PERFORMANCE DRIVESHAFT TECH

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Godfrey Russell
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PERFORMANCE DRIVESHAFT TECH A properly selected and set up driveshaft can safely and smoothly deliver all of a performance engine s horsepower and torque to the rear wheels, while the wrong

The driveshaft represents more than a simple connection between output power and final-drive.

There s an array of driveshaft materials from which to choose.

1 PERFORMANCE DRIVESHAFT TECH A properly selected and set up driveshaft can safely and smoothly deliver all of a performance engine s horsepower and torque to the rear wheels, while the wrong driveshaft can cause a number of problems. The driveshaft in any highperformance (street or race) application is a critical component that s too often taken for granted. The driveshaft represents more than a simple connection between output power and final-drive. In this article, we ll briefly discuss driveshaft-related topics, including shaft material choices and tips on determining and curing vibrational issues. There s an array of driveshaft materials from which to choose. Although mild steel tubing has been the traditional choice for decades, today s offerings for high-performance applications have expanded, including such materials as high-strength aluminum, chromoly and carbon fiber. Mild steel shafts, made from seamless DOM (drawn-over-mandrel) tubing, remain popular and are perfectly applicable to any restoration/custom/street rod target. This type of construction is the least expensive, typically averaging $150 to $300 or so. Aluminum shafts, by the nature of the alloy, offer lighter weight, which is a factor in competition applications. The nice thing about aluminum shafts for street rod or custom builds is that they can take a full polish for the sake of appearance. Aluminum shafts are typically made from 6061-T6 DOM, with aluminum tube yokes. Prices range from $300 to $400 and higher. Yokes are traditionally MIG-welded to the shafts. Mark Williams Enterprises has introduced a unique Accu- Bond process that adhesive-bonds the yokes to the shaft with a patented process that the company claims is 50% stronger than welding, and is similar to the method used for certain aerospace applications. Chromoly is a bit lighter in weight than mild steel, with extra strength Mild steel driveshafts are suitable for most street-vehicle applications. For performance use, consider a tubing wall thickness for DOM (drawn-over-mandrel) of.083 in. Steel shafts are typically offered in diameters ranging from 2 to 3.5 in. Chromoly performance shafts usually feature.083-in. wall thickness and are lighter but offer high strength. Aluminum driveshafts offer lighter weight (a factor in competition) and a slightly reduced harmonic frequency. They re typically made from 6061-T6 tubing featuring a.125-in. wall thickness. Aluminum shafts are generally offered in the 3- to 4-in.-diameter range. Photos courtesy Inland Empire JIM GIBSON 20

3:34 PM Page 2 as much as around $1500, depending on diameter, length and type of ends. These different materials provide the choices, depending on the customer s application and budget.

For transmissions that feature an overdrive (as many of today s OE and performance aftermarket transmissions provide), use of a lighter weight aluminum or carbon fiber shaft material is highly

is a good choice for high-horsepower applications in the 1000-plus range. Highperformance chromoly shafts are generally rated for around 2800 ft.-lbs. of torque. Prices float in the $400 range.

2 3:34 PM Page 2 as much as around $1500, depending on diameter, length and type of ends. These different materials provide the choices, depending on the customer s application and budget. Of course, if you re performing a true restoration, a mild steel-as-original will likely be your only choice. For transmissions that feature an overdrive (as many of today s OE and performance aftermarket transmissions provide), use of a lighter weight aluminum or carbon fiber shaft material is highly recommended, because the overdrive portion is much more sensitive to frequency inputs created by U-joint operating an- Photos courtesy Mark Williams Enterprises (usually made from 4130 tubing), and is a good choice for high-horsepower applications in the 1000-plus range. Highperformance chromoly shafts are generally rated for around 2800 ft.-lbs. of torque. Prices float in the $400 range. Carbon fiber driveshafts offer substantial weight savings typically 50% or more. For a typical street application, the weight savings isn t as critical a factor as it might be in a competition application. However, carbon fiber does offer superior vibration damping, since the woven and wrapped material is able to absorb more torsional force. Arguably, the most significant aspect Mark Williams Enterprises has developed a special Accu-Bond process to secure yokes to aluminum shafts (left) that s reportedly 50% stronger than welds. Carbon fiber shafts (right) are considerably lighter, while offering increased safety, since in the event of a failure, a carbon fiber shaft will disintegrate instead of punching through the floor. Circle #14 22 of this material relates to safety. In the event of a failure, an unleashed, wildly spinning metal shaft can wreak havoc on the underside of the vehicle, and can, in worst-case scenarios, rip through the floor or drop and contact the road, which could transmit force to the drive axle and play havoc with the chassis. If a carbon fiber shaft fails (failed U-joint or yoke), it shreds and tends to disintegrate, eliminating the catastrophic damage that might otherwise occur. Carbon fiber shafts are made by wrapping the fiber in specific angles with torsional stress in mind. Depending on the specific design, carbon fiber shafts may be torque-tested up to a whopping 3900 ft.-lbs. They re light and strong, but you pay for it to the tune of about $600 to gles. A lighter weight shaft (i.e., less mass) reduces the energy that s fed back to the transmission, transferring less harmonics. Driveshaft Balance Driveshaft balance is critical, since it s subject to radial forces (and a degree of axial forces due to shaft angle) during operation. A new performance driveshaft should already be balanced by the maker. If you re having a shaft built by a local shop, make sure they balance it. Even if the shaft has minimal imbalance, a vibration at a certain speed range can often fool you into thinking you have a driveline angle problem, so eliminate the balance variable at the outset. If the driveshaft features excessive runout, it won t be possible to achieve

3:35 PM Page 3 Illustration courtesy Mark Williams Enterprises Driveshaft Length 02 01 U-joint operating angles, shown in this side-view illustration, should be 3 maximum, with 2 considered ideal.

