I have a vibration in my 78 z.. this only happens 50 mph and above.. vibration is definitley in rear. vibration dissapears when i push in clutch or take out og gear i have replaced main driive shaft and right u joints.. also new diff mount.

MY QUESTION IS THIS........ WHEN DOES THE LEFT REAR AXLE START TO ENGAGE? I KNOW THE RIGHT DOES THE MOST WORK..:stupid: JUST CURIOUS NOW WETHER OR NOT TO REPLACE UJOINTS OR AXLES??

THANKS FOR ANY HELP...

O BY THE WAY JUST ONE 2ND PLACE AT Z ATTACK IN NASHVILLE TN..

How differentials work:

http://auto.howstuffworks.com/differential2.htm

Yours is an open differential, unless you've put a limited slip in there. Both axles are engaged all the time. The reason you thought they weren't is because solid axle cars load the right tire with the torque from the engine, so they tend to spin the left. IRS cars like the Z don't do that, so if you take off in a straight line you'll likely spin both. If you get into a corner and punch the gas you'll likely spin the inside. A limited slip will fix that inside spin. All of this has nothing to do with your vibration.

Not sure what your vibration is from, but it is doubtful that it has anything to do with the gears in the diff. I'd suspect a U-joint somewhere, maybe on the left side if you haven't changed those yet.

Thanks alot.. Great webpage.. What the heck.. I will replace the left side now,,,, if i have to take the axle out to check i just go ahead and replace the joints, if needed or not..thanks again....

How differentials work:

http://auto.howstuffworks.com/differential2.htm

The reason you thought they weren't is because solid axle cars load the right tire with the torque from the engine, so they tend to spin the left.

Actually it's the other way around. Unless you are in reverse of course.

Steve

Is the drive shaft new or just the U joints? Drive shaft could be out of balance.

Actually it's the other way around. Unless you are in reverse of course.

Thanks for the correction. I'm obviously not a muscle car guy...

:stupid:Both axles do the same amount of work correct??? Unless turning...im still on my 280

I'd say it's almost definately your driveshaft UJ's.

:stupid:Both axles do the same amount of work correct??? Unless turning...im still on my 280

Correct. Also that work is limited by the tire with the least traction. That's why when you break one tire loose with an open diff you pretty much kill your acceleration.

Steve

It seems strange that the vibration goes away when you press the clutch or take it out of gear if the vibration is in the rear of the car. Everything from the gearbox back would be spinning at the same speed when you are rolling in or out of gear. Does the vibration go away if you just coast while in gear? If so it only vibrates under acceleration. Are 78's a two piece tailshaft? Centre bearing might be a possibility if you didn't do it when you did the driveshaft.

Thrown wheel weights can cause vibrations that increase with speed but that wouldn't go away if you pushed the clutch in.

It seems strange that the vibration goes away when you press the clutch or take it out of gear if the vibration is in the rear of the car. Everything from the gearbox back would be spinning at the same speed when you are rolling in or out of gear. Does the vibration go away if you just coast while in gear? If so it only vibrates under acceleration. Are 78's a two piece tailshaft? Centre bearing might be a possibility if you didn't do it when you did the driveshaft.

Thrown wheel weights can cause vibrations that increase with speed but that wouldn't go away if you pushed the clutch in.

The torque on the drivetrain causes misalignment in the failing joint that is relieved when the clutch is depressed.

Steve