My road racing Dart had a whole slew of drive train vibrations, which I've been eliminating one by one. Low-speed rumble from the worn-out B&T was fixed by converting the shaft to a slip-joint and U-joint. Rumble under acceleration due to pinion misalignment was fixed by shimming the rear end. A lot of other vibrations went away after replacing a bent axle shaft in our junkyard 8.25. (That took a professional mechanic to find.) It also got a little cleaner when we bought all new tires and had them balanced.

Now the car drives so smooth it's almost uncanny. I was used to the thing always sounding like there was a bag of hammers tied to the drive shaft, but now it feel like you're floating on a cloud... up to about 75 mph. Then you can feel another rumble. Not a whine; you can feel it before you can hear it. When you get above 80, the whole car is shaking.

I know the passenger-side motor mount is sagging, and it'll be replaced as soon as the new one arrives. I don't know if that would cause this problem, though. (I realize I may need to redo the rear end shims if this changes the engine/transmission angle considerably.)

The vibration is only speed-dependent. It doesn't depend on applied torque, and it doesn't show up at the same engine RPM in other gears. The drive shaft was balanced when the B&T conversion was done, and the front U-joint is new. The rear U-joint was replaced right after the axle swap earlier this year.

Other things that I considered:
-Transmission tail shaft bushing could be worn out, but the trans was rebuilt by a good shop in 2005.
-Pinion shaft bearing could be worn out. The axle did come from a junkyard, and it apparently had one bent shaft in it.
-Other bearings in the diff housing, same as above.
-Our wheels are not hub-centric, so we could have a slight mechanical runout. My buddy has a dial indicator, so we're going to investigate that this weekend.

Any thoughts? It's possible this vibration has been around since before the axle swap but it's only now that we notice it, because all the improvements to gearing and engine tune are letting us regularly drive this fast now.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Driveline angularity (rear axle pinion angle).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

It sounds like you have thought of most likely problem areas...I had a similar prob in my 1970 Val. At about 80 there was a vibration,not bad ,but there,especially now I was running overdrive and cruising at 2200 rpm. It was a combination of a lot of things, my wheels were non hub centric and yes,one did have a runout. It's a long story,it's an Australian Borg Warner rear,but the wheels on it now are hub centric and the axle spigots are long enough to locate on them correctly. I would imagine a similar issue with racing rubber would be a lot worse...I also fitted an A518 overdrive and had the driveshaft shortened. I jacked up the rear and put it on stands and had the engine idling in drive so I could see what was going on,the driveshaft was running true,but,after a lot of checking the front and rear weld yokes were not in phase,the front yoke was slightly cocked over prior to welding ,the yoke was actually poorly machined where it located in the tube,anyways,new yoke,remade shaft now all good.remember you also have a splined slip yoke now,it is critical to have the front and rear universals in alignment.You mentioned a junkyard rear with a bent axle? I'd also have a check on the housing,it could be bent,also while you have the dial indicator out check runout on both axle flanges. Also remember pinion angle is only one plane,the centreline of the engine and pinion have to be correct as well.

When I converted from B&T to the truck style with bolt on flange, they balanced my new shaft (I think up to 2000 RPM) but afterwards it always had a 75-80 MPH rumble/vibe. I wonder if the flange was a bit off center or something. The shop said the balance job was not perfect and they did it again, which helped a little. It was not too bad, and I drove it that way until swapping to the 5spd, even on the roadcourse and up to 130 MPH a coupla times. Yours sounds worse.

I very much doubt it is wheel centricity. You would have to be pretty far off.

I agree with Dan that pinion angle is the most likely culprit.

Lou

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Home of Slant6-powered fun machines since 1988

It made a noticeable difference to my setup, that was just my experience..... And it wasn't miles out. I'd check everything then when I'm sure all is spot on I'd start shimming the pinion angle.

We broke out the dial indicator tonight and checked runouts on both rear wheels. Radial runouts were good, within about 0.005" as near as we could tell (it was hard to get the dial close enough to measure in that direction). The passenger side, which is the side with the recently replaced axle shaft and new wheel bearings, had about 0.005" axial (lateral) runout on the outer edge of the rim-not bad. The driver's side had an axial runout of 0.025". We popped off the driver's side wheel and drum and checked the hub face as well. It showed about 0.010" axial along the outer edge of the flange. That actually works out to a larger angle when you take the measurement location into account, but the flange face was also rougher than the wheel face, so I guess that could add to the apparent runout.

The hub also had 0.019" side-to-side play, which seems excessive, but I'm not really sure what it should be. It was getting late, and I didn't bother taking off the passenger side wheel to compare. I'm going to have the driver's side wheel bearing replaced tomorrow while I'm at work and see if that changes anything. Should have had it done at the same time as the other one, but I was being needlessly frugal. While the mechanic is in there, I guess he can check and see if that axle shaft is bent as well.

Motor mount also comes in tomorrow, so we'll replace it tomorrow night. Hopefully one or both of these things fixes the problem. We're racing at Sebring next weekend, and it would be really nice if we could drive faster than 80 on the long straights.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

.025" sounds pretty good !try rotating the rim by one or two studs and try again? It may even just be wheel balance..I'd have a good look at that driveshaft as well,if those units are not in alignment with each other when they build the shaft it gets pretty bad. I used a very reputable truck shop to do mine and still had issues, I'd even suggest measuring along the shaft between centres of the yokes on each side,that's how I discovered the error in mine.when your sure all is good then sure,play pinion angles. But from my experiences doing that first is basically trying to null out issues in other areas of the driveshaft,it may help,it may move the speed of the problem,but if everything else is good,then you know where you stand.