Driveshaft Angle/Phasing The angles of the front and rear Ujoints must be equal (ideally within 1 of each other).

If the U-joints centerlines are too severely Engine 3 Down Rear End 3 Up Driveshaft 0 angled, the U-joints will operate at different velocities, which will create a vibrational problem.

3 3:35 PM Page 3 Illustration courtesy Mark Williams Enterprises Driveshaft Length U-joint operating angles, shown in this side-view illustration, should be 3 maximum, with 2 considered ideal. The front and rear U-joint angles (here labeled 01 and 02, respectively) should be equal to each other. Note: In this illustration, all angles have been exaggerated. Driveshaft Angle/Phasing The angles of the front and rear Ujoints must be equal (ideally within 1 of each other). U-joints are designed to individually handle an angle of 1 to 3 (2 is considered optimal for street use). If the U-joints centerlines are too severely Engine 3 Down Rear End 3 Up Driveshaft 0 angled, the U-joints will operate at different velocities, which will create a vibrational problem. You cannot measure shaft or joint angles with the suspension unloaded. The vehicle must be at normal ride height and with the suspension compressed (as normally loaded). With the vehicle on a drive-on hoist, measure the angle of the engine/transmission and the angle of the pinion. Using an inclinometer/protractor or a digital level (a digital Spicer Anglemaster is a good example), check the installed angle of the engine at a machined surface such as the starter mounting boss (or the block s oil pan rail, if the oil pan is removed). Check pinion angle at the machined surface where the U-bolt holes are drilled. These two angles should be equal and opposite (e.g., engine angle down and pinion angle up). Next, check the angle of the installed driveshaft tube, then subtract the shaft tube angle. Ideally, the resulting joint angle should be 3 or less. Joint phasing refers to the clock position of the front and rear U-joints in relation to each other. The positions of the joint bearing cap eyelets at each end of the shaft should be in-line. For exam- Driveshaft Level Working Angle A = 3-0 = 3 Working Angle B = 3-0 = 3 Uncancelled Angle = 0 A B Circle #16 24 Transmission output shaft-to-driveshaft and driveshaft-to-pinion shaft angles are compared to determine joint angles. Ideally, these angles should be within 1 of each other. Illustration courtesy Inland Empire balance. Check for proper yoke-to-shaft centering at the front and rear. It s possible the yokes may have been slightly off-center when welded to the shaft. Most replacement driveshaft shops are accustomed to building and servicing driveshafts for heavy equipment, such as dump trucks and the like. Their driveshaft balancers may be designed to spin the shaft at around only 1500 rpm. While this may be acceptable for a vehicle that will be driven only at street-legal speeds, for any application where increased torque and engine speed is expected, the shaft should preferably be balanced at the anticipated shaft speed. If a low-rpm balance is performed (at, say, 1500 rpm), a high-horsepower/highspeed application may experience a vibration at speed. Again, if at all possible, the driveshaft for a high-performance application should be balanced at the anticipated engine speed (e.g., 6000 rpm).

3:36 PM ple, if you place a rod through the eyelets at the front of the shaft, and one through the eyelets at the rear of the shaft, the two rods should be in-line and parallel when viewed from the

when new, and the retaining tabs of the yoke are subject to wear over time and may loosen. To correct this condition, replace the worn or off-center pinion yoke or pinion flange.

