75mph vibration

[Dave145]

Hi everyone,

Just got the Dart out of storage for the first time in a few years. I took it to the tires replaced since they were from the 80's. Got the car back, all was well. Drives smooth enough at normal highway speeds (about 55-60 around here), until I get on the interstate. the second the car hits above 70, it develops a very bad vibration. Let off the gas and coast down to 60-65 and it's fine again. The vibration feels like it comes from rear to middle of the car.

Fast forward to now. I have now replaced the rear u joint (even though it felt fine turning by hand), pulled my tranny to have a look at the flex plate (all good no cracks at all and all bolts tight), and I greased every front end component that had a fitting on it. Someone suggested checking the brakes (4 wheel manual drums) to make sure they aren't hanging up and they are not.

I'm at a loss of what's next. Get the tires rebalanced? When I had them installed I was told by the shop they balanced kinda heavy (3+ oz offset to balance them apparently).

The car didn't do this 2 years ago when I last drove it, which is why I'm stumped.

Car is a 1976 Dodge Dart 4 door special edition with the 225 motor and 904 trans. Open rear end with 2.71 gears. If it helps.

 

[mguner]

Even though the tires are new it could easily be a tire issue.

 

[famous bob]

Even though the tires are new it could easily be a tire issue.

did it do it before you had the tires put on ?

 

[Dave145]

It only had a light shudder around 55 and up. Now it currently vibrates enough to drop your brain out your nose at 75 and up. I'm kind of suspecting tires, but wanted to get other opinions too so I wouldn't be overlooking anything.

 

[Inertia]

Have a look at the new tires balanced. The tires have red and yellow dots on the sidewall when new to help effectively balance the tire. I often see these dots anywhere but where they should be, with lotsa excessive weight added. You may have to look on the inside of the tire, cuz if the shop is respectable, they shoulda washed the dots off the outside. If you can locate either dot, it should be very close to the valve stem..

Regardless, I'd explain the problem to the tireshop, and ask if they could check the balance.. Most shops will do this no charge. I would.

It's also possible the wheel threw a weight causing an imbalance.

hope it helps

 

[daddio4048]

Jack up each tire & spin them by hand & see if you don't have a tire that is out of round or possible bent rim!

 

[TrailBeast]

Even though the tires are new it could easily be a tire issue.

X2
Take it back and make them fix it even if they have to use a different tire or two.
A tire need 3 oz isn't your problem and I am positive you didn't ask for a hard to balance tire when you bought them.

I have to make my shop rebalance tires all the time when I get them changed.
So much in fact that I'm going to look around for a different shop.
Point is, tire balancer people don't make a lot of money so the quality of the job is often in relationship to that.

 

[rich006]

If you're still having trouble after you get the tires checked again, put the rear on jackstands and "drive" it in the garage. If it still vibrates, then you've ruled out the front end as the source. To rule out the wheels as the source, remove the wheels and drums (put a block under the brake pedal) and try it again (you'll have to start the engine in neutral so you don't need to shift through reverse). If it still vibrates, then the problem is in the drivetrain somewhere. I assume you've already tried revving the engine while parked to rule out engine vibration.

 

[Dave145]

I will take the car back to the shop once I have the tranny back in (currently out for flex plate inspection). I suppose while I have the car on the jacks I can test it to rule out any front end noises. I have run the engine up and down the RPM range in both park and neutral and it's stable as it can be. I have a tach in the car, and when the car is vibrating like hell the tach is still reading rock solid at 1900-2000 rpms. I wondered if anything in or on the motor could be causing this, but I don't have reason to suspect the motor because this just started after the tires were installed and because the car only has 33,000 miles on it.

The tires I had put on the car were Cooper cobras, so I would hope they'd be quality pieces! I will check for the dots when I get home tonight.

 

[Mopar to ya]

Tires. Get the balance checked. Also, what tires do you run? I ran BFG Radial TA for years and they took a ton of weight, and I mean a ton. Anywhere from 6 to 9 ozs each rear wheel. I had to static balance them, and I hate doing that. When I bought a new set, they took a ton of weight. I thought I had bad wheels. Then I went to the Nitto NT555R and they took 1.75 oz total on one wheel and 1.5 oz total on the other. Not bad wheels, bad tires - three years in a row. I will never use BFG again.

 

[Dave145]

I originally had some Dayton on it from the 80's. Then I got them switched to the Cooper Cobra tires aabout a week ago.

 

[Mopar to ya]

I'm not a huge Cobra fan, but it's just the looks. Cooper makes good tires. We are very limited in what we can get for our cars. I work in a shop and we sell tires, so i have access to a lot of tires. What size front and rear, and how wide are the rims?

