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Discussion Starter · #1 ·
My 1966 convertible has a non-stock 9" ford rear end installed. The rear end yoke and the driveshaft yoke don't match, so I had to find a u-joint that had different lengths north-south vs east-west, if you know what I mean.

Anyway, I have a pretty severe vibration at about 50-55 MPH. A "strumming" that comes and goes varying with speed (frequency increases with speed).

My question is this: Since the u-joint isn't 4 way symmetrical, would this mean that the driveshaft would be way out of balance and might be causing this (assuming the driveshaft wasn't set up for this type of u-joint)? Or would you think that since east=west and north=south, that no special balancing would be required? The vibration is severe enough that something is seriously wrong (not looking at pebbles in the tires or a gram here or there!).

Anybody know?

Rich
 

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having your ds balanced at a shop is actually extremely cheap. I'd take it in and just have it balanced...then you'll have a piece of mind.

If it still vibrates after that, perhaps check pinion angles at the rear end, since it's not the oem setup
 

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You might try flipping the DS 180* on the pinion flange and try it again or invest in a DS set-up designed for the 9" like the OEM DS from a 289HiPo.

GT
 

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Discussion Starter · #4 ·
I tried the 180 degree flip, no change.

Last night I shimmed the axle housing to decrease the pinion angle...made a new vibration, and didn't remedy the existing. Then I reversed the shims increasing the pinion angle and that got rid of the new vibration ....but didn't remedy the old vibration.

Tonight I'm going to measure runout on the driveshaft to see if its bent or if a u-joint is binding. Time permitting I might swap tires front to back and see what a test drives might tell me.

But looking at that u-joint still causes me concern. I can't decide if equal weight on either side of a rotating shaft means it's balanced or not. I think I'm into harmonics and such....ugh.
 

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What you have is a conversion u-joint. It is one dimension across one set of caps (north and south as you put it) and another dimension across the other set of caps (east and west). This by itself will not cause a vibration and conversion u-joints are popular.

Tell me more about the original vibration. Does it come on at a particular speed or rpm? Can you drive through it? What happens to the vibration if you press in the clutch (or throw it in neutral if it's an automatic)? What happens to the vibration if you let off the gas while it's happening?

Someone earlier mentioned having the driveshaft checked, have you had that done yet?

Typically a driveshaft vibration will come on at a particular speed (not rpm) and in most cases, you can drive through it (i.e. speed up or slow down).
A bad pinion angle on the other hand will usually come on at a particular rpm (not speed) and could get worse if you let off the gas during the vibration.

My guess is it is not a pinion angle issue as you already tried shimming the pinion and it did not change the original vibration. Typically if it is a drive line angle problem, shimming the pinion and/or transmission will result in a change in the dynamics of the vibration (make it worse or better).

Check the driveshaft tube itself and make sure there is at least 1 weight welded to the tubing on each end of the tube. I have seen many times a weight that gets slung off and this will cause a vibration.
 

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Discussion Starter · #6 ·
Thanks for the reply.

To update, I rotated the tires front to back with no significant change. I measured run-out of the drive shaft. Less than 10 thou at the back, but almost 50 thou at the front. If I push the front u-joint up I can see the slip joint change angle slightly.

I removed the driveshaft and took out the front u-joint. I fitted the slip joint back to the trans and it seems a pretty nice fit. Not "tight" like its hard to insert, but pretty snug. I can get a visible wiggle though.

I put the slip joke on the shaft "backwards" to how it was (in case I messed it up the first time). Shaft has weights on both ends. Test drove and the vibration is still there.

Vibration: It's a 1966 Convertible, so it isn't that smooth at any speed......but, the vibration that I am fighting comes on at about 60 mph (indicated, likely a bit lower) which is 3000 rpm. It pulses faint at say 60 beats per minute, and as I speed up the frequency increases, as does the strength. By 75 mph (say 3800 rpm) its frequency is so fast that it's essentially steady and the vibration is extreme. It shakes pretty bad so I back off which brings it back to a pulsation which is gone at about 50 mph.

I've soft driven 2nd gear (automatic) through to 4500 rpm to see if it would appear, but it doesn't. So I think it is speed related. I have a 3.50 rear end in now, so I might swap to a 3.00 I have lying around. Maybe the 3.50 is no good, or if nothing else, it should shift my vibration higher!

I might try the 60 mph run on axle stands w/o rear tires and see if that tells me anything.......

Any ideas?
 

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Go and get the driveshaft balanced and then go from there. My local driveline shop only charged me $40 to do it. Make sure to include the slipyoke and the rear U-joint caps. The tires may need balancing, but the frequency is usually lower than the driveshaft frequency. Have you checked to see if the trans output shaft is running parallel to the pinion shaft? A binding U-joint will cause vibration also. As for the conversion U-joint, the added material on the two ends cancel each other out, there isn't a problem with that. As said, people use them all of the time with no additional problems from its use. It's cheaper to just buy the conversion U-joint than buying another pinion yoke. For some types of driving that's all that's needed.
 

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That's quite a bit of difference in the runout between the front and rear. Do you know that you can attach hose clamps onto the drive shaft using the screw portion of the clamp as a test weight? Attach one or two to offset the front runout. Using two clamps allows you to fine tune the amount of weight that is applied. Most service manuals for rear drive cars have a section that describes the procedure using the clamps. It is usually listed under "NVH" or Noise Vibration Harshness.
 

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Discussion Starter · #9 ·
I'm thinking about that front u-joint.

Would a slightly loose slip joint bushing cause front of shaft run-out when you are turning it by hand (turning the rear wheel)? I wouldn't think so. It's a brand new u-joint, but it did go back together kind of weird.

I think I'll get it replaced while having the shaft balanced and see what that does.
 
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