Two-piece drive shaft geometry

My car has irs and therefore a fixed differential angle(without lots of fab work). My drive shaft needs to be 65" from u-joint eye to u-joint eye. For that length I would need a drive shaft way too large of diameter to fit where it needs to. That tells me I need to set up a two-piece drive shaft, correct? Currently the differential and the output yoke are parallel but that can be changed. How do I lay out angles of a two-piece drive shaft?

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This is what I've got so far. I don't have a driveshaft yet so I don't have lengths figured out. 

So for a two-piece driveshaft, the angles have to add up to zero just like with a 1-piece, the only difference is that there are more angles involved. So your sketch with the joints at 0deg, 2.9deg up, and 2.9deg down would work fine.

I'm trying to recall that pesky math degree...

If the trans and diff are fixed angles, won't the angles between always add up to 0°?

Anyway, yes, I can make the front come straight off the trans and the rear come straight forward off the diff.

Then it'll be off to a junkyard looking for a donor

Yeah it's all about keeping the trans output and diff input shafts parallel.

U joint angles are trickier to measure with a two piece, depending on how it is laid out.  I would like to see some angle in the center joint myself, partly because U joints and CV joints don't like running at 0 degrees all the time, and partly to break up resonances.  Another good idea, on that note, is to ensure that the two shafts are of different lengths so that they have different resonant speeds.

Unless you are running 5000rpm+ driveshaft RPM, there is a lot you can get away with.

Thanks Pete, I was hoping you'd see this.

I have read that UJoints like a little angle to keep the bearings spinning a little bit. I can try and add in a little bit. Looks like my lengths will be about 25" rear and 40" front. Engine will be shifted at 4500rpm and likely won't see much time run in overdrive.

If the trans and diff flange planes are parallel, I think that's ensures your angles will zero out, so the next thing to check is your max angle per joint.

What trans and diff are you looking to connect? I did a bunch of junkyard 2 piece driveshaft scavenging for my Locost. Will your trans end have a splined connection or bolted?

I have a nissan trans with a 1310 yoke. Rear is a Mercedes diff with an adapter to 1310. So far I've only found a pre-1997 ranger long wheelbase to what I need. Although I could go for something like a 1350 driveshaft with a conversion joint.

I found Summit super helpful in my junkyard diving by looking up the fitment for a 1310 u joint. That narrowed the list down to a manageable volume to cross reference lengths off rock auto. Good luck!

Another thing, that I had forgotten, and will write a treatise with bad MS Paint images if need be, is that if you have three U joints, you need to treat them as two separate driveshafts, since the driveshafts will be 90 degrees out of phase with each other.

So you will measure the angle of the trans to the front driveshaft, which needs to be the same as the angle from the front driveshaft to the rear (sign does not matter).  Then the angle of the front driveshaft to the rear driveshaft needs to be the same as the angle from the rear driveshaft to the differential (again, sign does not matter).

So if you had 2deg in the front and 2deg in the back and 0deg in the middle, it won't be right.    You'd need to raise or lower the center bearing, and then adjust the diff angle.  

As I implied before, it can be "fun".

If you had to make a choice, it is best for the longer driveshaft to have the lowest angles. 

Having angle in the joint causes the driveshaft to speed up and slow down twice per revolution, more angle causes more speed variance.  This is why you have to clock the U joints, and why you want equal angles at both ends, so the speed change cancels out.

Some people hate them, but I really like double Cardan joints, they eliminate all this fuss

When I search 2 piece driveshaft alignment diagram, I find variations of this.

That's one way to do it, but not always possible for various reasons.  Sometimes you have to have the U joints change signs to keep them below 3 degrees or so.  Over that is pretty bad because of how much it forces the shaft to speed up and slow down, causes weird shakes and also causes horrible gear clatter when at neutral throttle because the mass of the driveshaft is higher than the mass of the pinion gear and the transmission shaft, so the driveshaft forces those to oscillate in speed instead. 

All that matters is that the angles of a given U joint pair (front and middle, middle and rear) are close to the same, whether up or down.

I had a Studebaker that required the pinion to point down, because doing it "that way" would have resulted in angles over 10 degrees.  Very bad.  Meanwhile, I have found that when you spin the driveshaft at 5000rpm on the highway, gotta keep the U joint angles below one degree, or it'll buzz when coasting.  That is why I was really impressed that Patrick's Skyline has 5.14 gears, and no drivetrain shakes.  Took me a very very long time to make 4.78s run smoothly.  Then I popped in a 9" with its much lower pinion height and had to play around with shimming the transmission down to get the U joint angles happier, and it's still not quite there...

Pardon my quick paint sketch. My middle sketch is what I want?

I'm also not against using a double cardon but not sure it helps here.