Drive shaft which is better?

[regulator522]

I have an '82 stepside shortbox, and everybody that has checked it out is amazed that it has a two piece drive shaft (actually so was i when first saw). I've heard that a solid (1 piece) is better for several reasons, some sound crazy and some logical like a two piece looses torque/power (assuming meaning more connections more loss??). I've also read some posts about two piece better? To help with my confusion (LOL) which is better over all? (My truck has a 383 stroker, with the SM465 manual trans) If I did go to a 1piece, was there a drive shaft that would be direct fit? thanks guys

[Captkaos]

This has been covered here, HD trucks typically used 2 piece from what I have seen.   Does your truck have 56" springs?

[regulator522]

I've heard the same, but couldnt imagine mine is HD? originally had the inline 6cyl and with the wood bed wouldnt think it to be HD. Not sure on springs, will check. So is there a reason that HD's ran a two piece? (sorry if covered in previous thread, didnt see)

[Irish_Alley]

ok heres my guess. first lets start with the a little math, your drive shaft rotating mass depends on the weight of the shaft. the weight depends on the diameter. the diameter depends on length (i know yours is a standard wheel base but just go with it) and the big kicker if you increase your diameter your rpms stay the same but the speed increases the farther you get from center.(much like tires) at a certain speed the shaft will just come flying apart.

soooooooo One reason for multiple shafts is, the longer the shaft the weaker it gets. the weaker it gets the more material in needs. the increase in material means the bigger the diameter(again the increase in diameter the increase in speed and catastrophic failure). the bigger diameter the more weight and thus the more rotating mass it has. (my physics term may be off idk) so the more mass something has, the more power it takes to spin it. so if you chop that shaft into two or three pieces you can keep the diameter smaller due to the length of each piece. is a two piece lighter than a one piece idk never put them side by side.

Reason number two and three. but lets replace the length of the shaft above with strength. if you want a stronger shaft you have to increase the diameter and material and you get the picture. so you can increase the strength of a shaft by cutting it into multiple pieces and since your rotating speed wont be increasing due to increase in diameter the higher the chances of a slight imbalance causing a wobble. which can lead to catastrophic failure

[rich weyand]

Close, Irish.  First, a driveshaft's diameter does not need to be greater in diameter for a longer piece in order to transfer the horsepower: the horsepower you can transfer through a shaft is totally based on the diameter and wall thickness.  The length doesn't enter into it.

Where the length does enter into it is bending moment.  A beam of any kind has a certain rigidity to bending.  The deflection for a given force goes as the square of the length, so a beam twice as long will deflect four times as much for a given side load.  This is why it is difficult to build clear spans in buildings: the beam sizes go up drastically as the span increases in order to limit deflection.

When a driveshaft deflects (bends) under load you get vibration.  The driveshaft can only be so balanced, and so true, particularly when built in an assembly line environment and maintained in local shops.  U-joints have tolerances, too.  Finally, engine and road vibration transfer to the ends of the driveshaft at the transmission and rear axle.

The end result is that driveshafts can vibrate at speed.  This speed is set up to be higher than the expected speed of the car.  When building high-speed cars, stock driveshafts have to be replaced with higher-tolerance and more rigid units.  Longer driveshafts vibrate at a lower speed than short ones, for the same reason that longer instruments make lower musical notes.  Their frequency of vibration is lower.

Excessive vibration in a driveshaft results in whipping, where the center of the driveshaft is moving excessively from side to side or up and down.  This creates huge side loads on the U-joints and will greatly accelerate driveline failure.

A two-piece driveshaft is used to increase the stiffness of the driveshafts by reducing the bending moment of each piece, thus increasing the speed at which excessive vibration sets in.  This is most important in longer vehicles, where the necessary length of a one-piece driveshaft will bring the speed at which excessive vibration sets in down into the expected operational range of the vehicle.

A two-piece driveshaft also has more weight, more complication, more components to fail, and greater horsepower losses in the drivetrain, so a one-piece driveshaft is preferred if the length of the driveshaft is short enough that the speed at which excessive vibration sets in is above the vehicle's operating range.

With specific regard to our trucks, a short-box K has the shortest driveshaft, and the least need for a two-piece driveshaft.  A long-box C has the longest driveshaft, and the greatest need for a two-piece driveshaft.

[Irish_Alley]

Rich as you know i respect what you say, especially if you compare it to my redneck explanation. 

