CF Monocoque - Info

[Nim-T]

Hi guys,

For our second car at Imperial College (London, UK) I am designing and building a CF monocoque for a class 2 entry in Formula Student. I'm nearing the end of the design stage and I intend to start pattern making within the next week or two after completing some FEA work.

Since most people don't seem secretive as hell, I thought I'd ask around for some info from teams that have also built monos. I'd really appreciate your responses! What kind of figures do people have for:

1) Monocoque weight excl. roll hoops,
2) Torsional stiffness (obviously including any rear frame structure),

How have people approached the safety equivalency document required for the use of composite materials in a chassis? How much detail have you gone into? A mixture of sensible hand calculations backed up by FEA?

Has anyone investigated using carbon hardpoints (referring to buying them in rather than making them yourselves)?

Whilst we definately won't do this, has anyone ever thought of extending the monocoque rearwards in order to bolt the engine on to the structure?

One last thing - please don't let this thread turn into a monocoque vs. spaceframe debate http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif

Cheers,

Nim
www.cgcu.net/icracing (http://www.cgcu.net/icracing) (see 'the cars' section for a CAD pic of the new car)

[Nim-T]

Hi guys,

For our second car at Imperial College (London, UK) I am designing and building a CF monocoque for a class 2 entry in Formula Student. I'm nearing the end of the design stage and I intend to start pattern making within the next week or two after completing some FEA work.

Since most people don't seem secretive as hell, I thought I'd ask around for some info from teams that have also built monos. I'd really appreciate your responses! What kind of figures do people have for:

1) Monocoque weight excl. roll hoops,
2) Torsional stiffness (obviously including any rear frame structure),

How have people approached the safety equivalency document required for the use of composite materials in a chassis? How much detail have you gone into? A mixture of sensible hand calculations backed up by FEA?

Has anyone investigated using carbon hardpoints (referring to buying them in rather than making them yourselves)?

Whilst we definately won't do this, has anyone ever thought of extending the monocoque rearwards in order to bolt the engine on to the structure?

One last thing - please don't let this thread turn into a monocoque vs. spaceframe debate http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif

Cheers,

Nim
www.cgcu.net/icracing (http://www.cgcu.net/icracing) (see 'the cars' section for a CAD pic of the new car)

[Tim Heinemann]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>

Whilst we definately won't do this, has anyone ever thought of extending the monocoque rearwards in order to bolt the engine on to the structure?

<HR></BLOCKQUOTE>

We're bolting the drivetrain directly to the frame:

http://www.ecurie-aix.rwth-aachen.de/graphics/rahmen/chassis2.jpg

As you might see the frame extends to the rear suspension mounts. That's because our engine can't serve as a structural member (sorry, there was a typo in my previous posting saying that it can be used). Actually I prefer to let the fibres run from the front to the rear suspension minimising interfaces. The only real thing to worry about is heat transfer and this can be dealt with by using the right matrices depending on location and different forms of insulation,



Tim

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Nim-T:

Whilst we definately won't do this, has anyone ever thought of extending the monocoque rearwards in order to bolt the engine on to the structure?
<HR></BLOCKQUOTE>

Yes http://fsae.com/groupee_common/emoticons/icon_wink.gif

Kansas State built a car using that approach as well.

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

[James Waltman]

Our design is not typical for a monocoque but we carry the chassis all the way past the engine. It creates some packaging challenges and we really had to pay attention during the planning stage because it is not so simple to add more things by welding on a tab. Overall, I think that it is a far better solution than space frame rears.
http://dot.etec.wwu.edu/fsae/SharedPictures/First%20Drive%20V35/DSC04835.JPG
http://dot.etec.wwu.edu/fsae/images/Headers%201_JPG.jpg

The safety equivalency is not a simple matter. I recommend submitting it before you start making anything. What you think is a reasonable solution (and probably is) may not work for the committee. We did some simple hand calculations for stuff like bolt shear strength and bonding shear strength (based on tests and bonding area). I wouldn't count on the judges to trust any FEA. In fact I don't think they should even accept it. It is one thing to FEA a steel A-arm or driveshaft and quite another to FEA a complex composite part. I do think that FEA can be a valuable tool. We have used it for composite parts but only as a qualifier and not a quantifier. It is helpful to see stress concentrations and build to accommodate them. Trusting the numbers is very dangerous. I may be wrong, some teams may have students who are capable of doing FEA on complex composite parts but I doubt it.

I know of some students who use steel or aluminum sheet in their FEA models and then want to make the fiber orientation of the composite part mimic those isotropic properties. This seems like a really bad approach. Composites have different properties in different directions – use it to your advantage.

