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Discussion Starter #1
Need the expert's help

Car Details:
  • 1969 Convertible
  • New rims & tires in 2016
  • Rebuilt Base 302 (mild cam, Edelbrock 4bbl, Pertronix Distributor / coil) - rebuilt in 2017; new engine mounts installed
  • C4 Transmission (rebuilt in 2016; new torque converter installed at the time) - new transmission mount installed
  • Arnie drop (2017)
  • Front & Rear suspension replaced in 2017
  • New 3rd member with 3:55 gears put in last summer (2019)
  • With car on the ground, differential pinion angle is 2.5 degrees up; Transmission pinion angle is 2.5 degree down
I developed a harmonic vibration in the driver's seat late last fall which I'm finally getting to now since I just pulled it out of storage. It's at it's worst at a steady cruising speed of about 35 mph. Not really felt on acceleration (gradual or hard) or slowing down.

I replaced u-joints last year when I replace the 3rd member, so thought I could potentially have one failing, so last night I replaced them again, and verified my pinion angles again and they are still what I mention above.

Took the car out for a drive after replacing u-joints and it's still there. I thought normally vibrations felt in the seat are drive train related so I'm confused now.

Unless I'm missing something my next steps I'll try are:
  • Verify toe on front suspension (although I feel no vibration on steering wheel)
  • Have the tires road-force balanced
  • Have the drive shaft balanced
Am I missing something? I feel like I'm missing something.......I'd love to hear the expert's thoughts on this.

Thanks in advance
 

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Discussion Starter #2
One other thing...when I did the front suspension in 2017 I replaced wheel bearings, and then I did the new 3rd member last summer I replaced the rear wheel bearings.

Probably only driven 250 miles over the past few years.
 

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I wonder if the tires are out of round. I had a problem exactly as you describe in my other car. I had the tread shaved, rebalanced the tires, and after that it was smooth as glass.
 

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Discussion Starter #4
I wonder if the tires are out of round. I had a problem exactly as you describe in my other car. I had the tread shaved, rebalanced the tires, and after that it was smooth as glass.
Good suggestion, I'll have the tire shop look at this
 

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Seat vibrations aren't always driveline per se. I was taught seat vibrations more likely rear wheels which includes driveline for rear wheel drive like the Mustang. Front vibrations are more pronounced in the steering wheel. To that end, check the rear tires for balance, out of round. Switch tires back to front to see if that changes anything, if vibration goes front, tires and wheels.
 

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Discussion Starter #6 (Edited)
We can check tires off the list of things to look at

I just got back from Discount Tires. They road-force balanced all of them, checked for out of round, and rotated them. I noticed on the drive home that if anything, now that the tires are freshly balanced the vibration in the seat is even more noticeable.

I think I'm going to have the drive shaft balanced next as I am out of ideas.

I'll still check the toe this weekend but that is more for my OCD to check it off the list.
 

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Check age of tires rubber gets hard as they age, also when checking pinion angle rotate 90 deg and take reading and subtract,view Youtube videos for exactly how to check. Tire pressure-experiment. Maybe too high of tire pressure.
 

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Discussion Starter #8
Getting closer...I think it may be tire related or possibly alignment related.

Had a new driveshaft made / balanced by a local shop, My old driveshaft had the rubber coupler (where the rear yoke assembly was pressed into drive shaft tube with a rubber sleeve around it)....the drive shaft shop owner thought that may be a potential source of issue since those assemblies slip / move over time so he made me a solid shaft (nice unit).

I Installed that last night an took the car out for a drive. The harmonic vibration in the seat is no longer there which I'm happy about, however, there is now a more noticeable vibration on the gas pedal and sometimes steering wheel as your cruising around 35-40 mph.

That is why I'm thinking tire issue or alignment. When time permits over the weekend or early next week I'm going to do some playing with tire rotation and air pressure to see if I can get a handle on where the vibration moves as I make changes. The good new is with the tire now being recently balanced, that variable is eliminated.
 

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I have found that is all other areas have been covered & the rumbling is only in the seat the likely cause may be
 

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This post is timely. I have the exact same issue with my 67 convertible. My vibration starts at 42mph and dissipates at 44mph, then comes back slightly at 72mph . Newer tires, wheels/tires balanced a couple times, front end redone about 8 years ago. Drive shaft was balanced about 12 years ago. Most vibration in steering wheel but some in the seat. This has gone on for years and I just can't seem to narrow the problem down. Maybe a new drive shaft is in order.
67 Mustang.jpg
 

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Discussion Starter #11
I talked to Discount Tires today right after I updated my post. They are the ones who rebalanced the tires last week.

The tires have a date code stamp of 2014 so 6 years old, still well within the 10 year window where no tire place is supposed to touch them after the 10 year date, but the guy I talked to is wondering if I have a tire or a possibly a number of tires where the belts have reformed themselves since drive time on this thing is pretty limited (100-200 miles a year). Sits more than it's driven.

I'm a visual / need to touch it type of personality when it comes to figuring stuff out, so I'm still trying to get my arms around this. I watched them balance the tires last week, the tires are true (round, no wobbles on the machine, etc.) but I "guess" it could be where the internal belts have shifted but again without being able to visualize it and see if that makes sense I'm not sure what to think about that. After all, that is the point of balancing the tire right? I don't know...what do folks think about what Discount Tires said?

I do have other mustangs, so I may take the wheels off my 65 (that has Radial T/A's too with the same date code) since that thing rides like it's on glass and see if I can eliminate tires being the issue. If the 65's tire still cause the 69 to vibrate it ain't the tires...

If it ain't the tires, then I'm down to alignment or I suppose it could be the torque converter but that too is fairly new (it was replaced when I had the tranny rebuilt.
 
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