Gear Contact Pattern - Help

What should change?

  • Pinion depth

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  • Backlash

    Votes: 0 0.0%
  • Both pinion depth and backlash (probably)

    Votes: 0 0.0%
  • Nothing, don't worry

    Votes: 0 0.0%

  • Total voters

Started to experience some gear whine about 300 mi after what I thought was a successful operation. I installed a set of FRPP 3.73 gears, Auburn ECTED 31-spline diff, Strange 31 spline axles, all new bearings/seals.

Backlash upon completion was .0075 inches (I think this might be my problem). Ford recommends .008 to .015

I'd like to mention my pumpkin is solidly mounted to the chassis at all attachment points with spherical bearings (Heim Joints), so all noises usually dampened by the rubber bushings of the stock quadra-bind are greatly amplified. The upper arms have been converted to a single mount made by an extinct company called Evolution Motorsport. It was referred to back then as the Tri-link, the predecessor to the current Mustangs single upper control arm. Attached is also a couple pics of that, and google has many more.

One of the things I was hoping someone of high experience could help with is the wear pattern in my pics. It seems to me that my pinion depth might be just one c**t hair too much. What I did do today is increase the backlash by .004, bringing the final reading to .0115

Bear in mind this is eleven thousands with another 5 ten-thousands (or half of a thousandth) tacked on, not 15 thousands.

I am reading more often that this is the ideal (anywhere between .010 and .012).

So with my backlash at eleven-point-five thousandth inches, I decided to take a break until tomorrow where I'll paint the teeth and run another pattern. I'd been working non-stop today on other junk as well and was pretty tired. The pics are of my original setup at .0075 in backlash. Tell me what you think/recommend aside from altering backlash which I did a few hours ago.



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Did some adjustments today and took away about .004 to .005 away from the pinion depth for a final depth of about .033

Backlash is at .011 with the occasional .009 reading (I put a dial indicator at 5 points around the ring gear just to be thorough this time).

With all that, I got the new pattern in the attached pics. I decided I'm going to run with this setup, but I haven't filled the pumpkin yet. Probably gonna fill it on Tuesday. RTV has to cure overnight anyway. What do you think? Kosher?


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Welp, after a couple days running the last setup, all seems well. I can hear a faint, almost imperceptible whining that happens every so often, mostly when coasting. Which is kinda weird because the coast side looked perfect when I ran a pattern check with the marking compound. But since I'm solidly mounted on all bushings, I'm going to let it slide. If I had stock suspension, I wouldn't hear anything at all.

Thanks for the comments, lol.
Your final pattern looks identical to mine when I did mine a few months ago. I had a little more backlash though, at around .012.

My results with it are identical as well. I can catch it just right on extremely slight decel in 3rd gear where I can just barely hear a whine. If I have the radio on at all, or the windows cracked at all, or if there's other cars around, I can't hear it. There's no way someone else could point it out.

FWIW, in my limited experience with doing gear swaps, "gear" whine is really more "bearing" whine. How much noise is made has more to do with how much pre-load is on the pinion and carrier bearings. Get those correct (which is pretty easy to do), get the backlash somewhere in the specified range (preferably higher up), and have a contact pattern than is reasonably accurate, and it'll be just fine. I see people fretting over a thousandth or two, or over how perfect a contact pattern is, when the reality is that if those are even close, and the bearing pre-loads are accurate, it will be fine. My experience anyway.
Hmmm, I never heard of bearing whine before. None of the tech write ups or forum junkies ever mentioned it, but it definitely makes sense. They're in constant contact just like the gears would be, and they're surrounded on all sides. Plus you're torqueing them down. I got mine (according to my dial torque wrench) to about 24 in/lb. A little bit on the high side but still well within spec (16 - 29 for new bearings). My thoughts now are that as time and miles progress, everything will wear in and I can expect to hear the whine diminish. Maybe not totally, but surely it will get better. I figure that if I was stock rubber suspension, it would be silent, so I guess I'm happy. I don't care to disassemble and re-assemble again, that much is certain. Once is enough anyway.

Thanks for that insight. I was solely concentrated on pinion depth/backlash as the sole source of whining.