nizzlemanu

So I've been reading the multiple posts about aluminum vs. steel driveshafts and haven't seen very many posts about the hard facts between the two. Not to mention the ones I have seen have been wrong. So here are a few comparisons between the two

-The steel driveshaft is 2.5" in diameter and the aluminum is 3". When calculating the torque to get the driveshaft spinning you have to calculate the mass moment of inertia (Not the polar moment of inertia, two different things). The mass moment of inertia is Im = m*R^2. Because in this case the thicknesses are negligible and they are the same length m can be taken as the density of the material multiplied by the circumference.
Applying this equation
Im(aluminum) = 0.911 Kg.m^2
Im(steel) = 1.50 Kg.m^2

So even with the larger diameter the aluminum shaft has a lower mass moment of inertia. +1 for aluminum

Next we can examine the rotational stiffness and bending stiffness. both of these equations use a similar formula. Rotational is G*Pi/2 * (ro^4 - ri^4) and bending is E*Pi/4 * (ro^4 - ri^4). G is the modulus of rigidity and E is the Young's modulus ri and ro are the inner and outer radii. Unfortunately I can't find the thickness information for any of the driveshafts or I could calculate these values as well. I played with some numbers and it only takes the aluminum to be about 0.075" thicker to make up for it being a weaker material. That is if the aluminum driveshaft is 0.075" thicker it will be just as strong (no bending) as the steel.

Finally the load on the aluminum will be less than the steel (assuming that thicker wall) because of its larger diameter. Using the equation t = T*c/J where T is the torque, c is the outer radius, t is the shear stress, and J = Pi/2 * (ro^4 - ri^4).

Adding the fact that aluminum is lighter than steel it seems that aluminum is the better choice in this instance. Now I doubt this advantage would show up in huge power numbers but by using an aluminum driveshaft you are lowering the load on the motor and that's never a bad thing when it comes to fatigue strength of your drivetrain.

The three advantages steel holds over aluminum is that
1) It takes less material to be stronger so if you are working with a small space that can't be enlarged (ie a pushrod) steel would be the better choice.
2) Steel is more ductile and workable than aluminum
Neither of these are a problem for driveshafts.
3) The fatigue strength of aluminum is less than steel so it is important not to have gouges or cracks in the aluminum or it will fail much faster than steel.

Hope this helps everyone

ASE doc

Wow! Ive never seen it analyzed to that extent. All that I know is that, at least in stock form, the steel driveshaft suffered harmonic vibration above 80mph. The aluminum driveshaft does not. I would assume that some of this has to do with the difference in density between the two materials. Most likely, the aluminum piece is also more precisely balanced. I also know people who have had the steel driveshafts balanced and have been satisfied with the results.

Where I think a large diameter steel driveshaft becomes a necessity is at very high power and/or torque numbers. I wouldn't want to trust my GM performance parts driveshaft, or the tiny stock steel driveshaft for that matter, behind an 800HP alcohol burning big block.

red rock

iTrader: (2)

This is something i found a while back. http://www.ws6.com/mod-1.htm

ASE doc

That's an excellent article. I also wasn't sure I felt any real performance gain but I definitely noticed the smoothness at high speed. Maybe that's not a good thing for me.

nizzlemanu

The vibrational differences are attributed to the mass per unit length and the stiffness mentioned above (E*I). With the higher mass per unit length of the steel it lowered the natural frequency into a range that resonates in the car, so as you mentioned that is another advantage of the aluminum.

3GZJerry

iTrader: (2)

Ok so 1 + 2= 3
Thanks, got it.

red rock

iTrader: (2)

Nizzlemanu, I don't care what anyone thinks, but anyone that has the education and takes the time to do the figuring that you did, has my complete attention. (I don't even know if what you figured is right or wrong, but it sounds good in the end) Thank you.