Well, the new wheel bearing changed nothing, which didn't surprise me much. The old passenger side motor mount was soft and partially collapsed, and it also had two shims under it. Not sure when that was done. Installing the new one also didn't change anything. If anything, it may have added a small vibration at lower speed that is sensitive to acceleration. Which I'm thinking means maybe I should put one or both of those shims back in.

I didn't think that the higher-speed vibration was due to pinion angle, since it didn't seem to be change under acceleration or coasting, but I don't have much experience with this. Aside from the drive shaft not being properly balanced (which I hope to check out/fix tomorrow), that's really the only possibility left.

I have a magnetic angle gauge. If my understanding is correct, the transmission output shaft and the differential pinion shaft should be parallel but not directly in line. The drive shaft should make an angle of around 3 degrees with both of these shafts. Where would you guys recommend taking the first two measurements? When we first installed the rear axle, I measured the transmission angle from the bottom of the pan and the pinion angle from a surface on top of the front half of the diff housing. My measurements may not have been accurate enough. How much room for error is there? 1 degree or so?

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

When I measure mine I did it on the machined face of the yoke on the rear end and on the rear face of the trans where the seal presses into,I think using the trans pan may be a little risky,but probably close enough . Remember you also need to allow for the rear end pinion to rise when under load of driving,I allowed around 2 degrees of additional nose down on the pinion,that is assuming you have leaf springs. I found it a little trial and error to get it right but eventually found that to be about the right angle....weird thing is some guys fit stuff in with all sorts of strange angles and with little regard for alignment and never have issues my luck never seems to roll like that.

Okay, so I removed the drive shaft and dropped it off to be rebalanced this morning, or yesterday morning at this point. Picked it back up this afternoon. The guy said the slip joint had some radial play in it, so he replaced it. The shaft supposedly had "a little bit" of runout, but wasn't too bad. Apparently this shop doesn't do high-speed balancing. They just do something he called "heat balancing," where they slow-roll it while heating it up and then just wait for the runout to get down below 0.005", or something.

Before reinstalling the shaft, I measured the pinion angle at the yoke and the transmission output shaft angle at the flange. I was an idiot the first time and didn't think of this. Apparently the pinion angle was 4 degrees lower than the transmission output shaft, with the shims I had installed previously. I left them in place when renistalling the shaft and took it for a test drive. Vibration was still there starting at about 75, but it seemed like it was a little reduced in intensity.

I removed the shims from the back of the axle perches, which should have reduced the pinion angle drop from 4 to 3 degrees, then test drove it again. Vibration seemed about the same. Because it was 3 AM at this point and the vibration wasn't quite as bad as it had been before, I decided to see how bad it got. It seemed to be at its worst between 80-85, but lo and behold, above that it just drops out completely. Smooth sailing from 90-95 mph, which was as far as I wanted to push it on a public highway. When coasting back down, the peak vibration occurs at a lower speed, more like 75 down to 70.

I'm tired. Someone please tell me what this means. It's probably good enough for now, but I'd like to know whether we can make it better. Is it still the pinion angle? We're going to finish packing the truck and leave around dawn. (There are drivers sleeping now, don't worry.) We should be arriving in Sebring around midnight tonight. We're driving in a LeMons race this weekend, if I hadn't mentioned that before.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

I bet you'll be fine. I have driven vibrating drivelines for 10s of thousands of miles, and they usually peak at some RPM and drop out at higher RPM. Go for it and good luck in the race!

Lou

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Home of Slant6-powered fun machines since 1988

After years of chasing racing 4x4 driveline issues in an ever evolving suspension/power environment ...I learned to " roll it over " regularly.

With the chassis at ride height ....weight on the springs (jack stands under the shock plates) .....wheels off......turn one wheel then the other(s) . turn the shaft ...

Rotate as much drivetrain as you can at a time. Listen and look at everything. Engage and disengage everything in turn.

In my mud drag 4x4 there were 2 drive shafts, a tansfer case and 6 u joints to go wrong...as well as shaft angles , lockers , trans case planetary. By rolling it all over I could usually hear where all the bind or resistance was occurring. When it was right it all just whirred.

Rolling it over allows you to see wows or bad pinion bearings ...

You can really hear/feel when shaft angles are off. Best to rotate driveshaft by hand and feel for any flutter or binding.

Bad bearings feel like gravel....

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Yeah....Im the one who destroyed this rare, vintage automobile.....

The pinion angle will be different on drive to deceleration due to spring movement....the fact you get it at a specific rpm both accel and decel would lean towards a balance problem rather than a pinion angle prob in my book. It's likely not going to give you any probs unless you sit in that range for extended periods... I'd take the shaft somewhere else to get balanced properly. They heat the tube to get it running true....never heard of "heat balancing" ...but it should have balance weights welded onto it (unless it's perfect)after they spin it up ...no different to wheels really.....have you checked your wheel balance?

I remember reading in some of my old repair manuals a way to balance the driveshaft yourself with hose clamps............

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Here's one:

http://www.popularmechanics.com/cars/ho ... /1272541-2

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes