Colt

My car has always had a vibration that i have never been able to figure out, but i think I'm finally on the verge of pinpointing where the vibration is coming from.

My vibration starts around 80mph and gets worse as i go faster. Its also accompanied by a low but loud hum which also gets worse with speed.

I have a t56 in my car, and i dont think thats the issue since dont feel any vibration in the shifter. I have had the driveshaft and tires rebalanced and am confident thats not it. I now think its the rear end because i finally noticed that the sound changes a little if im on or off the gas; so it changes with the slight load.

I'm thinking that the sound and vibration is something to do with the ring and pinion, though, i had a new diff carrier installed last year and they didnt note anything wrong with the them.

I was wondering if anyone else has ever had an experience like this and if the rear end was the culprit.

big gear head

About 98% of the time it's the drive shaft. You may have had it balanced several times, but at low speeds the shaft is fine. When you spin the shaft faster it will develop a bow and cause the vibration. There are drive shaft shops that have high speed balance equipment that can check the shaft, but you may need a new shaft made of larger diameter tubing.


There really isn't anything in the rear end to cause this problem unless the pinion bearings are loose.

Colt

wouldn't I be able to detect the bow in the from of runout? Surely if its bowing at high rpm, its got to have some thats detectable with an indicator? The shaft is a factory 4th gen aluminum shaft. How much runout is too much?

big gear head

The bow would only be there at high RPMs. It would go away when the speed went back down. A high speed balancer would show this. Most drive shaft shops only have low speed balancers. Some of the better shops like Denny's have high speed balancers.

Colt

I'm wondering if i should just order a new shaft vs send it out. Summit has a new aluminum shaft for only 350 bucks or so. It sure would be easier to order a new shaft rather than send it out and wait to have it come back.

http://www.summitracing.com/parts/su...model/firebird

it has pretty good reviews... I dont need a beefy shaft.

big gear head

It's only 3 inches. I wouldn't use any aluminum shaft that was less than 3 1/2 inches, and 4 inches would be better. You may not need a strong shaft due to HP or torque, but you do need a rigid shaft due to higher RPMs. I'm guessing that because you have a 6 speed transmission you also have something like a 4.10 gear in the rear end. This causes the drive shaft to spin at very high RPMs at highway speeds, even though the overdrive transmission brings the engine RPMs down. You need a shaft that will stay straight at high RPMs even if you don't have a lot of power.

Colt

It's a 3.42 rear gear. I have been in factory 4th gens doing over 130 with no issues. I think my junk yard drive shaft is just bad.

big gear head

Did you have to change the length of the shaft when you installed the T56? If it's stock length then you might try swapping in another 3rd gen drive shaft just to check it.

Colt

No, my shaft is the stock length. I would like to try another shaft, but its not like they are just laying around. I have seen some on ebay, but there's no guarantee that those shafts dont suffer the same same issue I'm experiencing now. Those shafts are being sold for a reason, so I ordered the summit one.

a 3.5" shaft would be nice, but wow, those get pricey.

I still have a feeling that the issue is from worn out ring and pinion gears, but i have no experience with the failure mode of those so im not sure.

But this whole thing doesn't make any sense. If the shaft has no runout and is in balance, in theory, it should never bow. But i do know my shaft does have runout. I just dont remember how much. Its been years since i put an indicator on it.

Colt

Well, I'm running out of of ideas. I just put in a new drive shaft and the vibrations are still there. I have no idea what it could be.

big gear head

I'm sorry, but like I said before, most of the time it's the drive shaft. I guess you have already checked the transmission mount, rear suspension bushings and all of that stuff.

Colt

Yeah, checked all that. Im certain its emanating from the diff. The sound and vibrations change with the load of the engine. It actually gets worse in neutral.