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Old 10-27-2019, 05:03 PM
Grand73Am Grand73Am is offline
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Default 60mph-70mph shaking ideas

79 TA 403 auto, t-top. I've been fighting a shaking problem a long time, and decided to see if anyone else has had the problem and fixed it. It's fine up to 60 mph. Then I get a little shaking in the steering wheel. It feels mostly up front from what I can tell. It does this up to 70 mph. It smooths out above 70. I've already gone through the list from the Pontiac service manual below. I have new rebuilt front suspension and steering linkage parts. All new front and rear springs and shocks. New body mounts. I've used a couple of sets of different wheels and new tires, re-balanced a couple times. Balanced the drive shaft. It used to be worse, and some of these things I did made it better. I used to think the problem was only in the wheels and tires. I'm using 2 different sets of original Snowflakes and 2 sets of new BFG Radial TA's. The first tire store I used didn't seem capable of doing a good balance job. I switched to a local family owned tire store who does a better job of balancing and that's where I got a lot of improvement. Both sets of wheels/tires get the same shaking at the same speeds, so I'm now thinking there's a little something wrong with the car. It's tolerable, but it should be smooth riding at those speeds after all I've done, so that little shimmy is annoying. I'm out of ideas, so wondering if someone knows another reason that could cause this, that I haven't already thought of? Thanks!

Untitled by grand73am, on Flickr

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  #2  
Old 10-27-2019, 06:25 PM
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Was the drive shaft balanced with the same installed angles, as in car???

Maybe find someone that balances tires, on car. Might need to ask at truck shops...

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  #3  
Old 10-27-2019, 06:45 PM
Grand73Am Grand73Am is offline
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Good thoughts, thanks!

I might have discovered a good reason for the problem. I remembered the other day when changing out wheels (again), that the rotors were dragging. I thought at the time a little drag is normal, with new rotors and pads. But this may be too much drag. The rotor only goes about a half turn with a good turn. I checked the rotors on my 75 Lemans, which has basically the same brakes, and there's no drag and the rotor spins freely multiple turns with the same effort.

I've been getting alot of brake dust on the wheels and that must be it, and it can't be good to be dragging like that at speed.

So, I might have a situation where the new booster is putting a slight pre-load on the master cylinder causing the drag. I'm going to loosen the master and move it away from the booster to see if the calipers will retract to free up the rotors.

That would be a relief if that's all it is.

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Old 10-27-2019, 07:15 PM
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Front rotors???

I would suspect a rear rotor potential problem more than a front rotor problem, with those wonderful ratcheting calipers on the back...

I assume you must be talking about front rotors, nothing 75 had rears...

Are all the brake components stock? Maybe a master cylinder issue??? Especially if you installed one manufactured in the last 12-15 years. All that stuff is imported.

The new booster could be a problem.
My old trash truck is a 85 F150. Bought it from my old employer, cheap, it's a pile...
The M/cyl rod on it is adjustable. The pedal has never felt good, even when we used it at my job. We fooled with it some there. I have fooled with it. I had to install a new booster....it's still not right, BUT I have talked to others with these Garbage is job One, vehicles, and their brakes are all the same.
I have used a caliper and a small 6" steel ruler to make measurements, to make sure rod is adjusted correctly (as in, not too short.
I plan to check it again as the master cylinder is leaking (again) when I replace the cylinder soon. THis is like the 4th NEW m/cyl since my company bought this maintenance hog, around 2000.

I'm sure you can check rod length (for being too long), like I did on pickup...

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Old 10-27-2019, 07:20 PM
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Were the tires road force balanced? Some of the newer GM trucks with 20" tires have had some vibration issues, and this type of balancing helps.

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Old 10-27-2019, 07:31 PM
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check the tail shaft bushing on the transmission as well as the rear wheel bearings

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Old 10-27-2019, 07:41 PM
Grand73Am Grand73Am is offline
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Quote:
Originally Posted by 77 TRASHCAN View Post
Front rotors???

I would suspect a rear rotor potential problem more than a front rotor problem, with those wonderful ratcheting calipers on the back...

I assume you must be talking about front rotors, nothing 75 had rears...

Are all the brake components stock? Maybe a master cylinder issue??? Especially if you installed one manufactured in the last 12-15 years. All that stuff is imported.

The new booster could be a problem.
My old trash truck is a 85 F150. Bought it from my old employer, cheap, it's a pile...
The M/cyl rod on it is adjustable. The pedal has never felt good, even when we used it at my job. We fooled with it some there. I have fooled with it. I had to install a new booster....it's still not right, BUT I have talked to others with these Garbage is job One, vehicles, and their brakes are all the same.
I have used a caliper and a small 6" steel ruler to make measurements, to make sure rod is adjusted correctly (as in, not too short.
I plan to check it again as the master cylinder is leaking (again) when I replace the cylinder soon. THis is like the 4th NEW m/cyl since my company bought this maintenance hog, around 2000.

