Carbon fibres orientation effects

[ham2000]

Hi

For carbon fibre wishbones for Formula student car -
How does the orientation of the fibres affect a cars suspension?

- does the way they are orientated affect the amount of twist on the wishbone when it is loaded, would anyone else agree with me when I say this would be the major design consideration.

Is the wishbones usually fully carbon fibre with a aluminium piece at either end for the bearing/ tie rod end to screw into?

any links to articles when be much appreciated.

Thanks

[flyboy2160]

As respectfully as I can: If you are asking simple questions like these, forget about a composite wishbone and make it from thin wall welded 4130.

Didn't your faculty advisor explain to you that there is a logical progression in engineering? You shouldn't be worrying about composite fiber orientation when you don't even have the basic loads in your structure.

And you should do your homework before asking questions. There are at least 2 threads on this subject on this website, which include links to very good papers, but it appears you haven't even done a simple search of this website before making assumptions and asking some one else to search for you.

[thepowerofnone]

As flyboy2160 says, if you have to ask, its probably not something you want to be looking at doing. However for the sake of humouring your interest, assess what you know and would have been taught in structural analysis about torsion and the equations of EBT. If you need a recap on that, look here: http://www.roymech.co.uk/Useful_Tables/ ... rsion.html.

Carbon fibre is a well known orthotropic material, or at the very least a partially anisotropic material. So in short, yes, the direction of the laying of fibres will certainly affect your torsional modulus of rigidity, which will in turn affect how much your suspension will twist, and the shape of your wishbone will affect this too (think second moment of inertia). However if I am honest, that is about as far as my knowledge extends - carbon fibre is notoriously tricky even under static loads, however under dynamic loading (which is basically the constant state for suspension) it is exceptionally difficult to model. The 787 is famed for being built mainly from CFRP despite the Boeing engineers not fully understanding the response under dynamic loading. Please also consider - do you have the facilities to reliably lay up CFRP suspension to a high tolerance? The last thing you want is two suspension struts on either side of your car which behave in different ways.

My advice would be to listen to flyboy2160 and look at an aluminium or (more likely) steel thin wall structure, with welded blocks at the ends to reinforce the pickups. I think I a right to say that F1 is virtually the ONLY racing series in the world to use CFRP suspension and for good reason - not only is it ultra-stiff it allows them to make aerodynamically neutral suspension (to gain a benefit from it would contravene the rules) which is important at their speeds but much less so at Formula Student speeds. I have worked on cars with carbon monocoques (which I would be very surprised if your car was) and they don't use CFRP suspension - its too complicated and it is not that much of a gain. There are much better ways to improve your car.

[Lycoming]

A round tube is aerodynamically neutral too, in the sense that it doesn't make any net lift/downforce, but profiled wishbones like what F1 teams run will have less drag. To be fair, you can get 4130 tube in the same profiles, but you will be questioned as to why you did that since it's inferior to a round tube of equivalent weight under compression loads.

To be fair, carbon fiber tubes loaded axially are relatively simple to model, as compared to, say, a monocoque; but if you're at the level where you don't understand the effect of fiber orientation and couldn't find out on your own (it's seriously not that difficult; either use google or find a good composites book in your university library), you'd better stick to steel.

also, I'm curious; have you decided that whatever weight improvement you can gain from carbon fiber arms is indeed worth the time and effort to do all the research and development work? Or are you just doing it because you want to use carbon fiber somewhere? And why would a suspension arm ever see anything torsion loads?

[Jersey Tom]

Twist..? Twist in the control arm? The design intent is that each leg of the A-arm be axially loaded.

Is fiber orientation a major design consideration with composites? Yes - that's Design 101.

Could you make control arms out of pre-made carbon tube with bonded aluminum inserts? Sure - particularly if you have good design engineers with CNC machining experience. But you can make some damn light arms out of 0.049 or 0.035 wall tube.

[riff_raff]

Most engineering student race car competitions (Formula SAE, etc.) don't permit use of composite suspension parts for reasons of cost and safety. All the ones I have seen used welded steel. The idea behind these competitions is not really for students to create their own version of a no-holds-barred F1 car. Instead, the goal is to give students experience in all facets of designing, building, and tuning a race vehicle. Even doing a proper job of designing, analyzing, constructing and inspecting a welded steel a-arm is not a simple task for these students. And the team gets judged on their ability to perform these tasks just the same as their track results.

While the question in the OP was indeed a bit naive, we should also try to be patient when responding. Any of us that are engineers probably asked similar dumb questions when we were freshman undergrads. By giving a harsh response to questions that are obviously posted by a young person, you do them a bit of a disservice. Wouldn't it be better if we simply gave a polite explanation of how to do a better job of posting their question next time, and then maybe point them to some resources that they can utilize to research the subject on their own? Seriously, did anyone here understand how to design and analyze a composite laminate when they were a freshman undergrad?

[krisfx]

Our FSAE car uses composite suspension arms from a sponsor so it's not impossible.

Afaik you want the plies running along the strut from cockpit to wheel to give maximum strength along the horizontal. You could also have it twisted like a driveshaft I guess.

Apologies if this is a pointless refresh of info, it's 8:30am and I'm skimming

[Jersey Tom]

Any of us that are engineers probably asked similar dumb questions when we were freshman undergrads. [...] Seriously, did anyone here understand how to design and analyze a composite laminate when they were a freshman undergrad?

I had plenty of dumb questions and ideas about things outside of the curriculum at that time, sure. But Freshmen aren't designing critical FSAE car parts either. Or any parts, typically. By sophomore year.. static and materials science.. I'd say you'd have a good idea of what a truss is, what direction the loads go, and that composite plies go in the direction of load.

In any event, carbon arms are certainly doable. Really the most challenging aspect is the design and machining of the end lugs, whether they be solid billet aluminum pieces themselves or molds for something more slick...

[riff_raff]

JT- Granted, maybe my undergrad engineering curriculum was not quite as rigorous as yours. But I did not have the ability to properly analyse even a simple composite laminate structure until well into my junior year. Even a basic composite a-arm structure is more than simple tension/compression. There are buckling effects in the compression member to consider, and how the fiber orientations within the laminate affect buckling. There are also the complex conditions where the composite laminate transitions loads to the metal part through an adhesive bond joint.