So my K swap on the FRS has been great but I think the 1 piece driveshaft I have is creating vibrations. I was quoted a fairly inexpensive price to shorten a stock two piece and am now considering trying that out.
The only problem I see is that the front section of the driveshaft is going to be pretty short. I need 5 1/2" removed and there's not a lot to work with. See pic below. I need to place the front U-joint flange flush with the left edge of the blue tape. Honestly I imagine this is probably me overthinking things but anyone see a problem with it? I can't easily move the the bearing mount rearward or I'd just remove some from the rear section.
Note: Not an expert.
It looks to me like that front driveshaft would end up awfully short. Probably too short.
Noise from the single piece driveshaft is likely due to a mismatch in the pinion vs. output shaft angles.
In your installation, do you have any leeway to raise or lower the back of the transmission, and/or the nose of the differential?
https://www.therangerstation.com/tech_library/pinionangles.shtml
I dropped by the shop that I planned to have do the shortening and he said it should be fine even in a shorter state as long as it's well within the 20 degree angle of the U joints. Hopefully I'll have the shortened unit in a couple days and test out that theory soon enough. Gonna be a pricey mistake if that's not the case but I think based on his experience it seems like it should do the trick. If anything I'll need to space the center bearing mount down a little lower to reduce the angles which shouldn't require much effort.
Also not an expert but I thought that the shorter the shaft, the less of an issue it was (unless you're articulating, which you're not).
I'd try not to move the center bearing because that controls the front ujoint angle on the rear shaft which I think you want that to match the rear ujoint on the rear shaft.
Small angles don't matter, it's more important that the front and rear ujoint angles on any shaft match.
In fact, you might want to check that on your current single driveshaft first? Maybe that's the issue.
In reply to Robbie (Forum Supporter) :
One way or another I'll know if it was an expensive mistake or not as I've already approved the work to be done. Should have it in a couple days.
I do agree there may be a misalignment with the one piece, however whatever adjustability the kit currently provides will possibly create rubbing with the trans tunnel. I haven't tested it yet so I'm just assuming here. This is obviously what I should have tried first.
The main reason I wanted to get rid of the one piece was because it seems all aftermarket ones that are for the BRZ/86/FRS etc have issues with vibration. Something to do with critical speed if I remember correctly. Most folks toss them in their car and live with it or they are more focused on track work so it doesn't matter. The center support bearing seems to be taken into consideration for this with the factory with NVH added in as a bonus? I hope. Before I bought the kit I wanted to shorten the stock unit fearing I would end up with the same issue. I'm pretty sure that's what we are looking at.
As to rear U joint angles I haven't considered that. I assume it also only requires a 20 degrees or less mentality? Here's a pic of the full two piece setup.
I did a quick drawing since I think it explains what I'm trying to say best. You want the input and output of a u-joint driveshaft to be parallel. If they are, the angle isn't very important.
If they aren't parallel, even small angles are bad.
Aaaaaand just looked at therealdeals link. It lays it all out really well.
In reply to Robbie (Forum Supporter) :
That makes more sense now. I think the angles should be fine and still stay parallel.
I also just looked at the link and don't think I'm seeing that aggressive of an angle but would need to check not only alignment between the two but the angle of both.
I found with the RX-7 during all of my drivetrain shenaniganing that U-joints really do not like operating at more than about 2 degrees. A shorter shaft will make the angle more bearable, but the issue is that with angle the driveshaft is going to be accelerating and decelerating twice per revolution. The more mass the shaft has, the more it's going to want to wag the dog drivetrain-wise, causing gear clatter in the rearend and transmission unless you have enough load on it.
What's your final drive and tire diameter? IE what is your normal expected driveshaft speed on the highway?
4.30 rear gear with a 25.7 tire diameter.
From my measurements the trans has been lowered 1.25" and moved rearward 5.5". I can't say with certainty how extreme that will effect the angle but hopefully not too much.
It is kind of a shame that the chassis uses a Toyota diff instead of a Hitachi diff like a traditional Subaru. Traditional Subarus have a CV joint in the center. You just need to make the U joint angles straight and take up any angular difference at the CV joint. But the rear flange is different, so you can't start off with the rear half of an Impreza driveshaft.
