Vibration ONLY at 75 mph

[65a833Dart]

I have and interesting issue with my 65' Dart, 170 two door Sedan daily driver. First of all I am am a new member to Fabo, but I do post on Fbbo once and a great while for the shop I work for. I'm looking for educated answers on my problem, so please read all my post before answering so there wont be any confusion.

My Dart is a factory 4-speed car, with a mild; 225 s/6, (stock low milage short block, ported head/larger 1.7/1.44 size valves, that's been ported and milled, shorty clifford headers, Offy 4-bbl intake with a 600cfm edlebrock (AFB wanna be) carb and a Comp 264S cam. The trans is a 3.09 first gear non-o.d trans, that has the 68-up 30 spline main shaft and later large out-put shaft tail housing. The car uses a stock cast iron flywheel, and stock replacement 9-3/8 clutch disc, and a Chevy S-10 2.5 four cylinder pressure plate. The clutch was balanced prior to installation. The drive shaft is a custom built unit with 7290 solid Spicer u-joints front and rear, connecting to an A-body 8-3/4 axle with 2.94 gears and a Sure Grip (cone clutch style). Pinion angle for street use is at negative 1-degree in back (pinion points down ward in relation to the drive shaft) and zero at the front. The car also has 10-inch A-body brakes all the way around, 340 torsion bars, custom made six leaf springs in back (with new poly bushings, and new gas shocks all the way around. The front suspension has been rebuilt with all new poly bushings, and Moog ball joints, tie rod ends, idler & pitman arm. As of now, the car has a fresh trans, diff, balanced driveshaft and freshly balanced 14-inch tires all the way around.

The problem; ever since I put the car together over a year ago there has always been a slight vibration between 72-77mph, any slower or faster it is a smooth as glass. But at 75, it feels like a driveshaft issue, enough to realize that there is something is not right. The vibration can be felt through the entire car; steering wheel, dash, seat, floor, roof, sun visors, etc. Accelerating, or deceleration doesn't change it at all, and neither does driving with the car in gear propelling itself or even coasting (trans in Neutral, with the engine at an idle)the same vibration is present. Since the car began to run, over the last 40,000 miles and almost one year of driving I have changed the following:

-Tires, front and back
-Wheels, front and back
-Driveshaft, (had two different ones) and the current one was just balanced
-Complete trans
-Rebuilt the entire front suspension, inspected all parts for wear, play, cracks etc. Installed new front shocks and t-bars
-Rebuilt rear suspension, new leaf springs & new bushings, new shocks
-Tried a different pair of SBP axles
-Tried a different pair of rear drums
-Tried a bunch of different driveshaft angles
-Complete drop out section of diff, including different flange, case, gears, sure-grip unit etc
-Tried a different pair of front brake hubs and drums
-Tried a different clutch assembly all together (first clutch set up slipped while power shifting)

Since the vibration isn't terrible, I just usually avoid driving at that speed. 70 its perfectly smooth, and at 80 its perfectly smooth. After everything driveline and suspension has been changed the vibration has not gotten worse, better or different it remains the same. This car has been very reliable, as it has been driven from Southern California to Northern Oregon (1,000-miles, in 18 hours of straight drive time at 70-80mph) three times, and it has gone on at least two dozen 300-mile drives with ease. What has me puzzled is that everything but the engine and the body has been changed over time, one part at a time and nothing at all changes. Again, to clarify; the car does vibrate at 75 mph coasting even if the engine is off, engine rpm and/or load has no effect on it.

This car when it first got running, got a bunch of drag strip use after I already was aware of the vibration, with lower gears the vibration was still present at the same traveling speed, regardless of how fast the drive shaft is spinning, from 4.10's to 2.94's still the same. The car endured over 100-drag strip passes, clutch dumps, wide open throttle power shifts etc.. The problem again never changed..

So if you actual read all of this, please any insight as to what it could be would be greatly appreciated. Thanks in advance, and I've loved getting to read others post on here for years, and its great seeing all of the enthusiasm for these cars is still alive and well.

