Dale Amy
January 11, 2012

Like Rodney Dangerfield, a driveshaft gets no respect. Or even much thought, for that matter. Until we exceed its design limits, that is, at which point the extreme importance of this delicately balanced, high-revving power transmitter comes suddenly, and sometimes frighteningly, to the forefront. Consider this: the factory driveshaft under your vintage Ford was engineered to withstand the rigors of OEM power levels, traction limits, and speed/rpm capabilities for many years, which it likely has. But we'd never go broke by betting that all those factory power, grip, and speed parameters have long since been exceeded in most of our readers' rides, 'cause we don't call this Modified Mustangs & Fords for nothing.

Forcing a stock driveshaft to work with increased power levels is a recipe for disaster, as a broken U-joint or twisted driveshaft can damage the chassis, drivetrain, and in extreme circumstances, even pole vault the car at speed. So it may be time to give more than a passing thought to that long, thin shaft spinning furiously mere inches away from your posterior. Is it up to withstanding your modified Ford's current horsepower and torque? Do you spend any time on the 'strip (or street) launching on the latest and greatest sticky tires? Does your quarter-mile trap speed now far exceed your ride's original top speed? Have you just replaced your tranny with one of a different length? Heck, even if you haven't (yet) powered up your classic, does it simply wear a rusty 40-year-old driveshaft? We see your collective heads nodding, so we turned to the experts at DynoTech Engineering in Troy, Michigan, for some driveshaft facts and advice.

Limiting Factors

The ideal driveshaft is both light and strong. A lighter shaft consumes less power to rotate and therefore passes on more engine power to the drive wheels. When it comes to strength, driveshafts are rated and limited by two factors: torsional strength (resistance to twisting) and "critical speed." The latter being the velocity at which a normally straight and true driveshaft begins to look somewhat like a banana and plots a rotational path not unlike that of a skipping rope, causing much vibration and possible shaft failure in the process. As a rule, for any given construction, the longer the driveshaft, the lower its torsional strength and critical speed limit. Otherwise, these strengths and limits are principally determined by driveshaft material and wall thickness, overall shaft diameter, and such details as degree of balance, location of balance weights, and U-joint phasing and clearance.

Despite the name, driveshafts are not actually solid shafts, but instead constitute some form of hollow tube of steel, aluminum, composite, or hybrid (aluminum with carbon-fiber wrap) construction. Let's look at each:

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Steel
Odds are your Ford's factory driveshaft was made of "seam tube" (rolled, and welded along the seam) steel, the cheapest and most common OEM construction, and perfectly adequate for standard duty and light high-performance use. On the downside, seam tube is heavy, forms the weakest of the steel driveshafts, and also has the lowest critical speed.

Next up the steel hierarchy are DOM (drawn-over-mandrel) driveshafts which are seamless, slightly more expensive than seam-tube shafts, but notably stronger, and with higher critical speeds. The DOM process work-hardens and strengthens the steel, making it suitable for nearly any high-performance application.

Most expensive of the steel driveshafts are the chrome-moly variants. These have extremely high torsional strength, but are available only in 3- and 3.5-inch diameters, and are relatively heavy and pricey.

Aluminum
Most aluminum driveshafts are constructed of 6061 T6 alloy. These are light in weight, never rust, and have higher critical speeds, but less torsional strength, than steel shafts, making them suitable for moderate horsepower and/or lightweight vehicle applications. If you happen to have a mirror under your car, they also look kinda cool.

Composite
Like most everything made of carbon fiber, composite driveshafts are very strong, very light, and very expensive. Though extremely well suited for race applications, they're probably overkill for your restomod Ford. They also ooze high-tech coolness, so it's almost a shame to hide one beneath a car.

Hybrid
Take an aluminum driveshaft, wrap it in a thin carbon-fiber layer, and you have a hybrid that's stronger than aluminum, cheaper than full-on carbon fiber (though still not inexpensive), and with a very high critical speed.

And that about covers the gamut of basic driveshaft materials in the DynoTech Engineering lineup, but really just scratches the surface of what makes a good driveshaft. Proper U-joints are obviously also critical, but perhaps what most separates a great driveshaft from a merely average one is the precision with which it's built. Remember: a driveshaft does its work while spinning madly. A vibration-free driveshaft requires minimal "tube runout"--a measurement of the deviation from straightness or trueness of the shaft as it rotates. Smaller runout needs fewer balance weights, so DynoTech claims to maintain runout at 0.020 inches or less over the entire tube length.

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Orientation (or "phasing") of the U-joint assemblies at either end also demands precision construction techniques, as does the centering of those U-joint assemblies on the driveshaft tube (if a U-joint is mounted off-center, the entire weight of the driveshaft is then rotating off the driveshaft centerline--a huge imbalance.) Performance companies like DynoTech often use special fixtures to ensure extremely accurate U-joint phasing and centering.

The faster a driveshaft rotates, the greater the effect of any imbalance; this imbalance force increases as the square of rpm increases. In other words, as shaft rpm doubles, imbalance force increases four-fold, or if shaft speed triples, imbalance force increases by a factor of nine. The solution is to balance the driveshaft at high rpm. While many driveshafts are reportedly balanced at 1,800 to 2,800 rpm, DynoTech's are balanced at a minimum of 5,000 rpm.

Anyway, now that we know some of the theory, let's see the practical side as the folks at Livernois Motorsports install a DynoTech driveshaft on a customer's '67 Mustang fastback that has had recent power upgrades that dictated replacement of its original factory driveshaft.

DynoTech's Formula For a Strong, Straight Shaft
No matter how strong a driveshaft, if it's not extremely well built and balanced, its longevity--and your enjoyment--will suffer. Both materials and manufacturing processes are of critical importance. Michigan's DynoTech Engineering currently manufactures about 85 percent of the driveshafts used in NASCAR, and also acts as a consultant to Detroit's Big Three on driveline issues, so it must be doing something right...

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