Few things aggravate Corvette enthusiasts more than a driveline vibration. Vibration disturbs the balance of nature and the genuine smoothness of Corvette cruising. When a vibration surfaces and a peaceful road trip turns to mental gymnastics to determine where the vibration is coming from, it takes all of the fun out of a nice getaway.
Driveline vibration of any kind is evaluated by frequency, more specifically low frequency (shake) or high frequency (groan or buzz). Low frequency vibration is typically associated with the tires and wheels because they rotate at a lower speed than the engine and driveline. A higher frequency vibration is more likely related to the engine and driveline because these components spin at a high rate when you’re on the road cruising at speed.
C1 Corvettes have an articulating driveshaft with a solid rear axle. C2, C3 and C4 Corvettes employ a fixed driveshaft with independent rear suspension. C5 and newer Corvettes have typical driveline issues engineered out to where vibration is rarely an issue because of the enclosed driveshaft within a tube that runs the length of the driveline to a rear transaxle.
Because C2, C3 and C4 Corvettes are equipped with an independent rear suspension with a fixed differential bolted to the chassis, there is a fixed driveshaft mid-ship with two halfshafts located between the differential and the rear hubs. Because the halfshafts rotate at the same speed as the tires and wheels, their vibration frequency will be much lower than that of the driveshaft, transmission and engine.
If you’re running a vintage Turbo-Hydramatic or four-speed the driveshaft and engine turn at the same speed as the engine when you’re in final drive. When you notice a higher frequency vibration associated with the driveshaft most of the time it can be traced back to worn universal joints or the slip yoke at the transmission. The vibration can also be caused by excessively worn pinion bearings causing side play at the pinion and oscillation around the driveshaft’s centerline. There can also be issues with the transmission output shaft and its support system (bushings or bearings), which can also cause an oscillation around the driveshaft’s centerline.
If you’re running an overdrive transmission, the engine and driveshaft will be operating at different speeds, with the driveshaft spinning faster than the engine. In overdrive where engine speed and transmission output shaft speed differ, you can get into driveline harmonics that can be hard to trace.
Types of Driveline Vibration
To understand why you have driveline harmonics (vibration), pay close attention to when it happens: under acceleration, deceleration or coasting. Is there noise associated with the harmonics, such as a worn universal joint, wobbly slip yoke or bad pinion bearings? If there is vibration under acceleration, the chances are that you have worn universal joints. If you get harmonics during coasting and deceleration it indicates driveshaft oscillation because there’s either no load on the ’shaft or a load against the ’shaft.
There are several types of driveshaft vibration issues you should be aware of. According to Inland Empire Driveline Service, torsional vibration occurs around the centerline of the driveshaft at the driven end of the transmission yoke and at the universal joints at both ends of the ’shaft. Torsional vibration causes a driveshaft to “whip” from side to side, eventually weakening the ’shaft and its components. Catching the most stress from torsional vibration are the universal joints.
Think of torsional vibration as double trouble because the ’shaft vibrates twice in a single revolution. It whips every 180 degrees of rotation, or every halfway around. Over time the ’shaft and related components deteriorate, and it will only get worse.
One reason torsional vibration happens is the operating angle of the universal joints and yokes at each end of the ’shaft get out of spec. The transmission tailshaft and differential pinion must each have an operating angle of less than 3 degrees, yet within one degree of one another for smooth operation. The ends (trans tailshaft and diff pinion) need to be parallel to each other, or darned close to it. As long as both ends of the ’shaft are within 1 degree of each other, you should be in the clear.
Another reason for vibration could be because the tube and its ends are not perfectly aligned. Inland Empire Driveline Service can take your driveshaft or halfshafts and check them for proper runout to ascertain straightness. Unless the ’shaft is significantly distorted, Inland Empire Driveline Service can work the ’shaft with heat and mechanical manipulation to get it true. If distortion is beyond a predetermined amount, the ’shaft is beyond repair and should be replaced.
This type of imbalance is caused by an improper dynamic balancing of the driveshaft. Think of driveshaft dynamic imbalance like you would balancing a tire and wheel assembly. You’d never consider installing new tires without dynamic balancing. You wouldn’t have an engine built without dynamic balancing. Why should your driveshaft be any different? Dynamic balancing gets the shake out. Centrifugal force is always at work. And when everything is in balance a driveshaft works quite well. When one side of the ’shaft is a pinch heavier than the other, the heavier side takes control causing vibration, or shake, from side to side. The result is that “whipping” from side-to-side we were addressing earlier.
