[pggto]

I have a 70 GTO convertible that has had a vibration in it years! In that time frame I have replaced everything that moves. I have changed rear ends, engines, trans, drive shafts, brake rotors and drums, wheels and tires, and suspension bushings and related parts. The vibration is still there, it's like a pulsating harmonic type vibration that resonates through the car that kicks in at about 2600-2800 rpm and gets worse with more revs and mph. Now that I put a more powerful engine in it this summer it's worse. It was just annoying with the other engine, now it just pisses me off with the new engine. I had everything balanced with the new engine. It vibrates so bad you can't see anything in the rear view mirror! Could the pinion angle be causing this????? What should the angle be? How do you actually measure the angle? I was going to install adjustable rear upper control arms, but I'm not sure where to start. I've noticed this vibration in other A body convertibles but not as bad as my car. Any input is welcome, thanks.

[Blued and Painted]

you did not montion the drive shaft.

[pggto]

Originally Posted by Blued and Painted

you did not montion the drive shaft.

Yes I've tried those as well, currently has a new one, with joints.

[HWYSTR455]

Does it go away when you put it in N and coast?

.

[Bobalong]

Originally Posted by pggto

Yes I've tried those as well, currently has a new one, with joints.

Without a doubt you have given this a lot of thought and $$. Hopefully someone here will help. Bet it doesn't vibrate in neutral at the given RPM, that's too easy. If the mirror / steering wheel are shaking but not so much in your butt it's an indication of front wheel- tire- rotor balance. Can you find an old school tire balance machine that uses spinner and strobe light. The wheel-tire stays on, and is spun on the car ? You can individually check each wheel.
Put it on a dyno ? Where you can tour around the car as it revs the desired RPM.

[Tim john]

Here's a thought. Are the factory "cocktail shakers" in place. These were installed for a reason. Convertibles are notorious for vibration and the "vibration dampeners" (cocktail shakers) were the solution. Your symptoms are exactly why the factory installed them. Higher rpms and speeds cause the harmonics, the shakers balance/cancel them out. Your car should have two in the front and two in the trunk (one per corner).

Swap wheels front to back, side to side, etc...confirm each wheel is balanced accurately, also confirm your new driveshaft is "balanced".


Tim john---

[pggto]

Originally Posted by HWYSTR455

Does it go away when you put it in N and coast?

.

I haven't tried that, but I will.Thanks

[pggto]

Originally Posted by Bobalong

Without a doubt you have given this a lot of thought and $$. Hopefully someone here will help. Bet it doesn't vibrate in neutral at the given RPM, that's too easy. If the mirror / steering wheel are shaking but not so much in your butt it's an indication of front wheel- tire- rotor balance. Can you find an old school tire balance machine that uses spinner and strobe light. The wheel-tire stays on, and is spun on the car ? You can individually check each wheel.
Put it on a dyno ? Where you can tour around the car as it revs the desired RPM.

I haven't seen one of those style balancers in decades, I used to work at a Goodyear dealer and we had one in the 80's. I've had the tires balanced, and I've put different wheels on the car, same problem. I've replaced the rotors and wheel bearings. I don't feel it in the steering wheel, it's in the chassis. The dyno is a good idea, there's a place not to far from me that does that, I think I will check into that, Thanks

[pggto]

Originally Posted by Tim john

Here's a thought. Are the factory "cocktail shakers" in place. These were installed for a reason. Convertibles are notorious for vibration and the "vibration dampeners" (cocktail shakers) were the solution. Your symptoms are exactly why the factory installed them. Higher rpms and speeds cause the harmonics, the shakers balance/cancel them out. Your car should have two in the front and two in the trunk (one per corner).

Swap wheels front to back, side to side, etc...confirm each wheel is balanced accurately, also confirm your new driveshaft is "balanced".


Tim john---

As far as I know the A bodies didn't come with the cocktail shakers, I believe they were only on the 1st gen F bodies. But I will double check, my car currently doesn't have them. Thanks

[HWYSTR455]

Originally Posted by pggto

I haven't tried that, but I will.Thanks

If you put it in N and it doesn't vibrate, it's before the trans. If it does, it's after the trans or chassis related.

Another thing to check is to rev it to the 2600-2800 RPM while parked, see if it vibrates. That will also tell you if it's engine or chassis related.

Usually, if it's RPM related, it's engine related, if it's speed related, it's a chassis issue.

I've seen people have pulleys and fans out of balance that they kept swapping for config to config, car to car, and it followed them.

.

[Keith Seymore]

Here is the information that I use for both driveline angles and vibration diagnosis:


























If you follow this thought process you can fix any vibration. It's all about separating those areas that don't require any attention from those that you need to focus on.

Having said that - I'm beginning to believe there are some inherent beaming modes in A body convertibles. I worked on a '70 Buick convertible that could not be improved until the owner added a roll cage.

