If the rearend is out of alignment, could it cause a vibration that starts at about 60mph or should I be looking for it somewhere else?
I just re-did the whole rearend and everything from the transmission back, besides the 4.11 differential is brand new.. I put the same amount and thickness of shims on each side of the trailing arm bushings and leveled the tires to zero (to at least get me somewhat straight for the 40 mile drive to the alignment shop).. Anyways, I took it for a short test run and it starts to vibrate at 60mph.. The new parts I installed were:
1. New driveshaft u-joints (driveshaft was balanced when they installed the u-joints)
2. New balanced 3" half shafts with spicer u-joints
3. Rebuilt trailing arms with new bearing and bushings
4. New Fiberglass mono leaf
5. New Adjustable trackbars
6. New Shocks
7. New Tires and wheels that were all just balanced

I purchased the differential from a forum member and the yoke end-play is less than .030... Before rebuilding the entire rearend, there were allot of extremely worn out parts that caused a ton of rear movement but I had it up to 90+ and never had any vibration issues.. Just wondering if I can narrow this down myself.. I have a drag race in a few weeks and plan on running 90+ (1/8 mile) so I need to resolve this...

Thanks for any help!

 

look at your transmission / rear-end alignment.... it should be like a "Z" not a "U" ..... ie, if the transmission is pointing slightly to the passenger rear tire, then the rear should be pointing slightly towards the drivers side front tire. .... if not it will do as you described... i went through this for a very very long time....




this of course if your half-shafts, wheels, and drive shaft are in balance, ..... and make sure you have new rubber on the rear end and transmission mounts.

 

An out of alignment issue CAN NOT CAUSE VIBRATION! This is the oldest myth in this industry. Did you check the tires for Out of Round, or tread sep?

 

I never did anything with the trans or tans mount and Prior to re-builidng the whole rear end, as mentioned, I didnt have any vibration issues but I will check the trans/rear alignment to see if its a Z pattern...

Thanks again

 

I just installed these tires before I rebuilt the rearend last month and didnt have this issue..

Not being an alignment issue would at least rules that out then..

 

..... Myself and many others have been down this road... read the following thread





you dont have to do anything... just during your reinstall sometimes it happens, it happened to me and i changed everything, balanced everything and fought it for years.... finally ran a lazar line down it, and it was a U , realigned to a Z, problem went away.

took me a long time to find and fix this. think of the U shape as a jump rope, always out of balance to one side..

here was my thread, it explains it all... good luck..

http://forums.corvetteforum.com/c3-t...lso-still.html

 

Have you tried putting the car on Jackstands and letting it run in gear? I'd look at the tires and U joints. The alignment shop may also be able to see something, but camber and toe shouldn't cause a vibration unless the thrust angle is really off. You can check the thrust angle with a straight edge across a tire pointing a laser at a ruler set up against the front wheels bearing hub, (find somthing to set the ruler on which is the correct height). My Z-06 was 1 inch off, but didn't vibrate.

 

Oh man, do you actually believe that?
U-joint alignment absolutely can and does cause vibration, U-joint vibration happens at 4 times the shaft speed. this is because a u-joint travels in a ellipse meaning it changes velocity as it rotates. If the front and rear u-joint angles are not equal and opposite, the shaft will twist as it rotates and thus vibrate.

Never run a Vette on jackstands the half shafts are at an extreme angle and you will destroy something, like a u-joint.


so to summarize, u-joints need to be equal and opposite, this means up and down AND side to side.

Side to side is the one people miss. The rear end needs to be square to the transmission or it will drive you crazy!

 

I don't understand this piece of advice...?

My '68 never had any issues with vibration; after I rebuilt the rear suspension & diff it vibrated like crazy at in-town driving speeds. Had it aligned to Gulstrand specs and the vibration was gone.

 

Anyone have any idea how much pinion angle I can actually run? If I'm going to change it, I might as well set it up for a better bite at the strip.. I know they recommend 6-7 degress of pinion angle on a leaf spring set-up, and I used to run 8 on my Camaro drag car for maximum traction of the line, but I have never worked with an IRS system before so I dont know what the max pinion angle even is?

 

On a vette with a rear end that dosn't move, 3 degrees is usually recommended

 

I do not remember the up/down angle, but what I was talking about for vibration was left/right angles of the transmission tail shaft and rear end pinion ....

 

 

I'm going to check that while I'm under it..

How do you change pinion angle, by changing the pinion snubber thickness?

 

Again, you are looking at up/down, (which is also important) but i am talking side to side.

if you read my thread for this vibration problem you will see i ran a lazar line down both the transmission drive line and rear-end pinion direction. loosened all mounts for trans/ engine, and all mounts for rear-end then changed angle slightly..... 95% of my shifter vibration went the way of the doe-doe bird....

of course make sure everything is balanced correctly and the u joints are seated properly. But i fought (or just lived with it most of the time) this battle for like almost 20 years, just after my first (of about 5) rear-end blow outs. Along with 2 super T-10 transmissions and 5+ motors... (I am kind of hard on them i suppose)

I knew it was something different for me because of the symptoms, I could rev my engine from idle to 6KRPM and it was as smooth as a babies but...(have a new one of those so i can tell you that it is smooth, his name is Alex) .... Anyway, and I could take Baby to 100mph, through her in neutral and coast to any speed and again, smooth. BUT with power on somewhere around say 70ish the vibration would come ZZZZZZZZZZZZZ AAAHHGGGGGHHHHH!! I found that there was a sweet spot, If i would put on the gas slightly, not to speed up or to slow down, but in that middle area the vibration would smooth out and even go away...

Change angles as someone suggested to me, and I have not had the vibration since....