i did a search and posted on other forums but with no luck. thanks


I have a 68 fastback c code with 289 converted to 4v carb, auto trans, 8 inch rear end. this car was a complet restoration finished in 02 it has now 2000 miles since resto. it rides very well, it is very tight for a 68. but i have a vibration starting at 50 mph and it get's worse at higher speeds. it's still there when i put in neutral when coasting at 50-80 mph. i've read a few treads on this matter cause i've been trying to find the problem since i bought the car 4 months ago. first it had a bad left rear axle bearing making noise so i changed that with no results, then i checked the car on a jack and i tought the axles were bent cause when off the ground and rolling at 55 mph with no weight on the ground it seems that the wheels are wobbling ( i'm sure it's the axle flex from the heavy wheel rotating, cause we checked another 67 fastback and had the car rolling at 55 also on a jack and it did the exact same thing)so i changed the axles to find out it was not that.it has brand new rims and tires on it now that have been checked, rotated, balanced twice (it was vibrating with the other wheels it came with) i checked the drums,had the drive shaft balanced, checked the u-joints and they are fine, checked the pinion angles and they seam ok, checked the trans mount it seams ok. i dont want to end up with a few members here never finding the problem. please let me know what you think. thanks

 

might want to check the trans tail shaft busing. This is the bushing the supports the driveshaft at the slipyoke.

The best way to check it is with the car up high enough that you can get under it. Push up and pull down on the DS. Watch for movement right were the slipyoke enters the tailshaft.

-Gun

 

i'd replace u-joints again.
its usually one of the first items but if the problem is ongoing, you can toast a set in no time. but because they were ruled out before no one thinks to go back to them.

when you jack the rear now are the wheels smooth? did you replace the rear axles with anything better than stock? were they new or used? your tailshaft guy should be able to test the run out in axles as well. I went thru several pairs before i finally resolved the issue.

you might also want to look at rear axle alignment - either the housing itself (unlikely) or the way it seats on the spring sets. Are your spring bushes in good condition and rear shocks ok.

good luck.

 

I have a similar problem and have yet to find the issue.

i will keep watching though!

 

Are you sure it's coming from the rear? I would repack the front wheel bearings, and make sure they are adjusted properly.

 

I would also suggest trying new U joints.

 

I wouldn't worry about the u-joints at that speed.

 

First off, we need to translate into text the frequency of the vibes. Is it a high enough frequency to be considered a "buzz", sliding a screwdriver down the teeth of a hairbrush? Or slower, like quickly slapping one hand on the desk? Tires and wheels are the hand slapping speed - driveline multiplies that by a factor averaging 3 which is related to the comb scenario. Figure that one out, and you've narrowed it down to who we'll pick on next.

 

[QUOTE=KMatch;7059462]First off, we need to translate into text the frequency of the vibes. Is it a high enough frequency to be considered a "buzz", sliding a screwdriver down the teeth of a hairbrush? Or slower, like quickly slapping one hand on the desk? Tires and wheels are the hand slapping speed - driveline multiplies that by a factor averaging 3 which is related to the comb scenario. Figure that one out, and you've narrowed it down to who we'll pick on next. the higher frequency one. i can feel it in the floor it comes and goes every 4-5 seconds.

 

OK, it's a high frequency, intermittent (cycling) resonance to be a touch technical. Hmmm... One thing jumping out at me is something making contact that normally shouldn't, ie, exhaust on the frame, rear trans mount grounded out, engine mount collapsed. I can't rule out pinion bearings and such, but it's rare that anything from the driveshaft back will come and go at 4-5 seconds. Check mounts and exhaust and get back to us. Let's don't play that "Looks OK" business, we need to put some pressure on the mounts and really check them. Really, you just described the classic collapsed trans mount.