What DriveShaft to get?
03-18-2007, 05:14 AM
#11
2nd Gear Member
Join Date: Nov 2005
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Posts: 176
RE: What DriveShaft to get?
CrazyAl, since you're a mechanical engineer, can you answer this question?
Being that the drivehsafts are made out of tube (all of the weight being on the outer edges), and that the steel ones like Denny's are 3", and most of the aluminum ones are 4", how much lighter than the 23 lb 3" Denny's driveshaft would a 4" aluminum driveshaft have to be in order to show in increase in HP, as it relates to rotational weight?
In other words, how light would a 4" DS need to be to have less rotational mass than a 23lb 3" DS, assuming all of the weight is on the outside?
Being that the drivehsafts are made out of tube (all of the weight being on the outer edges), and that the steel ones like Denny's are 3", and most of the aluminum ones are 4", how much lighter than the 23 lb 3" Denny's driveshaft would a 4" aluminum driveshaft have to be in order to show in increase in HP, as it relates to rotational weight?
In other words, how light would a 4" DS need to be to have less rotational mass than a 23lb 3" DS, assuming all of the weight is on the outside?
03-18-2007, 09:32 AM
#13
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Join Date: Feb 2006
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Posts: 374
RE: What DriveShaft to get?
ORIGINAL: Racefiend
CrazyAl, since you're a mechanical engineer, can you answer this question?
Being that the drivehsafts are made out of tube (all of the weight being on the outer edges), and that the steel ones like Denny's are 3", and most of the aluminum ones are 4", how much lighter than the 23 lb 3" Denny's driveshaft would a 4" aluminum driveshaft have to be in order to show in increase in HP, as it relates to rotational weight?
In other words, how light would a 4" DS need to be to have less rotational mass than a 23lb 3" DS, assuming all of the weight is on the outside?
CrazyAl, since you're a mechanical engineer, can you answer this question?
Being that the drivehsafts are made out of tube (all of the weight being on the outer edges), and that the steel ones like Denny's are 3", and most of the aluminum ones are 4", how much lighter than the 23 lb 3" Denny's driveshaft would a 4" aluminum driveshaft have to be in order to show in increase in HP, as it relates to rotational weight?
In other words, how light would a 4" DS need to be to have less rotational mass than a 23lb 3" DS, assuming all of the weight is on the outside?
First you would have to know the exact thickness of the driveshaft walls, but if you assumed all the weight of thedriveshaft was at the outer edge the rotational moment of inertial is found by this equation: I = m r^2 where m is the mass and r is the radius from the center.
So we ideally would want both moments of inertia to be the same so we would have:
I(steel) = I(aluminum)
or m(steel) * radius^2 = m(aluminum) * radius^2
we know the radus of the two DS so
m(steel) * (3)^2 = m(aluminum)*(4)^2
m(steel)* 9 = m (aluminum) * 16
m(aluminum) = (9 * m(steel))/ 16
we know the mass of the steel drive shaft is 23 lbs
m (aluminum) = ( 9 * 23 )/16 = 12.93 lbs
So about 13lbs is what the 4" aluminum DS would have to weigh to have the same rotational mass as the 3" steel driveshaft weighing 23lbs
If that answer sounds way to light,I would have to know the thickness of the driveshaft walls for both driveshafts to get a more accurate answer. Or maybe I don't know what I am talking about and CrazyAl will correct me.
03-18-2007, 01:12 PM
#14
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Join Date: Mar 2006
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RE: What DriveShaft to get?
Rock, your logic is good, but you made one mistake that I can see. You used the diameters in the equations, not the radius....but it acutally doesn't affect the end calculation.
So, the 4" aluminum driveshaft has a radius of 2", and the 3" Steel driveshaft has a radius of 1.5"
m(steel) * (1.5)^2 = m(aluminum)*(2)^2
23*2.25 = m (aluminum) * 4
51.75 = m (aluminum) * 4
m (alum) = 51.75/4 = 12.4 lbs
Of course, this calculation ignores the "ends" of the driveshafts and assumes they are nothing but a long, hollow, tube ofnegligeably thin wall. It does not account for the U-joints or the flanges that hold them, or the telescopic joints in them, and so on, so it is questionable how accurate this would be...but it DOES show that even though the steel driveshaft from Denny's (for example) is statically heavier than the 4" driveshafts (Spyder / Powerhouse), it at the very least has similar (or possibly better) inertial characteristics.
So, the 4" aluminum driveshaft has a radius of 2", and the 3" Steel driveshaft has a radius of 1.5"
m(steel) * (1.5)^2 = m(aluminum)*(2)^2
23*2.25 = m (aluminum) * 4
51.75 = m (aluminum) * 4
m (alum) = 51.75/4 = 12.4 lbs
Of course, this calculation ignores the "ends" of the driveshafts and assumes they are nothing but a long, hollow, tube ofnegligeably thin wall. It does not account for the U-joints or the flanges that hold them, or the telescopic joints in them, and so on, so it is questionable how accurate this would be...but it DOES show that even though the steel driveshaft from Denny's (for example) is statically heavier than the 4" driveshafts (Spyder / Powerhouse), it at the very least has similar (or possibly better) inertial characteristics.
03-18-2007, 10:44 PM
#19
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Join Date: Jul 2005
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Posts: 718
RE: What DriveShaft to get?
Al and Rock, you guys should look at Jeepforums.com !! I have a 84 CJ-7 rockcrawler and calculations on torque, tire size, axle diameter, HP, and traction coefficent are daily happenings. You guys would be in heaven. And you would be blown away by how precise some of the "hardcore" guys are dialing in their rigs (within a couple of pounds before breakage). And I'm sorry to everyone else, yes this is off topic but the engineers in this thread can relate.
