[CGBrown]

396 4 speed Muncie 12 Bolt Posi
I’ve put on about 1,000 miles since I bought the car. Everything was fine until the last several rides when I started noticing a vibration at 50-60mph. I have made NO changes to the drive train so something’s definitely wearing out.
I can feel the vibration in the shifter but it’s definitely coming from the rear somewhere… kind of feels like the rear end is driving down a worn down rumble strip. There’s absolutely no vibration at all up front - nothing at all on the steering wheel.

After reading the posts here on this subject, here’s what I’ve done so far:
· Rear tires rebalanced – no change.
· Swapped the front and rear tires – no change and the vibration did not move to the front.
· Replaced u-joints – no change and didn’t notice any slop at the pinion bearing or tail house bushing
· No evidence the driveshaft threw a weight – is it possible for the driveshaft to get torqued out of balance?

I think I’m pretty much down to the axle. I don’t think I bent it – not sure if this matters but it’s got traction bars so I’m guessing it would be pretty hard to bend the axle with those things bolted on. I don't drive it that hard and don't take it to the strip.

I’m lucky to have a very good local shop but before I bring it in is there anything else I could check? I’d hate to take it in only to find I overlooked something obvious.

Thanks in advance for the help!

 

[Mike]

Explain " traction bars" .
Your car has a 4 link suspension and ,sounds like ,you now have a 6 link which causes suspension bind.
Unbolt them and hope you haven't bent/damaged/broken and of the suspension components.

 

[CGBrown]

Mike - thanks for the reply; here's a pic

 

[Mike]

Are the original 4 bars still connected?
Can't tell from the pic.
If so ,unbolt those ladder bars and see if your issues resolve themselves.

 

[CGBrown]

Yep - both the upper and lower control arms are still connected. The traction bars are connected along with the lower control arms to the lower mounts. Thanks for the suggestion - I'll get some help next weekend and post back!

 

[chrispicide35]

Not sure if the traction/ladder bars may be causing the problem,there is only one way to find out. remove them and if your vibration goes away then your set. However if after they are gone and you end up with wheel hop on the launch then you need to invest in some quality boxed upper and lower control arms with quality bushings.

 

[CGBrown]

Thanks Chris. Any other ideas in case the ladder bar removal doesn't help? Also, if I end up removing the ladder bars permanently, I'll need a rear sway bar; right?

 

[CGBrown]

UPDATE - I haven't unhooked the ladder bars (yet) because since my last post:
1) I swapped the right front and right rear tires suspecting a right rear bad wheel/tire. When I did so, the vibration went away. Front tire is 215/50/15 rear is 255/60/15. that led me to...
2) purchased 4 new tires, figuring the 16 year old Dunlops were shot, but the vibration came back...
3) while doing all of this, I noticed if I tug on the rear tires, they slide out maybe 1/8 of an inch each. They don't wobble and there is absolutely zero runout - but if I jack the rear up each one can slide in and out a little bit.

I'm guessing that's not a good thing so I'm down to doing what Mike told me to do over a month ago and unbolt the ladder bars
or
quit screwing around pretending I know what I'm doing and take her in to someone who can handle what appears to be a rear end problem

 

[Mike]

I don't think 1/8" axle play is excessive ,but worn posi clutches can/will cause excessive axle play.

 

[StupidSexyFlanders]

Your problem is in the pinion bearing. Mine has been doing the same thing as yours for 5-6 seasons now, ever since I replaced the seal and over-torqued the pinion shaft nut without replacing the crush-sleeve. The only real cure is to have someone tear it down and properly set-up the pinion depth, bearing preload, etc.. (New bearing, of course)I'm too lazy to dork with it, so I just live with the pulsating rumble-strip vibrition at 50-60 mph..

 

[CGBrown]

thanks again for the help!

 

[the heckler]

my 67ss396 automatic car used to do that when I first bought it. drove me crazy. my friends used to say my rear end was hopping going down the highway. turned out to be worn out control arm bushings. replaced them all. fixed it.

 

[ElCameeeno]

I had the same 50 - 60mph vibration that caused my shifter to rattle like crazy. Drove myself nuts trying to figure it out. Turns out the pinion yoke itself was worn. There are two little tabs that center the joint in the saddle, mine was worn which allowed the driveshaft to go off center just a little bit. Replaced the yoke and it has been perfect ever since.

 

[pearl]

Im doing the same thing now. Some meathead used the long stock crush sleeve instead of the short one for replacement gears

 

[CGBrown]

I don't think 1/8" axle play is excessive ,but worn posi clutches can/will cause excessive axle play.

Mike, you were right. First round's on me! :beers:

I ended up taking it to a local shop who came highly recommended. Turns out the posi was toast (probably thanks to all the burnouts I did as soon as I bought the car). Anyway, the gears were fine but replacing the posi with a new Eaton unit solved the problem. There's still a little vibration in 4th and the clutch is worn so next year I'll replace the clutch, throwout bearing, etc. Also figured I should have the flywheel rebalanced - any thoughts on that? Makes sense to me to get it done while she's in the shop but figured I'd ask...

 

[Ed habuda]

Your problem is in the pinion bearing. Mine has been doing the same thing as yours for 5-6 seasons now, ever since I replaced the seal and over-torqued the pinion shaft nut without replacing the crush-sleeve. The only real cure is to have someone tear it down and properly set-up the pinion depth, bearing preload, etc.. (New bearing, of course)I'm too lazy to dork with it, so I just live with the pulsating rumble-strip vibrition at 50-60 mph..

I had all of this done and still have this problem with my 1965 impala ss 12 bolt