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Discussion Starter · #1 ·
A week ago or so I posted about a vibration. It was there at about 2600 rpms, more in third. I then discovered it at idle. After replaceing trans recently, thought maybe t/conv issue. Well, turns out my two rear motor mounts were a little loose. I went over everything twice and found that.
Idle vibration is gone. Now I read a few searches here. One titled pinion angle in which Rick L gives some good descriptions. One thing he mentioned was how "jacked up rears would vibrate".
Well, heres my issue. I still have osillating vibe in third at 2600-3000 rpms. Interestingly, I still have air shocks (hate them) and keep rear up a little cause I like stance. More interesting is after trans install I went to my work/shop and cahnged rear diff fluids, chk up etc. At end of service I added air to shocks because my home unit cant get enough air in. Its not real high, but I forgot I did this. Well, this weekend it hit me. So, I pulled over, let some air out, and vibration now seeming crept up to a higher rpm in third and was almost not felt in 4th. Now Im assuming more air changes the angle of the d/shft?
Anyway, during all this, I put car on a flat 4 post lift and raised it. Sitting level on it, the trans seal measured 1 deg down. The shaft measured 5 deg sloping downwards from trans to diff. The diff sat right at 0 deg.

So here's the question... Where do I want the pinion angle set for vibe free with this setup? Im assuming I need to shim it upwards, but haw much?
Thanks...Phil

Here was that post...http://www.trifive.com/forums/showthread.php?t=47635&highlight=pinion+angle
 

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With the weight on the rear and everything at the ride height you want to run, the transmission shaft angle and the pinion shaft angle must be equal. The two shafts must be parallel at ride height.

Even then, if the angle between the driveshaft and transmission shaft exceeds 4 degrees, you'll risk a high speed vibration. The quick way to fix that is to lower the ride height.
 

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Discussion Starter · #3 ·
So then to have this look...
Land vehicle Vehicle Car Classic car Coupé


I would need to lower the front end, and the rear end evenly? Because of the 4-5 deg downward angle of the shaft now, lower all around will change this angle too, correct? As you can see from the pic, it doesnt seem I am crazy high in the back, but I guess its been wrong all along.
 

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I'm going to follow this one close. Not trying to hijack you, but I am having a similar problem. Mine is very smooth up to about 55 mph and then a little vibration at 60 and gets worse as you get up to interstate speed. It makes it not fun to drive on any lengthy trip. It definitely feels like a drive-line vibration, but I am not sure where to start. My pinion angle and trans angle are very close to parallel, but I have not checked the angle of each to the drive shaft. I was suspecting the torque converter, but really don't know. It is a 200-4R, behind a 283. I had my tires rebalanced last week, helped very little.

Good luck getting it resolved.
 

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The height of the front won't matter because the engine is aligned to the frame. Also, 1 deg down trans and 0 deg at the differential will be ok because under most any load there will be 1 to 2 degrees up because of spring wrap (with a stock rear suspension).

Put some weight on the rear and remeasure how much each of the u-joints are angled. This should help you to see how much you need to lower the rear end to get 4deg or less on the driveshaft difference. Other means would be to drop the rear of the trans and shim the spring mounts to the axle.
 

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Discussion Starter · #6 ·
The height of the front won't matter because the engine is aligned to the frame.
Yes, but if I was to lower front end lets say 2". The frame would also come down, meaning the tail of trans would be lower to street. Pinion yoke is always the same height of the street, so I would think this would lower the overall "slope" or angle of driveshaft, no?
there will be 1 to 2 degrees up because of spring wrap (with a stock rear suspension).
another good point. I do have traction bars and also new 5 leaf springs so I think everything is pretty tight back there. Im mainly looking for good streetability. Love cruising.
 

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Yes, but if I was to lower front end lets say 2". The frame would also come down, meaning the tail of trans would be lower to street. Pinion yoke is always the same height of the street, so I would think this would lower the overall "slope" or angle of driveshaft, no?

another good point. I do have traction bars and also new 5 leaf springs so I think everything is pretty tight back there. Im mainly looking for good streetability. Love cruising.
Pinion angle is in relation to the engine centerline and nothing more. The attitude (nose down - up - or level) has nothing to do with it.

In the interest of keeping any other issues introduced, it is, if possible, best to maintain the chassis in a level attitude, loaded on the suspension. Then what you measure at the engine and at the pinion and do the math with doesn't have to either have othe angles added or subtracted.
 

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But if you can add and subtract and keep it all straight, it's not necessary. Just easier to keep it all straight.
 

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So where does the vibration come from then? The Pinion gear is held into place tightly with the bearing and crush sleeve. The tranny shafts aren't loose. So what is vibrating and why?
 

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So where does the vibration come from then? The Pinion gear is held into place tightly with the bearing and crush sleeve. The tranny shafts aren't loose. So what is vibrating and why?
Harmonics from missmatched angles at the Ujoints.
 

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Harmonics

its Kind of like holding a top in the end of your finger when it spins it also rotates at the top, the rotation is the vibration you feel, but it only rotates at certain speeds and its because of the length of the drive shaft that makes it feel like a vibration. Kind of hard to explain this effect. But the angles should be equal and opposite, parallel to each other. the spring wrap is a factor for pinion angle but usually only under hard acceleration at speed the deflection of spring should be minimal.
 

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so then how do you actually measure the angles then?
 

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drive angles

Get a small protractor with a small arm attached. (The cheapies from General in yellow and black are good enough.) . Put the movable arm on the drive shaft next to the u -Joint and put the protractor head on the ujoint cap until it is flat on the cap and read the angle in degrees. Seems to me one of the race manufacturer's 6made a large fancy version 6 or 7 years ago.
I remember a few years ago one of the rear end builders talking about 55-57 rears. they would start the mount the spring angle at -7 degrees as the stock springs changed the angle 5 degrees so you had a net of 2 degrees down. Good luck
 

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The best way to check everything is with a digital angle finder. The prices are steadily dropping on these, and there's no use for the little analog ones any more, most of which sit there and let the needle swing for a long time.

I saw one at Lowe's yesterday for $25. Sears has had one for a couple of years at $35, and it's a bit nicer than the Lowe's one. Fifteen years ago these tools were $150.

To use it, you just need to find a surface that's parallel or perdendicular to the thing you want to measure, put the angle finder on it and get a reading. Sometimes this is easier said than done, particularly on the rear axle. But there's always a way if you give it enough thought.
 

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To use it, you just need to find a surface that's parallel or perdendicular to the thing you want to measure, put the angle finder on it and get a reading. Sometimes this is easier said than done, particularly on the rear axle. But there's always a way if you give it enough thought.
When i checked mine, I took a deep-well socket that was just a hair smaller diameter than the u-joint cap and bolted it in the rear end yoke and put the digital angle finder up against the socket. I have an old transmission yoke that I saved for checking the trans tail-shaft the same way, except you just have to hold that one in place since there are no u bolts there.
 

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Good stuff, my mind was kind of blank on specifics when I replied. There are many more similar things that you can use to do these things.
 

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I am in the process of replacing my stock 55 rearend with a 68 Camaro rearend and this information on spring perch angles is just what I needed. Thanks guys! Brian :bowtieb:
 
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