Vibrations at speed

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rustyswinger

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Hi All,

I have a 1974 dart swinger. I tore this car down to nothing rotisserie sandblast, the works.

Engine is a 318, presumably stock, bought it locally on a gamble, have finally managed to make it run smooth and seemingly well.

904 transmission, and 81/4 rear end literally farmers field parts car parts that I have drained, cleaned and put new fluid, trans filter kit, input/output seals into.

New rear leaf springs, manufactured using my stockers as template
4 New cheapo OEM style blue shock absorbers
4 new tires, balanced and correctly inflated
All new front end parts including pitman arm, Idler arm, upper and lower bushings, offset moog bushings for UCA
1.03 torsion bars and ride height and alignment done locally ( very little faith in local shop, they measured height using sheet metal.)
All brand new brakes and brake components including rotors and drums.
I have checked and rechecked bolt torque on everything and anything.

Now the question.

Car runs and drives very well around town and has as much power as a 318 stocker should have.

Get it on the highway and hit 77MPH ( according to my needle) and she starts to shake and vibrate pretty good.

I do not feel it in the steering at all, and it seems to handle well.

My question is.... how do I even begin to investigate this? What should I look for? Any common issue that may cause this?
My fear is that when I turned this car into a skeleton, I DID NOT brace the car, when I was welding new steel onto her, it was pretty obviously not 100% square and symmetrical.

any thoughts would be welcome
thank you all
 
Driveshaft?? Maybe the driveshaft is a little bent, or has a U-joint going out?

Did the alignment shop do a 4 wheel alignment, meaning did they put the sensors on the back wheels too, so they could see if something was way out of whack?
 
Driveshaft?? Maybe the driveshaft is a little bent, or has a U-joint going out?

Did the alignment shop do a 4 wheel alignment, meaning did they put the sensors on the back wheels too, so they could see if something was way out of whack?
Good call on the drive shaft. U joints are new all around, drive shaft looks undamaged but could well be out of balance.

I can't guarantee they did a 4 wheel alignment. like I said I dont 100 percent trust the alignment shop. My wallets worn thin.....I'm trying to chill out the spending, dont want to throw good money after bad
 
Put rear up on jack stands, (BLOCK FRONT ETC SAFETY FIRST) and run car in gear, look for any wobbles anywhere, have someone get it up to speed and look and feel for vibrations.

MY assumption is that you have no vibration in neutral at the same RPM the engine would run at 77MPH.

All this MIGHT rule out drive train as issue.
 
Put rear up on jack stands, (BLOCK FRONT ETC SAFETY FIRST) and run car in gear, look for any wobbles anywhere, have someone get it up to speed and look and feel for vibrations.

MY assumption is that you have no vibration in neutral at the same RPM the engine would run at 77MPH.

All this MIGHT rule out drive train as issue.

thank you.

I agree Safety first. I like this idea. I am also going to change the driveshaft clocking and take it for a ride. I probably cant lose having a driveline pro do a balance job on the drive shaft.

I would love to take it to a mechanic that I can trust to do a first rate job on ride height and alignment. Both shops in my town assured me they knew how and measured from the lip on the fender.

even one look at the fitment issues I have on my car should tell you not to trust the sheet metal.....
 
My 67 has a shimmy at 44-46 MPH but it is front end for sure. entire cars shifts side to side and steering wheel shakes as well. from your description it sounds like a balance issue.

Another possibility is a bad NEW shock (doubt it BUT???) can you have someone follow you looking for bouncing tires?

Was the drive shaft modified in any way?
 
look in your Factory shop manual for measurements and points
I have the measurement points, and method down from the FSM. My garage floor is so uneven and broken I don't trust it. They wont let me back into the shop to show them how to measure and when I asked how he did it based on fenders i plain got mad and left.
 
My 67 has a shimmy at 44-46 MPH but it is front end for sure. entire cars shifts side to side and steering wheel shakes as well. from your description it sounds like a balance issue.

Another possibility is a bad NEW shock (doubt it BUT???) can you have someone follow you looking for bouncing tires?

Was the drive shaft modified in any way?

Drive shaft was also right off the parts car and was previously mated to the same trans and rear end it is on now. Only mods done was I cleaned it, and painted it with tremclad and pressed new U-joints onto it.
 
Sounds like you covered all angles, now just get'r in the air and up to speed.

also check the pinion and trans angles
 
Also check the rear pinion yoke and make sure the centering clips have not worn a slot. The u joint may be off center.
 
Also check the rear pinion yoke and make sure the centering clips have not worn a slot. The u joint may be off center.
Interesting. I did not notice any wear or when I was installing U-joints. I will carefully inspect.

thank you!
 
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