[Z06] Harmonic vibration in chassis - 08 z
12-13-2014, 09:10 PM
#1
Racer
Thread Starter
Harmonic vibration in chassis - 08 z
I've had my 08 zo6 (hci car w kooks lt and catted x, 17k miles) for a couple of months now and I've noticed a vibration (vibration cones for a half second or so every other second) in the chassis at 80+ mph, it's noticeable in the seat as if it was from the rear of the car. I've swapped tires and wheels and and the vibration is unchanged. The vibration is in 5th or 6th gear. Is this a harmonic issue with the exhaust or is something worn out? Where to start if it's not normal?
12-13-2014, 11:07 PM
#3
12-14-2014, 09:20 AM
#5
Pro
The vibration you have described is known as a "Beat frequency".
A beat frequency will "come and then go". So to speak. A beat frequency is caused by two separate forcing frequencies that go in and then out of phase. Many of us have stood beside an 3600 rpm Ac induction motor and felt a slight increase and then a decrease. That will be a beat frequency. 3587 rotation going in and out of phase with the 60 Hz incoming line frequency.
Imagine two separate sinusoids. When the sinusoids are "in phase", they add together and can become very noticeable. When the two sinusoids' go "out of phase", the forces cancel each other out.
Imagine it as two people pushing a large mass from the south side with a force of 125IBs per person. Total force will be 250IBs of force and if this will overcome the static inertia of the mass, it will begin to move. Hence, displacement.
Now, same scenario. Same two people push on the same mass. One on the north side while the other is pushing from the south side. Both still apply 125 IBs of force. Total force applied, 250IBs. Total displacement? None.
To form a beat frequency two things are needed. 2 separate forcing frequencies and a slight delta between the 2. The period between the "peak to peak" vibration will be the difference in the two frequencies. If it seems to be .5 of 1 second from the highest vibration period to the next cycle, the separating frequency will be 2Hz or 120 CPM.
With all that said, If you can get an FFT analyzer or a Vibrations Analyst, this can be solved. I've done it on numerous occasions.
Keep in mind this also can be a natural frequency of some component being irritated by a force that will coincide with the natural frequency. This is then a resonance. However it still will be due to two separate frequencies and summation. This is not a Harmonic!
Tires and balancing?
I've written this before but I'll write it once again. A simple static correction will not properly address an out of balance situation due to the width of the rims and tires. It's called the L/D ratio. Ours borderline that value. When having these balanced ask for a dynamic solution. Place one weight near the inside edge of the rim and the other near the spokes on the inside.
Static, (Vector solution) assumes the extra mass (Uneven distribution), is at 1 location in relation to the 360 degree circumference. Very seldom will this happen.
Dynamic actually measures both planes and makes the correction in that plane opposite the heavy spot of the assembly.
Let's imagine we have a 19" roll that is 14" in length. On one end at 0 degrees we have a 1 ounce mass. On the opposite end we have a 1 ounce mass at 180 degrees. Turn the roll and find the heavy spot. Well it seems to be in balance! It sits stationary at any position. This is true until centrifugal force becomes relevant.
Now we have 2 separate unbalance forces acting to create a twisting effect. When the tires are as wide as these, 3 types of unbalance can be present. Force or static. Couple. Dynamic. Dynamic will most often be the case.
Harmonic? A "Harmonic" is any direct whole number multiple of a forcing frequency in vibration terminology.
E.g.- 100, 200, 300, 400, etc.
Not 100, 125, 285, etc. Harmonics result from amplitude severity. Only way this could be possible is if a component has a natural frequency of "X". A force has a frequency of .5 x "X". Then it would be possible to have a Harmonic of a sub Harmonic force. However, the response is resonance.
The pulsation is key. FFT analyzer will identify and eliminate with the right person interpreting the data.
In either case, elimination of the force will eliminate the response.
A beat frequency will "come and then go". So to speak. A beat frequency is caused by two separate forcing frequencies that go in and then out of phase. Many of us have stood beside an 3600 rpm Ac induction motor and felt a slight increase and then a decrease. That will be a beat frequency. 3587 rotation going in and out of phase with the 60 Hz incoming line frequency.
Imagine two separate sinusoids. When the sinusoids are "in phase", they add together and can become very noticeable. When the two sinusoids' go "out of phase", the forces cancel each other out.
