About Us

pontiacmark · 03-02-2008, 10:25 PM

Been tracking down a vibration with my GTO and I think it is down to a the driveshaft/pinion angle. The car has an M20 4-speed with a 10-bolt rear. It is not modified but the driveshaft angle is off, maybe due to a tweaked frame. With the driveshaft being considered at a 0 degree angle, the trans is angled 0.2 degrees up from there, and the pinion is pointing down to the driveshaft 4.3 degrees. From what I have read, the difference between the trans and pinion should be less than 1 degree. The difference between the driveshaft and the mating units should be less than 3 degrees. The motor mounts are brand new, so the only two ways I know how to change the angles are to shim the trans mount and to change the upper control arms to adjustable ones from BMR, Hotchkis or another source. Does anybody have any other thoughts or ideas?

getmygoat · 03-02-2008, 10:40 PM

I think you have it.... I know many of the aftermarket trans mounts are too thick, but in your case it sounds like you don't have enough thickness..... Are you sure your trans mount is in good condition... maybe you just need a new one?

Usually transmissions sit too high, and the fan hits the bottom of the shroud, etc.

I wonder if there is some natural frame aging process that results in these angles getting out of whack.

6D6GOAT · 03-02-2008, 11:17 PM

did you rule out all other options. after i converted to a 6 speed i had problems with vibrations due to pinion angles.. i got the angles a little bit better, in fact i dont think any better than what you are running now. it helped a little but i still had vibrations. it wasnt until i replaced my cragars (which i bought new and were properly balanced) with torq thrusts that my vibrations went away.

if you do go the adjustable upper rear control arms (i went with edelbrocks and am happy with them) just remember that you need the pinion angle to be pointing down. when i adjusted mine up it wheel hopped so bad i about shook the car apart.

pontiacmark · 03-03-2008, 07:56 AM

Yes, I ruled out other options. Tires and wheels are balanced well. Driveshaft is new. Rear drums are new. I measured runout on the axle flanges and the axles appeared straight. It is in the pinion angle or something internal in the rear end. I've pulled the driveshaft out and ran it to an indicated 75mph with no vibration, so I know it is something behind the trans. It looks like I will need to raise the transmission mount a little and then rotate the pinion up a few degrees. Were the Edelbrock control arms of good quality?

Kyflier · 03-03-2008, 12:46 PM

I think you missed part of his post. He said his wheels and tires were well balanced as well. You might try a buddies wheels and tires before you mess with anything else to rule that out as well. Just a thought, FWIW

getmygoat · 03-03-2008, 01:49 PM

I've also heard that balancing the tires is not a sure fire way to rule them out... apparently some tire/wheel problems get past balancing.

Also - when you ran without a driveshaft, did you end up with oil everywhere? I've considered that but assumed without a yoke the trans would leak like crazy.

Thanks - I've got the same problem, so I'm watching this intensely to see if you can sort it out.

Keith Seymore · 03-03-2008, 04:43 PM

Quote:

Originally Posted by getmygoat

I've also heard that balancing the tires is not a sure fire way to rule them out... apparently some tire/wheel problems get past balancing.

Well, if the tire is "egg shaped" then that will create a vibration, but a twice the wheel speed (ie, "2nd order of tire rotation"), whereas an imbalance is once per rev.

Quote:

Originally Posted by getmygoat

Also - when you ran without a driveshaft, did you end up with oil everywhere? I've considered that but assumed without a yoke the trans would leak like crazy.

I have a plastic shipping plug that I put in the trans tail for this kind of work.

Quote:

Originally Posted by getmygoat

Thanks - I've got the same problem, so I'm watching this intensely to see if you can sort it out.

Hey, I thought you were going to start swapping parts between your "good Goat" and your "bad Goat". You need to get to work!

This one sounds like too much pinion angle to me. The driveline will tolerate a bit more working angle than one degree IF the front and rear joints are closer to being equal. I'd try to get that pinion working angle closer to 1 degree IMHO.

K

Grant Miller · 03-03-2008, 05:43 PM

Quote:

Originally Posted by pontiacmark

and to change the upper control arms to adjustable ones from BMR, Hotchkis or another source. Does anybody have any other thoughts or ideas?

If you want some good quality and inexpensive adjustable uppers, search for dshop2000 on ebay. I bought a set from him last year. I don't see any listed currently for the A-body, but shoot him an email and he can hook you up.

