Composite wishbones design considerations

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ham2000
0
Joined: 14 Jul 2009, 21:01

Composite wishbones design considerations

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Hi,

I want some help with major considerations to be taken into account during the design of Formula student composite wishbones and processing methods.

I know their are
- stiffness and weight requirements
- is it possible to have a mixture of metal and composite eg. alum and carbon fiber

How are these wishbones made?
Is the carbon fiber wrapped around a steel rod?

If there is any links to websites with useful info that you could direct me to, much appreciated.


flyboy2160
84
Joined: 25 Apr 2011, 17:05

Re: Composite wishbones design considerations

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A's paper is an excellent starting point. Also look at this thread http://www.f1technical.net/forum/viewto ... 06#p295806

The construction method in A's paper is not the only way to make such a structure. As shown above, you can fully wrap the nose and over bond the trailing edge. You also have the choice of an internal or external mandrel. In your situation, for making just a few pieces in a school project, a extractable steel mandrel is too expensive to machine, as is an expanding internal bladder. You can use a multi-piece extractable, machinable PU tooling board or a wash-out salt mandrel, but these are labor intensive. The quickest, dirtiest way is an internal foam mandrel (or "core") that you leave in when done.(This is the method championed by Uncle Burt.) This requires very skilled layup (and an external caul plate) to get a nice external surface finish and not have ply wrinkles. But it does add some weight. PM me if you want detailed advice.

A's paper mentions loss of performance due to fabrication imperfections. In the case of buckling critical parts, the 0 degree buckling resistance can be improved by using pultruded graphite rods in the layup. These eliminate any manufactured-in wrinkles in the 0 direction fibers. They are commonly used in very high aspect ratio structures, such as sailplane wings

Have fun and learn a lot with your school project! As part of my Masters thesis, I did design, analysis, fabrication, and materials testing of a little missile fin.

[edit] Don't put aluminum fittings directly against plies with carbon fiber. There is a severe galvanic corrosion problem.

Lycoming
106
Joined: 25 Aug 2011, 22:58

Re: Composite wishbones design considerations

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I should ask what's the point. Carbon fiber tubes are extremely expensive compared to chromoly. It takes quite a substantial development effort, including lots of physical testing of tubes and adhesives. And how much weight will you save by doing it? If you have a well made set of steel a-arms, going to carbon probably won't save you more than 1.5 pounds, perhaps 2 pounds if you do a really good job on your first attempt. Could you have saved more weight using the same time and money doing something else?

marcush.
159
Joined: 09 Mar 2004, 16:55

Re: Composite wishbones design considerations

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I´d think it does only make sense when your suspension design cannot be fabricated in steel or titanium .The question is also hows the rest of the car done ? so if you have to weld up a space frame as chassis anyways per reg you would be well advised to stick with similar technology in suspension fabrication to not drain your recources too much in learning fabrication methods ...experience is hard earned most of the time .

Dragonfly
23
Joined: 17 Mar 2008, 21:48
Location: Bulgaria

Re: Composite wishbones design considerations

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Not wishbone but push rod, still interesting IMO. I remembered and dug out this thread
http://www.f1technical.net/forum/viewto ... =6&t=11557
F1PitRadio ‏@F1PitRadio : MSC, "Sorry guys, there's not more in it"
Spa 2012

flyboy2160
84
Joined: 25 Apr 2011, 17:05

Re: Composite wishbones design considerations

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Lycoming wrote:I should ask what's the point. Carbon fiber tubes are extremely expensive compared to chromoly. It takes quite a substantial development effort, including lots of physical testing of tubes and adhesives. And how much weight will you save by doing it? If you have a well made set of steel a-arms, going to carbon probably won't save you more than 1.5 pounds, perhaps 2 pounds if you do a really good job on your first attempt. Could you have saved more weight using the same time and money doing something else?
I'm a composites guy, but I agree 100% with Lycoming and Marcush. Part of engineering is picking the right material for a given situation.

You're not on an F1 team, so use welded 4130 if you're just trying to get the suspension made. If you're trying to learn about composite design, analysis, and fabrication, that's different.

"Composites" are now PC amongst know-nothing aerospace managers. Part of the reason I got laid off recently was for designing an aerospace structure from welded thin wall 4130 tubes (under a strict budget), when all the big shots wanted a composite design. Composites in that case wouldn't save much weight (or might even be heavier!!!), would be 200-1000 times more costly, and take 200-1000 times longer to fabricate.

I'm designing an experimental airplane at home and picked welded 4130 thin wall tubes for the fuselage truss instead of composites.

But I'll give you advice if you go with a composite design.

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