aluminum drive shaft - anyone?

[Ultramaroon]

Originally Posted by industrial

As I've said over and over, you need modeling software specific to this application

Bullshit.

[Poodles]

Originally Posted by Dimman

My buddy had a CF driveshaft grenade on him. Wasn't pretty. Not sure but we think it was from a bad fit between the CF tube and metal yoke part covered by too much adhesive.

Ever since then I've been a bit leery of half-engineered driveshafts. Learning that the harmonic stuff is weird, aluminum's fatigue properties, and how welding it adds stress risers while removing initial heat treat strength has been enough for me to get uncomfortable looking at a few aluminum DS in the past.

Are these things you're interested in discussing, Stu (in your inside voice)?

Keep in mind that the failure mode of a CF driveshaft is by far the BEST out of all the materials used. It effectively becomes a very large broom

Originally Posted by industrial

Yes but if there wasn't a real performance benefit, it wouldn't exist on the OEM level in such ubiquity. Most OEMs would just pass the savings (from a steel driveshaft over megabucks cf) on to profits. I find it hard to believe that any OEM would go to any length to spend more money on the driveshaft for marketing. It's under the car and over the exhaust. The customer will never see it or feel the difference (because a steel one was never installed). Also, just because you can't trust the marketing department at Audioquest cables doesn't mean you can't trust all corporate sources.

This discussion is beyond futile here. There are countless books and sources that say reducing rotating mass (to include the driveshaft) is beneficial for a performance car. Almost every OEM does this. There are aftermarket driveshafts available for almost every performance car foreign and domestic, that doesn't come with one. It's not like lightweight driveshafts are something new that came along with the ft86. People in other car communities pay 2-5x more for lighter and more exotic driveshafts.

Since we are beyond the polite stage, all the math in this thread is nearly worthless for this discussion. As I've said over and over, you need modeling software specific to this application that the OEMs have to demonstrate mathematically whether this mod is "worth it". I'm not sure why these few guys are so vehemently defending their position that this mod is not "worth it" since none of them have any real world experience with this mod. Don't know why I'm still here.

The same reason they still put crossdrilled rotors on performance cars...

[Koa]

Originally Posted by Manic

I'm starting to get the feeling that your computer might have some sort of autoblock feature, setup to block certain words or phrases.

No one has disputed the fact that reducing rotating mass, including the driveshaft, isn't beneficial. The only argument being made by the heretics who believe in math and science is that the benefit of a light weight drive shaft is negligible in the practical scope of the car.

It's akin to taking a heavy shit and driving your car naked in hopes that the reduced sprung weight is going to improve your lap times. Technically you've reduced the sprung weight of your car. Does it have a practical benefit? Nope.

If the OP wants to buy a light weight drive shaft, good on him. But stop trying to convince yourself that you've purchased a miracle drive line upgrade which'll make you .05 seconds faster in the corners. It just won't.

Couldn't have put it any better. Full points old chap. @industrial it's ok.

[Dimman]

Originally Posted by Poodles

Keep in mind that the failure mode of a CF driveshaft is by far the BEST out of all the materials used. It effectively becomes a very large broom





The same reason they still put crossdrilled rotors on performance cars...

This one busted of at the connection between the carbon tube and the metal end, wrapped up his ebrake cable ending up with no power and instantly locked rear brakes. The carbon end was frayed where it cracked, but most of the tube stayed together. Didn't cause any major damage or hurt anyone, but CF aren't necessarily super safe either.

This was an aftermarket one, though.

[Dimman]

Also, 44 milliseconds at 100 mph is over 6 feet if my not NASA-level math is correct.

So can we do the fun math again for distance between cars at the end of an acceleration run? Then also braking? Then hard accelerating and hard braking many times over, say a road course?

[industrial]

Originally Posted by Poodles

The same reason they still put crossdrilled rotors on performance cars...

Yes, the same reason. Because everyone can see your nice carbon driveshaft. Also because F1 and Ferrari uses them too. Did you comprehend the paragraph I wrote or are you a sentence by sentence kind of guy?