Imagine it as two people pushing a large mass from the south side with a force of 125IBs per person. Total force will be 250IBs of force and if this will overcome the static inertia of the mass, it will begin to move. Hence, displacement.
Now, same scenario. Same two people push on the same mass. One on the north side while the other is pushing from the south side. Both still apply 125 IBs of force. Total force applied, 250IBs. Total displacement? None.
To form a beat frequency two things are needed. 2 separate forcing frequencies and a slight delta between the 2. The period between the "peak to peak" vibration will be the difference in the two frequencies. If it seems to be .5 of 1 second from the highest vibration period to the next cycle, the separating frequency will be 2Hz or 120 CPM.
With all that said, If you can get an FFT analyzer or a Vibrations Analyst, this can be solved. I've done it on numerous occasions.
Keep in mind this also can be a natural frequency of some component being irritated by a force that will coincide with the natural frequency. This is then a resonance. However it still will be due to two separate frequencies and summation. This is not a Harmonic!
Tires and balancing?
I've written this before but I'll write it once again. A simple static correction will not properly address an out of balance situation due to the width of the rims and tires. It's called the L/D ratio. Ours borderline that value. When having these balanced ask for a dynamic solution. Place one weight near the inside edge of the rim and the other near the spokes on the inside.
Static, (Vector solution) assumes the extra mass (Uneven distribution), is at 1 location in relation to the 360 degree circumference. Very seldom will this happen.
Dynamic actually measures both planes and makes the correction in that plane opposite the heavy spot of the assembly.
Let's imagine we have a 19" roll that is 14" in length. On one end at 0 degrees we have a 1 ounce mass. On the opposite end we have a 1 ounce mass at 180 degrees. Turn the roll and find the heavy spot. Well it seems to be in balance! It sits stationary at any position. This is true until centrifugal force becomes relevant.
Now we have 2 separate unbalance forces acting to create a twisting effect. When the tires are as wide as these, 3 types of unbalance can be present. Force or static. Couple. Dynamic. Dynamic will most often be the case.
Harmonic? A "Harmonic" is any direct whole number multiple of a forcing frequency in vibration terminology.
E.g.- 100, 200, 300, 400, etc.
Not 100, 125, 285, etc. Harmonics result from amplitude severity. Only way this could be possible is if a component has a natural frequency of "X". A force has a frequency of .5 x "X". Then it would be possible to have a Harmonic of a sub Harmonic force. However, the response is resonance.
The pulsation is key. FFT analyzer will identify and eliminate with the right person interpreting the data.
In either case, elimination of the force will eliminate the response.
Last edited by JWingo; 12-14-2014 at 09:33 AM. Reason: Add one additional statement
12-14-2014, 10:55 AM
#6
Racer
Thread Starter
The vibration you have described is known as a "Beat frequency".
A beat frequency will "come and then go". So to speak. A beat frequency is caused by two separate forcing frequencies that go in and then out of phase. Many of us have stood beside an 3600 rpm Ac induction motor and felt a slight increase and then a decrease. That will be a beat frequency. 3587 rotation going in and out of phase with the 60 Hz incoming line frequency.
Imagine two separate sinusoids. When the sinusoids are "in phase", they add together and can become very noticeable. When the two sinusoids' go "out of phase", the forces cancel each other out.
Imagine it as two people pushing a large mass from the south side with a force of 125IBs per person. Total force will be 250IBs of force and if this will overcome the static inertia of the mass, it will begin to move. Hence, displacement.
Now, same scenario. Same two people push on the same mass. One on the north side while the other is pushing from the south side. Both still apply 125 IBs of force. Total force applied, 250IBs. Total displacement? None.
To form a beat frequency two things are needed. 2 separate forcing frequencies and a slight delta between the 2. The period between the "peak to peak" vibration will be the difference in the two frequencies. If it seems to be .5 of 1 second from the highest vibration period to the next cycle, the separating frequency will be 2Hz or 120 CPM.
With all that said, If you can get an FFT analyzer or a Vibrations Analyst, this can be solved. I've done it on numerous occasions.
Keep in mind this also can be a natural frequency of some component being irritated by a force that will coincide with the natural frequency. This is then a resonance. However it still will be due to two separate frequencies and summation. This is not a Harmonic!
Tires and balancing?
