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Thread: CF monocoque designe

  1. #1

    CF monocoque designe

    Hello everyone,
    I am a student at BGU and a team member in our FSAE race car project.
    We are building our 5th car, first one with a full monocoque body.
    I need some advice about the assembly of the monocoque…
    Meaning: 2 symmetric molds (right and left) or 2 asymmetric molds (up and down).
    What were your considerations in this decision, apart from the obvious ones (like that the attachments to the monocoque should be away from the seam).
    Your help will be much appreciated,
    Thank you

    Alex lebedinsky
    BGR – monocoque team

  2. #2
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    I doubt that people will be too willing to share their secrets of monocoque construction, but from what I've seen in the past many teams favour the asymmetric mold route, with one mold for the top an a separate one for the bottom.

    This process allows for you to easily control things like cockpit size from above, (tombstone template requirements), and suspension, engine, or pedal cluster mounting, from below.

    On another note, many teams have been able to hide a seam running between top and bottom by covering it with logos; sidepods, faded paint, and such, whereas a seam running along the central axis of a car may be a little harder to disguise. This may no be important to you, but that's just my two cents.


    Find the seam on K-State's car here
    DSC07812.jpg
    DSC07797.jpg
    My views, thoughts and wording do not reflect those of Carleton University in any way, shape or form.

    CU FSAE Ravens Racing (Volunteer Team) - Composites Mentor/Bodywork 2014/2015
    CU FSAE Ravens Racing (Volunteer Team) - Bodywork/Manufacturing 2013/2014

  3. #3
    What is BGU?
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  4. #4
    Quote Originally Posted by Claude Rouelle View Post
    What is BGU?
    BGU - http://in.bgu.ac.il/en/Pages/default.aspx
    BGRacing - http://www.bgracing.com

  5. #5
    hey SkeH14,
    thank you for your quick reply.
    I guess i was trying to squeeze some tips from the more experienced teams.
    happy to get any information,
    thanks

  6. #6
    We are using a left and right mold and did so on our last chassis--but I would not necessarily recommend this route. By using a top and bottom mold you can set all suspension pickups with the mold rather than by aligning the two halves.
    Andrew Cunningham
    California FSAE

    Car Chief, 2013
    Team Lead, 2014
    Aerodynamics Lead, 2015

  7. #7
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    There is also a third option on how to make your monocoque: in one piece

    It is of course more difficult to lamiate a monocouque in one piece as you are always working on a closed mold and the access is only possible through the bulkhead and cockpit opening (and engine-opening if you have a full monocoque) but it is of course the best option if you are looking for the lowest weight possible since it is not necessary to bond 2 parts together.

    Our molds are splitted verticaly (so a left and a right mold) because our suspension department wants very complex geometry for the mounting points and this can only be achieved with a vertical split. We have a flange on the halves of the molds, bolt them together and fill any gaps between the two with epoxy putty and sand it down until it is perfect. Then we start laminating everything from the inside.

    I don´t know if you want to make your chassis with prepregs or if you do it with wet-laminating. If you use the latter I would recomend you a horizontaly split mold because you will be able to reach better tolerances on the finished part since all the suspension and engine mounting points will be on the lower half that has no seam in it.

    Some teams drill their holes into the monocouque when it´s finished but I would recommend you to position your hard points with dowel pins.

    Let me explain: When you have your positive molds (plugs) machined, you also drill the holes that you will later need for your suspenison and other parts. (we use 6mm dowels everywhere) Then you insert a dowel before you do your laminating of the negative molds (don´t forget to put some releasing agent onto them and you will end up with a mold that has all the needed positong holes in them. You should use the kind of dowel pin that has a thread in it on one side so that you can pull them out with a special sliding hammer once the mold is cured.

    We then insert freshly prepped dowels into the finished mold and positon our inserts with them (most of them are water-cut 5mm thick carbon, but we also use titanium and aluminium in some places) When your monocouque is cured, you pull out all the dowels, open the mold and you have a monocoque with holes in them that have more or less the same tolerances as the machine that drilled them into the positive mold in the first place

    Hope that helps,
    Regards
    Paul Mayr-Harting

    TU Graz Racing Team (TUG Racing)
    Chassis 2012
    Head of Chassis & Aero 2013
    Chassis & Aero 2014

  8. #8
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    Also here is a picture of our 2013 monocoque during production with the first layer in place, view from the bulkhead opening

    2013-03-09 15.29.58.jpg
    Paul Mayr-Harting

    TU Graz Racing Team (TUG Racing)
    Chassis 2012
    Head of Chassis & Aero 2013
    Chassis & Aero 2014

  9. #9
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    Paul,

    Oddly enough, that's nearly the same process we attempted last year... without much luck. One of the many things we failed to account for was the difficulty of conforming the core material to curved surfaces. In fact, we naively used some compound curves a swell. For our next attempt, we've made sure to design the chassis so that it's comprised almost entirely of planer surfaces... similar in appearance to a "cut and fold" Oxford Brooks style alloy skinned chassis. This will hopefully allow us to cut our core material into individual sections, fitting cleanly into the planar envelopes defined by the outer skin. -- Which is right about where we ran out of ideas.

    Could I ask you to comment on the methods you used to create continuity through your core material?

    The only solutions I've come up with aren't very appealing: ...
    a) Leave some core sections joined at an edge and bend -> crush the joint.
    b) Fill the voids between core sections with syntactic foam....
    c) Use a machinable core material (fill the honeycomb with wax) and attempt to mate each section directly.


    Thanks for the photo as well... I've always admired that chassis. Somehow, it just now occurred to me that your pan-down attachments improve kinematics (from a packaging perspective... in addition to the assumed compliance improvement).

  10. #10
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    Regarding the honecomb:

    A standard 15mm honeycomb can take quite a lot of curvature, especially the light ones that we use, but if single cells are colapsing because of the curvature, then this is no big deal. If there are smaller radii, we use Flexcore (http://www.hexcel.com/Resources/Data...lexcore_US.pdf) which can accomodate just about any surface you can imagine.
    But it is of course best to use flat surfaces for your sandwich panels, especially if you use thick honeycombs (everything above 15mm)

    And for the seams: We use small radiuses between the surfaces (5mm) in order to join the honeycombs together directly. You can machine a honeycomb without filling it with resin, we use air-powered tools with standard sanding disks for that. If you are carful you can do all the necessary edge filets etc. with that. Before we put the monocoque into the autoclave (for the 2nd curing process that bonds the hneycomb to the outer skin) We apply 3-4 layers of Flashbreaker- Tape over the joints of the honeycomb sections so that the vacuum bag cannot get inbetween (collapsing of the core because of the side-forces would be the effect)

    After the 2nd autoclave-process we fill the small gaps between the honeycombs with a white paste which is made up of resin and filling agent (we use glas-microballons) and we make sure that this paste does not enter any cells of the honeycombs, just the seam (weight reduction, bro!) In the end we sand off everything to get smooth surfaces and joints for the inner layers of Carbon. Also we take a vacuum cleaner to remove any dust from the honeycombs before we start with the inner layers (you have to take weight reduction serious if you are building a 150kg car)
    Paul Mayr-Harting

    TU Graz Racing Team (TUG Racing)
    Chassis 2012
    Head of Chassis & Aero 2013
    Chassis & Aero 2014

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