Driveline vibration...

[bealaddison]

I have a rear driveline vibration that starts @ around 50mph while accelerating and diminishes as you accelerate past 60. Let off the throttle and coast or just give it enough throttle to maintain speed and the vibration goes away. My ride is '87 Suburban with 6in lift and 37in tires. I recently swapped in a sm465/np205 combo. Prior to that it had a 700r4/np208. At the same time as the trans swap, I changed out the rear axle from a semi float 14bolt to a full float 14 bolt. I also got rid of the rear leafs/blocks combo and installed a set of 6in lift springs.

I realize that is a lot of parts to swap at once and chasing this vibration down will not be easy. I was just hoping to hear some opinions and narrow it down a little. I checked transfer case output shaft angle and it is around 5deg down. Pinion angle was around 1deg up, so I installed 3deg shims at the axle, making it 4 deg up. I had the vibration before and after installing the shims. Not much change.

I am also running a conversion U-joint, 1310 to 1350 at the pinion. Prior to install I had the driveshaft shortened and balanced by a driveline shop. Transfercase output is a fixed yoke with the slip yoke in the driveshaft.

Axle and trans seem tight. I believe I am chasing a working angle issue or driveshaft imbalance. Anyone want to offer some ideas?

[geezer#99]

You likely have a mismatch of your pinion and tranny output angles.
Study this to figure out what you need.
http://www.drivetrain.com/parts_cata..._problems.html

[slotard]

What's your driveshaft angle? How close to bottomed/topped out is the slip yoke?

[bealaddison]

Originally Posted by slotard

What's your driveshaft angle? How close to bottomed/topped out is the slip yoke?

Slip yoke is centered at ride height. I'd have to check my driveshaft angle again...but its a Suburban with 6in lift, so its not that bad.

[Keith Seymore]

Originally Posted by geezer#99

You likely have a mismatch of your pinion and tranny output angles.
Study this to figure out what you need.
http://www.drivetrain.com/parts_cata..._problems.html

Agree.

Imbalance is not normally load dependent (ie, gets better or worse on and off the throttle). A u joint working angle issue usually is load dependent.

Coupled with the fact that the truck is lifted points towards a driveline angle issue.

Bealaddison: here's a different version of what geezer posted

http://67-72chevytrucks.com/vboard/s...d.php?t=522180

Get very familiar with either his document or mine and you can fix any driveline vibration issue.

K

[bealaddison]

Thanks for the input everyone. I will check angles and go back through the info. I forgot to take into account pinion rise during acceleration. I may need to go the opposite direction with my pinion angle.
Posted via Mobile Device

[LostMy65]

Found this informative write up:
http://www.hurst-drivelines.com/file...ration-FAQ.pdf

[SkinnyG]

Vibration on acceleration, but not on decel, tells me the pinion angle is too high.

Since your truck is lifted, and since lifting the truck in increase "spring wrap" (lifted whether by blocks or by leafs), you might need to drop the pinion a bit less than you started with. Maybe try 1° down?

What is the operating angle of the u-joints?