Vibration

Questions about all other brand differentials.
Bobman58
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Vibration

Postby Bobman58 » Mon Sep 23, 2013 9:20 pm

I've had this problem for a long time now. It's a high speed vibration. I don't generally get into the 100+ range so it doesn't happen too often. I notice it at or near the end of a quarter mile run when my speed is approaching 100 or so. The gearshift lever gets pretty violent. Once it got so bad the the rear u joint broke out and I lost the driveline. I had to replace it with another so I don't think that is the prob because it still does it to this day. No noticeable vibration at lower speeds up through around 80-85 but at 95 things get pretty dicey. I'm wondering if the could be a bent axle? I've had the rear rebuilt with new bearings and crush sleeve and all but still have the issue.

Robert

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monzaz
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Re: Vibration

Postby monzaz » Wed Sep 25, 2013 8:01 am

wow... WE HATE VIBRATIONS AS THERE ARE a LOT OF MOVING ROTATING PARTS ON THE DRIVE LINE...LOL.

Things you can look at and hoping other will chime in here as i am sure many people have had these issues and with ANY 40 -50 year old car these vibrations are almost always common place. Especially at higher rpms.

OK well here is a list-
MOUNTS - from motor mounts to transmission mount to rear control arm bushings... they sometimes LOOK good but are all worn out just from age.
DRIVE line angle - If you installed a different motor, motor mounts transmission mounts rear end springs lower or jack the car up all these things can affect the drive line running ANGLE. There is a optimal angle for the front u-joint compared to the rear u-joint... under heavy loads such as drag racing and high speed runs the drive line angle will pitch upward and should only go to zero degrees making that a strong position for torque to be delivered to the rear end components...once these angles are compromised past that point they will cause bad harsh vibration and harmonics to the drive line. Extreme cases have caused u-joints and tail shaft of trans to beak, crack, fracture and even explode!
U-joints - Old u-joints - WRONG or improper installed u-joints will cause vibration also - sometime peopel will have a cap fall off and loose a few needle bearings and not even realize - THIS will cause vibration and harmonics again too.
SHOCKS - shocks are one of the biggest OVERLOOKED items for vibration... They look fine so they must be ok...NOT SO. most of the time shocks are damaged inside the shock and you will NEVER KNOW. Tire will bounce like basket balls on burn outs - high speed vibration - excessive body roll - squirrelly rear end - brake dip and more headaches...
TRANMISSION TAIL SHAFT BUSHING- worn out - loose again will cause low and high speed vibrations...
Tires- Belts broken, tire tread from burn outs, out of balance aggressive tread all might cause noise vibration and harmonics

I will add to this later. have to get back to the shop. :) Jim
J D Race

Bobman58
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Re: Vibration

Postby Bobman58 » Wed Sep 25, 2013 12:57 pm

Thanks for the reply Jim.
I guess I should've given some background on the car. It's a 69 Firebird that I have replaced, or swapped out, just about everything at one time or another. Motor mounts. trans mount(poly), swapped trans out with another M20, replaced all shocks with new KYBs(gas adjust). As I said I replaced the drive shaft at one point(I think I had it balanced before I put it in), obviously the U-joints were replaced(and greesed). I had the original rear end rebuilt by a reputable trans/driveline shop. I've swapped out a few axles to repair bad bearings over the years but always with used axles with new bearings installed. New tires and wheels(alignment done). About the only thing I have not replaced yet is the rear leaf springs. I have swapped them side for side trying to level up the car because the left side has a sag to it but side for side didn't fix it so that prob is somewhere else. I run Comp Engineering Slide-a-Links and a panhard bar. I've owned this car for close to 35 years and it has always had this vibration as far as I know, but like I said it's rare that I get into the 100mph range(mostly street driving). The car does sit low to the ground and I have never checked the pinion angle.
I wonder if there is a tutorial somewhere that explains how to go about doing that?
Also I don't think it's rpm related because it doesn't do it in the lower gears so pretty sure it's a speed specific thing.

Robert

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monzaz
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Re: Vibration

Postby monzaz » Wed Sep 25, 2013 10:56 pm

yES WELL IF THOSE SPRINGS ARE SAGGING AND WEAK ESPECIALLY FROM TORQUE being delivered to it hard... they would ne the next replacement--- it will change the drive line angle from the trans to the pinion yoke. WHICH may or may not affect the angle.

Remember with poly bushing- they are much less forgiving for vibrations as they are tighter and do not soak up the harmonics and vibration that stock rubber does.

Now too has the motor been rebuilt? If it does not have all the parts reciprocating parts balanced they will shake the car apart the fast the RPMs rev.

stated before vibrations are tough to get out of cars. years of gravity pulling on the chassis and frame any accidents where frame bends happened??
I have a 1971 Monte Carlo and too have vibrations harmonics at certain rpm ranges on decel and accel.

See how well the NEW cars we have on the road today drive in 40-50 years. I can almost guarantee you will not see 90% of them even around. :) Jim
J D Race


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