vibration

[Oldboy]

Hello,
First a bit of history. While my ford 9 inch was out for rebuilding, I did two things:
I had a different gear set installed. I changed from 3:01 to a 3.50. And while the axle was out I added. 1 1/2 lowering blocks .
Now i have a noticable vibration at certain speeds. It is only slight, but it is there.The local driveshaft buillder says that i have changed the angle and possibly "pulled" the driveshaft further out of the tail of the transmission.
Does any of this make sense? Does it sound logical? Can i adjust the pinion angle?
Additionally, while the gearing makes for more fun, i am on now wanting a 4 th gear, currently running a gm turbo 400.
thanks for any feedback. Russ

[Sam_Fear]

Not sure I understand the tailshaft comment, but I could see the change in angle affecting it. Have you checked the U joints and connecting bolts?

You could remove the lowering blocks and see if it goes away.

[Gottkts]

They make pinion shims to use with your lowering blocks, had to use them on my ranger minitruck way back in the day.

[shakey56]

Yeah, Oldboy, you have definitely changed your pinion angle and caused the vibration. And it probably did pull your driveshaft out of your transmision ever so slightly.

I lowered my 56 by moving the spring shackles up 4 inches. That gave me a very slight vibration. Then when I put a 9 inch in, it lowered it more and the vibration became worse. I changed my pinion angle with shims, and it stopped.

There are guys on here that can tell you the technical info to figure what size shim you need. I just took a stab at it and got lucky. Good luck.

Lynn

[Oldboy]

Thanks for the replies.
Going to check everything tomorrow for tightness. Maybe. I will pull the blocks out, if it goes away, then ill pickup some shims.
will post when sorted out. Thanks again

[05snopro440]

The used to make lowering blocks with an angle built in, but they no longer do. I never figured out the logic in that...

Any time you lower it you stand a chance of introducing vibration. That said, Usually anything 2" or less the difference in angle is very small and you shouldn't get much noticable vibration (this is from experience). If you do have a vibration, the shims can definitely correct it, but since you changed so much before the vibration showed up I would say you're on the right track trying it without the blocks out to see if that's the issue.

The comments about pulling the driveshaft out of the transmission are interesting. If you just have a regular slip-yoke then it won't matter, but some TH400's have the U-joint bolt directly to the transmission. In that case I could see there being some potential issues. Which type is your transmission?

[Sam_Fear]

Originally Posted by 05snopro440

The comments about pulling the driveshaft out of the transmission are interesting. If you just have a regular slip-yoke then it won't matter, but some TH400's have the U-joint bolt directly to the transmission. In that case I could see there being some potential issues. Which type is your transmission?

I was going to ask about that. Thanks for the info.

[shakey56]

OK...think about it guys. As you drive down the road, the body/frame of your vehicle moves up and down according to the smoothnes of the road....right? Because of the rear suspension, the differential moves up and down.....that is it's job. That is why there is a slipyoke in the rear of the transmission. When the diff goes up or down, the slipyoke....uhhhh,..slips in and out. If it couldn't slip....something would break When you lower a vehicle, you change the geometry and therefore the position of the slipyoke; in or out.

And actually, lowering will push the drive shaft slipyoke further in, not out. You have to make sure you still have enough travel in and out so the collar of the yoke does not hit the tailshaft....I think you need 1 inch of clearence.

Lynn

[kenny c]

another thing to consider is if the slip yoke is built into the driveshaft and it has been taken apart and put back together out of phase, meaning that the two yokes of the shaft itself are not in the same plane it will vibrate.

[endicott_jb]

The odds are, due to the amount that the op lowered it has caused the angles to change enough that the u-joints are now past their "workable" angles and now need to be adjusted as such. If a u-joint surpasses 4degrees, it is no longer working as intended. There are a few options out there. One would be to change the rear end angle to attempt to get it back within that 4degrees. Another would be to use a continuous velocity u-joint. And the third would be to use a continuous velocity driveshaft which are very expensive but you can have up to 48 degrees of working angle on them. I like things simple so I'd attempt to find the happy median for the driveshaft you already have which would be to raise the rear of the transmission or adjust the pinion angle.