Your other thread says that you are experiencing a noise (and not a vibration) under a de-accelerating load. I appreciate the difficulty here of isolating the problem, but it would seem to be somewhere else, based on your adjustments*. I have the same compound angle due to engine offset and it doesn't seem to aggravate anything.

Can you describe the noise and its characteristics?

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*somewhere else on this forum I posted modifications to the diff bracket to get the full rubber bushing back in place without screwing up the angles. Ditched the poly bushings/mounts I was using. That does mean more compliance and changes in angles while driving but it's been OK.

 

It sounds a lot like a rattle except for it's regularity. It's a speed related noise that's unaffected by rpm or what gear you're in. I think it does it whether accelerating or decelerating but that exhaust/engine noise masks it when accelerating. I've tried turning left and right while it's happening but it's unchanged by that. You can hear it slow down while decelerating down to about 15-20 mph when it seems louder though slower. I can make it worse by adjusting the pinion angle up or down although it seems to be attenuated about as much as it's possible to do by equalizing the driveline angles. I'd be really happy just to get the pinion angle to 0 degrees because I could use a C/V joint at the slip yoke at that point, but that's still almost 3 inches up from where it is with a stock front diff mount, and the front of the diff housing would run into the crossmember before I got anywhere close to 0 degrees. I've looked over the frame very carefully and can't find any damage, and I've got experience looking at bent frames from my 20+ years as a bodyman. Also, the previous owner of my car used the frame as a jig to build headers and sidepipes for C3's with LS motors in them and got no complaints of his products not fitting. I feel like it shouldn't be coming from the u-joints because the driveline angles are equal within half a degree but I can't get past the fact that the noise I hear changes with pinion angle. I also can't get past the fact that the differential is installed nose down and therefore shouldn't have correct angles whether there's an LS motor in it or not. I might be (probably am) missing something but I just don't know what.

 

You sound like you know what your are doing, but I will throw out a couple of things. Just disregard these if you have already checked. Worn slip yoke, bad U joints, and re-checking the joint angles. I thought in your other thread you were using a cell phone to measure - if so, that would not be my first choice. The cheap analog angle finder I use works OK. The good digital ones (above) would be great. I would re-measure the driveshaft to pinion and driveshaft to slip yoke and calculate each angle. I am not sure how you are measuring, but the pinion angle only matters in relation to the driveshaft.

 

I've checked and rechecked the u-joints, and I've looked at the slip yoke pretty carefully. It doesn't wiggle around on the output shaft so I'm assuming it's ok but that's good advice so I'm going to take another look at it. I feel like I've got the hang of checking the angles,
I started out using my iphone to measure the angles, and after getting inconsistent results I bought a digital angle finder. This gave me more consistent and repeatable results but was very nearly impossible to read! As soon as you'd get it on the pinion yoke for example, the display would just sort of disappear. The viewing angle on those displays is pretty small(it's all about angles isn't it) and the only way I could read it was to contort into a kind of human pretzel with the angle finder in one hand, a light in the other, and my head as high as I could get it underneath the car. After doing this a few times I compared the angle finder results with the iphone results and found that they were very close, with the iphone much easier to see. The key to consistent readings turned out to be always measuring from the same side of the car and always in the same place. What I've learned to do is put the car on jackstands, the jack the rear tires up until the suspension is loaded. Then I measure the driveshaft angle, write it down then remove the driveshaft. Then I measure the transmission yoke and the pinion yokes and write those down. The last time I did this I took the bottom part of the rubber mount out and experimented with the pinion angle until my driveline angles were equal, and then measured the gap between the differential and the mounting surface. Using that measurement I cut 5/8 of an inch from the bottom part of the rubber mount and reinstalled it. I remeasured and got a front angle of 2 degrees and a rear angle of 2.3, so less than 3 degrees and within .5 of each other. At that point I was pretty optimistic that the problem was solved, but it wasn't. At this point I figure that it's either compound angles on the driveshaft being incompatible between the horizontal plane (w-arrangement) and the vertical plane (parallel arrangement) or something unrelated that I haven't noticed. If it's compound angles then it's over my head, but it seems unlikely to me that it's that because the difference in the vertical plane angles is so small. This leaves something that I haven't noticed and I'm hoping for a flash of inspiration that points me in the right direction but failing that I'll just keep plugging away at it. Last night I was thinking how nice it would be if I could install a driveshaft with a c/v on either end.

 

If I was doing this I'd want to do fault isolation before replacing major expensive assemblies. And I thought (to myself) one way might be to try and hook up a remote microphone to my iPod and move it here and there under the car to see if I could zero in on the noise. The frequency of the noise would matter too, the half shafts would be different than the driveshaft. You'd have to do a little math.

I poked around on the internet a little yesterday but was pretty busy and didn't find anything. There're so many apps and gadgets some sort of analysis kluge might exist.

