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PostPosted: June 25, 2012, 10:52 am 
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Ran across this on a manufacturer's website.

Just to put some numbers out there. .. add in the go force and shock loads, I'd bet control arms still ain't such a good idea.

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We decided to test a sample of our stock tube 45402 ( 0.75 x 1.00 x 35 inch) under a simple bend test and found that this tube fail at an amazing 1700 lbs


http://www.rockwestcomposites.com/

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PostPosted: June 25, 2012, 1:03 pm 
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Their material has some impressive specs, but it loses 97% of it's strength at 90C. That would be worth paying attention to...

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PostPosted: June 25, 2012, 1:20 pm 
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Therein lies the major problem with the carbon fiber stuff. It didnt bend or kink that it wouldnt have returned to its original form.....it simply failed. We work with steel and after a "shunt" we could look at a piece and see if it was deformed in any way and redesign it to better suit our needs. The carbon fiber piece may end up way overbuilt for our needs or it could fail spectacularily with a small bump. The trucks that came with the carbon fiber driveshafts had a problem . When they had a U joint failure and hit the ground the impact would cause them to desintegrate.......all that would be left were the yolks.
Dont get me wrong I love this stuff and would like to use it everywhere once it becomes economically feasible, but I dont have the skills yet......DO YOU?

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PostPosted: June 25, 2012, 1:45 pm 
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egoman wrote:
Dont get me wrong I love this stuff and would like to use it everywhere once it becomes economically feasible, but I dont have the skills yet......DO YOU?


Assuming I have the skills (I don't) to build the part, I'd also have to have the engineering skills to design it properly, then have the money and facilities to test it. IT occurs to me that one can't simply make a CF design to be identical to steel / aluminum / magnesium / designs?

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PostPosted: June 25, 2012, 2:06 pm 
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oldejack wrote:
Ran across this on a manufacturer's website.

Just to put some numbers out there. .. add in the go force and shock loads, I'd bet control arms still ain't such a good idea.

Quote:
We decided to test a sample of our stock tube 45402 ( 0.75 x 1.00 x 35 inch) under a simple bend test and found that this tube fail at an amazing 1700 lbs


http://www.rockwestcomposites.com/


Note that they say that it was a bending test; that's amazing strength whether it's cantilevered or supported at the end with a center load.

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PostPosted: June 25, 2012, 2:09 pm 
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egoman wrote:
The carbon fiber piece may end up way overbuilt for our needs or it could fail spectacularily with a small bump.


Given the strength it would have to be a very large bump, which would make the failure even more spectacular.

The problem with CF is toughness, the ability to absorb energy by deformation.

Kevlar would be much better, but not being a composites expert I'm not sure it would be as good as steel.

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PostPosted: June 25, 2012, 2:14 pm 
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egoman wrote:
Therein lies the major problem with the carbon fiber stuff. It didn't bend or kink that it wouldn't have returned to its original form.....it simply failed. We work with steel and after a "shunt" we could look at a piece and see if it was deformed in any way and redesign it to better suit our needs. The carbon fiber piece may end up way overbuilt for our needs or it could fail spectacularily with a small bump. The trucks that came with the carbon fiber driveshafts had a problem . When they had a U joint failure and hit the ground the impact would cause them to disintegrate.......all that would be left were the yolks.
Dont get me wrong I love this stuff and would like to use it everywhere once it becomes economically feasible, but I don't have the skills yet......DO YOU?


Me? Oh heavens no. .. and you won't catch me hanging from an I beam by my helmet either. :mrgreen:

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PostPosted: June 26, 2012, 1:03 am 
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geek49203 wrote:
It occurs to me that one can't simply make a CF design to be identical to steel / aluminum / magnesium / designs?


When you get down to it none of those materials mentioned are directly interchangable with each other (from a pedantic engineering veiwpoint).

For this forums theme, CF, and I mean real CF, is a no brainer - the only application is to buy a mass produced CF box or tube and base your monocoque cockpit on that and hang all your front and rear components off it more or less like the first CF F1 cars were like.

As for CF components, again as in real autoclave components in difference to laid up CF (no more than glorified fiberglass), the design and cost is beyond this forum's theme.


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PostPosted: June 26, 2012, 1:17 am 
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I can imagine designing a frame with those tubes and being very conservative etc. Then someday you hit a wall and watch all those tubes deform. At first you're smug - look it didn't break! Then you watch as all those tubes straighten out and launch you back the path you came at what ever speed you hit the wall.

Years of watching Road Runner cartoons tell me what ever you do, don't buy ACME brand CF tubes!

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PostPosted: June 26, 2012, 2:02 am 
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cheapracer wrote:
geek49203 wrote:
It occurs to me that one can't simply make a CF design to be identical to steel / aluminum / magnesium / designs?


