Technical High speed vibration and pinion angle?

My '55 Nomad is experiencing a high speed vibration at between 72 and 77 MPH. I have balanced and re-balanced all the wheels/tires, and since it's speed induced and not RPM, I'm assuming it's not the driveshaft (which has been balanced twice now).

The interwebs suggest that the likely culprit is pinion angle, relative to the tranny/driveshaft. I've used the Tremec App on my iPhone to measure the angles and according to that, they are "within acceptable limits".

The vibration is most evident in the gear shift and floor of the car, and to a lesser degree in the steering wheel.

Here are a few details:
GM RamJet 350 crate motor
Tremec TKO 5-speed
Moser 9" rear
Wheel Vintiques steel wheels with hubcap style Moon discs
Coker/Goodrich Silvertown P225/70R15 wide whites
New, balanced driveshaft
New leaf springs with QA-1 shocks
Wilwood 4x power discs
About 11,000 miles on this current combination

Anyone have experience with a vibration like this and have suggestions to offer?

 

Had a friend that recently experience that exact problem. How far are you from having the crankshaft and the pinion parallel?

 

Check the bushing in the back of the trans, where the slip yoke rides.

 

I think it was about 1.2 degrees total angle. I'm going to have to measure it again and save a photo of the final screenshot, since the app doesn't save the readings...

 

I'm guessing you mean drop the driveshaft and check the yoke for runout?

 

he ended up with about .5 degree out of parallel with a driveshaft angle of almost 3 and his problem went away.
with a leaf spring car i would't think 1.2 would be bad if the pinion was down.

 

Let's get down to the nut cutting fast.
Put your car on a drive-on lift or jackstands with the car at normal suspension load.
Measure the angle of the engine/transmission at a position known to be parallel with the crankshaft, or 90 degrees from it. If an automatic transmission, the pan gasket face and pan bottom are not a good reference. I like a machined surface that's parallel or 90 from the crank.
For the sake of it, drop the rear of the driveshaft and take an angle measurement at the machined face of the pinion yoke or flange. Button the shaft back in, and also look to see if your universal joint has the ability to move before you hit the torque down.
Then, measure the angle of the driveshaft.
In all measurements, please. Call front up "UP, or nose UP" in relation to looking at one side of the car, car front is nose, back bumper is not the nose. If the angle slopes downward to the front, call it "DOWN, or nose DOWN".
Hit me with those angle measurements, and I will teach you and others how to make corrections, and also how to build correctly. There is a possibility that your car may have another problem.

 

Get some angled spring wedges and make the yoke to pinion angle a zero difference. Could try that in an hour. Then come tell us if it makes any difference. I bet it does.

 

Does it vibrate the same in 4th and 5th gear at the same speed?

Does it vibrate at that speed with the clutch pushed in?

Does it vibrate more on acceleration, cruise or coast?

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The vibration is consistent/constant with the clutch pushed in and at coast, and the same under acceleration as at coast. I first thought it was a clutch/flywheel issue but this would indicate otherwise I suppose.

 

I am going to get the car back on the lift this week and take all of these measurements, etc. and post them back here ASAP. Thanks for all the help so far!

 

Process of elimination. Tells you for sure it is driveline. Pull the driveshaft and articulate the u-joints to make sure it isn't binding. Then measure the pinion and crankshaft angles. An angle gauge helps here.... Local hardware store has them cheap.

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Very recently I attended a Driveline Seminar held by Inland Empire Drivelines (a very reputable firm) and one of the points made was a very common cause of driveshaft vibration was due to over tightening the U bolts that clamp the rear U joint to the yoke. Many people have the notion those U bolts have to be really tight and that IS NOT the case.

The U joint cups are captured in the machined half circle in the yoke and retaining them does not take much. Over tightening them 'egg shapes' the cup and binds the U joint. The presenter said "tighten the nut until the lock washer is flat and then another eighth turn".

In any case, it is a simple and no cost addition to the list of things to check out.

Ray

 

Guys, thank you tons for the help you've offered so far. So here's some more detailed information and answers to some of your questions that I hope will lead to a more narrowed conclusion:

1. I switched out the wheels/tires with a different set of the same size P225/70R15 but these are new Centerline aluminum wheels and Cooper tires which have been spin balanced. The change in wheels/tires made no difference and the vibration is present in exactly the same frequency, speed and tone.