I would regret buying a house in 2007 or hitting a kid with my car. Spending $400 on a well crafted, well thoughtout car part that I lovingly installed on my car to daydream about my .2 second improvement on HPDE day is just part of the hobby. This entire hobby is about setting money on fire. If you think otherwise, you are just deluding yourself. I laugh everytime I hear someone use the word "invest" when they talk about buying shit for their cars. Newsflash: Every aftermarket car part is a horrible use of money.

[Sypher]

Originally Posted by stugray

I simplified it as best I could:

Energy required to accelerate a 2800 lb car to 100 MPH
1270 kg @ 44.7 m/s

Total kinetic energy = ½ MV^2 = 0.5*1270 * 44.7^2 = 1268787.15 joules

Rotational kinetic energy of a spinning cylinder
E-rotational = ½ I w^2 (w is angular velocity in radians/sec I is moment of inertia )

30 lbs driveshaft = 13.6 kg 3" dia (1.5” radius = 3.81 cm = .0381 m) =
I = .0381 * .0381 * 13.6 = .01974 kg.m^2

Stock wheel = 1964.44 mm circumference

@ 100 MPH our wheels are spinning at
@ 44.7 m/s => 44.7/1.964 = 22.75 RPS @ wheels
Final drive ratio = 4.1:1

Driveshaft RPS = 22.75 * 4.1 = 93.275 RPS = 5596 RPM = 35160.7 radians/min = 586 radians/sec

Rotational energy of the spinning driveshaft = ½ I w^2 = 0.5 * .01974 * 586^2 = 3389.32 joules

Energy stored in moving car: 1268787.15 joules
Energy stored in spinning driveshaft: 3389.32 joules

Total energy (NOT COUNTING the energy stored in the REST of the rotating mass) = 1272176.47 joules

For a whopping ratio of 375/1 = .0027 or .27% of the energy in the system.

SO while the engine has stored up a total of 1272176.47 joules in the system, only 3389.32 joules are in the spinning up of the driveshaft.


BRZ gets ~ 16.2 seconds 0-100 mph
That’s 1272176.47 joules / 16.2 seconds or 78529.4 joules/second

OR ~105.36 hp Which for back of napkin calcs is in the ballpark enough (holy shit!).


If you had no energy stored in the spinning of the driveshaft at all: total system energy would be 1268787.15 joules at 100 MPH
If the car delivered the same 78529.4 joules/second it would achieve 1268787.15 joules in 16.156 seconds

So I guess you were all right!
If you reduce the 30 lbs driveshaft to 0 pounds (removing the energy stored in it entirely!) and do otherwise equal runs 0-100 MPH you WILL notice a 44 millisecond difference in a 16.2 second run! Congratulations you are superhuman!

And I was being generous by leaving things out such as the REST of the rotating mass so the actual difference of removing the shaft would be even less.

I never said that it wouldnt make a difference I said that a driver wouldnt be able to TELL the difference.


Shall I repeat the calcs for a 4 pound reduction of the drive pulley?
Which coincidentally would be MORE than the driveshaft since IT spins to ~6000 RPM

And @Shutter beat me to it:

i love the math here but i do have a question about the moment of inertia for the drive shaft. did you figure it as a hollow cylinder? when i figure it as a hallow cylinder with an internal and external radii i get a very different moment of inertia. 0.07574. of course this is with the assumption of a 3" external radius and a 2.875 internal radius and weighing 30lbs.

[Manic]

Industrial's so close. He almost has it.

[Captain Snooze]

Originally Posted by industrial

Yes but if there wasn't a real performance benefit, it wouldn't exist on the OEM level in such ubiquity. Most OEMs would just pass the savings (from a steel driveshaft over megabucks cf) on to profits. I find it hard to believe that any OEM would go to any length to spend more money on the driveshaft for marketing.

Originally Posted by Poodles

The same reason they still put crossdrilled rotors on performance cars...

This has to be one of the best examples of marketing over function.

[Sypher]

Originally Posted by Poodles

Axial strength is pretty pointless on a driveshaft. I'll say it again: DRIVESHAFT CRITICAL SPEED IS FAR WORSE.

