Thread: Mild vibration at speed

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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?