Strange Vibration

[greyghost]

Howdy (again)! When I get my '79 LeBaron out on the highway and I get up to 70 mph, I feel a cyclic vibration. The vibration is not constant. It runs in cycles. It would be like running over rumble strips every 200 feet. That's the best way I can describe it. At 65 mph there is no vibration. Since then I have replaced the front wheel bearings and front & rear u-joints but the vibration is still there without change. Any ideas as to what's causing this strange vibration?

 

[Oldiron440]

I had a set of copper tires on my 05 ram that would balance fine but vibrate at hyway speeds. Tire shop tried telling me that it was the OEM wheels but the vibration disappeared with a new set of BFGs.

 

[greyghost]

When I first noticed the vibration, I had an off brand tire (that you can't even find anymore) P245/60R15's all around on 7" rims. Since then I have now got Mastercraft Avenger GT P245/60R15's on 7" rims up front and Mastercraft Avenger GT P275/60R15's on 8" rims in the back (balanced OFF the car) and there was no change in the vibration. I replaced the front wheel bearings when I replaced the front calipers last year. The vibration was still there. Then I replaced the u-joints. The vibration is still there. The vibration is NOT constant. It is cyclic. In other words the vibration pauses every few seconds and only at 70 mph. How weird is that!?

 

[volare 77]

Hmmm.. hard to tell , sometimes vibrations are the hardest things to track down. trans mount in good shape?

 

[Aspen500]

A cyclic vibration is usually not caused by tires or bearings or an imbalance in any component, especially when it's only noticed in a certain speed range.

One possible cause is the frequency of the engine vibration caused by firing pulses (normal vibration, not an imbalance) matches the natural harmonic frequency of the chassis/body of the car. You may have seen those heavy rubber mounted weights on transmissions before. That's what those are for, to change the vibration frequency of the drivetrain. It's similar thing to an interior exhaust drone with certain mufflers. The frequency of the exhaust pulses in the mufflers matches the natural harmonic frequency of the car and,,,,,,,,,,DRONE.

Drive the car in 2nd gear at about the same rpm as 70 mph in drive and see if you notice the same vibration. It won't be as strong but would still be there.

So,,,,,,,,,,,,,first thing I'd take a good look at is your rear trans mount, and also the engine mounts. Make sure the rubber isn't collapsed, mushy or solidified. Look to be sure nothing is grounding out on the chassis or body from the engine/trans also, even the exhuast and it's insulators.
Reason I say this is, I chased the type of cyclic vibration you are talking about on my car. It came on about 55 mph and continued until 70 mph. Tried everything with no change. Then, just to be sure, I replaced the urethane trans mount insulator with a stock type rubber one from Carquest and,,,,,,,vibration gone. It was the drivetrain natural vibration frequency matching the car causing the harmonic resonance, which feels like a vibration and it can get VERY irritating. It was bad enough I never wanted to drive the car above 45 mph.

 

[greyghost]

Thanks for your input everybody! Honestly though, I think all of the rubber components for the undercarriage should be replaced. Suspension, mounts. etc. From what I'm seeing here in the forums, rubber is preferred over polyurethane. So is it advisable that I pursue rubber components?

 

[Aspen500]

IMO, it all depends on what you want the car to be. If you are going for the best handling, then urethane. If a more comfortable ride is preferred then rubber, except for the k-frame insulators. On those, urethane is best or if you're OK with a little more jarring ride and noise transmission into the car, then the solid spacers, like from Firm Feel.
If it's in the budget, the absolute best upgrade you can make for handling and general driveability is a Firm Feel steering gear. The Stage 2 is a good compromise for regular street use. Not too stiff, not too sloppy.

I'd recommend staying with rubber for the engine and trans mounts, mainly to prevent harmonic vibration, like you have now. Both the engine and trans have normal higher frequency vibrations and the rubber isolates them from the chassis and body.

This is all only my opinion and others may disagree.

 

[greyghost]

Thanks for the info "Aspen500"! Lots to consider! Feel, cost and timing! Thanks!

 

[BudW]

Places I recommend retaining rubber bushings:
Upper and lower control arm to frame bushings
Leaf spring bushings (front and rear)
Engine mounts (see note below)

Places I recommend urethane (or polyurethane) bushings:
K-frame to frame mounts (if not aluminum or cast iron)
Sway bar bushings (both frame to bar and end link to bar)

Exceptions:
Urethane torsion bar to K-frame and torsion bar to control arm bushings – I have not tried yet – so no comment there yet.
Sense rubber torsion bar bushings are almost impossible to find now, urethane or reuse original bushings might be your only choice.

I have heard from friends that when using a performance engine, replacing the drivers side engine mount with urethane helps to transmit more power to wheels – but it also transmits more vibration to car. Rubber passenger side and transmission mount will help the vibration down to a minimum - but it will still be noticable.
The other 98% of us should use rubber for all three engine/transmission mounts.

The rubber rear ISO-clamp pads need to go. To replace them yesterday is still "too long" ago.
Some members have replaced the rubber pads with urethane and has been happy.
Personally, I highly recommend getting rid of the entire system and either use an older factory shock plate or use one of the newer aftermarket versions. The metal brackets used in ISO-clamp system (using rubber or urethane) is not that strong to begin with (IMO).


Note: I changed the upper and lower control arm bushings in my ’69 Road Runner 30 years ago, to polyurethane (when they first came out). The car went from handling OK to a car that would shake your kidneys out over every bug or blade of grass you drove over. They would be good for road racing – but not for normal street usage (and that was back when I was in my mid 20’s).
If you are considering urethane upper or lower control arm bushings – I highly recommend you get a spare (used) set of 4 arms, and have them made up at same time. That way you have something to easily switch back if you decided that you had enough with your kidneys traveling to different locations inside of your body, from the road harshness.
BudW

 

[greyghost]

Bud, thanks again for the wealth of information! I don't want to be pissing blood for a week after going for a drive in my LeBaron. I just want to have a nice looking, peppy, and comfortable hot rod.