I've written this before but I'll write it once again. A simple static correction will not properly address an out of balance situation due to the width of the rims and tires. It's called the L/D ratio. Ours borderline that value. When having these balanced ask for a dynamic solution. Place one weight near the inside edge of the rim and the other near the spokes on the inside.
Static, (Vector solution) assumes the extra mass (Uneven distribution), is at 1 location in relation to the 360 degree circumference. Very seldom will this happen.
Dynamic actually measures both planes and makes the correction in that plane opposite the heavy spot of the assembly.
Let's imagine we have a 19" roll that is 14" in length. On one end at 0 degrees we have a 1 ounce mass. On the opposite end we have a 1 ounce mass at 180 degrees. Turn the roll and find the heavy spot. Well it seems to be in balance! It sits stationary at any position. This is true until centrifugal force becomes relevant.
Now we have 2 separate unbalance forces acting to create a twisting effect. When the tires are as wide as these, 3 types of unbalance can be present. Force or static. Couple. Dynamic. Dynamic will most often be the case.
Harmonic? A "Harmonic" is any direct whole number multiple of a forcing frequency in vibration terminology.
E.g.- 100, 200, 300, 400, etc.
Not 100, 125, 285, etc. Harmonics result from amplitude severity. Only way this could be possible is if a component has a natural frequency of "X". A force has a frequency of .5 x "X". Then it would be possible to have a Harmonic of a sub Harmonic force. However, the response is resonance.
The pulsation is key. FFT analyzer will identify and eliminate with the right person interpreting the data.
In either case, elimination of the force will eliminate the response.
A beat frequency will "come and then go". So to speak. A beat frequency is caused by two separate forcing frequencies that go in and then out of phase. Many of us have stood beside an 3600 rpm Ac induction motor and felt a slight increase and then a decrease. That will be a beat frequency. 3587 rotation going in and out of phase with the 60 Hz incoming line frequency.
Imagine two separate sinusoids. When the sinusoids are "in phase", they add together and can become very noticeable. When the two sinusoids' go "out of phase", the forces cancel each other out.
Imagine it as two people pushing a large mass from the south side with a force of 125IBs per person. Total force will be 250IBs of force and if this will overcome the static inertia of the mass, it will begin to move. Hence, displacement.
Now, same scenario. Same two people push on the same mass. One on the north side while the other is pushing from the south side. Both still apply 125 IBs of force. Total force applied, 250IBs. Total displacement? None.
To form a beat frequency two things are needed. 2 separate forcing frequencies and a slight delta between the 2. The period between the "peak to peak" vibration will be the difference in the two frequencies. If it seems to be .5 of 1 second from the highest vibration period to the next cycle, the separating frequency will be 2Hz or 120 CPM.
With all that said, If you can get an FFT analyzer or a Vibrations Analyst, this can be solved. I've done it on numerous occasions.
Keep in mind this also can be a natural frequency of some component being irritated by a force that will coincide with the natural frequency. This is then a resonance. However it still will be due to two separate frequencies and summation. This is not a Harmonic!
Tires and balancing?
I've written this before but I'll write it once again. A simple static correction will not properly address an out of balance situation due to the width of the rims and tires. It's called the L/D ratio. Ours borderline that value. When having these balanced ask for a dynamic solution. Place one weight near the inside edge of the rim and the other near the spokes on the inside.
Static, (Vector solution) assumes the extra mass (Uneven distribution), is at 1 location in relation to the 360 degree circumference. Very seldom will this happen.
Dynamic actually measures both planes and makes the correction in that plane opposite the heavy spot of the assembly.
Let's imagine we have a 19" roll that is 14" in length. On one end at 0 degrees we have a 1 ounce mass. On the opposite end we have a 1 ounce mass at 180 degrees. Turn the roll and find the heavy spot. Well it seems to be in balance! It sits stationary at any position. This is true until centrifugal force becomes relevant.
Now we have 2 separate unbalance forces acting to create a twisting effect. When the tires are as wide as these, 3 types of unbalance can be present. Force or static. Couple. Dynamic. Dynamic will most often be the case.
Harmonic? A "Harmonic" is any direct whole number multiple of a forcing frequency in vibration terminology.
E.g.- 100, 200, 300, 400, etc.
Not 100, 125, 285, etc. Harmonics result from amplitude severity. Only way this could be possible is if a component has a natural frequency of "X". A force has a frequency of .5 x "X". Then it would be possible to have a Harmonic of a sub Harmonic force. However, the response is resonance.
