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Well-known member
Aug 25, 2013
Pine bush, ny
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
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.
They make pinion shims to use with your lowering blocks, had to use them on my ranger minitruck way back in the day.
For sure...

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.:cool:

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
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?
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.
Up and down...

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.

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.
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.
Or it's simply an old u-joint and it's just unhappy working at a slightly different angle and higher speed due to the gear change, usually the rear one is the first to fail - take it apart and inspect it for wear, you may just need a new one. The 1 1/2" lowering you did is not that severe and is well within the u-joints normal operating range, very doubtful it would have pulled the driveshaft out of the tailhousing that much. Another possibility is that the higher operating speed of the driveshaft has uncovered a balancing problem in the driveshaft itself.
Yes thankyou for all the imput. I think i will first try raising the transmission slightly to see what that does. Next will be to change the pinion anle. Where can i buy shims? Jegs?Summit? Will look after this post.
how much should i move it? I do not own an inclometer, but perhaps a visit to sears is in order.
I noticed that the pinion is pointing slightly down, where as the yoke on the trans is nearly level.. Drive shaft has a slip yoke, and the whole assembly was recently gone through.
many thanks. Will post asap. Im also in the middle of a move
There has to be a slip joint, if not as part of the tail shaft it'll be part of the drive shaft, the length grows and shrinks with the movement of the suspension. When I've needed shims I found them at the off-road shop. Good luck.
Ok so i fabbed up some spacers this morning which will raise the transmission crossmember 1/2 inch. Dont know if this will work. If it does not work, and i buy pinion spacers, which direction do i want to move the pinion, up?down? And 2°,4°
I cannot visualize this.
thanks. Feeling ignorant on this.
Unless the lowering blocks are angled, they would not change your pinion angle compared to your transmission angle. They would only change the driveshaft angle. What is the history of that car or the rear axle? Race/drag cars often have a pinion pointing down so that it levels out under heavy axle torque.

Like was mentioned, lowering would push the slip yolk farther in to the trans. (but not much) Lowering would also cause your driveshaft angle to decrease. For a street car, you don't want the trans/shaft/pinion in a straight line - the u-joint bearings won't move properly and it will more likely vibrate.

This might help:

Start with your motor angle - carb base should be level. Check your trans output shaft angle and match your pinion angle to that.
Thanks for the responce. Rear end is rebuilt., driveshaft gone through maybe 1 year ago.
Im thinking the vibration must have always been there, but now with tbe gear change, it is present right at cruise speed. Im going to make prescribed changes, then will follow up post
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The slip-yoke being in the driveshaft is what I forgot about on some TH400's. If you're lowering the vehicle, whether or not the slip yoke pushed in or pulls out depends on where your rear end was in relation to your transmission before, and if it is closer or farther away. None of us can determine this from the info on this thread.

This is the same for which angle to move the diff or transmission with shims. You need to look at the U-joints and determine which way the angle has moved and which way you'll need to move it back to get it back to a better angle, if that's the issue.
I believe that as the rear went up into the body, that the angle of the universal joint increased. If this is true, then raising the tail of the transmission slightly uphill should counter that.Thats what i will find out when i drive the car saturday.
I believe that the angles were marginal when first installed. That blocks were installed, as well as a gear change at the same time, i think, only amplified the problem. As i said earlier, while only subtle, it obviously cannot vibrate.The slip yoke appears to have sufficient room for travel.
i appreciate all the help. I will post with the outcome.
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Im getting somewhere. I installed a spacer at the transmission crossmember. made the vibration worse. going to pull out the blocks and see what happens. Timing is bad on this I'm moving up on Tuesday. just want to get it quieted down until I can get a inclino meter, like I should have done from the start.
I'm not surprised. Let's say you started with figure A. The trans and pinion are parallel to each other, but not in line, thus making a 4˚ angle between each and the driveshaft. Figure B is what happened when you blocked the rear axle. It decreased the angle between driveshaft and pinion, but also equally decreased the angle between the driveshaft and trans. Less than 3˚ angle will increase chances of vibration. Then you tilted the trans/engine, represented in figure C. You increased the angles between the driveshaft and the pinion and also the trans, BUT since you didn't raise the engine front too, you angled the trans more therefore increasing the angle between trans and driveshaft without equally increasing the angle at the pinion.

If you don't have a inclinometer available - try Harbor Freight. Or at least use a level - you can at least get a good idea of how far off the trans and pinion are - they should be very very close to the same.


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