Thread: Mild vibration at speed

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I originally thought it was a tire or 2 out of balance, but now I'm thinking it may be in the drivetrain... ( I had the rear tires static balanced (weights on rear lip only and the front ones spun balanced with stick on weights towards the center and lead weights on the rear lip) car runs perfectly around town, but take it on the highway, and going up a steady grade with my foot on the gas, it seems to have a mild vibration... not terrible, but feels like a tire out of balance...I can sort of feel it in the steering wheel but also in the seat of my pants... it tracks down the road straight when i take my hand off the wheel. The vibration doesn't seem to go away if i go faster, basically does it from about 60 mph up. Going down the same stretch of highway the opposide direction, with a slight grade DOWNHILL instead of up, the vibration is seemingly much less noticable. Any guesses on what may be the cause? Driveshaft out of balance? Universal? Something else?

One thing I didn't do and should have, was throw it out of gear to see if the vibration went away... I'll try that tomorrow if the weather holds out, but could use any ideas or suggestions on what to look for in the meantime... thanks in advance guys! -Jb

My guess would be the driveshaft balance....U-Joints are all new, aren't they?

I'll go along with Dave on the driveshaft balance bit.
Check the runout as well.

Imo, statically balanced tires don't cut it.
You can have an imbalance between the inner and outer sides of the wheel.
Modern spin balancers will see this and that's why different size weights and locations between inner and outer sides of the wheel.

Another one that will drive you crazy - and it used to be common on bias plies - is the tire is out of round.
Even when perfectly balanced you'll still get a vibration at speed.

Don't overlook the engine either.
I'm not real knowledgeable about the SBC, but I understand there are different harmonic dampeners and if you have the wrong one....

Yes, the universals all appear new, but the driveshaft does not appear to have any weights on it at all, front or rear... is this unusual? As for the tires, I sota have them ruled out, at least for the most part, as the vibration definitely seems worse under load than when freewheeling in gear. The tires are currently radials, brand new BFG's actually. The fronts are spun balanced and the machine read 000 on front and back, so they are not likely the problem, however, the rears are still just static balanced. Think I should go get weights on the front lips too, or do you think its more likely the driveshaft? The engine is a crate 350, a bone stock 260hp, so I'd imagine the banancer is not the issue, although i will say it seems as though it could use some mild tuning, as it doesn't idle quite as silky smooth as I would expect a stock cammed 260hp small block to run. Also, not to sound nieve, but what is runout?

No driveshaft weights are somewhat common so no big deal there.
Runout will tell you if your driveshaft is straight.
Use a magnet and set up a wire pointer that's close to the middle of the driveshaft about 1/8" - 1/4" away.
Assuming you don't have a locker diff, get one tire off the ground - safely blocked, jackstand etc. - trans in neutral if a stick shift and spin the wheel by hand while you watch the driveshaft.
The distance between pointer and driveshaft shouldn't change to any great degree as the driveshaft revolves.
If I remember right the max allowable runout is 1/16".

If you have questions, pull the shaft and take it to a driveshaft shop where they have a driveshaft balancer.
Most cities have a driveshaft shop.

I've had some shafts require balance weights and some that do not.

If runout is excessive, most times the driveshaft shop can bring it into compliance . . . and if it needs balancing, they can do that as well.

If the driveshaft checks out, take the rear wheel/tire combo to a shop with electronic balancer and see what's needed.

I have 285/70R x 15 BFG tires on 10" x 15" wheels and they balance just fine with stick-on weights in the middle.

There's a chance the shop you were at did a static balance due to the large tires.
If so, go to a shop that sells 4 x 4 tires and they'll be able to balance your roadsters tires.

Double check your motor mounts to make sure one of them isn't loose.
Your difference in uphill/downhill vibration could be due to that.

Another one is to check the driveshaft angle at trans and diff.

As far as smooth idle goes, any chance you have #5 & #7 plug wires running parallel to one another for an overly long distance?
If so, one will induce a voltage in the other and fire when it shouldn't.

Along those same lines, are you sure the plug wire firing order arrangement is correct?


The taking the car out of gear and coasting will tell you several things.
One of them being the plug wire bit.

Thanks for the info... I know theres a driveshaft shop close by, maybe i'll just take out the shaft and let them check it out, balance, whatever. As for the rear tires, They're the same size as yours, except I have a bit narrower wheel, but i still think they should be able to use stick on weights near the center. I know the plug wires are right, but I will double check where the 5 and 7 run in relation to each other... one other thing, any chance it could be in the rear end? I know its just an old open rear out of an old ford truck or Grenada or something... factory gears and all... any chance it could be a bent axle or something like that?

Run a floor jack under the pumpkin and raise the tires off the ground a couple of inches. Chock the front wheels or run the nerf bar up against a big tree. Run a floor jack under the pumpkin and raise the tires off the ground a couple of inches. Run it up in gear to where you feel the vibration and note the speed for sure. Remove the tires and run it up again. If it still vibrates, it's not the tires. It's in the driveshaft, trans or motor.

Try this, while driving down the road, when you feel the vibration put the car in netural with the engine still running, if the vibration stops it's in the driveline, if not look at the tires or the front end.

Crawl underneath, grab the drive shaft and try to move it up and down at the gearbox. If it seems like there is excessive movement the yoke and tailshaft bearing are probably worn and thats where it's coming from. I've owned a couple of cars with that problem.

this is the first time I have EVER heard someone tell another to put there car against a tree :0

Well, sort of an update to the vibration dilemma... took it out again today, definitely notice more of a vibration under accelleration or load, let off the gas and it nearly disappears... pull it in neutral and the vibration also diminishes significantly, if not entirely. I'd say its definitely not the tires, most likely in the driveshaft or universals... I guess I will take out the shaft and see if i can find a place that can balance it, i heard the driveshaft shop in town doesn't do that (go figure) but that a semi-local machine shop does. In an unrelated "event", I snapped off my throttle cable right in front of my buddy's house... He egged me on into gagging on the throttle a little, and when i did, the Lokar cable pulled right out of the clip at the carb end. The return springs did their job, but I've decided that cabe is a pretty lousy design... did a band aid fix to get it home, but will be re-engineering that too I think.

I will check that, but the universals are both brand new, unless he overtightened them or they are defective, i think there's more of a liklihood the problem is in the balance or straightness of the driveshaft itself than the universals on the ends... I'd imagine its just a longer shaft that was shortened to fit a 32, I'm figuring there's a fair chance it may have been welded crooked, has a slight bend, or something along those lines...I didn't find a tree big enough to do the official driveshaft runout "test" , but I'll start digging into it soon and keep you posted on what i find (or don't find!) Thanks for all the tips. -jb

and if it's howling they probley threw away the inner cardboard tube, I did that one my first driveshaft shortening job, took forever to figure out where that noise was coming from.

I have taken a driveshaft out @ the rear yoke and rotated it 180 degrees and reinstalled it ... and the vibration went away ... more than once ...

Originally Posted by Deuce

I have taken a driveshaft out @ the rear yoke and rotated it 180 degrees and reinstalled it ... and the vibration went away ... more than once ...

I've heard this from someone else too... Sounds like a total long shot to me, but just the same, i plan to try it tomorrow... wish me luck!