aluminum drive shaft - anyone?

[stugray]

Originally Posted by cdrazic93

Stu idk why youre still wasting your energy.

But its only 0.27% of my total energy budget so I dont notice...

[zooki]

[ame="http://youtu.be/j4XT-l-_3y0"]http://youtu.be/j4XT-l-_3y0[/ame]

[CSG Mike]

Originally Posted by stugray

And I'll bet Mike can tell the difference between driving with all of the electrical accessories on vs off too! (not joking there, and not counting AC pump).

The AC compressor's clutch disconnects anytime you load the car past a certain threshold (changes with RPM).... so I can't tell on the FRS/BRZ

I can tell on my s2k though!

[fika84]

This was a fun read. It's nice when someone actually posts up fact and not crap. Cheers to a fellow Colorado Engineer @stugray

[CSG Mike]

Originally Posted by cdrazic93

This thread is hilarious.

I dont care if anyone thinks this mod is noticable or not. My physics professor and textbook prove that the math behind the process of lighter weight materials used as a substitute for a driveshaft are most definitely below the threshold of human sensation.

I would be more invested in this argument, as i love a good debate (without name calling and harsh words and blatant, '@$!# you' comments lol). But i have studying to do.

I will keep track of this thread though. It provides me with some comical relief

I'd beg to differ. In a blind test, I would be willing to bet I can identify the difference with statistical significance, and in the world of "professional driving", my talent/skill is considered mediocre at best.

I know guys that can tell you if the weight of the car changed by 5 lbs in one corner, or can tell you how new (or worn) a brake rotor is by feeling the car out as they drive it....

[CSG Mike]

@stugray

I wonder how much drivetrain loss is mitigated by eliminating the extra joint in the stock driveshaft... I'll need to try mounting a gopro somewhere that can see how much flex there is under the car.

Also, the harmonic resonance of the whole car is changed; I've always wondered if the stock driveshaft was for cost purposes or NVH purposes. This is something beyond the scope of my knowledge and anecdotal evidence...

[stugray]

Originally Posted by CSG Mike

@stugray

I wonder how much drivetrain loss is mitigated by eliminating the extra joint in the stock driveshaft... I'll need to try mounting a gopro somewhere that can see how much flex there is under the car.

Also, the harmonic resonance of the whole car is changed; I've always wondered if the stock driveshaft was for cost purposes or NVH purposes. This is something beyond the scope of my knowledge and anecdotal evidence...

You nailed it right there. I have been thinking (and keeping it to myself to not start another branch of the subject) that what everyone "feels" is either smoother operation from the balanced DS, or more harsh NVH due to loosing that joint.
Either way I'm willing to accept that you can feel something different (just not increased hp :-)

[wootwoot]

I agree. I have a carbon fiber drive shaft and whiteline trans insert. For a while I also had whiteline diff inserts and it made a CRAZY resonance at 3000rpm. It was seriously insane. After I took the diff inserts out it was drivable again.

[cdrazic93]

Originally Posted by CSG Mike

I'd beg to differ. In a blind test, I would be willing to bet I can identify the difference with statistical significance, and in the world of "professional driving", my talent/skill is considered mediocre at best.

I know guys that can tell you if the weight of the car changed by 5 lbs in one corner, or can tell you how new (or worn) a brake rotor is by feeling the car out as they drive it....

I was basing that sensation off of just speed, not resonance or something else.

Just like how aftermarket catbacks give no gain in terms of hp, the resonance frequency of the exhaust changes, also called drone, where it is at xxxx rpm. Or not at all.

[Poodles]

Originally Posted by industrial

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.

I think the point about marketing BS flew right over your head...

Originally Posted by Sypher

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

Depends, if they swap the diff ratio it could be an issue. Still need concrete specs on these driveshafts...

Originally Posted by Xinshadow

So I see this critical speed thing pop up a lot and I get the gist, but how unsafe are they really as it relates to problems with critical speed? I mean I have 50+ buddies who circuit and drag race imports and domestic RWD and use aluminum (specifically not CF due to the drawbacks of price, lead time and additional hardware ), driveshafts, and not a single one of them in ten years has ever said they suffered a failure in that way. If and when they do fail it's always because they can't hold enough torque and twist up.

Not trying to be a ****, I'm genuinely curious if this is something that any of us can reasonably expect to encounter in a performance application or if this is something thats mathematically possible but very unlikely given our platform.

Depends a lot on the car and setup. It's a combination of high speeds and diff ratio that causes the issue. I've seen guys with rock crawlers have failures from it because of their crazy diff ratios cause the driveshaft to be in the critical range at just north of highway speeds. A critical speed failure is pretty damn nasty compared to snapping a driveshaft off the line at dragstrip.

Originally Posted by cdrazic93

9.999999999/10 times, no. That .0000001% would account for possible defects, which are unlikely. The driveshaft critical speed is a speed that wouldn't be attainable with the power given to the car. Nor the transmission. This car tops at ~140 or so, so the critical speed of the driveshaft would be somewhere close to or north of 200mph to be safe. At the point of changing the gearing and aerodynamics of the car to get there, you would have already replaced the driveshaft anyway.

