Vibration Chasing

Hey guys,

Working on my '73 Barracuda. It has been through a re-power (LA to stroker Magnum), front end resto-mod, subframe connector/torque boxes welded in and a custom drive shaft. Brakes have been completely rebuilt with modern friction materials and new rotors/drums/front bearings, wheel cylinders and rebuilt slider calipers and they perform flawlessly. 3.55 8.75" with old school XHD MP leaf springs (Made in USA) with good bushings. 3.55SG with cones machined and all new axle bearings (standard outers) done 5 yrs/12K miles ago, endplay corrected in other vibration chasing.

I have got this thing as dialed in as I can, no binding in the suspension, solid alignment, nothing moving in any way that it shouldn't. Handles very well on the street. I had the chassis shop that did the SFC's and torque boxes check the body it for square and make sure everything was in spec dimensionally (with the improved K-frame installed) and was told it is good to go.

I'm down to a vibration that the car has always had (I've owned it since 1988, bone stock when purchased) that comes in at around 65MPH, gets worse until around 75 and remains minimal but constant as fast as I go, and goes away completely when I decel below 65. These speeds are on smooth highway. If I'm on bumpier roads the onset speed is lower, but everything else is the same. The vibration I believe to be drive-train independent as it is present regardless of gear. I've had 3.23, 3.55 and 3.91's over the years with the same vibration pattern at the same speed.

One other observation that is hard to explain, but I'll try is that the vibration escalates in intensity and settles down, then de-escalates and mirrors the behavior on decel through the speed range. Always. It doesn't just start and stop.

Any ideas on the brain trust on this one? Monday car

Thanks
Chris

"I've had 3.23, 3.55 and 3.91's over the years with the same vibration pattern at the same speed"

Engine or Vehicle speed?

Vehicle speed. Engine RPM *seems* to be completely independent. I've played around with big cams and steep(er) gears, high vacuum stockish configurations, and now this stump puller so different torque peaks.

Well that should eliminate a lot of possibilities. Ever check for a bent rear axle/flange/wheel?

Not specifically, but I have been in there on both sides chasing an oil leak, doing brakes, etc. and didn't notice anything. Wheels all bolt up nicely and everything spins nicely with no inappropriate drag or anything.

That reminds me though, when I put the 3.55 R&P in (dropping from a 3.91 may have been the event that made me realize I had grown older), I had a shop machine the SG cones, replace the bearings in the chunk and outer axles. One of my axles suffered a shop accident and the guy got me a proper length/spline, etc. axle of unknown provenance.

The way it worked out, the other axle was installed and I just took the guy's word that the provided one would work, I just picked it up after work one evening, slammed it home, set the endplay and went driving. I've never looked at the axles side by side thinking about it. I suppose some are heavier than others too. How much bend would an axle need to cause a problem? I did sight it and eyeball the splines before I had him put the bearing on. Also, would excessively worn axle or carrier splines cause such a vibration?

A further detail I neglected to provide, the vibration seems to be in the front end. The front clip does a dance in the worst of the vibration.

J had a 72 challenger that did that before and after complete front end rebuild. Changed wheels all gone.

Jack up the front end and put it on jack stands under the lower control arms and then check both front tires and wheels for run out left to right and for run out up and down also
The car is talking to you, now is the time to find out what it is saying is wrong to you
I would also put it on 4 jack stands and run it at the same RPM in high gear to see if is still doing it or not

Excellent, thanks Cab! Some tangible action I can take that I haven't already.

I'll to that after work one evening this week and report back.

You changed gears, but have always had the same axles? Maybe try a dial indicator on the rear axle flanges.

One of the axles was swapped out, but good idea.

With back to school and everything, I haven't gotten out to put it up on stands and see if it has the vibration when not moving.

It may be a few more weeks, but as soon as I have an opportunity I'll get back to this.

Thanks for the input!

If an auto trans, check the transmission mount. Known to be a source of vibration if the rubber is worn or weak.
Ron

lower ball joint. seen them come loose in their seat from a good bump.. the wheel try's to hop at speed. you can jack it up an grab it by the tire and lift it up and down to find them if its bad enough... smack it with a hammer on the spindle to see if it is hanging up it may be coming loose and dropping back in especially if it got bent some....

What wheels are you running?

I had similar problem for 10 yrs, had tires and drive shaft balanced multiple times.
Not to mention the tire, rims, drive line angle changes, tranny mount and multiple other experiments.
Finally, on the advice of someone here, changed the drive shaft yoke and solved my problem.

Hey guys, thanks for keeping this up on the list. Sorry for the delay in updates. I've been giving little attention to the interwebs of late.

I put it on stands and ran it through the vibration speed range, no vibe. To me, that isolated things to the front. I checked every pivot, attachment point, mount point and all tight.

Wheel and tire package is 235/65-15s (Firestone Firehawk Indy 500 (their take on BFG T/A) on 15x7 cop wheels. The tires are from like 2009, but in great condition with low miles. Few places around here will touch them except to remove them.

Anyhow, I called the shop that got my alignment dialed for me with the adj strut rods and other tweaks and was bending the guy's ear about it. He has the gear to balance the tires on the car, and invited me back to visit his establishment. Interesting process if you haven't seen it and actually took care of it, though I'm not convinced it didn't just cover something else up. When I commented as such, the guy who did the work said that it was covering up old tire carcasses.

I'll be committing on a brake/wheel combo before long and there is much more available in 17. I guess I shouldn't be surprised that it is from the only wear items on the car that aren't actually new at this point.

The test on stands does not rule out out of round wheels/tires or axle flange runout.

A balanced tire can still shake a car. It may be out of round or have a sidewall problem but still balances zero.

Cab mentioned checking for runout. Was that done?

Vibration still gone with 100 mi or so including highway trips with speeds in the 70's. I quit troubleshooting after the on-the-car balance took care of things. I've got a few other items ahead of it in line, but if the problem returns when I settle on a brake/wheel/tire pack and get it installed, runout will be the 1st check.

Thanks guys