DeltaElite121

I actually just ordered a 3.5" PST aluminum driveshaft last night, heh. I think it was YellowBullet that discussed the rotating mass difference being fairly important because some people were arguing that "weight is just weight". There was a huge thread on it with equally as much math involved. Certainly interesting.

Steel driveshafts are great, but I really enjoy how smooth aluminum driveshafts are.

87350IROC

iTrader: (2)

I am thoroughly happy there is another math man on here.

I do appreciate the effort. However as you know cars are a system of systems. Overall the mmi of the driveshaft is very small when compared with the rest of the drive train. Consider a 40lb crank with weights out at the ends. A 20-30lb flywheel / torque converter with a huge radius. 30lbs of small radius components in the trans. A big heavy ring gear. Big heavy brake rotors and very big heavy wheels and tires. That is a lot of rotating weight. IMO the effect of the driveshaft is negligible. Both designs, steel and aluminum, are quite efficient compared to the rest of the system.

IMO the biggest advantage of the aluminum shaft is its corrosion resistance.

Base91

iTrader: (1)

I think the best corrosion resistance is living somewhere warm and dry!

nizzlemanu

@Red Rock
Thanks for the vote of confidence

@87350IROC
Even the graph showed that there isn't much of a performance improvement, but an improvement none the less. And amen to corrosion resistance, I live in the midwest and have busted many of nuts trying to break rusted bolts loose. I actually got excited when I saw my wife's g6 had zinc plated caliper bolts. <--sad

DeltaElite121

performance for direct HP gain is probably negligable at best.. but there IS a gain to be had. If there wasn't - people wouldn't bother lightening cranks, rods, driveshafts, rear center sections, etc. All of that is rotational mass, and reducing the weight will allow benefits in transition of energy. Lighter isn't ALWAYS better by any means, but in this case - it does help.

Like that big discussion I mentioned on YB, rotational weight and "dead" weight are two different things.

I'm sure there's math for this idea, but I'm not as well versed in that area as that's not where my schooling went.. haha.


http://www.yellowbullet.com/forum/sh...d.php?t=456323

There's the thread I was talking about.

red rock

iTrader: (2)

This is a very informative thread. Thanks, cause i just bought a Lt1 shaft last week. Now if it would quit snowing, I would like to try it out.

nizzlemanu

Rotational energy = 1/2 m*rs^2*w^2
Translational energy = 1/2 m*v^2
m=mass; w=angular velocity; v=velocity of car; rs = radius of driveshaft;

Let's say that the rear diff is 3.73 then the angular velocity of the ds is w=3.73*v/(8.67*rs)
The 8.67 is the ratio of the wheel radius over the driveshaft radius.
It turns out that the overall energy needed to rotate a driveshaft is about 1/10th that needed to move it forward. Then compare it with a 3000 lb car and it's a tiny fraction of the energy.

The most energy loss in a drivetrain is due to low bending and rotational stiffness. For instance above I showed how the bending stiffness of the aluminum shaft can easily be greater than the steel resulting in slightly better performance.

I think the best advantage to using aluminum parts is that engines are high tolerance machines and a lighter part will make those tolerances less pronounced if they are used in the correct situations.

Keoman

iTrader: (3)

Dude I just had a physics final today, I'll take your word for it.
Aluminum over Steel

IROCZ1989

iTrader: (1)

Good post. But why do some use heavier flexplate or flywheels? It is said they may use hp to spin but make the car launch better in the 60ft. I assume it over comming intial acceleration of the extra weight. question being a heavier object moving at x speed takes x amount of hp less to keep it spinning because of rotational weight is higher than simlaer object of a lesser weight.

nizzlemanu

The purpose of flywheels is to store the energy of the motor so that when it is applied to the trans it doesn't kill the motor. The heavier the flywheel the more energy it can transfer. As you can see this would be great for times when you need to launch the car such as a drag race. Aluminum flywheels are better suited for track racing where it is more important to save weight and you're not starting from a stand still so the inertia of the car acts as the energy storage. Or if it were a DD I would definitely go with the steel flywheel.