Btw: Here's a picture of my car at Wildcat Auto Wrecking up in Oregon on one of my trips up north, most of the paint is original lol..

 

[/6 Matt]

Do the tires in the back wear normally? Maybe the rear axle isnt aligned just right. I really don't know this one beats the hell outta me.

 

[/6 Matt]

BTW even though im sure that slant runs (driveability wise) like a mild build, you amuse me with your definitionnof mild lol! I call that moderate.

 

[CUDA69]

Because the vibration only happens above 70 MPH it has to be coming from some where between the output shaft of the transmission and the wheels. I have been battling a similar vibration for years. With the drive shaft out try moving the yoke through it's full range of motion. If it binds even a little it will cause the shaft to vibrate at higher speeds. This will not show up when they balance the drive shaft. After changing U joints mine still was binding. It turns out the two flanges of the yoke that the U joint mounts in were not parallel, it was Y'd out. A new yoke solved the problem. It is an easy check.

 

[65a833Dart]

BTW even though im sure that slant runs (driveability wise) like a mild build, you amuse me with your definitionnof mild lol! I call that moderate.

Lol, well it is mild by my definition. The car does have issues with pinging above 4500-rpm's in 3rd and 4th gear on pump gas with the high rear axle gears but other than that it is insanely drivable. The vacuum is very high at idle (15-inches of Vacuum @750-rpm) with a nice subtle lopey idle. This engine was also in my 66' Dart with a pro shifted four speed, and a 4.56 gear spool and ran mid 13's in the quarter. In this green street car, best run is a 15-ohh with 3.23's and street tires. Compared to my previously installed 360 in my 66', this slant feels like a dog. (V8 spoiled me)I even have a 60' Corvair with a "mild engine" and a stick and it runs similar quarter mile times..

The rear tires have been on the car for the last 30,000 miles of the 40,000 traveled in this car, and they are wearing more in the center (burnouts + high air pressure) but the wear is uniform around the tires. The previously used rear tires for the first 10,000 miles got used up quick doing some street racing/burnouts and the early drag racing action. And the first pair of tires wore evenly across the tread with out any signs of a toe in, toe out/ camber alignment issue. Since the car got all of its drivetrain stuff swapped out, I've been nicer to it, (no more drag racing) and have been primarily driving it like a grandma. lol

I'm stumped too, hoping someone on here might aid in some insight. Next I'm going to accurately double check the wheel base from side to side as that is something I haven't done. The front aligned beautifully, and it steers perfectly straight at any speeds even with the brakes heavily applied so i doubt even if the wheel base was off a little bit as it would show up in the alignment or drivablity. Also I should add, that I'm sure this car hasn't been in any kind of accident of any kind, it was a one owner 79,*** mile car when I got it.

 

[65a833Dart]

Because the vibration only happens above 70 MPH it has to be coming from some where between the output shaft of the transmission and the wheels. I have been battling a similar vibration for years. With the drive shaft out try moving the yoke through it's full range of motion. If it binds even a little it will cause the shaft to vibrate at higher speeds. This will not show up when they balance the drive shaft. After changing U joints mine still was binding. It turns out the two flanges of the yoke that the U joint mounts in were not parallel, it was Y'd out. A new yoke solved the problem. It is an easy check.

Please read entire post, it only happens around 75mph, and at 80-110mph its dead smooth. The car has had four different drop out third members all with completely different components, and two different axle housings. Plus three different driveshaft; stock ball and trunion, a 7260 equipped stock style driveshaft and now the 7290 large U-joint driveshaft out of my 66' Dart that drives perfectly smooth. But thanks for your input.

 

[Coyote Jack]

Is it possible that you have an air problem? That is, at 75 MPH the air traveling over/under/around the car is buffeting and setting up a vibration through the whole car. Try changing the stance of the car, raise or lower the front and see if the vibration changes or disappears. I'm just trying to think outside the box, because it looks like you have checked or changed most everything else.

