Hi

I recently changed gears from 2.73 to 3.42 in my rear axle. I also threw in a new eaton true-trac and new polybushings in the controlarms. I could not afford new driveaxles so I had to reuse the ones I had. New bearings and seals from koyo and timken, and the gears was from yukon.

I have driven it for about 550miles carefully without going over 50mph and no spinouts or fast acceleration. Everything went smooth but some clunk between gears, I read up on that and it seems like some of the true-tracs have this. Alot of play between the driveshaft and driveaxles also seem to have been normal on some units. Anyways...

Today was the first time I passed 70mph and at 75ish it started to vibrate alot, but only while applying throttle. Right as I release the gaspedal it stops.

Any idea what this can be? I checked the transbushing and it could be that its worn but it didn't look that bad. Could it cause such vibrations? I know I did not have this problem befor rebuilding the rear axle and installing the polybushings in the controlarms.

Thanks

 

Did you do the gear installation yourself? If you did, how did you set the pinion bearing preload?


Many times this will show up after changing gear ratios. The stock drive shaft is built to handle the RPMs of a 2.73 gear, but many times they will not handle the RPMs of a lower gear. Now that you have installed 3.42 gears your drive shaft is turning faster than it did before. This can cause the shaft to bow at high RPMs. It may be fine at lower speeds and be balanced perfectly at those speeds, but the shaft is not rigid enough to stay straight at higher RPMs. You may need to upgrade to a better drive shaft to fix this problem.

 

Yes, I installed it myself and preload according to installation instructions.

Good theory but shouldn't the vibration sound different when going faster than 75mph, I mean wouldn't the sound change? Like with higher rpms the sound would be faster? I felt the sound beeing the same only gaining more vibration with speed. With the old gears I hit 120mph without problems except whining from the old gears. I don't have any calculator to calculate at which speed the driveshaft spun at these speeds though.

 

If the pinion bearing preload is correct then about the only thing left is the drive shaft. You can check the transmission rear bushing and see if it's worn, but if it's ok then see if someone will loan you a stronger drive shaft to test it.

 

I havn't checked preload since I mounted the axle but should I do it again?

Gonna try new trans bushing but with my luck it probably is the driveshaft. I've read about people getting vibrations when installing poly bushings and some people that have had success, so should I just go with rubber to be on the safe side?

I also thought about the driveshaft some more and was wondering if someone have tried balancing it or is that just a waste of time and money? Checked summit and they have aluminum shafts there but that is softer than steel or are they compact aluminum? Anyways, the cost of that is way to much for me this season with shipping and taxes to finland so if the trans bushing doesn't solve the problem I'll guess I just have to keep it below 70-75mph.

 

Most drive shaft shops only balance at low speed. Your shaft might balance perfect at low speed, but at high speed it may develop a bow that throws it out of balance. If you have a drive shaft shop that can balance at high speed then that would be a good way to check it. I would say that it needs to spin at about 3500 RPMs to see if it will bow. This is around the speed that yours is spinning at 75 MPH.

 

snap! I need to investigate further into this.

Thanks for leading me to the driveshaft!

 

I've read enough about this that I don't believe I'll be installing the Energy Suspension poly bushing that I got in. At least not right before my big road trip when I move in 6 weeks. 1200 miles! Woooo....