 

[Dave145]

I know the size is 225/75/r14 on all 4 corners. This I believe is a bit larger than stock size tires, but the PO put that size on and I like the way they fill out the fenders and don't rub. Rim width I'm not sure of though.

 

[twayne24365]

If it turns out not to be the tires could possibly be the driveshaft is out of balance

 

[Mopar to ya]

That's a very easy size to balance. Not too large to throw lots of weight. I'd have them recheck them, or better still, find a shop that has a road force balancer and they can check to see if the wheel or tire is out of round. If all is good, I agree with the driveshaft balance being off.

 

[TrailBeast]

The only problem with the driveshaft balance theories is that changing tires doesn't change the driveshaft balance.

 

[Mopar to ya]

It does not, but this is all assuming it's a tire issue and not something else. When My driveshaft was out I had it on a hoist in gear with the wheels spinning and could physically see the yokes were true and there was up/down movement in the middle of the shaft.

 

[twayne24365]

Dads car did same thing but at around 50... it was the driveshaft....just a possibility

 

[AJ/FormS]

You may have more than one problem, but
Wait your slanty goes 75mph at 1950 rpm on 225/70-14s? That calculates out to a rear gear of 2.05!!
Something is not computing.Unless you have an O/D tranny. And I haven't heard of an auto with O/D for a slanty.
I believe you have a bad driveshaft. I think somebody bent it for you.
And here's why;
A tire imbalance or a bent rim, starts vibrating right from the get-go, getting worse and worse as its rpm goes up.By 75mph, that tire is doing 955 rpm.
An out of-round tire starts hammering the car up and down,almost as soon as the car gets moving.
A blown cord will wag the tail and make you seasick
But if you truly are only spinning 1950 ish, that makes perfect sense for a bent driveshaft, and you said you thought it was coming from the center of the car, and you said you saw the D/S moving up and down.
It is possible to bend or kink a driveshaft when lifting a car on one of those in-ground single-post hydraulic lifts.
Now let's revisit your 75 mph/1950 relationship.
A typical rear gear is 2.76. With your new Coopers, they would put your rpm around 2635 with the converter locked up, Allowing 10 % for slip, with cruise throttle it would still be around 2372.
So Ima guessing your speedO is generous. At least you won't get caught speeding......
If you run the car with the rear tires off, and drums on, don't forget to secure the drums with all 5 nuts.

 

[diymirage]

Got a mopar buddy around?
Have him come over and slap your wheels on his car and see if it vibrates

 

[Mopar to ya]

AJ, while I agree with you for the most part, vibrations in tires come and go at various speeds. It's very common to see them come in around 60 - 65 and go away at 75 - 80. I see it in the shop on a daily basis. I still would get the balance checked as it may have been a bad balance or tossed a weight, but I believe there is another issue as well - likely the driveshaft.

 

[Dave145]

I will check the driveshaft spinning tonight once it's all back together again (almost there haha). The shop is reputable, and hell my dad works there and put the tires on the car in the first place.

As for freeway RPMs, I'll double check my speed with a GPS at some point, but according to my build sheet the car has a 2.71 rear end in it and even looking at them now the tires are in fact 225/75r14 as I thought I remembered. I have a small tach in the car mounted below the ash tray (ya know hat little $40 one at Advance Auto Parts) so it's kinda hard to read on the highway. But yes, it most definitely did not read past 2100 RPMs while doing 70. However, the speedo gear is the original one that's probably set up for the stock size smaller tires, so it may be off a bit. As I said, the car is a 100% original barn find car with 33,000 miles on it, and I'm the second owner. Besides the tires, everything else in this car is stock.

 

[Dave145]

Quick update: checked the tires by free spinning them all by hand (no driveshaft in yet). None of the tires appear to wobble as the spin. All are mounted tightly and can't be wiggled by hand. Watched the rims and didn't see them walking in and out either. As for the dots on the tires that were mentioned, I did not see any on either side of any tire. I'll check driveshaft movement once it's in. Otherwise I'll bring the car back to the shop for a rebalancing on Friday since it's supposed to rain up here tonight thru tomorrow. Anyone near Burlington, Wi a classic mopar fan?

 

[Dave145]

Well guys, looks like we found our problem. As initially suspected, it's the rear tires. With the car on jacks and in gear, it had a definite vibration/waddle. When I walked behind the car, I could watch the tire shimmy side to side! Back to the tire shop tomorrow when it's not supposed to rain. Hopefully it's just a tire thing and not a bent rim (though I don't know how that could have happened).