STi diff bushings and carbon driveshaft

[gpvecchi]

I made this thread looking for people with same setup, as I hadn't found anybody in other threads, just people with bushings inserts.
I would like to know if this configuration suffers of the same annoying vibrations of the inserts...

[Decep]

I would think if there weren't NVH implications they would be in there stock

[gpvecchi]

I don't mean slight vibrations, I mean of the entire body shaking like unbalanced rims.
Many are reporting this with insert in bushings, I would like to know if somebody tested it with STi diff bushings

[StraightOuttaCanadaEh]

I didn't even know STI diff bushings existed... You could always try. It's easy to get rid of the bushings if they do vibrate, and then you still have the driveshaft

[blsfrs]

I don't see how bushings would cause a vibration. They are non-rotating , stationary parts. Stiffer bushings will only amplify vibrations. Try putting you stock drive shaft back in the car. I've read where folks have had problems with the aftermarket carbon fiber and aluminum driveshafts.

[StraightOuttaCanadaEh]

Originally Posted by blsfrs

I don't see how bushings would cause a vibration. They are non-rotating , stationary parts. Stiffer bushings will only amplify vibrations. Try putting you stock drive shaft back in the car. I've read where folks have had problems with the aftermarket carbon fiber and aluminum driveshafts.

He's referring to a thread I started (or commented on with my findings) where I basically remove the diff bushings and get rid of driveshaft vibration. I was researching this issue in different car forums, Mustang, BMW, Hyundai Genesis, and every one of them had at least one thread where either the diff or subframe (or both) bushings caused the driveshaft to vibrate at certain speeds (~80mph for me).

[blsfrs]

Originally Posted by StraightOuttaCanadaEh

He's referring to a thread I started (or commented on with my findings) where I basically remove the diff bushings and get rid of driveshaft vibration. I was researching this issue in different car forums, Mustang, BMW, Hyundai Genesis, and every one of them had at least one thread where either the diff or subframe (or both) bushings caused the driveshaft to vibrate at certain speeds (~80mph for me).

I would say that by installing softer bushings, you prevented the vibration from being transmitted to the frame. The driveshaft is still vibrating. There may be some kind of harmonics involved with various bushing materials and the frequency of the vibration but that's beyond my pay grade.

[StraightOuttaCanadaEh]

Originally Posted by blsfrs

I would say that by installing softer bushings, you prevented the vibration from being transmitted to the frame. The driveshaft is still vibrating. There may be some kind of harmonics involved with various bushing materials and the frequency of the vibration but that's beyond my pay grade.

Yeah that's what I was thinking, but in all those threads, the words "pinion angle" were mentioned a lot, perhaps insinuating that the diff bushings somehow affect that angle

[gpvecchi]

It's not easy to replace the bushings, and it's not cheap to buy the shaft, so I would like to be sure before.

[StraightOuttaCanadaEh]

Originally Posted by gpvecchi

It's not easy to replace the bushings, and it's not cheap to buy the shaft, so I would like to be sure before.

I should add that mine were just diff inserts. Not sure if actual bushing replacements would behave differently

[Turdinator]

No bushes at all sounds like a bad idea. I have a carbon drive shaft installed and a pair of STi bushes to go in. My stock bushes have cracked and torn but installing the STi bushes looks like a big-ish job so I haven't got around to it yet. When I eventually do i will report back here.

[jflogerzi]

For most upgrades the STI bushing or mounts seem to be a happy middle ground. Stiffer than stock but not much NVH added but YMMV.

[Matt93SE]

Digging this one back up to provide some color commentary..
pinion angle is the angle between the driveshaft and the differential pinion flange.
basically your trans and differential are both aligned parallel to each other, but not necessarily perfectly in line. i.e.



Note that the angle ad the diff is the exact opposite than that at the transmission.
*typically*, as long as that angle is less than 3deg and both flange angles are the same, you should not have vibration problems in the driveline.
But if either of those angles isn't exactly the same and you have an overall misalignment, you're going to have vibrations in the driveline.

Sooo, what's this mean to you? in short, don't jack with the position of the differential. Diff insert bushings that don't move the diff? sure thing, no problem.
replacement bushings that move the diff and/or change the angle in relation to the transmission? you may have problems.

Watch this video for some great illustration:

When I bought my car, it had an aluminum 1-pc driveshaft in it. very slight vibrations over 100mph, but it was just barely there. no big deal, because racecar. I was having problems eating rear CV boots/ axles, so I installed some PBM diff riser bushings. Next race was at COTA where I should be doing >130 on the back straight. I couldn't get the car over 115, and the vibrations were so bad that I was waiting for something to break. The car was absorbing so much energy from the vibrations, it just didn't have the drive to reach proper speeds. I had other problems (engine related) in practice, so I packed up and went home before doing worse damage.

When I got home, I did some research online about the diff bushings and learned about the nasty words "pinion angle". did some measurements and I was at about 4.5* misalignment just from installing the PBM bushings!
the vibrations were also so bad that the aluminum driveshaft was bending and contacting a screw on the inside of the trans tunnel.. It machined a groove into it deep enough to compromise the integrity.. SHEEEIT. money wasted.. Throw the driveshaft away and work on adjusting the diff angles.
I carefully measured the diff angles and was able to re-drill the holes in the PBM bushings to et the diff to within 0.5* of original and installed a stock 2-pc driveshaft. Zero vibrations now.

I don't know if it was the 1-pc driveshaft out of balance or the misalignment itself that caused the problems since I replaced both at the same time, but I do know that both problems are related and can cause nasty things to happen. even if you don't have awful driveline vibrations at lower speeds, the constant wobbles on something like that can cause ring and pinion and/or trans bearing failures over time, so it's important to fix or avoid that stuff in general.

HTH.

[churchx]

My line of thoughts goes that diff riser bushings are not there with goal to just change said pinion angle for no reason. They are there "to fix"/revert pinion angle closer to stock on overlowered car (also reducing CV joint angles). That in turn probably means, that if car is not dropped very low, then their installment nets those mentioned problems instead of doing something good, and thus should not be used.