trying to troubleshoot vibrations while on jackstands?
 

So ive had some driveline vibrations of varying sorts for some time now. Ive replaced a few parts, but just havn't been able to figure out what the real issue is. MY new idea is to put the rear up on stands and run the car in gear around the rpms im getting the vibrations at while inspecting under the car for any obvious signs of oscillations or vibrating parts.

Obviously it's a 2 person plus job, but i just cant figure out how to troubleshoot this any other way. I know some of it is just normal roughness being a G37, but some of it is acceleration based under hard acceleration and some of it is lash out oscillations when slowing. I just can't tell if its a bad u joint, worn out tranny mount, out of balance driveshaft, or something else connected to the driveline. Ive done manual inspections of all the components and nothing looks bad, rusted, or obviously worn. The only thing that i could remotely say could be an issue is one of the diff mount bushings that was installed by a shop doesnt look fully set like the other side. I just keep forgetting to get under and try tourqing it down to see if its loose.

thanks

By "two person job" do you mean having someone *UNDER* the car while it is in gear, running at speed on jack stands?

I don't know if you will find the vibration, but there is a good chance you will find a Darwin Award.

Oh yeah i hear you on that one! I mean one "driving" and the other trying to look under without being under. I have a web cam with a long wire that i thought might work on a stick. the only reason i even thought it was a possibility was a bmw dealer told me they had my other car running on the lift trying to troubleshoot axle issues. Im just not wealthy enough to throw money at this problem and im growing more concerned about some sort of driveline explosion with the entire car vibrating like this.

If all tires are off the ground, you can put it in gear and let it idle, and the drivetrain components will turn. In 6th, you would be turning the driveshaft and tires around 20mph. But you could leave it in gear, and step outside of the car and look under. But if it's a busing, it would probably need to be at a specific speed to get the harmonic vibration. Or you might need someone revving it, to torque the driveline to see the movement.

By removing the load on the suspension you change the half shaft (diff to wheel axle) angle and that may prevent the vibration for happening. Also you won't be able to place the same load on the drive line.


There are vibration analysis apps for your phone that you can use to determine the frequency (and rotational speed) of the vibration.


Questions:
-Auto or manual tranny?
-what year & milage?
-Does the vibration happen when crusing on the highway?
-or just at low speed when full throttle is applied?