Close, Irish.  First, a driveshaft's diameter does not need to be greater in diameter for a longer piece in order to transfer the horsepower: the horsepower you can transfer through a shaft is totally based on the diameter and wall thickness.  The length doesn't enter into it.

Where the length does enter into it is bending moment.  A beam of any kind has a certain rigidity to bending.  The deflection for a given force goes as the square of the length, so a beam twice as long will deflect four times as much for a given side load.  This is why it is difficult to build clear spans in buildings: the beam sizes go up drastically as the span increases in order to limit deflection.

i think this depends on how you look at it, cause im confused lol. and i dont know if im comparing apples to oranges but lets say you are using a 3/8" torque wrench and your torquing on it it will flex lets say you apply 100ft. lbs.. now lets put the same amount of torque on a 24"x3/8s extension the extension will twist and your torque your applying will go into that twist so your 100 ft. lbs. is only 85ft lbs.. now lets upgrade that extension to 1/2" you wont get the same amount of flex with the same amount of torque.


When a driveshaft deflects (bends) under load you get vibration.  The driveshaft can only be so balanced, and so true, particularly when built in an assembly line environment and maintained in local shops.  U-joints have tolerances, too.  Finally, engine and road vibration transfer to the ends of the driveshaft at the transmission and rear axle.

The end result is that driveshafts can vibrate at speed.  This speed is set up to be higher than the expected speed of the car.  When building high-speed cars, stock driveshafts have to be replaced with higher-tolerance and more rigid units.  Longer driveshafts vibrate at a lower speed than short ones, for the same reason that longer instruments make lower musical notes.  Their frequency of vibration is lower.

Excessive vibration in a driveshaft results in whipping, where the center of the driveshaft is moving excessively from side to side or up and down.  This creates huge side loads on the U-joints and will greatly accelerate driveline failure.

A two-piece driveshaft is used to increase the stiffness of the driveshafts by reducing the bending moment of each piece, thus increasing the speed at which excessive vibration sets in.  This is most important in longer vehicles, where the necessary length of a one-piece driveshaft will bring the speed at which excessive vibration sets in down into the expected operational range of the vehicle.

A two-piece driveshaft also has more weight, more complication, more components to fail, and greater horsepower losses in the drivetrain, so a one-piece driveshaft is preferred if the length of the driveshaft is short enough that the speed at which excessive vibration sets in is above the vehicle's operating range.


why do some drag car use two piece and some use one? if you are using two you can keep the diameter small and the terminal velocity is raised higher than the one piece that has a larger diameter but then your adding weight and lose hp? but i guess that all equates to shaft length, weight, diameter, and rpm


With specific regard to our trucks, a short-box K has the shortest driveshaft, and the least need for a two-piece driveshaft.  A long-box C has the longest driveshaft, and the greatest need for a two-piece driveshaft.


so why would a hd truck have a two piece?

[rich weyand]

First, the bending moment.  Let's say you have a 2x4 above a 6'wide window as a header, and it deflects 1/4" with load. Go to a 3' window, and the 2x4 will deflect only 1/16" in the center of the window with the same load, because you have cut the span in half, therefore you get one-quarter the deflection.  The same thing applies to driveshafts.  For a given driveshaft, and given side loads caused by vibration, the deflection will be one-quarter as much at the centers of two pieces as compared to the deflection at the center of a single piece driveshaft.

Some drag cars use two and some use one because of trade-offs.  You lose more horsepower in a two-pieces, because the extra U-joint introduces greater losses as it pivots back and forth as the driveshaft turns.  You gain in resistance to vibration.  There are other ways to address vibration, such as greater precision in machining the shaft and u-joints and in installing everything.  But the biggest factor I think is length.  Some of it may just be that people go with what they know and feel comfortable with.

As for why a two-piece on an HD: Don't forget that driveshaft vibration is being driven by the vibrations of the engine and the axle.  Stiffer springs and a higher torque/lower rpm engine on an HD means the vibrations at each end of the driveshaft are greater, and those are what drive the vibrations of the driveshaft itself.  Also, an HD is more likely to be subjected to off-road or construction site conditions in which the driveshaft can be slightly dented or bent; these are less likely to cause excessive deflection in a two-piece where the pieces are only half as long because of the bending moment being reduced by 3/4.