When you get down to it none of those materials mentioned are directly interchangable with each other (from a pedantic engineering veiwpoint).


If there's enough margin on steel wishbones, why couldn't you use 6061-T6 at the same dia./wall thickness?

Seems to me they'd have ample fatigue life if they're designed to handle the large load cases like hitting a curb, which would be a very infrequent occurrence in the scheme of things.

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PostPosted: June 26, 2012, 11:14 am 
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horizenjob wrote:
I can imagine designing a frame with those tubes and being very conservative etc. Then someday you hit a wall and watch all those tubes deform. At first you're smug - look it didn't break! Then you watch as all those tubes straighten out and launch you back the path you came at what ever speed you hit the wall.

Years of watching Road Runner cartoons tell me what ever you do, don't buy ACME brand CF tubes!


:shock: (twaaaaang) :rofl:

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PostPosted: June 26, 2012, 3:19 pm 
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NoahKatz wrote:
If there's enough margin on steel wishbones, why couldn't you use 6061-T6 at the same dia./wall thickness?


I did say "pedantic" engineering!

But your'e right, in many cases of road cars, steel is generally used at 1.3 to 1.5 safety margin so at a 1.0 safety margin your aluminium could directly replace the steel. However at a 0.8 - 0.9 safety margin with steel that a top line race car engineer may take a chance with, then the aluminium at 1.0 can not compete - go look at what top race cars still use and used before CF suspension parts came along, ie; steel alloys.


NoahKatz wrote:
Seems to me they'd have ample fatigue life if they're designed to handle the large load cases like hitting a curb, which would be a very infrequent occurrence in the scheme of things.


I totally agree, the "aluminum fatigue" line is done to death by the 'Flat Earth Society'. Millions of MX bikes give their aluminium swingarns absolute hell and extreme stresses every weekend for many years, well proven but they have to be bigger in section than steel alloys and larger use of gusseting at joints and rod ends thus not suitable for most race cars.

By the way, Triumph (old) and BSA motorcycles both who use aluminum con rods since the 50's and there's not much in the automotive world that gets stressed in every direction and in every way possible more than a con rod. Their pushrods are aluminum also, big stresses there too.


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PostPosted: June 26, 2012, 9:54 pm 
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cheapracer wrote:
I totally agree, the "aluminum fatigue" line is done to death by the 'Flat Earth Society'. Millions of MX bikes give their aluminium swingarns absolute hell and extreme stresses every weekend for many years, well proven but they have to be bigger in section than steel alloys and larger use of gusseting at joints and rod ends thus not suitable for most race cars.

By the way, Triumph (old) and BSA motorcycles both who use aluminum con rods since the 50's and there's not much in the automotive world that gets stressed in every direction and in every way possible more than a con rod. Their pushrods are aluminum also, big stresses there too.


Of course, aircraft wings are aluminum, and are built to flex. However, at some point they too are at least inspected.

I supposes this is where we point out that both "steel" and "aluminum" come in various alloys -- and that neither seems to be used it its pure form for much of anything? And they both come processed in different ways too. But you obviously already know that, but I'm just saying, I'm betting that this is a bit more complex than we wanna cover here.

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PostPosted: June 26, 2012, 10:40 pm 
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geek49203 wrote:

Of course, aircraft wings are aluminum, and are built to flex. However, at some point they too are at least inspected.



Interesting subject, I watched a doco once where they stressed/flexed a wing of a Jumbo Jet 40 feet at the tip and had to make some intentional fatigue points (they made some cuts in the wing) before they could obtain failure - 40 feet!

They could make wings from steel alloys easily but they are too thin and don't absorb energy from impacts as well as soft metals/composites/fabric so the risk of failure is quite high from smallish incidents. But the worst part is the sparks .....


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PostPosted: June 27, 2012, 1:04 am 
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I totally agree, the "aluminum fatigue" line is done to death by the 'Flat Earth Society'.


I just don't agree. The problem is looking at things in black and white. If you design a structure in aluminum, fatigue can certainly be an issue. So you look up charts to derate the strength and design with that. You don't use the initial strength to design with if you expect millions of load cycles. Items like pistons and car wheels see millions of load reversals. That doesn't mean you shouldn't use aluminum or even that aluminum isn't the best material. It means you need to acknowledge the fatigue and design with numbers that represent the part's strength after millions of load cycles.

The issue of steel in airplane wings is not sparks, it just does not make as light a structure. Since it is heavier and stronger, when you use thinner pieces of it, it is subject to buckling. Planes always struggle to carry their payload and fuel, especially when there is money to be made. Planes are more honest about their physics then cars. They have to actually lift and carry their weight.

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