Driveshaft size

[69clone]

just wondering if anyone had any input on this.
I spoke to two different guys at strange engineering about a driveshaft. yesterday the guy told me a 3" was what I needed as I was keeping it under 7k rpm well today I called to place the order and the guy on the phone said I need a 3.5 to prevent any vibration in the higher rpm range as I'm close to 80% critical. I wound up ordering the 3.5" and a chromoly slip yoke. My driveshaft is 45" center to center or 54" from seal on tranny to pinion yoke. Is this 3.5" right or over kill for my car? 650hp 440 stroker street/strip. Was going to see if I can get ahold of the first guy I talked to tomorrow.

 

[Zeppelin264]

This might be of some help.

http://www.markwilliams.com/driveshafttech.aspx

And by RPM, I hope you understand this is driveline RPM, not engine RPM. You can calculate it out based on tire size, gear ratio. No offense ment

 

[Zeppelin264]

Here's the formula.

Math is simple:
convert the MPH"s" to inches per minute or IPM"s"
X Miles/hour * 1 mile/5280 ft * 1 foot/12 inches * 1 hour /60 min
= Y inches/min (IPM"s")

Find the height in inches of the tires on the rear = H

The circumference of said tire is Pi ( use 22/7) times the height (diameter)
Cir = H *Pi = H*22/7

Divide the IPM"s" by this circumference
this yields the RPM"s" of the wheel

Mutiply this by the axle ratio (4:11) - as the drive shaft spins
in this case 4:11 revolutions for each revolution of the wheel.

this will yield the drive shaft RPM

 

[69clone]

Thank you for the formula I was unaware of that. I'll have to do the math. I'm pretty certain he asked for engine rpm I believe they just converted it.

 

[mmissile]

If it's aluminum.....it'll be big.

 

[Zeppelin264]

Thank you for the formula I was unaware of that. I'll have to do the math. I'm pretty certain he asked for engine rpm I believe they just converted it.

yea, I guess you have to make the 1:1(trans/engine rpm) ratio part of the equation.

Personally I'd go 3" and never worry about it. I have a 2.5" on 3:91 gears, and 28" tall tire and it's been good up to 110mph so far.