Jack

 

[65a833Dart]

Is it possible that you have an air problem? That is, at 75 MPH the air traveling over/under/around the car is buffeting and setting up a vibration through the whole car. Try changing the stance of the car, raise or lower the front and see if the vibration changes or disappears. I'm just trying to think outside the box, because it looks like you have checked or changed most everything else.

Jack

You might be on to something with that... A friend of mine that I work with formally did some pit crewing on a NASCAR winston cup car some years ago, and he came up with the same theory. And what semi-validates that is that with the car loaded with people in the back seat the vibration is slightly less noticable-ish... Probably the only time I notice a change is when the car is loaded and sitting lower in the back.

Great reply, and much appreciated! Will give this suggestion from you, and my friend a try.. If it is that, it would be very interesting, and frustrating as I really like the way the car sits right now. I can say though now even when the suspension was stock (prior to rebuild) the car did sit like exactly the same height, so hence the common denominator. Will post after I try changing the ride stance, in the mean time any other idea's?

 

[Oldmanmopar]

Sounds to me like you have a axle vibration do to one of two things. The housing is bowed or not square in car. I had a 4x4 truck did the same thing . The rear housing which was a 9 1/4 was bowed. I found the problem after replacing the complete rear after the center pin fell out and destroyed the housing . After replacing the rear it was like a new truck. I dumped many dollars into it before this trying to fix the vibration. Check the housing and the 4 wheel alignment. This will be magnified with a sure grip mine was. try a open diff and see if it changes.

 

[bighammer]

I re-read your post twice, nobody has said it yet: flux capacitor

 

[Darter6]

This is just a thought..Harmonics.It was explained to me by a airplane mechanic years ago,, something about engines develop a vibration at a given rpm during flight. I don't know much about it but something that was addressed during WWII .Maybe google it ???
Again just a thought.

 

[65a833Dart]

Sounds to me like you have a axle vibration do to one of two things. The housing is bowed or not square in car. I had a 4x4 truck did the same thing . The rear housing which was a 9 1/4 was bowed. I found the problem after replacing the complete rear after the center pin fell out and destroyed the housing . After replacing the rear it was like a new truck. I dumped many dollars into it before this trying to fix the vibration. Check the housing and the 4 wheel alignment. This will be magnified with a sure grip mine was. try a open diff and see if it changes.

Again please Read the entire first post, the car has had two different axle housings. Both A-body 8-3/4 housings, one that had been trued up by a axle shop as I planned to use it in my 66' Strip Dart, even added a drain plug.
Thanks anyways for the input.

 

[65a833Dart]

This is just a thought..Harmonics.It was explained to me by a airplane mechanic years ago,, something about engines develop a vibration at a given rpm during flight. I don't know much about it but something that was addressed during WWII .Maybe google it ???
Again just a thought.

Remember the first statement pleeeease, read the entire first post, the vibration is present while coasting WITH the engine off, as in not spinning.
But thanks for the idea, if the condition was related to rpm it would change with different gear ratios as the rpm changes, but it has nothing to do with the engine, plus the engine was formally in my 66' and it was dead smooth.

 

[winstoninwisc]

my 1 used to do that , i used it for my speeedometer when it got shakinid either slow it back untill it didnt wich was about 72 perfect or run her up a bit kick the cobbwebs out ckecked about every thing took it to the tire shop they thru it on the balancer was good untill i told him crank her up the whole machine was shaking like it was gonna come apart thru on a new set of tires ballanced gone slipped belt in the tire prob from doing whole shots lost a wheel wieght 1nce too but solved what ended up being a cheaper 1 was time for tires anyways

 

[Robbie2734]

Let me take a shot at this, you said the pinion angle is -1 degree. I think the angle should be 3 to 4 degrees down. Maybe somebody could verify the numbers but I have a sense that this is your problem.

 

[t-granger]

Let me give some words that may lend some help to this matter.
I have a 2009 Challenger SRT-8 and for over a year I drove it at the ride height that came from the factory. Six months ago maybe more, I decided to change it up and put on coil over shocks and I lowered the car probably six inches or more. The fenders now sit at the tire and although it does not rub anywhere at 80 mph I have a small steering wheel vibration.

