The Stovebolt Site Forums The Shop Area The HiPo Shop Strange vibration

Hi All,
I've finally got my 41 chevy sedan on the road. Overall everything is running well.
It has a Chassis Engineering Mustant II front end.
T5, Nova rear end (2.73), 28" rear tire, 2000 rpm @ 70 mph.
New tires,balanced.
Alignment done.
The alignment guy was able to tell with his equipment that the font end and rear end were in straight.

Here's the issue, I have a bizzare vibration in the front end.
It starts to happen as I approach 60 mph, and pretty much stays there after that. There is not a specific mph that it happens. It seems to have a mind of its own. Here is the bizzare part, it's NOT a constant vibration, it happens in a wave. Vibration... smooth.... vibration.... smooth... vibration... smooth (you get the idea). In the front end, I feel it 100% in in the steering shaft (the front end does NOT shake), then move through the body from front to back (mildly).
Here comes the wave... steering shaft... rear windows rattle (very mildy).... rear end (barely noticeable, may just be my imagination). .. starts over in the front.... All in perfect time, just like a wave of motion moving through the car from front to back.

The vibration isn't severe.
I have had three different sets of tires and wheels on the front, all balanced, and it happened with all of them (I was actully trying different sizes in the front for appearance/fitment, it had nothing to do with the vibration).

It's not so bad that I don't drive the car.
The faster I go (over 60) the faster the vibration and the wave, but the actual intensity does NOT increase.
Acceleration/decelleration doesn't seem to effect it.
Going straight or around a curve doesn't seem to matter.
Really bizzare.

Thanks.
Joe

the 'wave' aspect = harmonics, prolly sympathetic, might relate to engine RPM more than MPH .... you might find more help from Chassis Engineering

Bill

Good idea. I'll drop them a note.
Thanks.

Are you sure you don't have a bent front wheel?

Thanks. I've updated my post to make it clear that there have been three different sets of tires AND wheels.

Perhaps a bad universal joint?

Pinion angle off?

Flat smooth road, 65mph, in the middle of the "wave cycle", If you throw in the clutch and let off the gas does it stop right away or taper off as the the vehicle slows?

I can share a similar vibration experience if worth anything for consideration:
I had a 77 Dodge van with a 360 ci and bad tranny and I had a good tranny off a 318 ci engine in storage. I swapped the trannies and the one from the 318 buckled right up to the 360!
The problem was, the harmonic balancer in the 318 (that I left in that tranny) wasn't balanced for the 360... so I had vibration until I replaced the balancer.

Good luck!
Jerry

Was the driveshaft balanced? Take it loose and rotate it around so it bolts up to the pinion differently. If that doesn't help, take the driveshaft to driveline shop to check for warpage and balanced.

You're describing pretty classic symptoms of a driveline vibration. Try this- - - -Find a screw-type hose clamp big enough to go around the driveshaft and secure a 1-ounce wheel weight to one end of the driveshaft, and mark the position of the weight with a sharpie, chalk, or a tire-marking crayon, etc. Test drive, noting the severity and speed of the vibration. Move the weight around the driveshaft in 1-inch increments until the vibration goes away or is at a minimum, then have someone mig-weld a flat washer to the shaft approximately the same weight and position as the test weight. Problem solved!
Jerry

Yes. Driveshaft was balanced.

Thanks Jerry. I will give that a go.

..... I work part time at Carolina Driveline and I see the drive shafts out of balance all the time. Not saying that is your problem but its worth letting a driveline shop give it a spin if they have the type machine that spins them. If its OK a good shop will probably not charge you anything ....or maybe only a few bucks.

I hope that Hotrod will reply about U joints being in alignment or out of alignment. I remember a Jim Handy ( or other) film about U joints and it showed that when the U joints were not in alignment the rear end side would go up and down in rpm as the engine side was at a steady rpm.

Cardan-type universal joints (the cross and cup types) change rotational velocity as they turn if they are the least bit flexed. As long as the front and rear U joint of a shaft is synchronized so they flex in the same plane, one joint speeds up as the other one slows down, so there is no gain or loss of speed from end to end. If the joints are slightly out of phase, like when a driveshaft tube gets twisted a little or the slip yoke of a 2-piece shaft gets installed one spline out of phase, there is a binding situation between the joints due to velocity changes. This creates bending stresses and also vibrations. Universal joint phasing can be checked with a couple of short spirit levels. Place a level on one end of the driveshaft across the bearing cap yoke and roll the tube to center the bubble, then place a second level on the yoke on the other end of the shaft. Both bubbles should level at the same time. This is best done with the shaft in a vise with the ends approximately the same height, but don't overtighten the vise. Collapsing the driveshaft tube is not a good thing!
Jerry

65mph, pushed the cluth in, no change in vibration.

That eliminated anything to do with the engine.
Also most everything to do with the transmission since everything went to unloaded stress.

So now it's pretty much wheels, tires, rear end and drive shaft asy.

Since you don't feel it in the steering wheel, eliminate front wheels.

Sure looks like you focused in on the drive shafy asy.

I had a universal joint seize. That caused a vibration

Sounds to me like driveline phasing. pull it out and see if it is in balance and phased correctly

Simple phase check is to look at each u-joint and make sure that each fixed end of the driveshaft is inline front to back. If you have a two piece driveshaft, then the second piece would have fixed ends 90 degrees off of the first driveshaft and again, inline front to back. Single piece will have the transmission yoke and rear axle yokes inline 90 degrees from the driveshaft fixed ends.

We had a electric 5 ton truck at work with a four section driveline. Two sections had the yokes bolted on out of phase, plus the slip joint out of phase. These were imported in and converted to electric, so a lot hands worked on them. The vibration was so bad, it blew the gear reduction out in the motor assembly, you really didn't notice the vibration since top speed is 52 mph. On a steady flat smooth road, it did just what yours does, smooth then vibrate, smooth then vibrate, etc......

Put it up on good heavy duty jack stands and run it up to speed, a driveline vibration doesn't really need a heavy load on it to vibrate, just rpm.

Actually, the steering wheel is where I feel 99% of it.

Does your steering shaft have a rag joint in it?

Block the front tires back and front.
Jack op the rear axel and put it on 2 good jack stands with the tires about 2" off the ground. Don't jack too high because it will change the balance of the car.
Start it up and put it in high gear.
Slowly run the speed up into the range where the vibration occurs.

If you can recreate the vibration, you eliminated the front end.

If nothing you can do will cause the vibration, start looking at the front end again. Swap the tires front to rear and do the test again. Still nothing?

Look for strange things to do with front end geometry. Are you running radials? does the recommended caster increase for radials.

bad or loose shock on one side? All the A frame bushings in correctly and tight?

No.

Just wanted to update this.
It has been over a year, but I recently go fed up with it so...

I did put the rear end up in the air and ran it without vibration. So it was definitely a front end thing.

I left the car with the guy who did my alignment (he does tires too). The next morning he called me to say he got rid of 95% of the vibration.
Both front tires were out of balance, AND one of the tires was way out of "road force" balance. He fixed both and I couldn't be happier. What little vibration that is left is negligible (and as nothing to do with the front end.) It cost me $20.

Thanks to all.
Joe

http://www.motorweek.org/features/goss_garage/road_force_balancing

To absolutely isolate the source I would pull the drive shaft and have someone pull (or push) me to 65mph. No vibration---drive shaft related; vibration---wheel/tire/balance/alignment related. Just a KISS approach.

You could strap the truck up to a dyno so there will be a load on the drive train but no input from the front end.