1965 Malibu Driveshaft - Bad Vibrations

Lincoln stands have rack bars made of investment cast-iron rather than more common, ductile iron, so they have an safety margin I like. While the 6-tonners hold more than the car's weight, I wanted their 25-inch height for extra working space under the back of the car.

Lincoln stands have rack bars made of investment cast-iron rather than more common, ductil

Chevrolets.We love 'em, right?

Well...not always; sometimes, they make us crazy, cost too much money, cause wives/girlfriends to leave, and even (sharp intake of breath) get us thinking about selling them.

My '65 Malibu is like that. Okay, my girlfriend didn't split on me, but it was no secret: she didn't care too much for "that ugly blue thing."

"Sweetheart, how can you say that about the 'Blue Bullet?'"
"Oh puh-lease. Bullet?! More like a slug-not even that, really, because slugs move. It doesn't."
Ouch. Tough crowd.

Of late, my Malibu has spent a lot of time in the garage. A while back, it developed a vibration I could feel through the seat-often a sign of a driveline or rear-wheel problem. By 70 mph, it blurred the rearview mirror and it excited parts of the car's structure to buzz like angry bees. This vibration limited speed to about 65, making my Malibu anything but a hot rod.

Here's the setup for measuring yoke free-play. Limited space around the tailhousing is a challenge when setting up a solid mounting for a dial indicator. We used a magnetic base on the car's center crossmember.

Here's the setup for measuring yoke free-play. Limited space around the tailhousing is a c

Strange vibrations and weird noises can be difficult problems for DIY enthusiasts to fix. My friend, Gary Peterson, and I chased my Malibu's vibration for a long time. We eventually solved it, but getting there was an ordeal.

The Chase Begins
First step: Rule out tire balance. I went to Tucker Tire Sales in Covina, California, and had the car's Goodyear Eagle F1 GSes balanced. A road test showed the vibration unchanged.

Next: Rule out rear brake rotors. For this, we needed the car in the air with the rear suspension at ride height. I use a Lincoln Automotive, 2-ton floor jack, because of its low pad height, 20-inch lift ability, and robust construction, as well as Lincoln jack stands. We set the front lower control arms on 2-ton stands and the rear axle housing on a couple of 6-ton stands.

We spun the rear wheels at 65 mph and felt the vibration. We did the same with the rear tires and brake rotors off, and the vibration was unchanged. We pulled the driveshaft and had its balance checked by Inland Empire Driveline Service in Ontario, California. Inland's Jeff Gilroy reported no problem.

A dial caliper demonstrates the problem with the first yoke. Splines are measured two ways: major diameter, between tops of the splines on the shaft, and minor diameter, between the bottom of the splines on the shaft. The nearly .020-inch difference between the major diameters is why the first yoke was loose.

A dial caliper demonstrates the problem with the first yoke. Splines are measured two ways

Suspecting a bad bearing, we sent the rearend to Tom's Differentials, which replaced all the bearings and shipped it back. The car still vibrated. We sent the axle to Tom's, again-250 miles later-to check for bent axles, which can also cause vibration, and they sold me a second set of new bearings and new axles. Later, I found there was nothing wrong with the original axles. After having the rearend rebuilt twice, up on the Lincoln stands at 65 mph, the vibration persisted. Frustration and expense were building.

In an under-car inspection of anything that moves, my friend, Gary, noticed the front yoke was loose on the transmission output shaft splines. I set up my dial indicator, and sure enough, free-play measured .008 inch. I pulled the driveshaft and temporarily installed a new yoke from Clippinger Chevrolet in Covina, California. It moved only .0015 inch.

Measurement of the two yokes showed their spines had the same minor diameter, 1.310 inches, but different major diameters, 1.395 for the bad yoke and 1.376 for new unit. We learned the .019-inch difference is a common, but sometimes undetected, problem for those using transmissions with the 32-spline output, such as Turbo Hydramatic 400s, 4L80-Es, later Muncie four-speeds, the ZF S6-40, and some aftermarket manual transmissions. That much free-play will cause a driveline vibration, and I thought we might be on to the problem. Unfortunately, we'll never know for sure, because we couldn't test the original driveshaft with the tight yoke.

Inland Driveline's Jeff Gilroy offers the results of driveshaft failure due to operation at critical speed.

Inland Driveline's Jeff Gilroy offers the results of driveshaft failure due to operation a

Driveshaft Doin's
Two problems prompted me to install a new driveshaft. The tighter yoke requires a larger U-joint than used by my stock shaft. Researching information provided by Bob Parquet, manager of education for Dana Corporation's Spicer Driveshaft Group, I learned that since modifications increased my car's top speed, my existing, OE-type, 3-inch diameter, 57 13/16-inch-long, steel driveshaft had a reliability problem unrelated to vibration.

Anything made of elastic material-and metals are elastic-has a resonant bending frequency. When a rotating shaft's speed approaches its resonant frequency, the shaft begins to vibrate as if it were unbalanced. If allowed to spin for very long in that speed range, known as "critical speed," the shaft will fail catastrophically. Don't be in the car when that happens.

To build a driveshaft, manufacturers need to know the distance between the transmission and the rear axle. There are a couple of different ways to measure this, and it is important to measure the way the driveshaft service suggests. Inland Driveline wants the distance between the pinion yoke.and the rear surface on the transmission extension housing. A tape measure is adequate, provided it can be stretched tight between the trans and axle.