I'm sure you can check rod length (for being too long), like I did on pickup...
Yes, front rotors dragging. I don't have rear discs. I am using one of the repro boosters with an adjustable push-pin. I thought I adjusted it accurately, but now I'm thinking I might have made it a tad too long, causing a bit of pre-load on my new master cylinder. I'll loosen the master from it tomorrow and see if the calipers retract enough to stop dragging on the rotors. if so, I'll correct that problem and see if my shaking goes away. Another clue is that I've noticed it produced a lot more brake dust than it should. Thanks!

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Last edited by Grand73Am; 10-27-2019 at 08:05 PM.
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Old 10-27-2019, 07:59 PM
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Quote:
Originally Posted by 77 Canamman View Post
Were the tires road force balanced? Some of the newer GM trucks with 20" tires have had some vibration issues, and this type of balancing helps.
Yes, I've taken them to 5 places, 2 places that check and repair rims, 2 tire stores and a general auto repair shop. At least 3 of them use the Hunter machine with Road Force. The others I couldn't see what machine they were using. One place(Discount Tire) couldn't seem to balance a tire, even with the Road Force. The current tire store I'm using, which is locally family owned, does the best job and that's what he uses. He actually found 2 of my new Radial TA's to have excessive Road Force measurements, and got them replaced with good tires. So, I think I'm getting good balancing work now, and after using the other places, I don't think I can get better work done. That's why I'm thinking this last bit of shake I have is something else. Thanks!

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Old 10-27-2019, 08:05 PM
Grand73Am Grand73Am is offline
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Quote:
Originally Posted by DOC View Post
check the tail shaft bushing on the transmission as well as the rear wheel bearings
Good thoughts, but those are good. The th350 is a fresh, completely rebuilt unit. And I installed new rear wheel bearings, and an axles. Also checked the axles in-out play, and it's good. Thanks!

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Old 10-28-2019, 12:01 PM
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A problem vibration I had ,that my alignment pointed out ,was that my rear drum was missing a weight, ordered new drums and the vibration was gone

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Old 10-28-2019, 07:21 PM
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A problem vibration I had ,that my alignment pointed out ,was that my rear drum was missing a weight, ordered new drums and the vibration was gone
That's certainly something to watch out for. Not on this car, but one time I bought a new drum that had a weight on it that was loose, and the drum just looked ugly with a poor casting, so I returned it immediately for a good one. I put new drums on this car and they looked very good to me. But, thanks for mentioning that!

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Old 10-28-2019, 08:00 PM
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I made some progress today. I unbolted the master cylinder and pulled it away from the booster a bit, but it made no difference. So the booster pin is not causing a pre-load, and I bolted it back up. Looking at the tight fit of the pads to the rotors, I remembered that the pads have silencing shims on the backs of them, those thin metal plates. I decided if I pull those shims off, I could use some CRC Disc Brake Quiet paste instead, which would reduce the thickness of the pads. It worked. The rotors spin freely now. I'll drive it tomorrow to see how it acts. Below is short video that shows how tight the pads were to the rotors. They were the same on both sides of the car. Click on the picture and it'll take you to the video.

002 by grand73am, on Flickr

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Old 10-30-2019, 10:57 AM
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Well, removing the drag from the front brakes did make some difference in the ride. I can tell there's less resistance going down the road, so maybe I'll get better gas mileage . Unfortunately, it didn't remove the shake though. So, still searching for the reason.

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Old 10-30-2019, 11:26 AM
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Have you mentioned replacing the front rotors?

It appears that since you notice it in the steering wheel, it most likely is coming from the front end. I don't know of anyone that balances rotors.

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Old 10-30-2019, 12:13 PM
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Quote:
Originally Posted by 77 Canamman View Post
Have you mentioned replacing the front rotors?

It appears that since you notice it in the steering wheel, it most likely is coming from the front end. I don't know of anyone that balances rotors.
Glad you brought that up. I keep a notebook of parts replacement and my work, so I looked back to see what I did about the rotors. It's been several years. I was thinking they're new, since I rebuilt everything, but now I see I kept the original rotors because they still measured good, within the wear specs. Even though that doesn't mean they're bad, it might be worth trying new ones. Thanks.

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Old 10-31-2019, 12:30 PM
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Might try putting the rear off the ground and run the motor in gear up to the vibration speed. This should at least tell you if the issue is with the front or back/drive train.

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Old 10-31-2019, 12:49 PM
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Quote:
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Might try putting the rear off the ground and run the motor in gear up to the vibration speed. This should at least tell you if the issue is with the front or back/drive train.
You're right. I do need to do that. Thanks!

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Old 11-16-2019, 09:39 PM
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Is the vibration tight or loose(wide)

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Old 11-18-2019, 10:36 PM
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Quote:
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Is the vibration tight or loose(wide)
Neither. I'd say it's medium.

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