2. I got the car on a flat lift and measured the driveline angles as suggested. The measurements are as follows:
Driveshaft angle is 2.6 degrees downward angle
Transmission angle is 2.3 degrees downward angle
Pinion angle is .5 degrees downward angle

3. The vibration is definitely most noticeable when at cruising throttle. I ran the car through the gears briskly to get up to 75mph in 4th gear and the vibration is still there, but not as noticeable. There's definitely an audible component to this as well. I can hear a noise that coincides with the vibration that comes in and goes out with speed.

4. The vibration is definitely high frequency like it's coming from the driveline, not low frequency like a bad tire, etc. and it's most evident in the gearshift ball/shifter and it shakes the floor of the car noticeably.

I've attached a screenshot of the measurement summary from the Tremec app.

 

It sounds like a bad u-joint.

 

Had a vibration at speed in a '55, sbc 4-speed. Unbolted the rear joint and rotated the driveshaft 180 degrees. Vibration went away.

 

If that these angles are based on downward, meaning REAR DOWN....
Your math says you have .3 degrees through the front u-joint.
In my math, that may get by to make the needles rotate.
Pinion angle is 2.6 down.
Your math says you have 2.1 degrees through the rear u-joint.
2.1 minus .3 equals 1.8.
Driveshaft math by folks smarter than me learned that u-joint angle at the front and rear need to be within or equal to 1 degree of each other. I follow this math on a two u-joint shaft in application, and it works.
This tells me that you need to lift the nose of the pinion in the neighborhood of 1 degree to help with some of the induced problems.
Back to how to make an equation. 1/4" of rise or drop at 1' of pivot =close enough to 1 degree for our math. That gives a tad of room for one hair, not two.

 

I just thought through something else...
If you have .3 angle, assuming all angles are front up and rear down, through the front and rear u-joint, and all in the same attitude....
Engine/transmission angle is too flat. This needs to rise another degree or so. Bring up the front of the harmonic balancer.
The top of the intake manifold will tell you if I'm right, or if your measuring device is OFF, or what the place of angle was measured from is OFF.
I triple check myself with expensive tools, that I learned from, to triple check, to make sure I got the same reading within 0.3 degrees repeatable in that area.
When you're dealing with high speed applications, or short driveshaft applications, you have to be that precise.
See a problem, mark it with a micrometer,mark it with chalk, cut it with a hatchet...doesn't cut and get the job done when the nut cuttin' gets that close.

 

#2 is your problem.

 

If you put it neutral at speed does it stop?

 

The pinion angle is wrong.

 

Went through this on my 57 when I went to a T5 trans. Don't recall what the angle was but I had a "nose down" pinion angle. Ended up doing a couple things to fix it. First was a couple angled shims under the springs pads to bring the pinion up. The second was a new rear bushing in the trans and driveshaft yoke.

 

Definitely pick the pinion up, I'd use a 1 degree shim at both leaf springs to start. I would clock the shaft 180 degrees first and I also go easy on the clamps.

 

How about a pic of the driveline, from your description it sounds like both transmission and pinion are angled down. The pinion should angle upward at same degree as transmission angle down to compliment each other.
A pic would help as I might be reading the description wrong.

 

This would be my first move too. Along with checking for a 'tight' u-joint. Although it does sound like your angles might be boggered to begin with.

 

I'm with oj, a picture or diagram would sure help. I also have questions about your driveshaft. Is there any chance the the u-joints could be slightly out of sync? That would give you the problem you have described.
Pete

 

No, the vibration remains if I push in the clutch at cruising speed, which I'm assuming is the same as putting it in neutral.


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I can try to diagram it, but you are all correct. The crankshaft line and pinion both angle downward. So the back of the tranny points down towards the back bumper and the pinion points down towards the front bumper. Hope that helps clear that up.


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Like I said waaayyyy up in post 8, try some wedges. Basically a minimal cost option that could be done in an hour on the lift.

 

I can try to diagram it, but you are all correct. The crankshaft line and pinion both angle downward. So the back of the tranny points down towards the back bumper and the pinion points down towards the front bumper. Hope that helps clear that up.

There is your problem. They need to be parallel.