Help me find a vibration

2002 Mustang, LS3/TKX, torque arm/panhard bar, coilovers.

After the new transmission install (TKX) I'm getting a slow pulsing vibration that is speed dependent. At 70mph it's a wub...wub...wub...wub. It seems to be much slower than wheel speed. I can feel it in the seat and steering wheel and it vibrates the whole car. At 100mph it's much faster and borderline violent. I just had my driveshaft rebalanced, they said it wasn't really bad to begin with. I put the car on jack stands with the wheels off and still exactly the same as if I'm driving the car. I put my hand on the axle housing while its running and can feel the vibration. I pulled the diff cover off and had no metal shavings or flakes and the wear pattern on the ring gear looks good. I have a Eaton Truetrac with about 300 miles on it since install. I installed the Eaton along with new 31 spline Moser axles, axle bearings, and rear brakes last summer and drove it about 20 miles with the old transmission and did not notice any vibration but it wasn't a great sample size and I don't remember if I ever had it on the highway. I reused the same ring and pinion I already had. They are Ford Racing 3.55's with a couple seasons of driving on them. The pinion did not come out when I installed the Eaton. I also rechecked pinion angle. The crank pulley and pinion flange are within 0.2° according to my digital gage. My driveshaft to pinion angle is 2.5°. Driveshaft shop said the driveshaft had no runout.

My possible scenarios;
-Bent axle (these are new)
-Bad Truetrac? (also new)
-Bad pinion, carrier, or axle bearing? It seems like a bearing would be a constant noise.
-Set it on fire.

How are the u joint angles?  
 

The change in trans (if it was a different model before the change) may have altered the u-joint angles so they no longer match up

See above.  They should be perfect.  The trans mount and torque arm have both been shimmed to get the angles correct.

Did you check backlash at four points on the ring gear?

Did the drivetain have Guibo couplers that got removed with the transmission swap?

Are all the mounts happy after the shimming?

I had an engine-swapped E30 whose PO and bodged the trans crossmember, which had the rubber mount itself all torqued around (er, twisted, IIRC) and it vibrated maddeningly until I made a new trans crossmember that held the mount correctly.

Are the U-joints in the correct orientation (EDIT: I mean are they phased correctly) after the visit to the balancer? Doesn't sound like they'd have been that far into it, but...

Nothing else wacky from the trans swap, like the tailshaft getting pushed left or right? (I have no idea what the old trans was or what the swap entailed, just wondering whether an angle got shoved in there via another axis...)

Pete. (l33t FS) said:

Did you check backlash at four points on the ring gear?

 

Did the drivetain have Guibo couplers that got removed with the transmission swap?

Backlash is .010", spec is .008-.012". 

I don't know what Guibo couplers are.  I'm using the same driveshaft with a different yoke.  I had the driveshaft balanced with the new yoke. 

I'm also using the same transmission mount and crossmember as the old transmission.

My transmission crossmember is bolted to my subframe connectors.  I ordered a Holley mount that bolts in the factory location to the sheet metal in the tunnel just to try something different.

In reply to Patientzero :

But did you check it at four points, so you know if the diff carrier has any runout or not?

Pete. (l33t FS) said:

In reply to Patientzero :

But did you check it at four points, so you know if the diff carrier has any runout or not?

I did not, but I can.  Would measuring radial runout tell the same thing?

In reply to Patientzero :

Not necessarily.  Runout can be caused by improper machining of the diff carrier, or a little bit of schmutz between the ring gear and carrier.  I've even seen it caused by Loctite getting between the ring gear and carrier while pulling it together if a press was not used and Loctite was put on the bolts before drawing it together.  (I try to use bolts to locate the ring gear and then use a press, but this is not always practical)

You're using the same ring gear and pinion as before, so it's not likely that you have a tooth on one and a tooth on the other making weird contact with each other every seventh or eighth axle revolution, but weird tolerance stackups can happen.  The cyclic nature of it unrelated to differential or driveshaft speed makes me THINK it isn't driveshaft or carrier related, directly.

I've had that happen a couple times on cheap aftermarket gearsets.  Heck the gearset in my RX-7 (4.87 9") does it now, a gentle whum-whum-whum.  I wouldn't expect it to crop up on a known good used set in the same case it was originally set up in, though.

A good general rule of thumb is if you can hear it, it's a driveshaft.  Humans can hear starting somewhere around 20Hz and a tire/axle doesn't go that fast (see below).   A vibration from anything at wheel speed is at such a low frequency that it is not audible.  Also, an axle itself has such a small diameter that it would take quite bit to vibrate.  I would say you really need to find out where it is coming from first instead of chasing everything.

I am a big fan of the Vibration App on my iPhone.  Unfortunately it only goes up to 50Hz so if it is the driveshaft you would not be able to measure it at higher speeds.  These apps are usually $5 or so and is cheap to save possibly hours.  It uses the accelerometers in the phone to measure the frequency and amplitude of a vibration.  You just set the phone on the dashboard or steering wheel and watch the graph.

