Hi guy,

I just got my 69 chevelle project on the road. I have a high frequency vibration from the rear of the vehicle at 65mph+. The combination is a rebuilt M20 with a 8.5" 10 bolt. 4:10 gears and a solid lifter 350. The u-joints apear to be in good condition. The car sits fairly high. Big block front springs and coil over helper shocks in the rear. It has 28" diamiter tires. 275-60 15. On 15x8 corvette rally wheels. My thoughts were that the hight of the car may be affecting the driveshaft angle causing the vibration. The control arms are original stock arms. This car has not been on the road in at least 25 years. It is still sporting its high 1970's stance. Would lowering the car help? Or should I replace the u-joints. If that does not help. Get the driveshaft balanced. Note the vibration is worse when coasting.

Thanks!

Tom:thumbsup:

Tom those 2 changes surely will not hurt anything...new joints and lowering. You mention a 4:10 gear in a 10 bolt so that could imply a possible issue there with set up....Drive shaft length and a rather large list of other possible causes could come into play here. If the car hasn't been on the street for 25 years its most likely gonna require a few shake down runs....Good Luck

In this case you may think u joints ar ok but may not be for 410 gear wht makes for very fast d-shaft rpm at higher speeds.

Do these 1st but this is assuming you can tell the difference between tire/s out of ballance vs true driveline vibration. I am not trying to be a wiseguy with that statement but i have hit more the one time over the yrs when people say they have driveline vibes just to find out it was a tire out of ballance all along when i would finally get the chance drive the car myslef to see what was going on which we cant do in this case here online in t/chevelle.

But if you thinks its truely driveline vibes then chk this out:

* Have a qualified tech chk ujoints to ensure ok,if any question with 410 gear causing high d-shaft rpm at highway speeds reaplce them.

* While checking ujoints with d-shaft still in car pull on d-shaft side to side back by rear yoke to check for worn pinion bearing ,if sloppy that could cause some vibes too and may need to be replaced too.

* Check yoke that goes into trans tial housing,if it shows wear replace it.

* Also,slide new yoke into trans tail housing to chk see if it fits snug in tailhousing bushing, too worn,if it snug great but if not that may need replacement too because worn trans yoke and or tail house bushing can add to vibes too.

* Then only after any parts have been replaced on the d-shaft like ujoint/s and or yoke have the d-shaft ballanced.

I have found doing the above will stop or at least greatly reduce driveline vibes .

I also find it not usually just 1 thing causing driveline vibes,with these 40 yr old cars it usually collectively 2-3 even 4 things at one time tha are collectively resulting in driveline vibes so keepthat in mind too that correcting only 1 issue may help but not stop vibes all together.

But if there are still vibes after doing the above then you will need to look into bad/worn trans or motor mounts,worn sloppy pinion bearing,improper drivelive angle/s,possible motor-flywheel-trq converter imballance that you think is drivelive vibes when it's really engine related vibes,etc.

Scott

Thanks for the info guys. I do plan on lowering it down to stock hight. It is a little high for my taist. The roads here in Michigan suck. So I won't be going protouring with the suspension. I just wasn't sure if this was a common problem with chevelles that are slightly higher than stock. The tail shaft bushing is new and the rear end gears and bearings are new. the u-joints are older-age and milage unknown.

Tom