4 3:36 PM ple, if you place a rod through the eyelets at the front of the shaft, and one through the eyelets at the rear of the shaft, the two rods should be in-line and parallel when viewed from the front or rear of the shaft. If an out-of-phase condition exists, this will promote vibration. Driveshaft Length & Runout much as.005 in. when new, and the retaining tabs of the yoke are subject to wear over time and may loosen. To correct this condition, replace the worn or off-center pinion yoke or pinion flange. It s important to note that shaft length is a factor in determining tube diameter. As shaft length increases, the diameter of the tube must also increase. For example, a 3-in.-diameter shaft may be acceptable for a Mustang, Dart or Camaro application, but a longer wheelbase vehicle will likely require a larger diameter shaft. Rather than guess, discuss shaft diameter recommendations with the aftermarket performance shaft maker. Determining the Source of Vibration Naturally, the wheel/tire assemblies need to be properly balanced. In some cases, certain tires may exhibit a phenomenon known as radial force variation, where a portion of the tire con- Photo: Jim Gibson With the vehicle at normal ride height (suspension loaded, do not support the vehicle on stands with the wheels hanging) and the shaft disconnected from the pinion yoke, push the shaft forward until it bottoms out into the transmission. There should be enough clearance to drop the rear shaft U-joint past the rear yoke by hand. As much as 3 8 in. clearance is acceptable. If you have more clearance, the shaft is too short; if you can t easily drop the rear of the shaft free of the pinion yoke by hand, the shaft is too long. Either condition Page 4 The Spicer Anglemaster digital level gauge can be quickly zeroed at any given angle, with the following measurement providing an angle difference between the zeroed index and the current angle. The gauge features a V-channel base for accurate resting on a driveshaft tube. Angle measurements are provided to within.1. will cause vibration. If the shaft length is modified, it must be rebalanced. With the vehicle on a lift, check shaft runout at three locations the center of the shaft length and within 2 in. of each weld (front and rear). The official (Spicer) suggested tolerance is.010 in. maximum allowable runout at the ends and.015 in. maximum allowable runout at the shaft center. Actually, you should shoot for tighter tolerances of.008 in. at the ends and.010 in. at the center. The less runout, the better. If runout is excessive, the shaft must be replaced. If runout is found at the pinion end, remove the shaft from the yoke, rotate it 180 and reinstall, then perform another runout check. Driveshaft runout should either improve or worsen, since splined yokes can be off-center by as struction features an inconsistent construction thickness and/or carcass hardness area that may not be revealed during conventional dynamic balancing. A noticeable radial force variation problem won t show up until rolling load is placed on the tire. There s much more involved in this subject, but if you suspect this condition, have the tire/wheel packages checked on a road force machine. A cheap and quick alternative is to try another set of wheels and tires during a test drive. If the vibration disappears, you ve isolated the problem to the original tire/wheel package. If you encounter a mystery driveline vibration, consider temporarily removing the driveshaft from the host of variables. With the vehicle on a lift, remove the driveshaft. Place a cap or plug Circle #27 25

3:37 PM Driveshaft Sources ACPT www.acpt.com Denny s Driveshafts www.dennysdriveshafts.com Inland Empire Driveline Service www.ieds.com Mark Williams Enterprises www.markwilliams.com Spicer www.

Suspect causes may include flywheel or flexplate imbalance, an incorrect weighted crankshaft damper (for engines where the crankshaft is externally balanced), an improperly installed converter,

5 3:37 PM Driveshaft Sources ACPT Denny s Driveshafts Inland Empire Driveline Service Mark Williams Enterprises Spicer at the transmission output and operate the engine and transmission. If you still feel a vibration, the driveshaft isn t the problem. Suspect causes may include flywheel or flexplate imbalance, an incorrect weighted crankshaft damper (for engines where the crankshaft is externally balanced), an improperly installed converter, off-center engineblock-to-transmission mounting or transmission shaft damage. If the vibration is eliminated with the Page 5 driveshaft removed, check for driveshaft joint angles, driveshaft runout and balance. If the engine has recently been built/rebuilt, talk to the customer about the build. If the engine was assembled by a novice who didn t have the crankshaft balanced, this could very well be the reason. If the customer has altered the angle of the transmission output shaft for any reason, don t focus your efforts only on the driveline. If the transmission now angles further downward, for instance, the engine oil level in the pan will change its surface angle. Depending on whether the engine oil pan is front-sump or rearsump, and on where the dipstick is located, you can end up with erroneous oil level checks on the dipstick. In severe cases, you might also starve the engine of oil, depending on the sump and oil pump pickup locations. By the same token, if you fiddle with rear axle pinion angle, you need to verify where the proper axle lube level needs to be to prevent input shaft bearing damage. This is an often-ignored variable. Circle #24 Photo courtesy Denny s Driveshafts This is an example of a 3.5-in.-diameter aluminum shaft from Denny s Driveshafts. Today s high-performance shafts that are intended for high-horsepower applications should be balanced for actual operating speed. Older balancing equipment used in local truck driveline shops may be able to perform a balance at only 1500 rpm, which is not suitable for today s high-performance street rod and custom car applications. Vibration-Chasing Tips If the vehicle experiences a vibration only during acceleration, try adjusting the pinion angle slightly downward (by about.5 ), which will help compensate for axle wrap-up (on a rear-drive vehicle). If vibration is felt only during deceleration, try adjusting the pinion angle slightly upward to compensate for pinion plunge. Overtightening the U-bolts at the joints can easily distort the bearing shells, resulting in bearing drag, vibration and bearing failure. According to Inland Empire, proper torque for 11 16and 11 8-in. caps is 14 to 17 ft.-lbs., and 20 ft.-lbs. for in. cap diameter. If you can t access the nuts with a torque wrench, Inland suggests using an approximation by tightening until a new lock washer flattens out, plus an additional eighth of a turn. Also, pay close attention to the fit of the U-joint bearing caps as they engage the pinion yoke. The fit should be snug. If there s a gap between the caps and sides, the caps are too small. If you find a gap between the caps and base of the yoke bearing seat, the caps are too large in diameter. Once fully seated in the yoke, the correct cap diameter should result in no gap whatsoever. This article can be found online at 26

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