You need to find the frequency of the vibration and its easiest to start with the tires to get the wheel/axle rev/second.  Take your tire size and put into a dimension calculator like this one:

https://tiresize.com/calculator/

 Here is an example:

Get the Rev/Mile number and then back into it to get Hz (rev/sec).  For every mile the tire rotates 787 times, at 60 MPH, the car travels one mile a minute so then the tire rotates 787/minute (RPM).  Take 787/60 seconds and you get 13.11/second or 13.11Hz.   You should be able to measure that with the phone app.  I usually just do some math in my head from there.

66MPH is 10% faster so the tires are now at 14.4Hz.  72MPH is 20%... and so on.

To get the driveshaft Hz, multiply the wheel/axle Hz times the ring/pinion ratio.

At 72 MPH the tire/axle is rotating (using the above tire size) at 15.72Hz.  The rear end ratio is 4.11 so the driveshaft is spinning at 64.6Hz (above the capacity of the 50Hz accelerometers in an iPhone).  But, you get the idea.  Using this example, the axle is at 15Hz and the driveshaft is at 64Hz.  You can't hear below 20Hz ( I can't hear that low anymore, I'm 59).   This is where the "if you can hear it it is probably the driveshaft" comes from.

>Scott

In reply to Scott_H :

Thanks for that.  I can not hear it.  It's just a vibration.  It still vibrates on jackstands with the tires and wheels removed so those are eliminated from the equation.  It would almost have to be something after the gear reduction, either in the Truetrac or the axles/bearings it would seem.

FWIW, I had a similar vibe in my '80 that turned out to be the bearing in the A/C belt idler pulley.  Thrummed cyclically all through the cabin and floor.  Deep whum-whum at about 1.5hz at 3000rpm.

In reply to Pete. (l33t FS) :

Mine is speed dependent, not RPM.  In gear, out of gear, clutch in, clutch out, doesn't matter.  It just gets faster/worse with speed.

In reply to Patientzero :

Point is, it may be unrelated to the obvious due to harmonic effects.  I wouldn't have ever expected a pulley spinning at roughly 7000rpm to make a 1.5hz vibration. 

It may be inside the trans, for that matter.  Given that the vibration started then, that may be the cause.

In reply to Patientzero :

I would still start out by measuring it.  If you can get its frequency, you can at least start down a path and make fewer guesses.

Time for a KT study.  What is it not?

If you get the car back up on stands, can you place the tip of a screwdriver up against the differential and the transmission, and try to feel the vibration in the handle, to get an idea where it is worse?  I've done that before to try to get closer to the source of a problem.
 

I went on eBay to check for some vibration analysis hardware from my old company, but I don't see any bargains.  Maybe there is some consumer grade stuff out there, though.

In reply to eastsideTim :

There's an app you can buy for about $400 that will, after you input tire diameter and gear ratios, do some serious math against the vibrations and tell you first, second, third, fourth order vibrations from whichever source.

You said your driveshaft to pinon angle is 2.5 degrees but what's your tailshaft to driveshaft angle?

When you've got the car on stands where are the stands? Under the rear axle or on the chassis?

When the car is at ride hight is the slip yoke pulled out a reasonable distance?

In reply to APEowner :

Also 2.5 degrees.

Jack stands under the rear axle and wood blocks under the front wheels. (sitting on the front wheels)

I would estimate 1 1/2" of the slip yoke is showing at ride height, it can't bottom out.  There is probably 4" inside the transmission.

Since it does it in neutral and it's speed dependent that would seem to rule out everything from the transmission input shaft forward. 

The fact that it does it with the rear axle sitting on the jackstands makes me think it's nothing in the axle.  If it was in the axle I would expect it to be bouncing noticeably on the jackstands before you could feel it in the car.

Am I correct in thinking that the slip yoke fit's nicely in the tailshaft with a light sliding fit?

Did you have to do any prying to get the transmission mount to bolt up?  It's unusual but if the engine mounting system has pressure on it you can get weird vibrations.

Do you feel it through the shifter more than the floor and seat?

APEowner said:

Since it does it in neutral and it's speed dependent that would seem to rule out everything from the transmission input shaft forward. 

The fact that it does it with the rear axle sitting on the jackstands makes me think it's nothing in the axle.  If it was in the axle I would expect it to be bouncing noticeably on the jackstands before you could feel it in the car.

Am I correct in thinking that the slip yoke fit's nicely in the tailshaft with a light sliding fit?

Did you have to do any prying to get the transmission mount to bolt up?  It's unusual but if the engine mounting system has pressure on it you can get weird vibrations.

Do you feel it through the shifter more than the floor and seat?

Slip yoke fits how you would expect it to.

Trans mount is slotted so no prying to get that to line up, motor mounts are solid anyway.  Trans mount is a Energy Suspension urethane mount.

You don't feel it at all in the shifter which makes me think it's not trans related.

Can you disconnect the driveshaft and run it?  If fluid comes out, can you plug and run it safely or is the yoke necessary to locate the output shaft?