cyclic / pulsing pibration at 55 MPH and up

[TheRestoGuy]

Hi all -



I have my car back together and am attempting to drive it. The problem so far is that I have a vibration at 55MPH and up ... until about 85 when I think it nulls itself out a bit. Until around 85, the vibration will speed up in frequency. I haven't gone much faster than that. I'm suspecting pinion angle possibly front wheel bearings right now.

Car in neutral coasting around the speed range changes nothing. During acceleration you still feel the vibration the same way, as well as during deceleration in and out of gear. Clutch position changes nothing and 4-6th gear all do it at the same frequency, speed dependent.

The vibration comes through the steering wheel, but it also transmits throughout the entire car.. my hood shakes as well as my seat.

My engine sits at 3 degrees crank pulley up (Hooker LSX mounts), the transmission is at 2.75 input shaft up (Hooker transmission cross-member), and the rear differential is at 1.75 yoke up. At this pinion angle I had the closest to smooth I'd had, but still had the pulsing vibration. I tried extreme up and down on the pinion angle (within reason, -1 from zero degrees and + yoke up to 3.2 degrees positive) both adding more vibration and groaning with harsher vibration at higher speed. The - 1 degree from the yoke seemed smoother, but still not right. I may try -2 degrees, but I'm not sure it'll help. I've been scouring the internet for some sort of lead with possible wheel bearings being something to look into. The car sat far a few years before I received it so I'll replace those and try again.

Had a T-56 that imploded in the car, it was used, and it vibrated the same with this new transmission in the car. The car now has a T-56 Magnum,new, and still vibrates.

Brand new 1350 spicer u joints on a new 3" steel driveshaft, sonnax yoke, still vibrates.

Wheel and tires have been balanced.

I have adjustable rear upper control arms on my 1983 Monte Carlo LS. It has a 15" rim and a 28 inch tall drag radial on the back, narrowed 12 bolt axle, and a notched frame. The rear lower control arms were replaced with a boxed aftermarket variant, but they were cut, bent, and welded to fit for the narrowed axle. I'm wondering if these aren't 100% equal, will this cause my cyclic vibration at highway speed?

I have a cube angle finder I can use. What are your recommendations? I'd really like to track this down, learn something, and not just throw parts at it.

Alex

[dontlifttoshift]

With those angle measurements, what is the angle of the driveshaft? We need to know the working angle of the U joints.

Transmission and engine are the same angle, they can not be different unless your bellhousing is bent so make sure you are getting accurate readings.

[TheRestoGuy]

My best setup that felt better but not right, still had the cyclic vibration starting at 55 mph was: Transmission: 2.75 Driveshaft: 2.9 Differential: 1.5

This was from last night. Today after work I can try any suggestions. Thanks!

Alex

[q67]

I had the exact same problem and chased it for years. One day I changed the light Springs to the heavy Springs in my MSD distributor and the vibration went away. It was the engine causing the vibration quite often the pinion angle has nothing to do with it. Long time ago I bought a brand new Savana cargo van and as soon as I took it off the lot and drove it on the highway the steering wheel vibrated up and down almost 2 inches this was a brand new truck they could not figure out the problem finally one mechanic installed a vibration dampener on the exhaust close to the muffler and the vibration went away.

[TheRestoGuy]

I had the exact same problem and chased it for years. One day I changed the light Springs to the heavy Springs in my MSD distributor and the vibration went away. It was the engine causing the vibration quite often the pinion angle has nothing to do with it. Long time ago I bought a brand new Savana cargo van and as soon as I took it off the lot and drove it on the highway the steering wheel vibrated up and down almost 2 inches this was a brand new truck they could not figure out the problem finally one mechanic installed a vibration dampener on the exhaust close to the muffler and the vibration went away.

I wish these things could help fix it.. but I've driven it open headers and have the same problem.. and it's an LQ4 engine, so no flyweights for me. Thanks for the input though.. Who knows though, maybe it's wheel bearings? Happy Friday.

Alex

[TheRestoGuy]

Any ideas? Should I change the pinion angle a bit? Any other ideas on the vibration?

[sccacuda]

I see lots of post on vibrations. If your willing to do a little math, I guarantee you can isolate where it is coming from. I was a dealer tech for many years, and long before Chassis Ears and Chassis Ears II, we diagnosed NVH issues with a Sirometer. The Sirometer is very low tech, but works! It will tell you the frequency (Hz) of the vibration, and then by applying some math, you can pinpoint which component(s) operate at the offending frequency. I've used it countless times, and in every instant, I found the issue. As my most memorable example, I had a truck with a vibration at 30 mph. Drove it, measured the frequency, did the math, and the only thing operating at the vibration frequency was the tires. Told the customer is was a tire. He told me he just had brand new tires put on it, was doing it before the tires were replaced, and didn't change after replacement. Told him I understood, but the math is correct, and it IS a tire. He replaced the tires again, and came back to thank me for correctly diagnosing. Could never have imagined that it had a bad tire, replaced with new tires, and got a bad tire again. I have found driveline, exhaust, and suspension issues with the Sirometer, but more importantly, I can eliminate possible causes by knowing what the offending frequency is and what other components operate at, engine, driveline, etc..

You can buy one here: https://www.amazon.com/Briggs-Stratt...ords=sirometer

This is a sample worksheet I use:



This is a sirometer:

[Ron F]

I had a similar issue with my 1970 Mustang. Two suggestions from my past experience.

(1) Check your shock absorbers - 3 of 4 of my shocks were not working on my car. After replacing the shocks my vibration was reduced dramatically.

(2) Check the roundness of your wheels and tires. I had my tires balanced twice at two different places. At the second place I was able to actually watch and saw my wheels and tires were out of round. Suggest using a dial indicator to check this.

I do like Mr Scholl's (sccacuda) solution too.