Driveshaft questions on T-5 swap

While inquiring about driveshafts at local shops I was told by two out of three that a drive shaft that is 60 inches long (approx six inches shorter than what is required on the t-5 swap) and 3" in diameter will reach its critical speed at 4000 RPM. Critical speed is defined as the rpm at which the driveshaft can fail if speed exceeded. I also checked on the internet under the heading "driveshaft critical speed" and saw the charts the driveshaft shops were using. My Volvo would be going less than 100 mph at this driveshaft speed. I would appreciate any input on this as I am ready for a driveshaft now.
I am leaning toward having the short shaft on my shortened and beefing up the carrier bearing.

use a 2 piece

RPM update

My car with 3.73 rear end would reach critical speed on the 3" dia, 6' long driveshaft at 77mph(4000 rpm) in fourth gear.

I don't know where you are getting this 66" driveshaft figure from, but on my car, the driveshaft is about 41" tube length. Add a few inches for center-center ujoints. It's a V8 conversion with the T5 trans, and from the pictures of Doug's car, the tranny looks like it's in about the same place. So figure around 40 something inches.

My D/S is 2" diameter, and I have a 3.73 rear gear. I should be safe until somewhere around 120 mph. I have been up around 115 at least, and I haven't gotten any vibration yet. If I were starting over, I think I would do 2.5", because if you start going much larger, then you can start getting clearance probs.

And we are talking steel here. The future upgrade is to carbon fiber, which is surprisingly not too bad in price, and it has lots of bene's, including a critical rpm high enough to not be an issue.

And I am very glad not to be dealing with a 2 piece shaft. Good-bye, center bearing, it's just too bad I have 2 other Volvos with them.

Greg

Driveshaft measurement

The 66 2/3" inch measurement comes from a Noah 244ti post in the T-5 article of Dougs'. He said he took his measurement from Doug.

he's talking about a 1 piece driveshaft.
Sean

CORRECTION! CORRECTION! CORRECTION!

The driveshaft on my 240 is not 66 inches long...
It is in fact 46 3/8" from center to center of U joint in a 3" diameter.
I dont know how 66" poped into my head when I was posting about my swap (I told you dont trust me :0 )... But yes, just climed out from under the car with a measuring tape.

I've had no problems with my driveshaft. I've been up to about 6k in 1-3 and just cruised at about 110 last night in 5th. No wierd vibrations and I'm sure if you checked the critical speed of a ** 46 3/8" X 3" **DS... Should be much higher than what you came up with for the 66 incher : )

Driveshaft measurement

Noah, thanks for the update/correction!! The calculator says if we keep the our cars under 9000 rpms we won't have any drive shaft problems. Sorry for any worry I caused!

Noah, did you have any clearance probs with the 3"? Seatbelt bolts?

Greg

Dont have any problems with the bolts from the seatbelts... If your car is lowered, you will want an adjustable panhard rod to center the rear end. The DS on mine is not centered and I get a tiny scrape under hard acceleration with some weight on the rear of the car (again wont be a problem once I get a panhard rod). Good luck.