Since replacing my rear axle with an 8.5 10 bolt i have had a cyclic vibration. The vibration occurs at 75 mph and above in fifth gear (tremec 3550) while cruising or accelerating. The vibration "cycles" about once a second- every second it goes from being there to not and then does it again. It is not terrible but can be heard and definitely felt through the shifter ball. The confusing thing is that it only happens in 5th gear.

Things that were changed:
Rear axle
ratio- 3.08 to 3.73
driveshaft was shortened and balanced
hotchkis 1.5" lowering springs

The tranny is the same. Another thing is that it and the driveshaft are spinning faster than they did with the 3.08 rear end. If it is a problem with the tranny in fifth gear, i wouldn't have noticed it until around 90 mph before. Still, it would seem like the problem would be with the driveshaft or rear end because those are the things that were changed. Thanks,
Dan

I would suspect the drive shaft is not straight or out of balance. Take it back to the shop and have them check the runout and balance at high speed. If they can’t do a high speed balance take it to a shop that can.

Bad bearings in the rear end can also cause vibrations like you describe. Put the rear end on jack stands and remove the rear wheels. Use the lug nuts to hold the brake drums on. Spin it up to speed and the rear end should be quiet and smooth. Any noise or vibration coming from the rear end housing would indicate a bad bearing.

I've done quite a bit of work on figuing out what's going on, but no luck so far, so I thought I'd update what I've found in hopes that someone might know something.....

I had the rear end checked out since I put it together myself. Mechanic says that everything is kosher in there.

I also had the driveshaft rebalanced and had the u-joints put into phase. Apparently they came from the factory 45 degrees out of phase to fix some geometry problem in some early Camaros.

One thing that I have done is run it up to about 90 and then turn off the motor and let out the clutch. The vibration when not under load stops cycling and is instead constant. This means that my fifth gear theory is bunk.

So, the only things that I am not sure about are the u-joints and the axles themselves. The u-joints were rebuilt and seem fine, but I think that I will replace those bearings next.

Do the wheels center on the studs or not? I pressed the studs in myself, and so the wheels could theoretically be offcenter on the axle.

I am at school, away from my car right now, so I'm pretty much just trying to work through this in my head and make sure there isn't something that I've overlooked...

Anyway, thanks for letting me write out my thoughts if nothing else!

Dan

I have witnessed when I can't find anything wrong with rear axle flanges ( bent), wheels bent or out of round , or drive lines or angles, that old headers will become fatiuged and and cause this harmonic vibration and or I have found that if you are running Centerforce II pressure plate, the little sliding weights will give a out of balance feel. This is also assuming that you have checked to see if your harmonic balancer has not slipped? Did you check axle flange runout after you installed axle?


Dont recall you saying if you declutched motor at hi speed and it still did this?

Yeah, I let out the clutch and the vibration was still there.

I really wish I could do some hands-on work on the car. I am getting suspicious of the axles... I think that I will call the axle company tomorrow and check to see if they have ever had problems like this or not...

There is a reason GM displaced the U-joints by 15 degrees, it is to always have a "loaded" driveshaft to eliminate vibrations.

Have the driveshaft set-up at the shop's machine and have them check the runout of the tube one inch towards the center at each end from the yokes, should be less than 0.006 inches, same for the axle flanges.

GM wheels are "hub-centric."

You should measure the phasing angles of the driveshaft, should be more than 1 degree difference.

Dont rely on what the axle people tell you because they will say your are the only person that ever had that problem... Go back and pull brake drum, set a jack stand close to the flange and roll it,, see how much runout its got ,, If you want to get hi tech,,, use a dial indicator and magnet stand.. Dont let stuff like this throw you, Do it yourself. Then if you got a bent flange,, call your vendor and tell him.

Have the rear axle and transmission angles been checked? Inland Empire Driveline's website has a good explanation of what needs to be checked, and done, to fix most driveline angle problem.

There is a reason GM displaced the U-joints by 15 degrees, it is to always have a "loaded" driveshaft to eliminate vibrations.
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What does that mean? Where is the 15 degrees you talk about?

thanks

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You should measure the phasing angles of the driveshaft, should be more than 1 degree difference.

What is a "phasing angle"?

thanks

Cyclic vibrations are usually due to an angle issue with the drivetrain. Randy's Ring and Pinion has some usefull info on vibrations.

http://ring-pinion.com/content/technicalhelp/default.asp?pid=114


Good article on driveshaft angles.
http://superchevy.com/technical/engines_drivetrain/driveshaft_rearend/0203sc_driving/

Basically, angle A should equal angle B, both vertically and horizontally.
http://carcraft.com/techarticles/p115739_image_large.jpg

Thanks for the replies guys, definitely helpful...

One thing that I didn't mention is that when the car is on stands the vibration is still there.

I've messed with the driveshaft angle a little bit but it didn't change anything significantly. I don't have a tool to use so that I could find actual angles. It could be that the Tremec and the Nova rear don't agree... It just seems like someone else should have delt with this before, but I guess every case is different.

Thanks all,
Dan

What does that mean? Where is the 15 degrees you talk about?

thanks
I tried to find the explanation on this from an earlier post, but could not find it.

When power is transferred through a "bent" U-joint, the speed of the U-joint is represened as a sine wave. The first 180* of rotation, the driving yoke leads, read spins faster than, the driven yoke. The last 180* of rotation, the driving yoke lags, read spins slower than, the driven yoke.

There are two U-joints per driveshaft. Same speed event happens on the rear U-joint as does the forward U-joint. Thereby, there are two points of no speed difference, 0* & 180*.

By offsetting one U-joint by 15*, the driveshaft is "loaded," the sine wave of one U-joint is lagging in transfer of power. The rear U-joint is the one lagging by 15*. Thus the shaft is always loaded.

One thing that I didn't mention is that when the car is on stands the vibration is still there.Did you remove the wheels for the jack stand test? Was the rear end making any unusual noise? If the rear end is the problem you will defiantly be able to feel the vibration on the rear end housing.

I would next remove the drive shaft and run it up in high gear. The idea here is to eliminate the clutch and transmission as the source of the vibration.

I still think the drive shaft is most likely the culprit.

I know how frustrating a vibration can be. I also had a cyclic vibration in my ’67. The rear end was vibrating as well as the drive shaft. The rear end had a bad bearing noise and you could really feel the vibration with the car on jackstands. The driveshaft (that was balanced at a local shop) :mad: was also vibrating. The two vibrations would cycle in and out of phase about once a second at 60 mph. The faster I drove the worse the vibration got.

When I changed the rear end to an 8.5”, before I took it off the jackstands, I hooked up the old driveshaft. It was a little long for the new rear end but I wanted to see if I had cured the vibration problem. Then I ran it up to speed. There was still a vibration, although it was not as bad and no longer cyclical. The next day I installed a driveshaft that I had cut to fit (at another driveline shop) and the vibration disappeared. :)

Now I can keep up on the freeway without rattling the car apart!

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You need to go to Sears get a magnetic protractor #939840 cost 10.00. Put it on end of transmission should be between 2-4 degrees.Then put it on rear end needs to be the same degree. Measure off a machined surface on both ends. This is pinion angle, if it is off it will give a vibration.

Just a thought I wounder if due to higher rpms if the engine could be sending vibs back though the car. If the timming curve is too advanced at the higher speeds it could misfire just enough to send vibs and not enough to really show much in performance.

Just a thought, but here's a nice .pdf document on chasing down vibration gremlins in a first gen Camaro: http://csgbenefits.org/camarovibration.pdf

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