reliable’ look as the 9-inch,” Shephard said. “It’s only usable with up to 300 horsepower, and it’s not an option for a heavy car like a Chevelle. But even for a ‘cruiser’ pony car such as a Nova or Mustang, an 8-inch is just fine.”

Even when building a heavier muscle car, picking the right rearend is all in the application, said Shephard.

“A couple might have a 700-horsepower car that they just putt to car shows, making a lot noise and never have a problem with a stock rearend,” he said. “On the other hand, you can put a young guy in a 400-horsepower car with drag radials and he’ll go out and scatter parts. Any car over 400 horsepower that the consumer intends on using in an abusive application needs to be examined and updates considered. An 8-inch Ford or GM 10-bolt shouldn’t be used in a real performance application at all.”

After the customer has chosen a basic rearend configuration, there are still options to be considered. Even a cruiser needs to choose brakes, gear ratio, and open or Posi operation.

“A performance car requires a performance rearend, and there are a world of options for a performance rearend,” said Shephard. “How strong does the housing need to be? What’s going to be the spline count of the axles? What level of nodular gear case, what type of gear carrier, and what’s the optimum gear ratio?”

In short, make sure that the unit accommodates the consumer’s needs.

Nifty 9-Inch Rearends

At the high end, it turns out, there are good reasons for the 9-inch Ford’s unparalleled reputation.

“The Ford 9-inch and 8-inch really have most of the advantages in the hot rod market over the GM 10- and 12-bolt,” Shephard said. “The Fords feature a better axle bearing design. It’s a larger bearing to start with and it’s pressed to the axle shaft rather than riding on it. And the Ford axle bearing is positively retained, rather than relying on a C-clip that can fall off and allow the axle to work its way out of the housing and pass you on the road.”

“Also, a 9-inch Ford can be back-braced for performance applications; you can’t do that with a GM rearend because they have the cover on the back,” Shephard continued. “The aftermarket following for the 9-inch Ford is 10 times that of any other rearend, [so] more aftermarket gear ratios, carrier types and brake kits are available for it.”

“Another point to consider is that the 8- and 9-inch Ford are all stamped steel, so you can weld on brackets and tabs wherever you need them,” he added.

The GM rears have cast centers which are not so suitable for welding.

Would Shephard ever recommend a GM 10- or 12-bolt rear? Yes, Shephard answered, for someone restoring a classic GM muscle car to authentic factory specification.

Currie’s newest rearend is the F9 (shown above), a fabricated 9-inch Ford housing that’s available in mild steel or chrome-moly.

“It’s the strongest housing we’ve ever built,” said Shephard. “It’s good in essentially any application, but excels in strength for high-horsepower vehicles.”

IEDS has released a forged 4140 Chromalloy 1310-series yoke for Ford 9-inch applications.

“It’s machined like billet,” Jeff Gilroy, a manager at IEDS, said. “We’re also developing a line of forged 4140 Chromalloy 1310 pinion and transmission yokes for other applications.”

An exclusive CNC-controlled heat treat provides greater strength without added weight, Gilroy explained.  If you’re considering a used rearend, Shephard offered a few words of caution.

“In the past, used 8-inch Fords and GM 10-bolts were inexpensive and plentiful, but none of the older axles used in hot rods are plentiful anymore,” he said. “If you do find one in a wrecking yard now, it must have just arrived recently, which means it has about a million miles on it, and a rebuild would be futile compared to the cost of a new unit.”

Shaft Selection

The driveshaft selection process comes down to what kind of power you’re making, how long the shaft needs to be, and how you want it to look, according to Steve Raymond, General Manager of Dynotech Driveshafts in Troy, Michigan.

“A lot of customers choose aluminum for the simple reason that it looks good,” he said. “These folks have spent hours making the underside of their vehicle look as good as the top side, and what better way to keep up the aesthetics than with an aluminum driveshaft? It looks as good, if not better than stainless steel.”

Typically, a 6061-T6 aluminum driveshaft with the industry-standard 0.125-inch wall thickness is as strong as a stock 0.083-inch wall-welded steel shaft, while reducing weight significantly.

“For example, our one-piece aluminum shaft for 2005 and newer Mustangs will take up to 900 horsepower and it’s 21 pounds lighter than stock,” Raymond said.

Street machines that need something stronger than aluminum can upgrade to DOM steel. “DOM” stands for “Drawn Over Mandrel,” a technique of cold-working welded tubing that relieves the heat effects of the weld while also improving finish and assuring more exact dimensions. Chrome-moly is stronger still. However, driveshaft manufacturers rarely deviate from the standard 0.083-inch wall thickness for steel.

“There’s not much choice outside of those dimensions because it becomes cost-prohibitive to have custom ends built,” Raymond said. “Not until you get to the 800-900 horsepower range do you have to be concerned about overpowering a well-built aftermarket shaft.”

But, Raymond said, “it’s easy to overpower a stock driveshaft, because it’s so easy to make huge power increases with a dead-stock-looking motor.”

While most original muscle car driveshafts are good for the stock power, it’s rare that they’re actually producing what the factory had intended for them.

