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Re: Driveshafts: Steel vs. Aluminum vs. Carbon Fiber
[Re: sgcuda]
#1898707
08/24/15 11:20 PM
08/24/15 11:20 PM
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Joined: Nov 2004
Posts: 6,890
North Alabama
Monte_Smith
master
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master
Joined: Nov 2004
Posts: 6,890
North Alabama
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A relatively stock chassied car, with a stock tunnel and a couple loops................when it breaks a steel or aluminum shaft, the whipping tubing literally destroys the rear of the car, if it doesn't break at the joint. The only way to remotely "contain" a broken shaft, is with some of the moly driveshaft "enclosures" like tube chassis cars have and those still take a serious beating.
Some may remember a few years ago the lady, Barb Nesbitt, who was trying to sue NHRA because her car broke a shaft, got in the car, wrapped her arm around it, and nearly pulled it off. If carbon, she may have had a splinter at worst
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Re: Driveshafts: Steel vs. Aluminum vs. Carbon Fiber
[Re: sgcuda]
#1898826
08/25/15 02:29 AM
08/25/15 02:29 AM
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Joined: Oct 2007
Posts: 4,209
New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,209
New York
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Re: "it's a law of diminishing returns"
The amount of power required to change rotational speed is exactly the same regardless of what speed.
Re: aluminum vs. steel
As MR_P_BODY said above, the diameter is too small for the rotational inertia to be much different. What you're left with is the actual "dead" weight difference, which is a very expensive way to save weight.
If I didn't have one, and I could afford it, I'd buy aluminum.
As Vizard points out, there is no way to tell whether a CF D/S is a great product or a bomb due to quality control issues.
Boffin Emeritus
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Re: Driveshafts: Steel vs. Aluminum vs. Carbon Fiber
[Re: Monte_Smith]
#1899187
08/25/15 04:52 PM
08/25/15 04:52 PM
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Joined: Jan 2003
Posts: 16,376
dogdays
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 16,376
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Theory says the engine has to accelerate two different things.
1. The weight of the car.
2. The rotational inertia of everything on the car that accelerates when the car accelerates. That means engine rotating mass, flywheel and all clutch parts, all transmission parts involved at any one time, the driveshaft, ring and pinion, axle shafts, wheels and tires, brakes.
In Mark's Mechanical Engineering handbook, 7th ed., there is an interesting statement that tfor a popular car of I believe the '50s, with a total gear ratio of around 14:1 the engine spend as much power accelerating the rotating parts as the static parts.
The issue is that rotational inertia is influenced by the distance of the mass from the rotational axis, I believe by something like the 4th power. Realistically this means that as the diameter decreases, the rotational inertia decreases rapidly. So the contribution of things like axle shafts is usually neglected. So with shafts in transmissions. Acceleration is change in rotional speed divided by time. If you think about it you see that the engine parts change rpm much more than say a driveshaft, which accelerates more slowly than any other shaft except the axles themselves. Plus the driveshaft is maybe 4: in diameter, which means that even though it has length, it doesn't contribute very much to the rotational mass the engine is trying to accelerate.
So the real contribution a lightweight driveshaft makes is to static mass reduction. This is, I believe, why the auto manufacturers are making them out of aluminum. Every pound of vehicle mass requires energy to move and the CAFE standards are tightening.
There's another factor that has been mentioned and that is stiffness. A driveshaft in a drag car is long and slender and unsupported on both ends, so it is prone to vibration. Building a stiffer driveshaft is easier when using a lightweight material like aluminum or carbon fiber, as it can be made larger in diameter for the same weight, and larger in diameter is better than wall thickness for increasing the resonant frequency.
Thus, using a lightweight driveshaft won't really make a difference in acceleration due to reduction in rotating mass, but it will make a difference in reducing static mass. It will also allow a stiffer driveshaft which will be safer. Go as light as you can afford.
R.