Another important aspect of transverse imbalance is dynamic balancing the ’shaft at the speeds in which it will be operating. Anything less isn’t getting the job done. A good balance job by Inland Empire Driveline Service demonstrates what the ’shaft will do at various speeds. It is important to remember ’shaft dynamics dictate what the ’shaft will do at different speeds. Vibration becomes more apparent at some speeds while operating quite smooth at other speeds.
There is another element known as “critical speed,” where a driveshaft can spin too fast for its length. This means the ’shaft is too long for its operational speed and becomes unstable. When the ’shaft reaches critical speed it can begin to whip, which makes it physically shorter (seriously) because the ends pull in as the center bows outward. Once this distortion reaches a given point, the ’shaft breaks where it becomes disconnected and winds up through the floorpan or in pieces all over the road.
If you have Inland Empire Driveline Service make a driveshaft or halfshaft, ask them to specify critical speed information. Critical speed is established when the ’shaft is made and balanced. Inland Empire Driveline Service will indicate critical speed to you when the ’shaft is made or when service is performed on an existing driveshaft.
Other Vibration Sources
Driveline vibration doesn’t come from just the driveshaft, but other elements between the engine’s crankshaft and the rear axle. Is your engine internally or externally balanced. If it is externally balanced do you have the correct flywheel or flexplate? Does the vibration occur with the vehicle in motion, at rest or both? If the vibration occurs with the vehicle at rest the source is likely the engine, flywheel, clutch, flexplate or the torque converter.
The main thing to remember when troubleshooting driveline vibration is to take the pursuit one element at a time until you find the source. Consider when the vibration occurs and take your search from there. Oftentimes, a vibration source will be very apparent. Otherwise, it’s a process of elimination and raw tenacity to find the fault and the fix. Vette
1. C2 and C3 Corvettes have three driveshaft assemblies: the driveshaft and two halfshafts, which are stubby driveshaft assemblies connecting the differential to the hubs and brakes. The halfshafts deliver power through conventional universal joints.
2. Here’s a typical C2/C3 halfshaft assembly consisting of two external clip universal joints and a four-bolt flange.
3. Halfshafts are straightforward to service, with a four-bolt flange on the hub end and a stub axle on the differential end.
4. Here’s another look at a halfshaft on a C2 Corvette roadster. The stub axle does not employ conventional U-bolts, but instead a cap, which keeps the inboard universal joint more secure in the stub axle.
5. Before you is the right-hand C3 halfshaft, stub axle, and flange from underneath. These halfshafts are straightforward to remove and service. Rarely do they cause vibration.
6. Here are the basic components that make up a typical driveline at each end of the driveshaft: slip yoke at the transmission, rear axle pinion yoke and external clip-style universal joints at each end.
7. Transmission slip yokes are available in a variety of strengths, sizes and number of splines. These are 1350 Spicer slip yokes in 27- and 32-spline. Most common in Corvettes are 1330 and 1350 sized universal joints.
8. Three basic types of driveshafts are available from Inland Empire Driveline Service (from left to right): steel, aluminum and carbon fiber. Steel is the most economical and surely the strongest choice, but it comes with a weight penalty. Aluminum costs not much more than steel and you benefit from weight loss, depending on expected horsepower and torque. Carbon fiber is expensive with the weight advantage. What’s more, carbon fiber looks cool.
9. The rear axle halfshafts take a tremendous amount of abuse because they’re in direct contact with the road. If you’re experiencing a shake at speed and all rolling stock has been cleared of defects, you could have worn universal joints or damaged halfshafts, which should be checked for runout and balance issues. Also check the stub shafts and flanges.
10. C2, C3 and C4 Corvettes have a conventional fixed position driveshaft between the transmission and differential. If you’re experiencing harmonics, determine when the harmonics occur. Does it happen during acceleration, coasting, or deceleration? Is there noise when the vibration occurs? Noise indicates universal joints, slip yoke or differential pinion bearings.
11. Here are two types of universal joints you can expect to find. On the left is a sealed-for-life Spicer 1350 universal joint. On the right is a 1350 with a grease zerk fitting. You get added strength from the sealed joint because the center is solid. You get serviceability from the greaseable zerk fitting but lose strength. The choice is yours.
