I have the same 60-65 mph vibration heard about frequently on the web. I've spent a fair amount of time playing with it and although I haven't fully tested my theory, I have one which the evidence supports.
Those of you who took (and remember) physics or dynamic control, will appreciate this. Any mass (weight) connected to a spring with minimal damping (underdamped system) will have a natural frequency and is governed (in a truely linear system) by the relation f=sqrt(k/m), where k is the spring rate and m is the mass. If set in motion it will continue to vibrate at that frequency until the energy is damped out of it by friction. Also this system can be induced to vibrate at a significant magnitude with only the slightest vibration driving it (a seperate source of vibration connected to the spring/mass in some way).
The motor/transmission is a mass and the rubber motor mounts are a effectively a spring (although not quite a linear system). It will have some kind of a natural frequency associated with it. Any tire imbalance or out of round at the right speed would become a driving vibration at the right speed. For stock tires (roughly 28.8" dia by the math, I haven't measured one) it apparently happens at 65 mph or 12.61 Hz. For 31' it would happen at 70 mph and for 33's it would happen at 74 mph. Does anyone have large (or small) tires and can verify this?
One of the things I've done to test this, is chain the motor to the frame, effectively increasing the spring rate and thus the vibration frequency. I didn't do a good enough job, because the chains went straight up and down allowing vibration side to side. The effect was that it changed the direction of vibration and could definately be felt. The whole dash shimmied side to side at speed. This adds credence to my theory that the motor is resonating and not some suspension component.
I also changed my motor mounts to something that felt a little stiffer. The frequency went up a little. I also notice that on cold days, it doesn't do it till the truck gets well warmed up.
The problem is wearing on me so I'll likely tinker some more in the near future. In the mean time I'd be interested to hear anything that has bearing on my theory.
Those of you who took (and remember) physics or dynamic control, will appreciate this. Any mass (weight) connected to a spring with minimal damping (underdamped system) will have a natural frequency and is governed (in a truely linear system) by the relation f=sqrt(k/m), where k is the spring rate and m is the mass. If set in motion it will continue to vibrate at that frequency until the energy is damped out of it by friction. Also this system can be induced to vibrate at a significant magnitude with only the slightest vibration driving it (a seperate source of vibration connected to the spring/mass in some way).
The motor/transmission is a mass and the rubber motor mounts are a effectively a spring (although not quite a linear system). It will have some kind of a natural frequency associated with it. Any tire imbalance or out of round at the right speed would become a driving vibration at the right speed. For stock tires (roughly 28.8" dia by the math, I haven't measured one) it apparently happens at 65 mph or 12.61 Hz. For 31' it would happen at 70 mph and for 33's it would happen at 74 mph. Does anyone have large (or small) tires and can verify this?
One of the things I've done to test this, is chain the motor to the frame, effectively increasing the spring rate and thus the vibration frequency. I didn't do a good enough job, because the chains went straight up and down allowing vibration side to side. The effect was that it changed the direction of vibration and could definately be felt. The whole dash shimmied side to side at speed. This adds credence to my theory that the motor is resonating and not some suspension component.
I also changed my motor mounts to something that felt a little stiffer. The frequency went up a little. I also notice that on cold days, it doesn't do it till the truck gets well warmed up.
The problem is wearing on me so I'll likely tinker some more in the near future. In the mean time I'd be interested to hear anything that has bearing on my theory.
