Another drivetrain vibration issue

gxnos · Apr 11, 2011

86 GT with 438rwhp and TKO trans / spec stage 3.
8.8" rear with Trutrac posi, moser 31 axles, FRPP driveshaft, Steeda L/UCAs.

Everything is brand new and the car vibrates like hell above 85mph. At 75, its perfectly smooth, and by 85, it feels horrible. It's a bit worse under hard acceleration, but it's there at 85 no matter what. I checked it on accel, decel, and in neutral, and it's always there.

It's hard to explain - the vibration comes intense and fades about every second. I've heard other people on here describing the same issue, and now I know what they meant!

Got the driveshaft balanced at Dynotech
Pinion angle is perfect -2.6 deg
Driveshaft runout is .015" in the back and .019" in the front (not sure on the spec)
Brand new tires were just balanced

I don't know what else to look at! Any ideas?

gxnos · Apr 12, 2011

Another guys on here that described the same problem said it was the U-joints. But mine are brand new. Hmmmmm

jtl · Apr 12, 2011

Does the vibration come through the steering wheel or the whole car?

gxnos · Apr 12, 2011

Its really the whole car, but more than anything it just feels like the seat is vibrating my ass.

Just re checked the driveshaft run-out in a few different positions. at the front, on the other side (rearward) of the weld, it was 0.018" (.047mm) which I believe is out of spec!

jtl · Apr 12, 2011

Have you checked to see if the driveshaft yoke is loose in the tail shaft housing? I know those bushings can wear and cause vibrations.

gxnos · Apr 12, 2011

jtl said:
Have you checked to see if the driveshaft yoke is loose in the tail shaft housing? I know those bushings can wear and cause vibrations.

I tried a couple different things and maybe noted something valuable.

With the car in gear the driveshaft is tight (rotationally) and only moves a couple of degrees.

With the car out of gear and the E-brake up, the driveshaft rotated freely 12.7 degrees (measured with a digital protractor). It seemed like a lot, and I also noticed while driving that it seemed 'clunky' ...meaning right at that spot between accel and decel I would hear a lot of clunking from the diff. Are we getting somewhere here?

jtl · Apr 12, 2011

gxnos said:
I tried a couple different things and maybe noted something valuable.

With the car in gear the driveshaft is tight (rotationally) and only moves a couple of degrees.

With the car out of gear and the E-brake up, the driveshaft rotated freely 12.7 degrees (measured with a digital protractor). It seemed like a lot, and I also noticed while driving that it seemed 'clunky' ...meaning right at that spot between accel and decel I would hear a lot of clunking from the diff. Are we getting somewhere here?

It's certainly possible that there is a problem in your diff. If you're hearing a lot of clunking in the rear it might be worth your while to check the ring gear backlash, I've also seen the splines wear in the spider gears and on the axles creating a lot of lash, though I'm not sure that would give you a vibration. Is there any other noise like grinding or whining coming from the diff? I wonder if it's possible that the gear mesh is off. And definitely check for play at the tail shaft bushing by pushing the drive shaft up and down to see if there is any movement there. Im sure there will be a little but it shouldn't be excessive.

gxnos · Apr 12, 2011

jtl said:
It's certainly possible that there is a problem in your diff. If you're hearing a lot of clunking in the rear it might be worth your while to check the ring gear backlash, I've also seen the splines wear in the spider gears and on the axles creating a lot of lash, though I'm not sure that would give you a vibration. Is there any other noise like grinding or whining coming from the diff? I wonder if it's possible that the gear mesh is off. And definitely check for play at the tail shaft bushing by pushing the drive shaft up and down to see if there is any movement there. Im sure there will be a little but it shouldn't be excessive.

hmmm I'm going to call the guy that did the rear end work and see what he says. No noise or whining at all, but interesting what you said about the backlash and gear mesh.

I just went under the car and pushed and pulled on the slip yoke trying to get it to move in the tail shaft and it was totally solid in all directions.

gxnos · Apr 14, 2011

So it looks like the output shaft angle is not quite equal to the pinion flange angle. I have to whether change my UCAs to adjustables and/or change the spacing between the crossmember and the tail shaft on the trans. This combined with the run out can cause a hefty vibration.

gxnos · Apr 14, 2011

Also, it looks like I may have a problem laterally too. IIRC, the stock crossmember was a bitch to line up with the TKO. I had to cut the welds and slide it back, make a spacer, etc.

jtl · Apr 14, 2011

Sounds like you might be on to something with the pinion angle. I'd say if that changed when you installed the trans. then there's a good possibility that could cause a vibration. I'll be curious to see how it all pans out.

fox04 · Apr 14, 2011

Make sure u check your harmonic balancer i posted my car was doin the same thing and it was my balancer took the vibration out as soon as it was installed

gxnos · Apr 14, 2011

fox04 said:
Make sure u check your harmonic balancer i posted my car was doin the same thing and it was my balancer took the vibration out as soon as it was installed

thanks for the help, but no, this has nothing to do with the harmonic balancer. this is speed related, not RPM related.

gxnos · Apr 14, 2011

jtl said:
Sounds like you might be on to something with the pinion angle. I'd say if that changed when you installed the trans. then there's a good possibility that could cause a vibration. I'll be curious to see how it all pans out.

And I was on to something! I got it!

For the hell of it, I took off the trans mount and put 2 washers in to lift the trans up about another 1/8". Went out on the freeway up to 100 and didn't feel a thing.

Turns out the tailshaft angle was not parallel to the pinion flange. I couldn't get a get zero on the trans to measure it, but looking at it close I could just tell. The pinion flange to driveshaft was -2.6 deg, and the tailshaft to driveshaft was between 0 and +0.5 degrees. Raising it up a tiny bit put the tail shaft to driveshaft angle negative, removing the vibration

jtl · Apr 15, 2011

Man, that's great! Good job troubleshooting. Enjoy your vibration free ride.

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