The pulsation is key. FFT analyzer will identify and eliminate with the right person interpreting the data.
In either case, elimination of the force will eliminate the response.
12-14-2014, 11:16 AM
#7
Pro
An FFT is "Fast Fourier Transform". It's calculated inside a Data Acquisition Device such as a Commtest VB7, VB8. Other vendors are DLI, CSI, SKF, Timken, GE, etc. An FFT app may even be available for your cell phone. Will need extremely high resolution.
A Data Acquisition Device separates multiple waveforms from amplitude(Y) versus time(X) called the time domain, to amplitude(Y) versus frequency(X) the frequency domain.
If you can get the data, I can analyze it for you. I am an ISO level 4 Vibrations Analyst. If you're near SC, I have all we need. GE Bently Nevada 140, Commtest VB7, SKF CMVA 65, Azima/DLI.
We will need both the waveform data and the FFT data.
The waveform will give the information on the fundamental frequency and the separating frequency. Then a list of possible repairs can be compiled while applying the most effective one or two.
12-14-2014, 11:27 AM
#8
Pro
I can promise you that for sure, SKF will have Vibration Analyst in Atlanta as will all the big vendors. Power plants, rotating machinery mills, aircraft turbine facilities, etc.
My offer still stands. I will go over the data for you just to help you out. As I said in my first response, you will need a good analyst and the data collector. Going to be a lot of information in the data.
If you feel like a few hours of driving time, I have all the equipment.
12-14-2014, 04:44 PM
#10
Pro
This is true.
Equipment and software normally around $40 - $50 k. Thank God I work for a large company. I have far more than that. A lot of real time continuous monitoring of the more critical equipment. 5 Portable multi-channel units for analytical. In todays industrial environment, shutting machinery down isn't an option until it's necessary and data driven.
I don't think most people realize exactly what a Vibration Analyst really does or if we apply ourselves what our full potential can be.
We train in many fields. Vibration, Damping, Isolation, Operation deflection shapes, Modal analysis, condition monitoring, fluid dynamics, power transmission, etc.
Not going to get into a debate over Tire balance and the quality grades. Everyone do what you feel good about on that issue. However I'd never tell you something that I know is wrong.
Did I mention I'm also ISO certified in portable in place balancing?
A spin balance machine is only as accurate as the grade at which it is set to. G2.5, G6, etc.
Hunter is fine equipment.
However.
If a tire/Wheel is balanced while under load, it will restrict the unbalance response.
Unbalance is unbalance. This is to say a measure of centrifugal force. Measured in ounce-inches or gram- mm's. That can be quantified to an ISO grade. We most likely hear the grade called G1, G2.5, G6, etc. Navy spec is 4w/n. Very tight tolerance to which turbines are balanced. If I add more resistance to any rotating machine, the response will be limited. Loading a tire does not redistribute the mass. Adding the correction weights to the correct plane and correct position simply cancel the uneven distribution of mass by introducing an even and opposite force. Unbalance forces increase by the square of the speed increase. E.g. An assembly running at 100 rpm introduces 20 IBs of centrifugal force. Same assembly is sped up to 300 rpm. Centrifugal force will now be 180 IBs. Guess what just happened? We just left the G2.5 and went straight up to G6 or above. A vector static solution is fine in most instances. I prefer a couple solution as if done correctly, will not be as sensitive to speed increase.
OP, I'll be happy to help you in any way I can.
Last edited by JWingo; 12-14-2014 at 06:50 PM. Reason: Typo
12-15-2014, 06:48 AM
#12
Racer
Thread Starter
Jwingo,
I greatly appreciate the offer and I may take you up on that. I am going to have the tires road force balanced first. I drove the car for roughly 300 miles yesterday with 35 psi in each tire vs 30 as called for. The initial miles did not have a vibration, once the tires were a bit warmer the vibration was evident. All the weights are inbound just behind the spokes. The vibration was evident no matter the gear or with the clutch pushed in.
Tim
I greatly appreciate the offer and I may take you up on that. I am going to have the tires road force balanced first. I drove the car for roughly 300 miles yesterday with 35 psi in each tire vs 30 as called for. The initial miles did not have a vibration, once the tires were a bit warmer the vibration was evident. All the weights are inbound just behind the spokes. The vibration was evident no matter the gear or with the clutch pushed in.
Tim