Need some help with my 1969 C-30
 

here's the rundown of my problem. I have a horrible vibration that feels like the rear end in jumping straight up and down at 50-55 mph and 24-2600 rpm.
History- bought truck 2 yrs ago- had been ditched on LF corner-bent lower control arm no frame damage-have had to frame shop- all inspected and measured righton.
Here's what already been changed(some were updates, some to help problem)
4 new front control arms, upper and lowers with all new bushings shaft kits, balljoints.
new front spindles, rotors, hubs, bearings, etc(converted to disc)
new steering box(old one leaked badly)
new steering parts, tie rods, etc(old ones worn)
6 new tires-good quality 10 ply
6 new steel dually wheels- all tires balance with 3 or less ozs of weight and no excess runout
Changed out eaton HO72 axle for Dana 70(changed from 4.56 gears to 4.11)
All new u-joints, and carrier-had driveline balance checked-good
recently found harmonic balancer broken, but the 427 is a nuetral engine, so no major effect there.
Installed all new mounts replaced flexplate with engine out, also replaced trans output bushing and seal(for leakage reasons.

The biggest change in the vibration was when changed gearing-it happens at same engine rpm, but now at higher speeds. can drive thru it, gets alot better nearer 65mph, but never goes away.

has anyone had anything similiar to this? I am at wits end and have already replaced or updated about anythig that spins! LOL.

PM me or reply here with suggestion, I need to be able to drive this truck.

Thanks in advance!

Re: Need some help with my 1969 C-30
 

Forgot to add- have replaced all four shocks with monroe Truck Magnums and seems to be a little better.

Re: Need some help with my 1969 C-30
 

This is just a guess, but my first thought was when you replaced the u-joints the drive shafts were put together out of phase.

Re: Need some help with my 1969 C-30
 

You say you replaced the rear axle,but what is the condition of the Dana 70? Are all the wheel bearings good,axle shafts straight,drums not warped,housing straight out of the center housing,spindles straight and not overly worn? That's where my mind went from reading your post. That wheel hopping has got to be something "at" the wheel.
On my '92 K3500,that I bought new,I was wearing rear tires out (outer tread) in 8,000 miles and had chronic axle seal leaks. I never had any wheel hop,though. I had a four wheel thrust alignment done and found the rear tires were towed in about 3/8" on each side. I figured the axle was assembled improperly from the factory. When I finally took the axle out to replace I found the axle tube had never been welded at the center housing and could be moved all over the place,once free. The spring was holding the axle tube in place. Actually,the spring hanger was twisted a bit making the spring sit cocked from lateral force. This probably isn't your problem,but you can see that it would be a good idea to check out that axle real thoroughly if you haven't already.

Re: Need some help with my 1969 C-30
 

The vibration was there with the old rear axle. Only thing that has changed since the switch was the mph where it occurs,which leads me to believe it is driveshaft related. I drove the donor truck home with the rear axle- it ran very smooth.The engine rpm is the same but the mph went from 40-45 to 50-55 which would make sence by going to a higher gear. Since I did not build the engine originally, starting to question the internal balance. The block numbers, head, etc all come up as a 427, which is a nuetral balance,(no counterweights on balancer or flywheel). Going to check crankshaft numbers next, see if someone swapped a 454 crank into it.

Re: Need some help with my 1969 C-30
 

I am not trying to say anyone is wrong with their suggestion, I sincerely appreciate any input, just haven't hit on anything new yet, and loosing hair daily!

Re: Need some help with my 1969 C-30
 

Either driveshaft out of whack or check your pinion angle.....Mine was off bad and at 55-60 would shake you out the truck....Checked the angle and it was 7 degrees when it should have been 3......Shimmed it to the right angle and its smooth again.....


Smitty

Re: Need some help with my 1969 C-30
 

Problem solved, FINALLY!! After checking over everything again, we tried lowering the rear tire pressure to 40 psi, vibration lessened quite a bit-so going with the idea it was in the rear suspension, we disassembled the rear springs, found the front bushing on the passenger side had deteriorated and the spring eye had about 1/2" of forward and back movement. This truck, being a Camper Chassis(not a Special) only has approx 24" of frame beyond the rear axle centerline, so not much weight on the rear axle. New bushings installed, springs back in and tire pressure at 55, life is good! Thanks for all that offered help and advise, all the ideas were good ones.

Re: Need some help with my 1969 C-30
 

Took your advise after we reassembled,and test drove. Changed angle from 5 to 3 degrees, made it even better! Thank you!

Re: Need some help with my 1969 C-30
 

Glad you found it. A lot of very smart folks in here, sounds like your one of them.

Re: Need some help with my 1969 C-30
 

input is good and you learn alot