Steel vs Aluminum driveshaft

[Bill Edgeworth]

I don’t think you can really make a blanket comparison between steel and aluminum drive shafts. Beneficial to some not to others, there is a lot of things to consider when swapping them out and seldom are the new and old physically comparable. One would first think that if its lighter it will take less energy to accelerate… well not necessarily if you take a small diameter steel shaft and replace it with a lighter large diameter shaft.
Here is an example
7000 rpm a 3” shaft will have a surface speed of 5498 feet per minute
7000 rpm a 4” shaft will have a surface speed of 7330 feet per minute a 33% higher speed
Using basic physics equations like Ke=1/2mv^2 if they weigh the same and most of the mass is in the tube not the end yokes the 4 inch shaft will take roughly 78% more energy to accelerate to 7000 rpm.
If you do all the math the 4” shaft would have to be about 40% lighter to take the same amount of energy to accelerate as the 3” shaft.
What this exercise shows is that most comparisons are apples to oranges you probably won’t see any gains unless you are replacing your current shaft with something lighter and the same or smaller diameter.
And this is without taking into consideration what’s going on in the rest of the car.

For something as simple as a drive shaft there is no simple answer.

[Eric Merryfield]

Originally Posted by Bill Edgeworth

I don’t think you can really make a blanket comparison between steel and aluminum drive shafts. Beneficial to some not to others, there is a lot of things to consider when swapping them out and seldom are the new and old physically comparable. One would first think that if its lighter it will take less energy to accelerate… well not necessarily if you take a small diameter steel shaft and replace it with a lighter large diameter shaft.
Here is an example
7000 rpm a 3” shaft will have a surface speed of 5498 feet per minute
7000 rpm a 4” shaft will have a surface speed of 7330 feet per minute a 33% higher speed
Using basic physics equations like Ke=1/2mv^2 if they weigh the same and most of the mass is in the tube not the end yokes the 4 inch shaft will take roughly 78% more energy to accelerate to 7000 rpm.
If you do all the math the 4” shaft would have to be about 40% lighter to take the same amount of energy to accelerate as the 3” shaft.
What this exercise shows is that most comparisons are apples to oranges you probably won’t see any gains unless you are replacing your current shaft with something lighter and the same or smaller diameter.
And this is without taking into consideration what’s going on in the rest of the car.

For something as simple as a drive shaft there is no simple answer.

Well, I just so happen to have a 4 inch steel in my stocker, which came with an aluminum.....so if I change it out of say a 3.5 aluminum, or even a 3" chromoly I will post the results.

Eric

[FED 387]

Mr Edgeworth some???? If a 4 inch diameter mild steel tube weighs about 4 times more than a 4 in diameter aluminum tube wouldn't there be some advantage in not having to accelerate that much mass either in HP required or ET resulting from the reduced weight??? Take for example that a .065 steel tube weighs about 3 pounds per foot vs a similiar sized alum tube weighing about .7 pounds per foot there just has to be something here and not just reduced vehicle weight dont ya think????
I can understand harmonics to an extent and the need for OEM vehicles to meet CAFE mileage requirements so the vehicles weigh less but the OEM guys used Steel Deriveshafts for millions of vehicles over the years and really never had any vibration problems so I cant believe that the OEMs replaced the Dshafts for that reason had more to do I bet with vehicle weight that any thing else.
Perhaps you can mathmatically calculate the difference in performance between the 2 or is it too small to actually make any difference---Forget about cost of a shft it is inconsequential just raw performance numbers if ya can please---Thanx Comp 387

[KingReptile]

Absolute BS that its not legal in stock with all the changes they allow as stated its safer and easier on rear end

[Bill Edgeworth]

Originally Posted by FED 387

Mr Edgeworth some???? If a 4 inch diameter mild steel tube weighs about 4 times more than a 4 in diameter aluminum tube wouldn't there be some advantage in not having to accelerate that much mass either in HP required or ET resulting from the reduced weight??? Take for example that a .065 steel tube weighs about 3 pounds per foot vs a similiar sized alum tube weighing about .7 pounds per foot there just has to be something here and not just reduced vehicle weight dont ya think????
I can understand harmonics to an extent and the need for OEM vehicles to meet CAFE mileage requirements so the vehicles weigh less but the OEM guys used Steel Deriveshafts for millions of vehicles over the years and really never had any vibration problems so I cant believe that the OEMs replaced the Dshafts for that reason had more to do I bet with vehicle weight that any thing else.
Perhaps you can mathmatically calculate the difference in performance between the 2 or is it too small to actually make any difference---Forget about cost of a shft it is inconsequential just raw performance numbers if ya can please---Thanx Comp 387

To answer your question yes. What you describe would be more of an apples to apples comparison. Same size shafts, same final surface speed half the weight half the energy required to accelerate to speed.