Like you it is only at 80 mph and at 82 it goes away I did not have it before but now I do. Being so low the car steering is not 100% aligned so it is my cross to bear for having the car so low. Perhaps your issue is in the suspension I know yours is not lowered but perhaps the front end is not 100% aligned.

You might want to start there. I do not think you said where you feel the shake? In the seat of your pants or in the steering wheel?

 

[65a833Dart]

my 1 used to do that , i used it for my speeedometer when it got shakinid either slow it back untill it didnt wich was about 72 perfect or run her up a bit kick the cobbwebs out ckecked about every thing took it to the tire shop they thru it on the balancer was good untill i told him crank her up the whole machine was shaking like it was gonna come apart thru on a new set of tires ballanced gone slipped belt in the tire prob from doing whole shots lost a wheel wieght 1nce too but solved what ended up being a cheaper 1 was time for tires anyways

I can't stress reading my entire first post enough, I have changed both front and back tires once already, and have tried two different sets of wheels on the back, and the front with no change. All three times the car got a different tire or wheel install, they were all balanced. But thanks for the thought.

 

[65a833Dart]

Let me take a shot at this, you said the pinion angle is -1 degree. I think the angle should be 3 to 4 degrees down. Maybe somebody could verify the numbers but I have a sense that this is your problem.

I've ran the pinion all the way from down 5-degree's, to up 3-degree's moving in one degree increments and this had zero effect on the vibration. Please read initial post as I stated all of this when I started the thread. But I appreciate the insight.

 

[65a833Dart]

Let me give some words that may lend some help to this matter.
I have a 2009 Challenger SRT-8 and for over a year I drove it at the ride height that came from the factory. Six months ago maybe more, I decided to change it up and put on coil over shocks and I lowered the car probably six inches or more. The fenders now sit at the tire and although it does not rub anywhere at 80 mph I have a small steering wheel vibration.

Like you it is only at 80 mph and at 82 it goes away I did not have it before but now I do. Being so low the car steering is not 100% aligned so it is my cross to bear for having the car so low. Perhaps your issue is in the suspension I know yours is not lowered but perhaps the front end is not 100% aligned.

You might want to start there. I do not think you said where you feel the shake? In the seat of your pants or in the steering wheel?

The car has had two different alignments done on the car, both at different caster/camber adjustments, and I've tried changing caster and camber in both directions while keeping the toe within the given 1/8-1/4-inch range. Still no effect on the condition.

 

[Darter6]

I have read and re-read your post.When I mentioned Harmonics it was not just about the engine.That was a example. Anything with movement can cause vibration. Loose siding flapping in a wind storm for one.Any imbalance in motion will cause a vibration be it rotational or directional. We understand your frustration and are only trying to "think" of things that may help.You got a tough one and I wish you the best at trying to find and fix the problem.Be thankful that you haven't had comments like "Don't drive it at 75 and it won't vibrate ya no mo". Again We are only trying to help.

 

[Oldmanmopar]

Did you try going 70 in reverse?

 

[moparlover]

most of the paint is original lol..

my guess is a loose body panel or part that vibrating at 75mph due to wind effects. when you are driving what area does the noise come from?

i know you said you can feel it everywhere... but is one place stronger than the rest? or could your passengers tell you where they thought it was coming from?

when you are coasting, the wind is still blowing. have you ever been driving with a headwind and have the vibration at a different speed? even one time?

also when coasting, the driveshaft and parts inside the trans are still moving. that's why you never coast down a mountain with the trans in 1st or 2nd gear even with the pedal pushed in, the clutch is still spinning. (people have exploded clutches that way, they are not made to spin 15k rpms lol)

i know you said you have changed everything. what things in the drive train have NOT been changed? it sounds like you changed every single drivetrain part.

it will be either a loose body panel or one of those parts in the drive train that have not been changed.