Vibration at cruising speeds

built67 · 02-22-2012, 10:54 AM

I have had my new 9" in for a little while now, but still havent hit the 500mile break in mark (in which i will syphon out the old fluid and put in fresh fluid)...I noticed the other day on the highway (around 55mph) a vibration coming from what sounded like the rear...i gave it a little more gas to slightly raise the rpms and the vibration went away...i noticed it happens only at certain speeds, but the rear seems fine (like i said, under 500 miles on it, no burnouts or beating on it yet. proper fluid and lsd additive was put in).

When i was installing the rear, i noticed one of the cups on the u-joint fell off and the bearings were kinda all over the place...some of them fell onto my dirty garage floor and i just picked them up, blew on them, and put them back into the cup and popped the cup back on. Could i have bad u-joints that would cause this vibration? Its more of a "noise" then an actual vibe...i don't feel the car shaking or anything. Please tell me there is nothing wrong with my new rear! haha thanks for the help guys

btw: the u-joints are the original ones that were on the car when i bought it (they're the greaseable kind)

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built67 · 02-22-2012, 06:07 PM

bump...

67 evil eleanor · 02-22-2012, 07:58 PM

Any dirt will cause the neeles to wear or damage very quickly, also one or more of the needles may have been missed when you picked them up. U-Joints are very inexpensive and easy to replace. My first step would be to replace the U-Joint.

68dustin · 02-22-2012, 08:44 PM

sounds like a u joint... or drive shaft needs balanced. did you go up in gear ratio alot? being that u knocked the bearings out the u joint, i really bet that is the problem

built67 · 02-23-2012, 11:25 AM

i did notice some of the needles fell out, and i could have either missed some or put them back in dirty (all i did was blow on them to get any dirt off lol)...i went from a stock open 2.xx 8" to a 69 Mach 1 9", 3.70s and a T-Lok, so i definitely made an extreme change. The u-joint did look pretty beat up and the cup fell right off and the needles went everywhere, so ill change that out first. should i get the driveshaft re-balanced?

68dustin · 02-23-2012, 12:23 PM

I wouldnt balance the drive shaft if you said the u joint looked like that..lol.. Sounds like that would be the problem. I just asked cause when i went from stock gears to 4:11's i had a pretty bad vibration and the fix ended up being the drive shaft needed rebalanced.

JMD · 02-23-2012, 01:04 PM

A 9" rear is going to make more noise than an 8", as will the 4.10 gears.

I have installed a few aftermarket rear housings, more often than not, the pinion angle is not set right as manufactured.

It can be dangerous to mention this here,

but check the pinion angle to see if it is in the ballpark, (pinion tilted up about the same degree as the tranny tailshaft points down) It doesn't have to be BALLS ON PERFECT, but they should be within about 2 degrees of one another.

built67 · 02-23-2012, 02:03 PM

i think the u-joint is the problem, but out of curiosity ill check the pinion angle. i know this is a stupid question but how exactly do i go about doing that haha. i thought the angle wouldnt have changed because all though it is a 9" replacing the 8", its a 9" out of a 69 mach1 (so i dont really count that as aftermarket)

jp1967stang · 02-23-2012, 07:07 PM

is the vib rhythmic. Idk if thats the right word but is it a repeating pattern if you let it happen or just random.

JMD · 02-23-2012, 08:22 PM

I measured the transmission angle with the straight edge off of the harmonic balancer, and the pinion off of the axle yoke with the caps removed and the straight edge resting across the machined surface where the caps bolt on.

The angle finder can be bought from Jeg's or Summit for a few bucks.