To build a driveshaft, manufacturers need to know the distance between the transmission an

Another noteworthy industry standard is "safe operating speed," defined as 75 percent of critical speed. Engineers (along with corporate risk management staffs, we figure) believe a driveshaft can run continuously at safe operating speed and be unaffected by resonant frequencies. Critical and safe operating speeds are a function of a shaft's length, diameter, stiffness, and tubing wall thickness. Using a calculator on the Spicer Driveshaft Web site (www.dana.com) I found that the safe operating speed of my Malibu's shaft was 4,370 rpm. Dividing that by 0.75, I got a critical speed of 5,827 rpm. A slide rule version of this calculator is available at many Spicer dealers.

For a given driveshaft speed, vehicle speed is a function of axle ratio (in this case, 3.73:1) and tire diameter (255/50ZR16 F1 GS at 26.06 inches). Using the formula: speed = (tire diameter x driveshaft rpm x .002975) (axle ratio), I determined at the shaft's critical speed, vehicle speed would be 120 mph. As I'd seen 120 a few times, it was a miracle my Malibu's driveshaft hadn't blown right through the floor.

To eliminate this danger, Inland Driveline built us a 57.25x3.5-inch driveshaft out of .120-wall aluminum tubing and fitted it with larger Spicer U-joints (PN 5-447X). Our new shaft's critical speed is 6,976 rpm, or 145 mph, and its safe operating speed is 5,232 rpm, or 109 mph. Though it was larger, as it was made of aluminum, there'd be no weight penalty. Inland machined the TH400 yoke to the correct length for the Malibu's transmission, a transplanted ZF S6-40 six-speed out of a '92 Corvette, and installed it on the new shaft.

The measurements customers make are transferred to the driveshaft fabrication process. Here, Inland's Tom Aragon verifies the length of our new shaft just before he fires up the welder.

The measurements customers make are transferred to the driveshaft fabrication process. Her

Bent Housing
We pulled the rearend a third time to install an Inland Driveline pinion yoke which fits the larger U-joints, so new suspicions I had about the axle's effect on our vibration could also be addressed. After the second rebuild, pinion rotation felt rough, as if there was a bad bearing. Tom's Differentials told me to drive the car 500 miles, and if the trouble persisted, call again. I believed there was a problem right then-time to try a different service.

Jim Cook at Performance Differentials in Ontario, California, tore down the axle. As I suspected, the rear pinion bearing was damaged due to either incorrect preload or improper installation of the bearing race. Worse yet, Cook found my axle housing was bent by .125 inch, which could cause a vibration.

Cook straightened the housing and installed new pinion bearings and the new pinion yoke. We put the rear back in the car, filled it with Red Line Heavy Shockproof Gear Lubricant, installed the new driveshaft, and went for a road test. I didn't get very far when, crossing a railroad track, I heard a knock on the floor.

Aragon heliarc welds driveshafts on a machine specially designed and built by Inland Driveline.

Aragon heliarc welds driveshafts on a machine specially designed and built by Inland Drive

Early Chevelles have a stiffening rib running across the body, under the floor, just aft of the front U-joint. As we'd lost 1/4 inch of clearance between that rib and the driveshaft when we upgraded to the 3.5-inch unit, the additional 1/8 inch of the front balancing weight protruded, and the weight was nicking the rib when the powertrain moved after the car hit a bump. I took the shaft back to Inland to have the weight moved. I was ready for a longer road test. Long story, short: the darn vibration was still there. Was I ready to give in? Nope.

Transmission Trauma
That we'd been on this job for nearly six months and had either repaired or replaced everything from the transmission yoke back without solving the vibration started me thinking transmission; maybe it had a bad output shaft bearing.

We pulled the Malibu's ZF and I took it to ZR51 Performance in Cave Creek, Arizona, outside of Phoenix, which is approved by ZF Industries to do overhauls on '89-96 Corvette six-speeds.

Here are the old yoke (left) and the "tight" TH400 yoke. It's easy to see the difference in size of the required U-joint. We did a little deburring work, then marked it for Inland Driveline service to cut.

Here are the old yoke (left) and the "tight" TH400 yoke. It's easy to see the difference i

Upon removal and cleaning of the transmission's five bearings, ZR51 Performance's owner, Bill Boudreau, did not find a problem. Though it wasn't necessary, I had Boudreau install a new set of genuine ZF Industries bearings (PN 1052BRGK) and a seal kit (PN 1052RBK) we ordered from one of the better sources for ZF six-speed parts, Rockland Standard Gear.

A week later, with the transmission back in car, we took a road test. The vibration was still there. Rats.

The Right Angle
Most driveline vibrations are either one excitation per revolution, typically coming from imbalance or run out, or two excitations per revolution, which is usually caused by a problem with universal joint angles.

I began to wonder about the car's universal joint angles. Gary and I found that the car's rear upper control arms were not Chevrolet items. They were stamped "PONT" which I figured was an abbreviation for Pontiac.

Not having a set of stock Chevy arms as a standard and figuring these Pontiac arms could be of a different length, which would change the rear U-joint angle, I ordered a new set of reproduction Chevrolet arms (PN AU64RP) from Year One. I had them reinforced by Global West Suspension, painted them with Eastwood's Chassis Black paint (PN 10025Z), and installed them. The U-joint angle was unchanged. Several months later, I found out by checking the angles on a bone-stock early Chevelle at Auto Craft, a shop in Rialto, California, that specializes in restorations, the rear U-joint angle on my Malibu, 2.8 degrees, was nearly the same as the OE 3.0 degrees I measured at Auto Craft.

Thanks to the generosity of the Autocraft guys in letting me crawl around on their floor for a while measuring the driveshaft angles of their Chevelle with a Dana AngleMaster II, I found the vibration was not being caused by a driveline angle problem.

Well, it was nice to know what wasn't the problem, but that's really all I had at this point: a lot of potential causes that were not causing the trouble.

I'm going to keep beatin' on this problem until it's fixed because I believe persistence always pays off. Next month, in Part Two, you'll find out whether it pays off or not.