OK, I have an '89 Iroc with the T-5 & 9 bolt axle. I've been chasing a vibration for a good while, and after taking some pinion angle measurements, I believe that I may have found the problem. The vibration is at 65+ mph, and does not go away when the clutch is pushed in. I've tried two driveshafts(1 is al), then had them both balanced, put on new tires, had them balanced, replaced the tailshaft bushing, replaced wheel bearings, and checked the pinion bearings(no play).

Finally, I got an inclinometer, and checked my pinion angles. What I discovered is that my transmission/driveshaft angle is about 3 degrees. I then checked the axle/driveshaft angles, only to find that my axle is actually angled further up than the driveshaft. Hopefully the picture will explain.

After reading some, and checking my measurements, I am now stumped as to how my pinion angles could have gotten this way. In looking over the car, I have learned that the axle in the car is most likely not the original. The RPO codes denote a 3.45 ratio, and the axle tag shows the 3.27. Both the optioned axle and the axle now in the car are the BW 9 bolt.

Anyway, the torque arm does not look damaged and from what I have read and seen, it is not adjustable. So, does anyone have any ideas on where I should go from here, besides spending $300 on an adjustable torque arm(Spohn)? Am I missing something? Is my theoretical drawing off? The adjustable TA is an obvious option, but I would rather spend the money elsewhere, if possible.

Thanks in advance for any ideas.

 

What trans mount are you running?

 

Yeah first off, if you run a poly mount, your vibrations will never go away, im convinced lol...

2ndly, when setting the ds pinion angle, you want to set it negative (that is the pinion points below the transmission. The pinion angle is also set with the vehicle at RIDE HIEGHT. very important, that the vehcile is sitting on its suspension when this is done.

The reason for the angle you have found is simply suspension travel. when purchase an adjustable torque arm, they typically recommend setting the angle to -3 - -4 degree's. stock i believe is -3*. the reason for this is because on a launch from a stop, where the most torque is pushed through the driveline, the back of the car squats, causing the pinion angle to actually go to zero. This helps with parisitic loss in the universals, since they are not taking up any angle, and it also allows for them to be at the strongest point- not trying to distribute power around a corner.

this happens not only because the rear end raises in the chassis (or the body lowers on the rear end, either way), but the lower control arms interacting with the torque arm cuase the rear end angle to also pivot some (very small amounts tho).

also, when you measure pinion angle, be sure to measure the angle of the pinion, not the ds. the ds angle is semi irrelevant, because th emotor sits tilted in the vehicle as well. so when even the pionion angle is 0, the ds angle could be 1-2*

Hope this helps, as for your vibration problem, i think you have the same problem i do, which i have not been able to confirm as anything but harmonics just coming together at that peticular speed. mine gets worse the faster you go however, which leads me to believe i think the rear end gears are the cause of it. i'll only know when i break this one and have to swap in a moser 9"

Steve

 

Thanks for the replys.....Sorry I forgot to mention the transmission mount. Currently, there is a stock mount installed. I purchased a Prothane mount and installed it, only to make my vibration much worse. Therefore, I put the stock mount back in, until I learned a little more about my problem.

As for the vibration...the car cannot be driven at interstate speed for any period of time. Also, I should have mentioned earlier, when the Prothane mount was installed, I had a vibration under hard acceleration at most any speed.

My measurements were taken with the axle on jackstands, therefore the weight of the car was on the suspension. What I did not check, though, was the axle to frame dimension given in the service manual.

Anyhow, thanks for the responses....I think I'm really just thinking outloud at this point......These vibrations sure can be frustrating.

 

better word i think is irritating!!! lol


best way to put the car up enough to check it is have the wheels on ramps or whatnot, and the car level.if you just have the back of the car raised up, the full wieght is not on it.

have you checked the wheels for being bent? like was this something that just started? or always been there....

some places will balance wheels and never watch them while on the machine for wobble... that i am thinking could also be one of my problems, since i have never trried a different set of wheels completely on my car...

lemme know what ya find!

 

Get the bushing in the back of the trans replaced, where the driveshaft slides into it; and check the pinion yoke. They get bent sometimes.

Shouldn't be too hard to tell the difference between a wheel-speed vibe and a driveline-speed vibe. Definitely get that sorted out before playing around with angles and stuff.

The stock suspension setting on these cars puts the pinion angle about 1 - 1.5 degrees below the drive shaft angle. That's usually a pretty good place to set it for street use, even with an adjustable. On the other hand, with real sticky tires and an otherwise properly set up suspension, 3 - 4 degrees often works better for drag racing launches. I would not set one up that way for the street though.

I have my adjustable set at about 1.5 degrees negative.

 

Did you ever find out what the problem was?
I have the same problem and can not seem to find out what is causing it.

 

Sorry for the lack of an update. I've been waiting on some warmer weather, as I have no garage heater. I've decided to concentrate more on the axle for now. I put the car on jack stands, removed the driveshaft, and ran the car....vibration went away. I've been trying to follow some advice from a Chevy Service Manager I know, and therefore removed the tires and ran the car on jack stands. Vibration was still evident. He thinks I should remove the rotors and verify that the vibration still exists. I don't think that it's the rotors, but I'm probably going to do it anyway1. He keeps telling me that internal problems in the axle make more sounds than vibrations. Anyhow, I'm basically tinkering my way through things. That's all I know at this point. Still open to suggestions...

Jason

 

Same problem with mine. I think I didnt tighten my pinion nut enough..

Pinion yoke bent eh?

 

I agree that problems in the rear will make sounds instead of vibration. That has always been my experience.

I'd try running it with no rotors and see what happens. People used to balance the tires on the car for exactly this reason. It allowed them to adjust for rotor, hub, and axel irregularities. Most rotors come balanced, but you never know.