One of the more common balance mistakes is using a 28 or 50 oz-in unbalance flywheel on the wrong crank. If I did my math correctly this morning, at 600 RPM this produces an unbalance force of 14 pounds which doesn't sound like much but try shaking 14 pounds back and forth 600 times per minute and you get a sense of the real forces involved. At 2100 RPM, the force is now 172 pounds and at 4200 RPM the force is 688 pounds. The unbalance forces increase at a rate of the square of the speed. When there is a noticeable vibration at 2100 RPM that improves when you go above 2100, then it is highly unlikely that unbalance is the root cause of your issue. Balance can always be improved but the difference between a motor destined for service in a Lincoln versus a Mustang would be measured in gram-inches of residual unbalance. So, a new flywheel, assuming that it is better than the old one may make it a little smoother but it won't correct the condition at 2100 RPM. If the pressure plate was not balanced with the flywheel as an assembly, you have the same situation - slightly rough through the entire RPM range but not at just one speed. Running the numbers, you can actually see the difference in unbalance forces if you neglect to put a washer on one of the flywheel bolts or if you use one bolt slightly longer or shorter than the others.
In a resonance condition, which is more typical when there is a specific RPM where vibration is high, you still need a force to excite it. A bell won't resonate unless you strike it. So, a perfect balance may lessen the amount of vibration at 2100 but it won't eliminate it. Also, at 2100 RPM, unless you are able to measure the frequencies that are present with a spectrum analyzer, what you are feeling may be right at engine speed, a multiple of engine speed such as reciprocating unbalance at 2 times running speed, or engine firing frequency, or even a fraction of running speed which can be related to the crank or other forces related to engine firing. So, unless a vibration spectrum shows that the only significant force and vibration amplitude is a 1X engine speed, replacing part by part that will only affect the rotor's balance may be an exercise in futility.
So, we can look at things that haven't fixed the problem - the damper can influence rotor balance and also torsional vibration of the crank. In some cases, the outer ring can slip which would change the balance relationship between the crank and the damper/balancer. If the damping material has deteriorated, it usually gets stiffer which would alter its ability to properly dampen crank shaft torsional oscillations that are caused by engine firing. Since the problem is unchanged, we now now that it is not a damper/balancer problem.
By disconnecting the exhaust, this will likely eliminate the pipes/mufflers as the source. Why I say likely is that I was assisting on a problem with a small truck in South Africa and the lifted the entire body off of the frame and there was still a severe noise in the cab even with no mechanical connection because the source was airborne due to a resonance of the side of the engine block we refer to as oil-canning. So, sometimes parts will still vibrate when disconnected from the engine since there is still a path through the frame.
There are three other potential issues that come to mind since we now now what it probably isn't. The first is the transmission crossmember for your T-5. I have seen several that are poorly designed so that they don't properly support the back end of the transmission. When they provide a cantilever support rather than a direct support, they can twist and produce a natural frequency of the entire engine/transmission even when mounts are good. This may show high vibration at the shifter but depending on how all the mass is arranged and supported, there may be a bounce at the front of the engine and the shifter may feel fairly smooth. A check that may help reveal this would be to use a jack and piece of wood and slightly lift on just the end of the transmission somewhere near the rear mound. Apply a little lift with the car fully on the ground and see if the RPM of the problem shifts up or down. If it remains the same then the rear crossmember can be taken off the list.
Next, although related to unbalance, the forces are a little different if the flywheel has axial runout. This can be caused by previous overheating due to clutch slippage, and imperfect surface of the flywheel flange or crank flange, incorrect or uneven torque on the flywheel bolts or something as simple as a small amount of dirt between the flywheel and the crank. You may need to pull the bellhousing to check this properly. Mount an indicator base on the engine block and indicate axially near the outer rim of the flywheel and check the TIR.
Last, for now at least, could be a problem with the crank itself. I don't have much experience with this other than talking to people who have had this happen. When a crank is cracked, it changes all of the torsional natural frequencies of crank and can lower them into much lower RPM ranges. From what I have been told, this can produce speed related vibration when engine firing at a certain speed excites the crank. This may be remote but when all other attempts have failed.... A crank doesn't have to be abused to crack. It can be a simple fatigue issue related to the age of the crank and total miles coupled with slight casting imperfections. The casting imperfections become the seeds of the crack origination.
I am not sure if I was following some earlier posts but if I read them correctly, the previous owner of the T-5 your have also had vibration issues. Was it just the trans or did you end up with the engine/trans combo. If it is following the trans and you are using the same crossmember then that could be a common link. I don't think this is related to an input shaft or pilot bearing issue as this would produce unbalance-like symptoms which would worsen with speed/RPM.
Last, I wanted to show some readings that I took on an an electric motor. The motors at the manufacturer did not meet the customer's vibration requirements and they threw a lot of parts at it with no improvement. When I looked at the design I told the owner of the company that it was highly likely they had a resonance problem. He said it can't be - because they had never had a problem before. In my career when I am told "it can't be" it often is exactly what the problem is. Long story short, I performed an impact test to find the motor's natural frequencies. The motor speed was 3600 RPM and the plot below shows a whopping natural frequency at 3750 just above operating range. Above or below, the motor vibration would be fine but right near its designed operating speed, the vibration was unacceptable. After we made some structural modifications we moved the natural frequency well above running speed and the vibration dropped by a factor of 6. I'm just including this to illustrate speed sensitivity and resonance. In this case, the rotors were being balanced down to nearly nothing on an ancient but excellent balancing machine but there was still enough residual unbalance to excite this natural frequency. If the motor speed was right at 3750, the vibration would have been 20 to 50 times higher than if it was not resonant.
Offline
Thread Starter