Drivetrain Vibrations

[mfain]

No, You don't want to put the yokes out of phase -- that was Chevy's crutch for a situation where the crank and pinion were not parallel. I would just drop a couple of plumb lines, one from the front/center of the crank pulley and one from the center of the transmission output shaft and make sure the line between them was at a true 90 degree angle to the rear axle (which means it is parallel to the pinion). If the crank and pinion are parallel, but offset laterally, that is not a problem as long as it is within the tolerence of the u-joint -- 2 degrees is no problem. If they are not parallel, I would investigate how to move the tailshaft left or right to correct the problem. This may not be your issue, but it is just one more thing to eliminate as the culprit.

Pappy

[GregWeld]

If you read a lot about drivelines -- and I think even a post from David Pozzi - they DISAGREE with the Parallel and Zero degree driveline "rule". They say if you have a 'down' 3 degree at the engine - to set your rear end at ZERO...

I've personally always gone with the "rule" of parallel... but with a 4 link - there is no (or minimal) climbing of the pinion.... so I'm going to reset mine at zero when I get the 37 off the rack.

There is also driveline harmonics... so while you have a "new" driveline... there is MATH to be done to figure out the rpms to see what the CRITICAL RPMS your driveline diameter and length can handle. With overdrive trannys - and big tires etc... this can start to come into play. Although I would certainly not expect it at the speeds you say it comes in at... since you don't know what the 'cause is -- might as well investigate everything while you're at it.

[mfain]

Reference Greg's points about driveshaft vibrations, you might want to look at the Driveshaft Shop site below. It has a tech section on setting driveshaft angles as well as a "vibration" section that includes a couple of techniques for isolating vibrations. I had them build my half shafts and driveshaft (3.5 inch dia/24 inch long). I had the driveshaft balanced for an 8000 rpm critical rpm (actually 7926 rpm -- 6500 engine rpm divided by .82 overdrive) and the half shafts for 2600 rpm (8000 rpm driveshaft speed divided by a 3.08 gear ratio). I also balanced the stub axles and the posi carrier (with ring gear, bolts, and safety wire). My earlier angularity discussion was in reference to lateral, not vertical alignment of the crank and pinion. There are a lot of opinions about the vertical alignment, mostly dependant on the pinion wrap-up or pinion angle change induced by suspension geometry changes during travel. The objective is to get proper geometry when the suspension is "loaded" -- easy for us independant rear suspension guys, since nothing moves. Also note as Greg pointed out, that while the pinion angle does not change significantly with a 4-link, IF the links are parallel and equal length, but the pinion angle will change during suspension travel if, for example the upper links are much shorter than the lower links, as is often the case. I assume that since you have experimented with a wide range of pinion angles, you have eliminated that as a vibration issue.

http://www.driveshaftshop.com/driveshaf-menu.php

[70rs]

I am sorry to hear about all of the vibration issues, but I am really glad you posted the question. All of the information and tips given here will be a big help to a lot of people.
Thanks to everyone who has contributed here.

I hope you get it figured out.

[7008ccGTO]

Late model GTO's have experienced a few driveline vibration problems that have ultimately been traced back to the output shaft on the T-56. Have you checked there?

Several have chased many other parts before finding it, which sounds similar to where you are.

[The WidowMaker]

visit this site http://www.vibratesoftware.com/. it has a lot of useful info on setting up drivelines and how to get rid of vibrations. if youre spending a bunch of money on replacing parts, it may be cheaper to take it to an expert to figure out. they can use a vibration frequency device to rule out possible issues.

the above site also lists all of the acceptable driveline angle combinations, and includes reasons why the common "equal and opposite" or "parallel" setups arent always ideal. i believe that most driveshaft companies state that those are the only way since it makes it less confusing. i used one of the alternative methods and ended up with working angles between 1* and 1.5*. according to what i have read, it doesnt get much better than that. the traditional setups had me closer to 4*.

heres the link to the part about driveshaft angles. http://www.vibratesoftware.com/html_...gurations.htm# remember that there is no standard reference point for what is level. you can choose your frame, how it sits on jack stands etc.... it really doesnt matter. its all about the relationship of the three parts and the angles (working angles) between them.
Tim

[RSZ28]

Originally Posted by Payton King

on a friends car. He went from a GM 12 bolt to a 9 inch ford. The Gm is hub centric and the ford is lug centric. Meaning the wheel fits on the raised portion of the axle to center the wheel on a GM. On a Ford, the style of lugs center the wheel. What happened was the same wheel and lugs were used when he switched to the 9. The wheel would move and get off center causing a vibration at high speeds....basically wheel run out.

You can either change lug style and make sure the wheels is sitting flush on the axle pad or machine a thin spacer with a hubcentric centering ring.

Does this make sense?

Makes sense to me. Have been through that one.

sfenco did you spin it up to 80mph both with and without wheels?

And..have you had someone drive along side the car when you are experiencing the vibe to see if the wheels are perfectly stable.

Have seen wheels oscillate vertically from weak shocks.