“Today, most of these cars are producing well over the factory-issued 400-450 horsepower,” said Raymond. “Then, if the customer is drag racing, autocrossing or even just running hard on the street, the combination of big power, new tires, and today’s ability to get these cars to hook up [make the risk of breaking a stock shaft even greater].”

“We recommend that you upgrade from a stock driveshaft, even if you’re making only 50 or 100 horsepower more than stock,” Raymond continued.

“For $329 Dynotech can build a DOM steel shaft that will take twice the abuse that the stock shaft could survive,” he added. “That’s pretty inexpensive, when [you] consider that when a driveshaft fails it almost always takes something else with it.”

“Stock was adequate when the car was new,” Gilroy agreed. “Merely adding modern tires will make ‘stock’ inadequate because of the added traction they provide.”

Balance & Resonance

If it’s material and processing that give a driveshaft strength, and wall thickness is standardized, then the factor that determines its diameter is its length, Raymond explained.

“Every driveshaft has a critical speed, determined by its length, diameter and material; and you have to make sure that your operating speed is outside of this speed range,” he said. “A driveshaft that runs at or near critical speed will suffer severe vibration, and if it spends too much time at critical speed, it will fail.”

The mathematics can get pretty complicated, which is why Raymond said Dynotech goes through them for every customer to “calculate driveline speed and critical speed, and to make sure what we are building is safe.”

Frick of IEDS pointed out that, of all the major driveline components, only the driveshaft is free to move as it rotates, allowing changes in distance between the transmission and the third member while correcting misalignment between the two.

“And with that freedom comes instability and sensitivity,” he said.

While Frick agreed that length, weight, diameter and rpm all affected critical speed, he added that, in the real world, critical speed can also be reduced by U-joint angles, shaft mounts and even the pulses in power as each of the engine’s cylinders fire. An overdrive transmission amplifies this last effect, feeding more energy into the driveshaft because of the additional torque required to operate at lower rpm.

“Keeping safely away from critical speed affects decisions about tube diameter, and whether to use a two-piece shaft set and add support when bridging long spans,” Frick said.

To understand how U-joints affect critical speed, Frick suggested first looking directly down at a dinner plate: It looks round. Then pick it up and tilt it, and see how it appears elliptical.

“The ellipse is the way the driveshaft ‘sees’ the universal joint, and for the joint to rotate through the ellipse, it must speed up and slow down twice per rotation,” he said. “And as the angle of operation increases, so does the abruptness with which the shaft must change speed. These pulses excite the natural frequency of the shaft, producing a shudder at one-half the critical rpm.”

Fortunately, this effect can be minimized by mounting the two ends of the driveshaft at equal and opposite angles. That is, the angle of the driveshaft relative to the transmission tail shaft and the angle of the driveshaft relative to the rear-end pinion shaft should always add up to zero. It should be pointed out that you can save yourself a lot of grief if, before you even begin to build a car, you make a sketch of the car running down the road at the attitude you want, with the wheels and tires drawn to scale.

Then, when the frame is built, set it on stands, once again not necessarily level to the ground, but angled to the ground the way you want it to run down the road. Remember to measure all angles relative to the ground, not to the frame.

“While each vehicle is different and no hard-and-fast rules apply to all cases, start asking questions when the distance from the tip of the transmission output shaft to the centerline of the third-member U-joint is 51 inches or more,” Frick suggested. “Then available crossmembers, frame obstacles and U-joint angle cancellation will all influence how you divide the span.”

Frick added that a 40/60 front/rear split is customary, and neither shaft should measure less than 18 inches. As with a one-piece shaft, all working angles have to add up to zero; however, now you have not two but three of them.

“The easiest way to achieve this is to mount the front shaft section at 0 degrees through the joint at the transmission,” Frick explained. “Then treat the rear shaft as if it were a one-piece unit.”

Sometimes, however, this just isn’t possible, so it’s good to allow for some vertical adjustment at the center support so the angles may be fine-tuned once the car has been driven. Of course, any driveshaft should be high-speed balanced before it’s installed. A driveshaft is a pretty big piece of fast-spinning metal, so even a small imbalance can create a significant force.

“Everyone has seen a washing machine start the spin cycle with an unbalanced load,” Raymond said. “It sounds like the house is coming apart.”

Today, production car driveshafts are balanced at between 3,200-3,500 rpm and might turn as fast as 3,700 rpm on the freeway. Yet some custom builders balance their driveshafts at only 1,500-2,500 rpm.

“Why would you ever balance so far below operating speed?” asked Raymond, especially when the majority of today’s hot rodders expect OEM-level ride comfort.

Looking Down the Road

Shephard of Currie concluded with some good advice about planning for future modifications (read: more sales) to the customer’s car.

“We interview each new prospective customer,” he said. “We ask the make, model and weight of the car, of course, and what the customer plans to do with it. But even more important, we ask what they plan to do with it in the future, ‘What is their ultimate goal for the car?’ because we want to sell them parts that are good for the long road.”

“Don’t sell the customer a unit today that’s going to require 50 percent of its cost in upgrades when the customer installs a supercharger four years from now,” he continued. “Build for the supercharger now.”

“You’ll earn more money up-front, of course, but you’ll also earn the long-term loyalty of your customers,” Shephard added.