Good luck on your vibration issue, I know your pain all too well.

pontiacmark · 03-03-2008, 09:47 PM

Yes, I did swap out the wheels and tires with a known good set. Thanks for the feedback so far.

tempest455 · 03-03-2008, 10:08 PM

Mark,

If you are using the stock control arms and rear end, the angle cannot be that far off. Additionally, the frame cannot be moved that much to create a significant deviation.

It's many times the small things that are the problem. Also. I have a buddy that bought a brand new high dollar racing driveshaft once and had a vibration. Turns out it was not balanced right.

Is it driveable?

pontiacmark · 03-03-2008, 10:37 PM

Eric,
Thanks for the reply here and the other forum. I have been chasing this for a long time. It is a vibration that comes on at 55 and then really starts resonating at 65+. The car is driveable at low speeds, just not on the highway. I have tried one used driveshaft and then had a new one made. I have put the car up on jack stands and run it up to speed and it shook the same. Just to rule out the new driveshaft, I just borrowed one from Tom Hart's '69 and it was still the same. I haven't ruled out something in the rear end yet.

I do suspect the car has been hit in the rear at some point as the rear bumper is angled up some.

Old Man Taylor · 03-04-2008, 12:02 AM

I'v had that problem twice. Once it was a bad pressure plate, the other time it was the drive shaft. I would be very surprised if your pinion angle would be causing a vibration that bad at a constant slow speed.

lust4speed · 03-04-2008, 12:23 AM

When you had it up on jackstands, did you remove the wheels to eliminate them from the equation? I would also remove the brake drums as well.

rod cole · 03-04-2008, 12:50 AM

I do not realy understand youre descripsion of youre angles. But the service manual says the engine\tranny and rearend should be in the same plane I.e. a line through them both should be parallel under power. So at rest the rearend can be up to 2 degree down from the engine to compensate for the rearend rising/rotating upwards under load. So youre angle finder pointing the same way on the bottom of the tranny u joint should be the same as the rearend u joint within 2 degrees down on the rear. So usually you will see with car level 2-3 degrees on the tranny and 31/2 to 4 dgrees on the rear end the angle of the shaft is irrelavant unless you are way jacked up as in 4 wheel drives.

lust4speed · 03-04-2008, 04:04 AM

From the company that designed the universal joints - seems like they should have the best idea of what is necessary: http://www2.dana.com/pdf/J3311-1-HVTSS.PDF

The problem that I am still trying to figure out is all the experts say to have the trans and pinion shaft in an equal plane. If the rear of the trans is pointed down slightly, then this should mean that the pinion should be pointed up to maintain the parallel relationship. Then everyone says that the pinion shaft should always be facing down a couple of degrees. At this point I'm confused on which advise is the best to follow.

pontiacmark · 03-04-2008, 08:00 AM

I'll create a drawing today showing the angles I have.

Remember guys, I have installed multiple driveshafts, multiple sets of wheels and tires, and a new set of rear drums. It is not in the transmission or forward either, as I tested with the driveshaft out.

Like Eric (Tempest455) said, I am not sure how the angles are off so much with stock components, but there is something out of line.

tempest455 · 03-04-2008, 08:51 AM

The more I think about the less I am conviced it's the angle. There obviously is something wrong but I really do not think that is your problem.

Start from sqaure 1. When did this problem start? What changes did you (if any) make when this happened?

I'd be happy to look at it if you can get it here.

rod cole · 03-04-2008, 03:04 PM

Yes the diff points up. But is down slightly from the tranny to compensate for the pinion rising under load. So that under load they are parallel.

bluebandit · 03-04-2008, 07:58 PM

Just a thought but in case you have not looked at this yet check the tailshaft bushing in your tranny. It fixed the vibration problem in my 78 T/A that was like what you are describing.

pontiacmark · 03-04-2008, 09:15 PM

Eric,
The car has had the problem since I bought it in 2000. I have just avoided highway speeds in the limited driving I have done with the car. It is about an 1 1/2 drive for me to Hendersonville, probably more if I have to limit my speed to about 55 mph. We can talk more about how we can get together to look at it.

Bluebandit,
I had thought about the tailshaft bushing. How did you check it? Did you just feel for play with the yoke or did you measure the inside diameter to see if it was oversize?

Menu

About Us