Quote:
Originally Posted by 4mul8ion
Setting aside the other design requirements, as far as axle noise is concerned, it takes three things to get axle noise. A source of the vibration to generate the noise, a path for the noise to take and a responder – or speaker – to make the noise audible to the driver.
The sources of the noise can come from the gear meshing but can also come from bearing faults as well. There are different mechanical vibration frequencies that can be the source of the noise depending on what is happening.
There are two paths for the noise to take and the associated speakers: through the suspension into the body that vibrates a body panel or forward into the driveshaft and that set off motions in the driveshaft that travels through the air into the vehicle cabin.
The classic gear noise that drives everyone crazy generally comes from vibrations generated by the first harmonic of the gear meshing that either travel through the suspension into the cabin or into the driveshaft and rings it like a bell. It generally occurs when either accelerating through the 45 – 65 MPH range or decelerating 65- 45 MPH. But not always.
Looking at the driveshaft path first, it is spinning hollow and is now commonly made out of aluminum with various OD’s and tube wall thicknesses depending on the manufacturer. Bare with me here – I’ll try to get to my point quick. It’s supported at one end at the axle and on the other end by the transmission. One end of it has to be able to translate in the vehicle fore/aft direction to allow the rear axle to move in the path dictated by the suspension. It has to take the engine torque, amplified by the transmission gear and sent it through to the rear axle through universal joints. The U joints are optimized to run an included angle between the transmission and driveshaft of no more than 3 degrees and, for best performance, an equal included angle of no more than 3 degrees to deliver smooth rotation from the transmission to the rear axle.
The point is that the driveshaft is allowed to move in its mounting and doesn’t deliver smooth power to the axle. The rotation accelerates and decelerates as the u joint rotates. The ring and pinion don’t mesh smoothly by themselves either. These accelerations / decelerations set up the gear mesh vibrations that forces the driveshaft into various resonant frequencies that you hear depending on the geometry of the tube and amplitude of the acceleration / deceleration of the rotation. The OD, wall thickness and length for a given material determines the bending frequencies and shell modes – or bell ringing – frequencies and the torsional frequencies of the driveline. Changing these dimensions move the resonant frequencies up or down in vehicle speed. Naturally, the OEMs try to increase these frequencies to mask the gear noise by other noises like wind or engine noises that aren’t so annoying or damp out the noise using tuned torsional dampers or inserts in the drive shaft tube. Neither eliminates the source of noise – it just moves the energy around. No free lunches.
The later 4th gen f-body driveshaft had an aluminum driveshaft with a larger OD and had both a tuned torsional damper that was pressed onto the transmission side slip yoke to damp out the torsional resonant frequencies. If you were to tap your knuckle along the axis of the driveshaft, there’ll be spots that will ring and spots that will go thud. Inside the tube, there are inserts to damp out the bending and bell modes of the driveshaft.
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Lot's of good thoughts here. OEM's spend millions of dollars making their parts mesh for the consumer. We slap some aftermarket parts on our 40 year old cars and wonder why it doesn't work out as we'd hoped. This is exactly where I was going with my original post. Four sets of gears and the noise is still there. It's time to start looking elsewhere for a solution.
I thought my Moser 12 bolt, 33 spline, true trac, with bolt in axles was quiet but to be honest, my exhaust was so loud who the hell knows. I will say that there was always a vibration in high gear in only a 300 rpm range that I could never completely get rid of the life of the car. I swapped out engine mounts, trans mounts, changed driveline angles. I finally just gave up as it was at like 85-90 anyway. I ended up with a rubber passenger mount and that made the biggest difference.
Greg, I told you not to put that POS TKO600 in your car. Nobody every listens.
