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Update: I shifted the rear subframe toward the driver’s side as well, although it only had maybe 2mm of adjustment in it. Drove the car on the freeway again. The vibration doesn’t come on until ~5.2k rpm in 6th now (~110 mph?) and is no longer super jarring. I’m going to reinstall the ppf brace this week as that seemed to help mute it a bit. Will try to align the trans tailshaft left to right as well using Rich’s technique.
I’m surprised you can get your subframe to shift, mine has little to no play at all
i followed your lead and got a laser
if you make a centerline of the car by splitting the distance between the pinch welds f/r, it looks like my engine and trans are almost exactly on the centerline.
the diff is shifted 16-17mm over to passenger side. The trig puts the driveshaft at .9* side angle. That matches my up down angle.
I’m no closer. Either my driveshaft is still just trash (I know that it is off) or I’m missing something. I just can’t accept that all the shafts both you and I have tried, even your nice one, are the only thing causing my vibe.
I may have cheated a bit with the subframe. Had the car on a lift and a powertrain jack supporting the subframe, then took a big pry bar and wedged it between the subframe and frame horn. It definitely moved over 1/4" or so. Can see an outline where the bolts and nuts were clamping it before. I think that's all the adjustment I have, though.
I'll have to get the centerline measurements for my trans/diff like you did. I'm spitballing here, but I want to say my total offset looked like less than 16-17mm. I didn't measure it, though, so I could be wrong.
I'm with you on the driveshaft bit. I was surprised to find that the new shaft only moved my resonant frequency up ~15mph, and my old shaft was f*cked.
Going to try and put the PPF delete brace back in my car today and see what effect that has. Swapping from the brace back to the PPF caused the vibration to come on quite a bit earlier, so hopefully I can alleviate some additional vibration by reverting back to the original setup.
I drove the car a LOT yesterday, and the vibration seems to be present starting at a similar speed as before (maybe 75mph), but is 80% less robust now. It's almost undetectable until you start getting north of 100mph, and even there it's still not nearly as violent as it used to be at 70mph. If I weren't looking for it acutely, I probably wouldn't suspect anything was wrong driving it at regular highway speeds.
So pardon my ignorance here, but is there a reason a CV joint style driveshaft could not be used? Something less sensitive to driveshaft angle and phasing than a cross style u-joint.
I see no reason why it couldn't. Honestly, given my current situation finding that decreasing the total necessary driveline angle helped the problem, I'm starting to think it'd help. FNFMC made a good point, the stock BMW setup incorporated the giubo and a two piece driveshaft with a center support bearing.
Maybe I'm just holding out false hope, but I don't see why my car would need a setup like BMW's when Six's car doesn't. I'm sure chassis wear and other structural factors will cause different resonant frequencies in each of our cars, but I still can't help but feel like there's something I'm missing that would make my current setup work.
I pulled out my PPF and reinstalled the KPower brace on my lunch break today. Vibration doesn't feel like it changed much on the lift, but then again, I didn't feel the improvement from the subframe adjustment until taking it on the road. While I had the car in the air, I took it up to 6k in 6th gear just for sh*ts. The vibration goes away at about 5.8k rpm. Not sure what good that knowledge does me, but it's data nonetheless.
Just spoke to Jamie at KPower. We chatted about a couple things with the swap. It was suggested that I confirm my current driveshaft was balanced to a high enough speed. Jamie said the shop that was originally making driveshafts for one of their 86 kits didn't balance the shafts at a high enough rpm and they ran into vibration issues until they specified they needed the shafts balanced at said higher speed.
I still have some more diag to do, but I was told KPower is switching suppliers and will be using more expensive driveshafts in their ZF kit from here on out. I believe Rich is getting his first. Seeing as I already got a shaft made, Jamie was kind enough to welcome my request to return my supplied driveshaft for credit towards a Getrag swap kit. Sounds like the new shafts are probably landing at KPower sometime next week. I told Jamie I'd call back Monday and get my shaft sent back to them. Will keep this thread updated.
Yeah, that is accurate. We’re trying an aluminum driveshaft with a different style yoke. I have not seen the design yet but the k power folks have described the yoke as being similar diameter to the driveshaft, with little to no slop.
Aluminum has 1/3 the stiffness as steel, so its resonance frequency will be different. We’ll see if this has any effect, we’re not really isolating variables here, and certainly will not care if this thing runs smooth.