RX8: Kinda carbon fiber (carbon reinforced plastic)
350Z: carbon fiber
370Z: same fing car...
GTR: Carbon fiber.
Z06: Doesn't count as it's a torque tube setup (tranny is in the rear)

Most of those are carbon fiber. Carbon fiber is used because it allows them to run a single piece driveshaft without having driveshaft critical speed issues. It's also fragile and has to have shields to prevent foreign objects from hitting it.

I don't see anyone breaking the stock driveshaft. It's also NOT stronger as (god I feel like I'm beating a dead horse here) driveshaft critical speeds will occur.

As none of the driveshaft companies want to give out their specs of the shaft (big surprise as it's probably unsafe), I'm guessing at the specs.


If someone wants to get me length, diameter, and wall thickness, it's easy to figure this stuff out...

good point. my guess is the aluminum drive shafts are 1/8" thick. which with a fancy calculator i found online (lol) says 1/8" thick 6061 aluminum drive shaft critical speed is just under 9000RPM. im guessing most of us with this car dont need to worry too much about hitting drive shaft critical speeds

http://www.wallaceracing.com/drivesh...critical_speed

[Dimman]

Originally Posted by Dimman

Also, 44 milliseconds at 100 mph is over 6 feet if my not NASA-level math is correct.

So can we do the fun math again for distance between cars at the end of an acceleration run? Then also braking? Then hard accelerating and hard braking many times over, say a road course?

For the love of god can someone pleeaaaase scientifically confirm that the hypothetical 0-100mph gap is over 6 feet???

I'm just absolutely, positively, metaphorically, dying to make a snarky yet scientifically correct comment about my superhuman ability to differentiate between 6 feet and zero without using a Sciencemaster Mathotron 9000.

[Jonsey]

Originally Posted by industrial

Actually, it is complex. Here is an excellent write up which demonstrates the lengths necessary to disprove or prove the point you guys are making. Please don't skim the article and fixate on a sentence and quote it here. Just read it.

http://www.stockcarracing.com/techar..._reducing_moi/

Thanks for sharing this article, very good for adding perspective on the effects that moment of inertia can have on acceleration. For me, it reaffirms my decision that a lightweight driveshaft should be one of my last mods when trying to extract performance out of the car.

The articles notes that removing 15 pounds from a flywheel (where the weight is farther from the center point and the moment of inertia is higher than a driveshaft) can be expected to drop only .08 seconds from a straightaway where acceleration from 60-130mph occurs. While I try to extract as much performance out of the car at the track, .08 seconds is well within my margin of error on any single lap and the difference from 15 pounds off the driveshaft will be much less than .08 seconds since the moment of inertia is lower. Now granted, this is ignoring the cumulative effects of constant acceleration/deceleration that occur each lap that a lower MOI would have. However, it has been mathematically proven in this thread that the MOI delta between the stock and aluminum driveshafts is rather insignificant, so I don't expect the cumulative effects of an insignificant change to become very significant.

[s2d4]

In the beginning, there was rain. The people thought the gods must've been angry so they sacrificed their kids as offerings and the rain stopped. Some people realized the offerings did SFA and told others to no avail, so the killing of their kids continues until they wipe themselves out.
The end.

[stugray]

Originally Posted by Sypher

i love the math here but i do have a question about the moment of inertia for the drive shaft. did you figure it as a hollow cylinder? when i figure it as a hallow cylinder with an internal and external radii i get a very different moment of inertia. 0.07574. of course this is with the assumption of a 3" external radius and a 2.875 internal radius and weighing 30lbs.

I used the very simple equation of: I = R^2 * mass
This simplification works for a perfect cylinder assuming a "thin" shell you dont have to "go all integral on it" but you will get better accuracy.
It also was within a few percent of an online calculator:
http://www.endmemo.com/physics/momentinertia.php
and the link posted back a couple pages.

30 lbs driveshaft = 13.6 kg 3" dia (1.5” radius = 3.81 cm = .0381 m) =
I = .0381 * .0381 * 13.6 = .01974 kg.m^2