Thread: Driveshaft Question

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Originally Posted by RF Overlord

Originally Posted by

Goto Jundi to know more.

It's a legit concern, under the right conditions.

Since you mentioned a formula, I take it you read the article on Driveline Critical Speed posted by Dennis Reinhart, and written by Jerry Wroblewski. Jerry was a transmission engineer for Ford who was principally responsible for the 4R70W, and knows more about them than anyone alive. If he says it's a potential issue, then it is.

Having said all that, if you aren't into triple-digit speeds for prolonged periods, then you should have no worries with the stock driveshaft, but just pay attention to the tailshaft where the yoke plugs in. If you start to see fluid leaking, you know the bushing is being damaged and you should watch your speed until you get it addressed. You won't necessarily feel any major vibration because the issue is one of harmonic flex, not balance.

The short version is. Critical speed is the RPM at which a driveshaft fails regardless of horsepower. They fold up in the middle if they exceed/reach critical speed. The longer the driveshaft, the lower the critical speed, the greater the tube diameter the higher the critical speed. This is why as a single piece driveshaft gets longer the tube diameter increases. Believe it or not, tire size and gear ratio comes into play when calculating safe operating speeds for a driveshaft. For example, a truck with 3.73s and 35" tires, the driveshaft RPM will be less than a truck with 33" tires at the same MPH. Tubing wall thickness is for the most part irrelevant.

Having said all this, some guys have built 4" steel driveshafts for CCSB and ECLB chevy trucks and been OK on the street. But if you go by the calculator, a chevy CCLB with 33s and stock gears will be at the limit of the safe operating speed @ 85 mph. As a driveshaft builder i stick real close to that safe operating speed as it figures in a safety factor for moderate wear in the end yoke, u-joints that will some day be loose ETC.

I can build a 5" steel shaft for the CCSB guys, and i have done a a few upgrades for chevy pullers as well as a few one off custom pieces for some 3.0 guys. Also, ide like to point out that the Stock aluminum driveshafts in the GM CCSb and ECLB can be had complete for $400 new from Dana. In fact i stock the complete shaft because for what dana charges for the tube its not really worth it for me to retube them.

Sorry for the novel, if anyone has any more questions on the subject hit me up, im pretty familiar.

Start here by reading some of this info with dana's critical speed calculator. Use the calculator also to see how many RPMS your shaft is good fo. http://www2.dana.com/expert/wc.dll?cvsp~Decrits~ The short version is. Critical speed is the RPM at which a driveshaft fails regardless of horsepower. They fold up in the middle if they exceed/reach critical speed. The longer the driveshaft, the lower the critical speed, the greater the tube diameter the higher the critical speed. This is why as a single piece driveshaft gets longer the tube diameter increases. Believe it or not, tire size and gear ratio comes into play when calculating safe operating speeds for a driveshaft. For example, a truck with 3.73s and 35" tires, the driveshaft RPM will be less than a truck with 33" tires at the same MPH. Tubing wall thickness is for the most part irrelevant.Having said all this, some guys have built 4" steel driveshafts for CCSB and ECLB chevy trucks and been OK on the street. But if you go by the calculator, a chevy CCLB with 33s and stock gears will be at the limit of the safe operating speed @ 85 mph. As a driveshaft builder i stick real close to that safe operating speed as it figures in a safety factor for moderate wear in the end yoke, u-joints that will some day be loose ETC.I can build a 5" steel shaft for the CCSB guys, and i have done a a few upgrades for chevy pullers as well as a few one off custom pieces for some 3.0 guys. Also, ide like to point out that the Stock aluminum driveshafts in the GM CCSb and ECLB can be had complete for $400 new from Dana. In fact i stock the complete shaft because for what dana charges for the tube its not really worth it for me to retube them.Sorry for the novel, if anyone has any more questions on the subject hit me up, im pretty familiar.

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