Low-Speed v. High-Speed Driveshaft Vibrations
Low-speed vibrations (typically categorized by under 30 mph) can be as a result of mechanical issues such as:
High-speed vibrations can be a result of:
Very high-speed vibrations can be a result of:
Approaching Critical Speed
So, let’s discuss these in a bit more detail.
Phasing is a term that describes the alignment of the single-cardan joints on opposite ends of the drive shaft. A single-cardan (or u-) joint doesn’t rotate at a constant velocity if the operating angle is non-zero. The driveshaft speeds up and slows down slightly as it rotates, due to the nature of the joint. The joints at each end of the driveshaft should be properly aligned. When the yokes on each end of the shaft line up with each other (as seen indicated by the light blue line in the figure below),
the affect will be that the two joints tend to cancel out the speed variations from each other. (In most 4×4 applications, the driveshaft will have a slip yoke in the middle to allow for changes in length.) Phasing, then, is the speed variation of the joint, related to its operating angle and angle of rotation. In order to get the most effective cancellation, the joint yokes “must” be aligned exactly with each other and the operating angles “must” be identical. Any variation in either angle will show up as an un-cancelled vibration. While unequal operating angles result in a vibration that increases with shaft RPM, phasing problems may be felt at lower RPMs and higher loads, like when accelerating from a stop.
For a double cardan driveshaft, phasing is not an issue, although you may want to try and line up the bearing caps anyway.
A frequently asked question is about driveshafts and angles: “How much shim do I need for X” of lift” or “Is Y° shim too much?”. Honestly, there’s no general answer to these general questions, rather the right answer is what works for that particular situation. For example, assuming that the driveshaft is aligned properly in a vehicle with stock suspension, if it’s lifted with a block or spring lift, then everything should still be lined up, at least with a single-cardan driveshaft. It’s like a parallelogram, the angles change, but the sides remain parallel. So the correct answer for how much to shim an axle to correct the driveshaft angles depends on how far off the angle is to begin with.
The issue of straightness as a cause of driveshaft vibration is either due to a new shaft not being built straight or a used shaft being bent.
- You’d need a dial indicator to measure the radial runout, which would probably best be done at a drive line shop, because after straightening, the shaft will likely need to be balanced as well.
- It could also be due to a u-joint not being installed properly or a driveshaft flange not fitting properly, causing the shaft to not line up with the output shaft or pinion gear.
- And, lastly, realize that driveshafts are typically straightened by use of heat and cold. The mere process involving the heat/cold scenario is typically best served and dealt with by a driveline shop, due to the equipment used and knowledge of the instrument required.
Much like tires on a vehicle, driveshafts require weights to balance them, too, which are usually welded on. An imbalanced driveshaft is typically the result of a dented or bent shaft, throwing the mass off-center and causing vibration. This is more common in rear-wheel, all-wheel, or four-wheel drive vehicles, while less common in front-wheel drive. Technically part of the suspension since it moves the wheels, the driveshaft isn’t always considered as such. A driveshaft vibration can mimic typical suspension issues. These vibrations may also come and go, due to the speed as you move in and out of resonant frequencies of the driveshaft.
Vibrations at very high speeds may be due to approaching the drive shaft critical speed, which is (essentially) the resonant point where the shaft begins to whip and vibrate violently. If this occurs, only a new driveshaft design will help — usually making a call or trip to the driveline shop a necessity. That being said, these sorts of RPM limits are usually up in the 8000-10,000 range — normally reserved for the race vehicle variety.
Vibrations that get worse when transitioning from acceleration to coasting to deceleration (like when you back-off the gas over the top of a hill and before you go into ful- on engine braking) may be due to loose or worn parts.
Vibrations that get worse going uphill or accelerating at a greater speed than when going the same speed on the flat road or downhill (or vice versa) may be due to an alignment issue.