In place of FEA we did a lot of testing of our parts and components. We tested our adhesives extensively. We built sample foot boxes and crushed them. We crushed our side impact structures. We pulled out our safety harness mounts several times. We crushed candidates for our crush structure and choose the best. We built a representation of our bulkheads and crushed it. When we started I resented the fact that we had to prove everything. Now I am glad we had to. We have a better car because of it. I would be disappointed if teams could get by with less.

Tim,
That looks great.
Do you have more pictures that you are willing to share?
What are you using for core material?
What matrices are you using (pre-preg or wet layup and what resin)?
What have you done for inserts where the drivetrain bolts up?

James Waltman
http://dot.etec.wwu.edu/fsae/
Vehicle Research Institute at
Western Washington University

[Jon @ Electromotive, Inc.]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by ben:

Kansas State built a car using that approach as well.

Ben

University of Birmingham
[URL=http://www.ubracing.co.uk<HR></BLOCKQUOTE>

I feel kind of silly making this point, but there are two Universities in Kansas. One has built several composite cars (University of Kansas aka KU) and the other has been to Detroit only 3 times in the past 7 years (Kansas State University aka KSU). Not surprisingly there has been confusion before. But because we (KSU) are working to become a regularly serious contender I would like to get people more aware of the difference. As a note, KSU was the team that had an all aluminum sidewinder that weighed in at 361 lbs (164kg) at Detroit last year. We're hoping to make an impression by actually driving this year.

Sorry for the off topic but this seemed as a good a time as any to note the difference.

Jon Rawlings
jwr9777@ksu.edu
Kansas State University Formula Motorsports
Electronics Team Leader

[Nim-T]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>I wouldn't count on the judges to trust any FEA. In fact I don't think they should even accept it. <HR></BLOCKQUOTE>

I'm not in the right position to cast judgement on whether the judges should accept an equivalency with FEA in it or not, but I have literally just gone through the report for our first car. It is a careful combination of hand calcs and FEA and there is sufficient validation on the composite shell elements that were used. The judge passed it without any trouble.

Setting up the material orientation for a composite analysis in a program is a little tricky, but I've recently worked it out and I'd be happy to share... http://fsae.com/groupee_common/emoticons/icon_smile.gif (Abaqus)

It sounds like the testing work you performed has done wonders for your confidence in your structure, and your layout is quite interesting - all the best.

[Kevin Hayward]

I would have serious doubts as to the transfer of FEA calcs to actual structures. For example the level of voids in your structure can have a significant effect on the ability of your structure to handle loads. Only through experimentation and manufacturing can you find materials and processes that help to limit such problems as well as assessing how much they reduce the strength of components.

Another BIG example is how effective is the bond between the core and the skins?

I definitely agree with Tim on previous posts when it comes to passing structures. Getting it past the judges should not be our primary goal, rather it should be to make it as effective, safe and predictable as we can. When designing and building under these guidelines "getting it past the judges" becomes a formality.

Kev

Suspension guy who seems to consistently get into Composite discussions
UWA Motorsport

[James Waltman]

Kevin, you said what I was thinking. Actually making the parts changes a lot of stuff.

Nim-T, does a class 2 entry mean that you aren't building a car yet? Do you just submit designs? Is a safety equivalency required for those entries? Do you have to submit another one later when the car is ready to compete? Sorry I don't know anything about the different classes.

James Waltman
http://dot.etec.wwu.edu/fsae/
Vehicle Research Institute at
Western Washington University

[Nim-T]

James,

For our class 2 entry we will have a partially completed car - it should be a 'rolling' but won't have the engine or any peripheral components installed. I don't think a safety equivalency actually has to be submitted unless you are running in class 1 (dynamic). However, i'm going to submit this soon to make sure there aren't any obvious mistakes.

So we aim to enter class 1 in 2005, and i believe that another safety equivalency is only due if any major changes have been made structurally. For example between our class 2 and 1 entry for our first car, we added composite side arm protectors, and duly gave the judges some updated calculations.

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by SamuraiSteve:

I feel kind of silly making this point, <HR></BLOCKQUOTE>

Not silly at all. It must be very frustrating to be confused with another team, particularly when FSAE breeds some of the most overt pride of any uni teams.

I apologise for not checking my facts.

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

[Jon @ Electromotive, Inc.]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by ben:

It must be very frustrating to be confused with another team,

<HR></BLOCKQUOTE>

No problem, it's an easy mistake to make without the two in front of you.

Jon Rawlings
jwr9777 at ksu.edu
Kansas State University Formula Motorsports
Electronics Team Leader