I had a similar experience with the dust cover on one of my wheels. It would only touch under certain conditions and sounded like it was coming from just about anywhere.

What fun!

 

the angle of engine/tranny, the line thru the dampner/tailshaft, cannot cross the angle of the in flange on the rearend..where the driveshaft u-joint bearing are mounted. The result of doing so results in vib. as well as short u-joint life. That's why some cars/trucks have the third u joint in the middle of the driveshaft.

 

Not trying to argue - but this is not entirely correct for every application. The speed of the two joints has to match.
Perhaps you should read through this thread from the start. The way you are proposing - should have the trans and pinion parallel. And as to center bearing two piece shafts - many times it is required because of engineering calculations. The harmonics of the shaft are dictated by the length, material and thickness of construction, and the speed of the shaft.

 

In any event it would benefit me to isolate the source. That seems to be the logical next step here. Mike Dyers (tracdogg) told me there's a tool that uses several microphones that can be useful, I'm going to try to find out who makes it and if I can afford it. I also sent an email to driveshaftshop.com to see if it's possible to make a driveshaft with a C/V on both ends.

 

I found this:

 

That's like the one I told you about but this one is better, no wires.
Mike

 

Kinda pricey but at least it comes with batteries! I spent an unconscionable amount of money at a shop years ago on a problem in the drivetrain to no avail so if that's the trade you'll be ahead of the game. Maybe you can play it into the radio and get cool exhaust noises when you're done.

Some of the complaints are about the wireless but on a fiberglass body there's less likely to be a problem.

I'd want to be able to get a signal waveform with some sound app too, but that's just the engineer in me. Maybe that's just a cable. Anyway keep us informed

 

First I'm going to replace my poly trans mount with a rubber one as the driveshaftshop.com just suggested.

 

Thanks Mike, if the rubber mount doesn't work I'll get it. I guess I should put one each on the transmission and differential and both trailing arms?

 

So I got the wireless chassis ear and the weather got nice enough for me to give it a try. I put the alligator clip mikes at the front and rear of the driveshaft and at the left inner and right inner halfshafts and went for a drive. To my surprise and delight the driveshaft was very quiet, so was the left halfshaft. The right one was making a lot of noise, so I put another mike at the outer halfshaft and went for another drive. The noise I've been hearing in the car is most easily heard at the outer halfshaft mike, but I think that bearing noise is competing with the mechanical noise at the inner halfshaft. In any event the right side yoke is wobbling so there's a good chance in my mind that the noise originates there. I talked to Mike about this and we agreed that the next step is to replace the side yoke, so I sent the differential off to him yesterday. I'm pretty happy that the problem isn't from driveline angles, and that I've localized the noise. Hopefully replacing the side yoke solves the problem, and it needs to be done first anyway so I'll post my results once I get the differential back.

 

Very admirable perseverance, hope that's it!

 

Well I just want it to be right, also I learn from these kinds of things. That chassis ear sure is a great tool!

 

Found this link over at the autoX forum.

http://www.macsmotorcitygarage.com/2...e-and-phasing/

Highly exaggerated angles for demo purposes.

 

That's a pretty effective video!

 

It's been some time since I posted on this thread, and in that time I've eliminated everything else but the driveline angle as the source of the noise. I did discover that improperly machined stub axles were contributing to my confusion over this and Mike Dyer replaced them with rebuilt stub axles which are turning without wobble so that problem is eliminated. Using the chassis ear I'm not getting any noise from any of the driveshafts, but it's there all the same. Here's what I know, the noise which is best described as a knocking sound speeds and slows with the speed of the car but the loudness remains the same bearing in mind that it's best heard when decelerating. This fact tells me that it must be a driveshaft noise. I've adjusted the pinion angle by raising the car and placing jackstands under the lower control arms both front and rear so the suspension is loaded and supporting the car. Using my digital angle finder I zero it to the driveshaft, then remove the driveshaft and measure the transmission angle. I then adjust the pinion to equal that angle. So in this case the driveshaft is zero the transmission is 1.8 degrees down and so is the pinion. Now I know that the w-arrangement should work, but I'm still getting the noise so I'm still missing something. Lately I've been considering changing the engine mounts to something like the ones on this page: http://www.brphotrods.com/products/6...tte/index.html, which would lower the engine in the front to stock height but this would require some changes that I don't want to make unless I have to and I'll exhaust all other options before I do this. I have to say also that I'm scratching my head that the non-swapped stock Corvettes don't have this problem, with the engine at 0 degrees and the pinion pointing down how do they not all make noise or vibrations? In any event the next and last step before changing motor mounts is to incrementally move the pinion up and down and record the changes in noise to see what happens. I just thought I'd update those of you who offered suggestions in this thread, man what a bitch of a problem!