as stated in a previous thread, i have adapted a bmw transmission into my rx8. i have modified the flanges on both the transmission and differential, and am using a custom driveshaft. both ends of the shaft now have a 4 bolt flange very similar to the rx8 rear flange. i have been making the shaft with a slip yoke to accommodate installation, but it seems like these are a bit heavy, and so far the driveshaft failure i have experienced has been at the slip yoke. what i was wondering is if i were to make a shaft with no slip yoke, just the u joint and flange at each end and ran it this way, would the over stress the bearings in the differential or the transmission. anyone have experience and care to share? or maybe even a well thought out theory? i have access to aluminum shafting and was really wanting to use that to save weight, but by the time a get a slip yoke in there i dont feel i save any rotating weight.

 

if you have access to a driveshaft shop they should be able to make one without a slip yoke, but likely it can be done without direct access there if you can measure it accurately yourself.

possibly the slip yoke one you had wasn’t built strong enough though
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that is likely. i had used an aluminum slip yoke from a ford ranger driveshaft. i feel pretty good about the machine work i did in the lathe, and even my redneck balance act wasn't too bad, but i am a novice self-taught tig welder. i will contact a driveshaft shop but i am pretty sure that is going to end up being cost prohibitive

 

It’s fairly common for that situation, but I’m not really sure what cost number you consider to be prohibitive. I’ll need the same for an upcoming Nissan CD009 conversion & ppf delete as well.
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Team, what engine are you pairing with the Nissan CD009?

 

REW turbo

 

update, had a driveshaft shop make one for me to the tune of 550.00. at 3500 rpm driveshaft speed it begins to vibrate. they told me that sucks, we balanced it there is nothing else we can do.

 

are you sure the driveshaft is aligned properly wrt angle between the transmission and differential? You may need to make some adjustments there depending on how you have them installed without the PPF.

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i will double check that, using a digital angle finder they were parallel in the vertical plane, the horizontal i measured with a tape measure and a flat bar across the flange. is very possible that i got that wrong but thought we had it.

 

not saying you don’t, but if it vibrates then it has to most likely has to be either the alignment or the balance. Maybe they thought it was balanced properly and it wasn’t. It would have to be checked through another source maybe.

There may be other reasons though, but my brain isn’t coming up with anything else. Conferring with a drivetrain specialist may be necessary.
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any more progress on this?
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i have not had a chance to get the car back in the shop. after a lot of reading, a lot of mental exercise, i plan to build a two piece shaft with a carrier bearing in the center. my thought is that the short section will be much easier to balance, and a lot less likely for there to be deflection in the shaft. i think the shaft i am using is too long for the small diameter

 

I’ve never heard of anyone having to do that on a PPF conversion, but ok.
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and i have never removed a ppf before so i dont know what is normal. i did use the smallest diameter driveshaft i could, and with the shortness of the bmw transmission the shaft is 13" longer than the original. it just "feels" wrong to be so long at such a small diameter. we will see

 

what diameter?

 

2" od, steel tube