K

[pggto]

Originally Posted by HWYSTR455

If you put it in N and it doesn't vibrate, it's before the trans. If it does, it's after the trans or chassis related.

Another thing to check is to rev it to the 2600-2800 RPM while parked, see if it vibrates. That will also tell you if it's engine or chassis related.

Usually, if it's RPM related, it's engine related, if it's speed related, it's a chassis issue.

I've seen people have pulleys and fans out of balance that they kept swapping for config to config, car to car, and it followed them.

.

New engine has new balancer, water pump, ps pump, and alt. I was just playing with the timing up to 3000 rpm and it runs smooth while setting the timing. But i did transfer over the pulleys, they were bead blasted and repainted, and look to run true on the engine, but I give them a closer look while running. Thanks

[HWYSTR455]

I had a vibration issue with the LeMans once, and it ended up being the yoke on the rear, the 'nubs' that center the u-joint caps were worn, and the u-joint would shift making it 'wobble'. It took a very close visual inspection to catch that.

Keith - What's the source of that diagnosis chart, etc? I'd like to have a copy on hand...

.

[Keith Seymore]

Originally Posted by HWYSTR455

Keith - What's the source of that diagnosis chart, etc? I'd like to have a copy on hand...

.

Looks like some things got packed away during my last office move; I believe it is from a Chevy S/T truck service manual.

I also have a (very interesting) Pontiac Vibration manual from the mid 60's but am hesitant to post that in case there are copyright issues.

K

[2002Z4CSS]

Since the vibration got worse after the new engine was installed, how good is the engine and trans mounts? Sounds like one may be grounding out or the exhaust is grounding out.

[pggto]

Originally Posted by 2002Z4CSS

Since the vibration got worse after the new engine was installed, how good is the engine and trans mounts? Sounds like one may be grounding out or the exhaust is grounding out.

The mounts are new, I put in the polyurethane motor mounts from Bulter and the trans mount was replaced a few years ago and is still in good shape. This vibration is definitely rotational due to the harmonics that I feel. I was an auto mechanic for 10 years and have fixed my share of vibrations, but this one has me stumped.

Maybe the reason its worse now with the new engine is because of the stiffer motor mounts, but that still doesn't explain the cause.

[Keith Seymore]

Originally Posted by pggto

Maybe the reason its worse now with the new engine is because of the stiffer motor mounts, but that still doesn't explain the cause.

Engine firing frequency!



When you say "rotational due to the harmonics you feel" does that mean it comes and goes?

If you are feeling it increase and decrease: that's a "boom/beat", which indicates you have two root causes. The increase and decrease is a result of the two out of phase amplitudes adding and subtracting. You would need to isolate and eliminate one of the root causes.

Does that sound like a good description of what you are experiencing?

K

[2002Z4CSS]

Originally Posted by pggto

The mounts are new, I put in the polyurethane motor mounts from Bulter and the trans mount was replaced a few years ago and is still in good shape. This vibration is definitely rotational due to the harmonics that I feel. I was an auto mechanic for 10 years and have fixed my share of vibrations, but this one has me stumped.

Maybe the reason its worse now with the new engine is because of the stiffer motor mounts, but that still doesn't explain the cause.

Get a syrometer (sp) and find out what HZ you get at what RPM or vehicle speed. From there you should be able to calculate what it is.

[pggto]

Originally Posted by Keith Seymore

Engine firing frequency!



When you say "rotational due to the harmonics you feel" does that mean it comes and goes?

If you are feeling it increase and decrease: that's a "boom/beat", which indicates you have two root causes. The increase and decrease is a result of the two out of phase amplitudes adding and subtracting. You would need to isolate and eliminate one of the root causes.

Does that sound like a good description of what you are experiencing?

K

Engine frequency? I have had 3 different engines in this car over the years and the vibration is the same.

It starts out as a pulsating vibration and as speed and rpm increase so does the vibration, I guess at higher speeds and rpm the frequency of the pulsations increase making it feel like one steady vibration, this car at 90-100 vibrates a lot, just feel it through out the car. Because I have changed out all the drive train components over the years is why I was questioning the drive shaft angle or some thing related, It's a new drive shaft but who says its balanced correctly, I'll rotate 180 and see if any thing changes. this vibration has been with this car since I restored it back in the 80's, I was really hoping it would go away with the new drive train, and it has not. Maybe it's time to tear in to the rear end as well?? I bought it rebuilt from a guy that does rear ends, 12 bolt 3:55 gears. Thanks Keith

[pggto]

Originally Posted by 2002Z4CSS

Get a syrometer (sp) and find out what HZ you get at what RPM or vehicle speed. From there you should be able to calculate what it is.

I had to Google that! Sirometer is a vibration tach generally used on small engines. Not sure exactly how that would help? What would the frequency tell me?