12. Here are the two types of universal joints you can expect to find: internal and external clip. What you will see most is the external clip style found on the right. The external clip style provides security as long as the clips are firmly seated.
13. If you’re experiencing mid-to-high frequency vibration with the vehicle at rest, the problem is most likely engine and/or transmission related. The causes could be any number of things, including clutch failure, out-of-balance flywheel or flexplate, torque converter, a bent or broken transmission input shaft or a failed transmission input shaft bearing.
14. The harmonic damper can be another source of perceived driveline vibration. Over time, the rubber rings dry up and wear out, causing a vibration. When the vibration occurs with the vehicle stopped and the engine running, the harmonic damper is another potential source of vibration and is easy to replace.
15. Overdrive transmissions, be they automatic or manual, can create driveline harmonics all their own in overdrive gear, making them challenging to troubleshoot. Take your transmission out of overdrive and modulate vehicle speed. Do you still have vibration? Put it back in overdrive and vary vehicle speed. Does the vibration show up in overdrive? Perceived vibration can also be a resonance that comes with certain engine rpm ranges and vehicle speed.
16. Muncie and T-10 four-speed transmissions are easier to troubleshoot because they are 1:1 straight drive at speed. When you’re in Fourth gear at speed, the engine and output shaft are turning at the same speed. Try shifting into Third gear and see what the vibration dynamic is. If it becomes higher at the same speed, the vibration is coming from the flywheel, clutch or transmission input shaft. Also try putting the transmission in Neutral at speed with the clutch out. This is where you will feel driveshaft harmonics most.
17. This is a C2/C3 differential with the driveshaft and halfshafts removed. Differential vibration alone normally comes from the pinion bearings or the driveshaft. Faulty backlash can also cause its own vibration dynamic in cruise or coast. Halfshafts rarely cause vibration unless they were never dynamically balanced or they are damaged.
18. Pinion bearings require precision installation. If the bearing races are not fully seated in the case you can wind up with a cocked bearing race and pinion gear/driveshaft harmonics.
19. Driveshaft manufacture begins here with each end of the ’shaft. These are the starting points for a competition use, heavy-duty steel ’shaft with 1350 universal joints. Inland Empire Driveline Service will pull steel tube stock from the rack, cut the tube to specifications and make a driveshaft.
20. Rocky Maldonado of Inland Empire Driveline Service finds the appropriate sized steel tube and measures overall length short of the end yoke with a little extra to spare for fitment purposes.
21. The tube stock is cut to size based on the customer’s measurements on the order sheet.
22. The tube stock and end yokes are leveled and pressed together. Once together, the ends are tweaked to get the ’shaft perfectly straight prior to welding.
23. Before welding, the driveshaft is checked for runout at each end and in the middle. Runout can never be more than 0.010-inch.
24. Runout is checked here in the middle of the ’shaft, which requires careful manipulation of the end yokes to get it within specifications.
25. The driveshaft ends are precision welded on a press welder in constant rotation to achieve a perfect bead. The heat of the welding process can distort the ’shaft to some degree. Runout will be checked again during dynamic balancing.
26. The hot weld is quenched and cooled following the press welder.
27. Runout is checked after welding and before dynamic balancing. Most of the time the ’shaft is straight after welding, requiring very little manipulation.
28. When runout is excessive, heat is applied to the ’shaft as shown along with a cooling process to minimize driveshaft runout.
29. Dynamic balancing is next, eliminating any vibration. This is just like balancing a car tire and wheel. Inland Empire Driveline Service finds the lightest spot and adds weight as necessary to that spot.
30. A balance weight is attached to the driveshaft and welded in place. Runout is checked again along with dynamic balancing.
31. Differential pinion yokes come in three basic flavors depending on how you intend to operate your Corvette. From left to right are billet aluminum, billet steel and cast iron. Steel billet and cast iron are best for street/strip use.
32. Have you checked the driveline angle? The pinion and engine crankshaft angles must be the same or within 3 degrees of another. This applies mostly to C1 Corvettes and is rarely an issue with C2s and C3s. Vibration can become an issue when the driveline angle goes beyond 3 degrees. Ideally, you will have 0 degrees at both ends of the driveshaft.
Photos by Jim Smart, illustraition courtesy of Inland Empire Driveline Service