That said you MAY need to space the center support down a little. Maybe. Either way, step 1 is connecting the trans to the diff, step 2 is figuring out where to shim this and that in order to make the U joints happy.
So I knew about the trans and diff yokes being parallel rule, but is that rule still important with a two piece shaft? Just curious.
In reply to gearheadmb :
Parallel is not really the rule, the U joint angles have to be equal. Can be up or down as long as the trans to driveshaft and driveshaft to diff angles are the same.
Old Studebakers had a really weird steering mechanism. Someone put a Chevy V8 in one and the drivetrain had to be at like a 7 degree angle to clear the steering. The driveshaft was fairly short. To make the diff parallel to the trans would have therefore needed 7 degrees of up angle which would result in something like 20 degrees of U joint angle because of the shortness of the shaft and how low the back of the trans was to clear the floor. I moved the pinion DOWN so that the U joint angles were complementary (mental notes suggest around 3-4 degrees each) and it worked great.
Quick googles tells me to space the carrier bearing with the trans and as long as the diff is parallel to the trans it should be gravy?
In reply to captainawesome :
As long as the U joint angles are not crazy. May need to space the carrier bearing to find a happy medium there, which also may require shimming the diff nose down to compensate for that, depending on how it all plays out.
Remember, you changed the drivetrain angle and height. You may have to tweak other things as a result.
This is cake work for me but I'd need to see the metal in front of me to see the way forward. It WOULD be a ton easier with a single piece driveshaft with double Cardan joints or a CV joint at each end, but "easy" is not the world we inhabit...
For what it is worth, I had my 2 piece driveshaft shortened about 6" when I put a Volvo 6 cylinder engine in my Volvo 1800. It worked out great. And in regards to keeping the flange angles equal, the car's 4 link solid rear axle with it's anti-squat rotates the pinion during suspension travel, so I think that there is some leeway as far as misalignment is concerned.
I have also read that for a solid rear axle, the pinion should be aimed lower than equal because when you accelerate, the torque rotates the pinion upwards (drag racing). Also, the driveshafts should never be aligned in a perfectly straight line, there has to be an offset in one direction or another of the shafts so that the needle bearings rotate a little with each revolution and don't sit in just one position all of the time and wear out the u-joint.
Edit: Ok, disregard what I just said above, it was covered in the above link.
https://www.therangerstation.com/tech_library/pinionangles.shtml 
I don't know much about driveshafts, so just consider my input anecdotes.
A lot of Ford Super Duty owners ditch the two piece rear driveshaft and go to a one piece. Apparently they experience less vibration and shuttering during acceleration. I just had the front half of my two piece driveshaft lengthened by about 3.5" and the front shaft shortened by the same amount to accommodate a transmission change. Cost about $200 total. I don't have many miles on it yet, but so far I haven't noticed any difference in vibration. The machine shop owner didn't have any concerns about extending the two piece, but was hesitant to do a one piece conversion due to overall length.
I picked up the shortened shaft yesterday but stars did not align for me to install and test out yet. Hopefully I will get a chance to get that in this weekend and report back. Pretty sure it's a 1/2" longer than requested from my tape measure so I'm paranoid it's not enough room at the slip yoke. Pretty sure if I'm spacing out the support bearing any that shouldn't matter and in fact may be helpful. Fingers crossed.
Just wanted to follow up on this one. Vibes are reduced dramatically all throughout the rev range now. The rearview mirror was blurry most of the time before but now I can see traffic under acceleration. More stuff to address but this was a monumental change towards a better daily driver.
I spaced the carrier bearing with 5/8" thick spacers I had laying around. Oddly enough I think this put everything perfectly aligned, or at least in this scenario "good enough".
Nice!! Usually angle related vibes go away under load and are most noticeable at neutral throttle. Sounds like you have it handled neatly.
Looking at the trans tunnel pic, I got an idea for the Mini, as I have to figure out how to support the carrier bearing Real Soon Now.
In reply to Pete. (l33t FS) :
Honestly I probably am doing a terrible job of explaining how and when the vibes were happening. I do believe it would reduce under heavy throttle while steady state RPM in that rev range was it's worse. Sounds like it matches your description.