Keep me posted nonetheless! I'd imagine the weight will have an effect on the vibration as well. Maybe less weight being thrown around will help out.
If the alu shaft fixes your issue, I'll just run one of the new shafts. Hope it makes a difference.
did you validate the left/right flange parallelism? curious as you've seemed to reduce the vibe quite a bit but not rid the car of the issue, even at the lower speeds. wondering if there is still an alignment issue.
I also don't have a great understanding how much tolerance there is for misalignment
I haven’t had a chance to dig into the left/right parallelism yet. Had about 20 minutes free before the shop doors closed yesterday so I just experimented with the tailshaft positioning before I left. I’ll have to check left/right angles like you did when I have some more time.
I did, however, have a potential “aha” moment after work today, though. I completely forgot I have a random passenger side poly motor mount that’s shorter than the ones currently installed on my car. I used to run a hybrid Mazdacomp/poly motor mount setup, then got this mount from a friend and ran it for a while (four months), not noticing that it made my motor/trans lean/shift a bit towards the passenger side until I took a close look while it was on the lift one day.
The vibration seemed to lessen after moving my motor/trans toward the passenger side of the vehicle. I’ve got my front subframe shifted full right, which lessened the severity of the vibration. I don’t think it’s a stellar idea to run long term, but I can install this mount to get the powertrain to sit even further right and see if that has any effect on the vibration.
I also don’t know how much tolerance there is for misalignment. Kinda just bumblef*cking my way through this, but this seems like it may confirm or deny my convictions about the driveline angle being too great due to excessive lateral distance between the motor/trans and the diff.
Pic I took when I went to pull my motor back in August. Trans situated pretty far to the passenger side of the car with that mount setup.
Dropped in that other passenger side motor mount. Had to hog the crossmember bolt holes out more to allow the bolts to reach the trans mount, so it moved the trans over to the passenger side a decent bit. Maybe a half inch or so. No change in the vibration. This leads me to think that if there's an alignment issue, it's on the diff side. Or I'm way off in the weeds. Probably the latter.
Dropped in that other passenger side motor mount. Had to hog the crossmember bolt holes out more to allow the bolts to reach the trans mount, so it moved the trans over to the passenger side a decent bit. Maybe a half inch or so. No change in the vibration. This leads me to think that if there's an alignment issue, it's on the diff side. Or I'm way off in the weeds. Probably the latter.
well, it takes two to tango. The alignment is just a relative thing between the two flanges.
Should have the new shaft in hand this week, hoping for the best. I am also going to take some pics this evening when I get back in the shop of what I've found with my car.
Yeah, true. At some point, the alignment is just gonna be within some acceptable range or it’s not. I think it can now be safely assumed that mine is within that acceptable range.
Anxiously awaiting the result with that fancy alu shaft. That’ll be the deciding factor for my next move. Interested to see what else you’ve played with on your car too.
I just grabbed a cheap rubber mount off Amazon. Mainly for the next day shipping. Seems like it’ll hold up, though, and it definitely helped with overall NVH. Link here:
I'm still amazed at the problem. I really don't know why you would have an issue and I don't. But you're finding changes that make a difference so you are obviously landing rounds on the target even if you haven't hit the bullseye yet.
My general M-O is to just fire enough rounds that at least a few of them land. Hopefully I can keep coming up with ideas on this.
Ever since swapping out the bad driveshaft, the vibration can be felt exclusively in the diff. It doesn’t seem super likely to me, but I wonder if the vibration/original driveshaft with tons of runout damaged or accelerated wear in my diff. The two month-old fluid I drained last Friday came out black, although I couldn’t find any particles in it. I’m gonna drain it again this week and see what I find. The pinion still spins freely by hand and doesn’t grind or exhibit any serious drag, though, so maybe it still is a balance issue.
Will continue throwing darts at the board until hearing back from Rich.
Dang, proper stuff on those measurements man. I guess with that considered, it’s pretty safe to say that your driveline alignment is within spec, or that one/some of the previous alignment settings have been within spec.
I haven’t felt any play in my pinion but haven’t really got in there and manipulated it by hand. I’ll try this week.
If you wanna try dumping fluid, I can send you a couple quarts of Motorcraft 75W-85. I got a major surplus of the stuff.
04-21-2024, 12:13 PM
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