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Re: Driveshafts: Steel vs. Aluminum vs. Carbon Fiber
[Re: dogdays]
#1899205
08/25/15 05:22 PM
08/25/15 05:22 PM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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Theory says the engine has to accelerate two different things.
1. The weight of the car.
2. The rotational inertia of everything on the car that accelerates when the car accelerates. That means engine rotating mass, flywheel and all clutch parts, all transmission parts involved at any one time, the driveshaft, ring and pinion, axle shafts, wheels and tires, brakes.
In Mark's Mechanical Engineering handbook, 7th ed., there is an interesting statement that tfor a popular car of I believe the '50s, with a total gear ratio of around 14:1 the engine spend as much power accelerating the rotating parts as the static parts.
The issue is that rotational inertia is influenced by the distance of the mass from the rotational axis, I believe by something like the 4th power. Realistically this means that as the diameter decreases, the rotational inertia decreases rapidly. So the contribution of things like axle shafts is usually neglected. So with shafts in transmissions. Acceleration is change in rotional speed divided by time. If you think about it you see that the engine parts change rpm much more than say a driveshaft, which accelerates more slowly than any other shaft except the axles themselves. Plus the driveshaft is maybe 4: in diameter, which means that even though it has length, it doesn't contribute very much to the rotational mass the engine is trying to accelerate.
So the real contribution a lightweight driveshaft makes is to static mass reduction. This is, I believe, why the auto manufacturers are making them out of aluminum. Every pound of vehicle mass requires energy to move and the CAFE standards are tightening.
There's another factor that has been mentioned and that is stiffness. A driveshaft in a drag car is long and slender and unsupported on both ends, so it is prone to vibration. Building a stiffer driveshaft is easier when using a lightweight material like aluminum or carbon fiber, as it can be made larger in diameter for the same weight, and larger in diameter is better than wall thickness for increasing the resonant frequency.
Thus, using a lightweight driveshaft won't really make a difference in acceleration due to reduction in rotating mass, but it will make a difference in reducing static mass. It will also allow a stiffer driveshaft which will be safer. Go as light as you can afford.
R. Thats why I'm glad my DS is short... I dont need the big diameter or thick wall... yes the other shafts would be lighter BUT at double the cost... and as a bracket car I dont worry about that .. cost vs weight.. yes I could have knocked another 50# out of my car but the cost didnt justify it 
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Re: Driveshafts: Steel vs. Aluminum vs. Carbon Fiber
[Re: sgcuda]
#1899404
08/25/15 10:40 PM
08/25/15 10:40 PM
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Joined: Jan 2005
Posts: 4,243
Charlotte, North Carolina
sgcuda
OP
master
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OP
master
Joined: Jan 2005
Posts: 4,243
Charlotte, North Carolina
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Well, I haven't done any pricing yet. How much are aluminum drive shafts going for these days. How much are steel ones in comparison. Can't be too cheap. My Mark Williams slip yoke wasn't cheap when I purchased it last century. At least I can reuse that for the new shaft. Already set up for 1350 u joints.
[image][/image]
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Re: Driveshafts: Steel vs. Aluminum vs. Carbon Fiber
[Re: sgcuda]
#1899919
08/26/15 07:33 PM
08/26/15 07:33 PM
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Joined: Sep 2008
Posts: 656
Florida
CJD AUTOMOTIVE
mopar
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mopar
Joined: Sep 2008
Posts: 656
Florida
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CF shafts can be made to have a much higher critical speed and more importantly, they store energy, like a spring. They wind and unwind, taking some of the driveline shock away that tends to break parts. They absorb a great deal of harmonic energy as well, i.e vibrations. Best of all they "broom" when they fail. The cost to fix any tunnel or trans damage, even with loops, is far more than an upgrade to a CF shaft. Just my .02. Maybe not quicker, but easier on parts, less vibration, and no huge damage when broken.
Craig Scholl CJD Automotive, LLC Jacksonville, FL www.CJDAUTOMOTIVE.com904-400-1802 "I own a Mopar. I already know it won't be in stock, won't ship tomorrow, and won't fit without modification"
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