Your torque discussion of the wrenches has more to do with a low-speed event, such as ductile failure of the driveshaft tube on launch.  I used to repair driveshafts for trucks where the drive shaft was twisted in half, like twisting a tootsie roll in half, what we used to call a taffy failure.  One-piece or two-piece, a low speed launch failure like that is dependent only on the cross-section of the tube and not its length: the tube is too small for the torque being transmitted.  Length only enters in for high-speed failures of the driveline due to excessive deflection of the shaft due to vibration.

[Irish_Alley]

i see now, thanks

First, the bending moment.  Let's say you have a 2x4 above a 6'wide window as a header, and it deflects 1/4" with load. Go to a 3' window, and the 2x4 will deflect only 1/16" in the center of the window with the same load, because you have cut the span in half, therefore you get one-quarter the deflection.  The same thing applies to driveshafts.  For a given driveshaft, and given side loads caused by vibration, the deflection will be one-quarter as much at the centers of two pieces as compared to the deflection at the center of a single piece driveshaft.

Makes sense, i can understand that

Some drag cars use two and some use one because of trade-offs.  You lose more horsepower in a two-pieces, because the extra U-joint introduces greater losses as it pivots back and forth as the driveshaft turns.  You gain in resistance to vibration.  There are other ways to address vibration, such as greater precision in machining the shaft and u-joints and in installing everything.  But the biggest factor I think is length.  Some of it may just be that people go with what they know and feel comfortable with.

and this


As for why a two-piece on an HD: Don't forget that driveshaft vibration is being driven by the vibrations of the engine and the axle.  Stiffer springs and a higher torque/lower rpm engine on an HD means the vibrations at each end of the driveshaft are greater, and those are what drive the vibrations of the driveshaft itself.  Also, an HD is more likely to be subjected to off-road or construction site conditions in which the driveshaft can be slightly dented or bent; these are less likely to cause excessive deflection in a two-piece where the pieces are only half as long because of the bending moment being reduced by 3/4.

and this

Your torque discussion of the wrenches has more to do with a low-speed event, such as ductile failure of the driveshaft tube on launch.  I used to repair driveshafts for trucks where the drive shaft was twisted in half, like twisting a tootsie roll in half, what we used to call a taffy failure.  One-piece or two-piece, a low speed launch failure like that is dependent only on the cross-section of the tube and not its length: the tube is too small for the torque being transmitted.  Length only enters in for high-speed failures of the driveline due to excessive deflection of the shaft due to vibration.

the tootsie roll failer i can understand its dependent on the cross section, but isnt the cross section dependent on the length?

[rich weyand]

Cross-section is only dependent on inside diameter and outside diameter.  Some manufacturers make longer driveshafts larger in diameter as well, to try to reduce deflection, rather than go to a two-piece solution.  That's a design choice, not a dependency.

[nlauffer]

I have read and it makes sense.  Quick question.  A friend of mine had a 72 Chevy 2wd LWB.  He built a monster 350, manual reverse pattern valve body TH350.  Needless to say there were a lot of hard launches and high RPMs.

Why would he have chosen a solid bar, one piece driveshaft, which was maybe 2" thick.  I don't remember exactly, but it looked like a little pencil underneath there.  Chose it because of hard launches completely.  Do you think it has any other benefits.  I would assume it weighed more than stock.

[rich weyand]

A solid driveshaft is more resistant to taffy failure than a tubular driveshaft of the same diameter when deflected, such as during axle hop.

[Engineer]

Also remember that a shaft that has its mass closer to the center of rotation the easier it is to accelerate.

[Engineer]

This has been covered here, HD trucks typically used 2 piece from what I have seen.   Does your truck have 56" springs?

His truck is a short box. In all likelihood it will have 52" rear springs.

[Engineer]

OP-

I prefer a one piece driveshaft but if your is working I would leave it be.

[Irish_Alley]

so all in all the only standards i can find from talking to driveshaft makers its all about opinion. one piece shaft makers seam to say theirs are better while two piece say theirs are better. there is nothing written in stone that says one way or the other except about the length of a shaft.

Cross-section is only dependent on inside diameter and outside diameter.  Some manufacturers make longer driveshafts larger in diameter as well to try to reduce deflection, rather than go to a two-piece solution.  That's a design choice, not a dependency.

so maybe dependent wasnt the right word, but it wouldnt make sense to make a driveshaft .120" thick wall (thats meant for lets say 30" length) and use that thickness for a 60" shaft you would have to thicken the wall to compensate for the length difference. not necessarily outside dia. but you could decrease the inside dia. and still strengthen the shaft.