just put on a 3" trailmaster sus. lift on my 94 yj.
I am really pleased with it but...
when I shift into 3rd gear from 4th or 2nd there is a slight vibration if I give it gas it goes away, if I let off gas then it starts to vibrates.
what is up.
PS I got some stock YJ leafs if someone wants them [email protected]
When you installed your lift, your driveline angle most likely changed. Installing shims between the leaf springs and axle to tilt the pinion up or down (may have to experiment) can alleviate this (you may not be able to get rid of it totally).
The angle at the transfer case should match the angle at the pinion and they will cancel each other out, if not you get a vibration.
My rear YJ driveline vibration didn't go away until I did the following (2" OME lift):
1) removed transfer case drop kit; install slip yoke eliminator and CV rear drive shaft
2) have rear axle spring perches removed & rewelded for optimum rear pinion angle (off shelf shims weren't enough)
3) balance rear drive shaft; the shop that made it hadn't balanced it!
Vibration was finally gone. I have a friend with a 4" lift on his YJ, and he has no vibration. It musat have to do with the manufacturing tolerances of the rear axle.
What's happening is that at speed, the driveshaft is turning fast enough to cause the vibration - this is most likely why it only occurs in 3rd and 4th. When you are on the gas, the torque being transferred to the rear axle keeps the pinion at the right angle, or at least close enough, thus giving you no vibration. When you remove your foot from the gas, the engine is braking the Jeep through the back tires. This causes the pinion to point down slightly more then it would normally. This is increasing the angle just enough to give you the vibration that you are noticing.
You could put a 2-3 degree shim between the leaf spring and the axle to point the pinion up a little more, but I would suggest you install a Slip Yoke Eliminator, especially if you 4 Wheel at all.