You are 100% more likely to twist the shaft from the resulting torque of the motor, or break the transmission.

When a driveshaft reaches critical speed; it reaches the natural frequency of the material it's made out of, making it unbalanced and you will quickly lose control of the car. Not something you want to have happen at that critical speed.


Folks who engine swap or build the piss out of this car end up replacing the stock unit with a stronger one anyway (in this case, a faster spinning one).

Wrong wrong wrong. With my guesstimates, and the top speed of the car at 140, you could very well have a shaft failure. This speed is lower if people are swapping final drives.


You're not likely to break a stock driveshaft from torque on this car...


Also, driveshaft critical speed doesn't result in loss of control, it results in a driveshaft coming apart at high speeds (shaft and road) and lots of damage that could cause a loss of control.

Originally Posted by CSG Mike

I would 100% claim that a lightweight driveshaft is noticeable.

That's why CSG sells them.

Could you get me specs on the driveshaft? Length, diameter and material thickness would be great.


I'd rather do the math and make sure people are safe with the actual data instead of trying to guess specs based off of similar cars...

Originally Posted by jsimon7777

Regarding critical speed...
Assuming top speed of 143mph in 4th gear...
Driveshaft RPM = (MPH)(1056)(Differential Ratio) / (Tire Diameter)(Pi)(gear ratio)

So...
Driveshaft RPM = (143MPH)(1056)(4.1) / (24.1)(3.14159)(1.213)

619132.8/91.839042947 = 6741.5rpm at 143mph in 4th gear, everything stock. That means, at most, a stock car needs a drive shaft that can handle a bit over 6741.5rpm.

So what is the critical speed of the AL drive shaft? I don't know how long the tubing is. ~6700rpm+ is my best guess for 52".If it's 50", more like ~7250rpm.

Yeah, it's a bit of a pain math wise without the actual specs. Stock tire size should be 24.6, and you're adding the gear ratio to the equation which is skewing the results.

Originally Posted by wootwoot

I agree. I have a carbon fiber drive shaft and whiteline trans insert. For a while I also had whiteline diff inserts and it made a CRAZY resonance at 3000rpm. It was seriously insane. After I took the diff inserts out it was drivable again.

Vibration should be speed related not RPM of the engine if it's from the driveshaft. Though it's possible it's a resonance as you said of the engine and driveshaft at the same frequency or a harmonic there of. CF driveshafts generally don't have these issues because of it's material properties.


Also of note is that there will be harmonics from the driveshaft critical speed at lower and higher speeds than the critical speed that could cause some discomfort. Keep in mind this is stuff that balancing cannot get rid of, it's because the U-joints aren't constant velocity joints and move in a figure 8.

[cdrazic93]

Originally Posted by Poodles

Also, driveshaft critical speed doesn't result in loss of control, it results in a driveshaft coming apart at high speeds (shaft and road)

The end result from reaching driveshaft critical speed is still loss of control. It doesnt have to explode all the time. It can perminently bend without exploding, which will still cause loss of control...

http://www.freestudy.co.uk/dynamics/...vibrations.pdf
Not exactly wrong, but I worded it weird.

[burdickjp]

Originally Posted by Captain Snooze

@stugray
This question isn't aimed at this thread in particular; it is a physics question. I can't remember too much from my study days

If one is using an inertia dyno is one actually measuring power or is power calculated?
That is, a known mass is accelerated in a known time; is this force,work or power?

Originally Posted by stugray

In this case (simple inertia dyno) the only things that are measured are Drum RPM, and time.
Using those two numbers (drum angular velocity) and elapsed time (∆t), you can derive the power.
Power is the rate of doing work.
The work done is the change in rotational energy of the drum.
If you start from a stop, w = 0 so rotational kinetic energy is zero
The power is the final kinetic energy of the drum (in joules) divided by the time it took to spin the drum to the final speed.
So rotational kinetic energy is E = 1/2*I*w^2 ( where E is in joules, I is drum rotational Inertia in kg-m^2, and w is angular velocity in radians/second).

So total energy is: (Efinal - Estart)/time-to-reach-Efinal (in seconds).
This will give an average "Joules per second" which is the definition of Watts.

Note: this will only give you average power over the whole "pull" and will not provide the nice dyno charts we typically see which represent instantaneous power across the RPM range.
This precisely how I calclulated an average hp draw of spinning just the driveshaft.

I actually have a different answer for this than stu.

inertia dynos are drums of known inertia. RPM of the drum and time is logged. Acceleration of the drum is calculated. Newton's second law is used to calculate torque:
F = m * a therefore t = I * a.
Torque and RPM gives power at the wheels.
With a known gear ratio between the tires and the flywheel gives torque at the engine. This is why having a good engine RPM pickup is necessary, or you end up with goofy numbers. Power will be correct, but torque will not.

[burdickjp]

Has anyone mentioned the damping and spring characteristics of steel versus aluminum versus CF?

[Ultramaroon]

<raises hand> Ah, if we're calculating driveshaft angular velocity, why do we care about the diff? It's upstream.

(Omega engine)(gearbox ratio) Unless you expect to go faster than redline in top gear, no?