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Thread: Frame Design - First Car IFS

  1. #1

    Question Frame Design - First Car IFS

    Hello everyone my name is Tiago and I'am the new Frame responsible from IFS Team ( ISEL Formula Student ), Lisbon, Portugal. We are a second year team and participaiting for second time in a formula student competition, the first one was this year in FSUK class 2. This is our first car. I have been reading this forum attentively from some months now, i've looked for various areas and almost every time found most of the answers to my questions already answered, even more, i've seen errors in our design that we would never think about, by reading posts here. Now I would like to request you help with your experience to tell me if you find any triangulations errors or others apart from those, in our frame.
    This is the last design of my collegue, I'am now trying to better is work.

    One thing i've understood is that our frame is heavily over "reinforced" probably more than necessary, and we are "scoring" very low in terms of mass. The torsional rigidity of the frame is about 3000 Nm/deg ( from a FEA model ) i've read that the values can some times be 50 % lower than the real ones and we have set the target for 2000 Nm/deg, after looking to the total roll stfiness FRT and Rear and also looking to the lateral load transfere. During the design we have paid attention where the forces go and trying iteratively to understand how the system is behaving and which triangulations and tubes offer more or less contribution to the torsional rigidity, and how the energy is dissipating thorugh the strucutre.

    Tubes colors vs ODxt are: Green (25.4x1.6) , Yellow (25.4x1.25) and Red (25.4x2.4)

    Isometric View.jpgSide View.jpgUpper View.jpgMeasurments Frame.jpgView_2.jpg

    Thank You, Best Regards

    Tiago
    Last edited by Tiago-IFS; 09-29-2014 at 08:58 AM.
    2013 - Chassis/Body Group ISEL FS
    2015 - Dynamics Head

    - Lisbon Higher Institute of Engineering

  2. #2
    Hey Tiago, can you upload a frame which includes a sketch of your suspension (A-Arms, Bell Crank)? It should be easier to see if your frame is stiff enough to react the suspension forces.

    What kind of engine you will use? What's your weight goal for the frame and the hole car?

    Think about the tubes above the SIS - are they all needed? And secondly if the driver is handicaped by them while get out of the car at driver egress?

  3. #3
    First of all, thank you for answering Ben. Of course here it is, i've made a section view of the frame with suspension lines so it is easier to understand. This is our first car, and we have had some sponsors, our financials are very low, one of the sponsors was KTM that offered us the engine: KTM Duke 390cc . And our overall mass goal was between 230 and 250 kg which is quite high for a small engine. Our Strongest point in terms of knowledge is undoubtedly the engine and we are studying the case to go from naturally aspirated to a turbocharged ( I also must account with a possbile extra power in case we go ahead with this decision ). The Tubes above de SIS is something that has been worring me, about the 5s rule.

    Front Suspension Lines

    Isometric View FSusp Lins.jpgUpper View FSusp Lines.jpgFront View Suspension Lines.jpg

    Rear Suspension Lines

    Rear View RearSusp Lines.jpgIsometric View RearSusp Lines.jpg

    Hope You can clearly see, is that fine to you to understand ?

    Tiago

  4. #4
    Sorry, I haven't got used to the image Shrink of the forum. Here it is, files with greater quality. https://www.dropbox.com/sh/wstnf4fqz...rvNKaclUa?dl=0
    2013 - Chassis/Body Group ISEL FS
    2015 - Dynamics Head

    - Lisbon Higher Institute of Engineering

  5. #5
    Senior Member
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    Quote Originally Posted by Tiago-IFS View Post
    ... This is our first car,
    ... our financials are very low,
    ... KTM Duke 390cc.
    ... our overall mass goal ... 230 and 250 kg,
    ... we are studying ... to go ... turbocharged.
    Tiago,

    As with all other Teams that have ever entered these competitions, you have the possibility of WINNING OUTRIGHT. (Yes!!!)

    But, your posted designs and the selected quotes above suggest that you will likely finish towards the bottom of the field.

    YOU ARE MAKING IT TOO COMPLICATED !!! (<- This is intended as helpful advice. )
    ~o0o~

    Specifically:

    1. Your "financials are very low" means that you have an advantage. Your low financials should discourage you from wasting time on silly stuff. Exploit this advantage by building a very simple and efficient car.

    2. KTM have given you an adequate engine. So your "engine choice" problems are solved! You can now move on to the more important things.

    3. Does your "overall mass goal" include the car AND driver, or just car alone (!)?

    I suggest mass targets of:
    * Engine+Transmission = 30 kg.
    * Front-Axle (ie. 2 x tyres/wheels/hubs/uprights/brakes/suspensions) = 30 kg.
    * Rear-Axle (as above) = 30 kg.
    * Frame = 30 kg.
    * Everything-Else (bodywork/aero/seat/seat-belts/steering-wheel/electronic-junk/other-junk???) = 30 kg.

    So, Total-Mass-of-Car = 150 kg.

    4. DO NOT "go turbocharged" until you have won at least one competition.

    5. Bottom line here, your frame is TOO COMPLICATED. Too big. Too many tubes. Too much work. Too many (Edit: UNNECESSARY!!!) wishbones, and (unnecessary) pushrods, and (unnecessary) rockers... And too much (unnecessary) CADing...
    ~o0o~

    Suggestions:

    1. Build an "ergonomic-mock-up" of the car. This is a simple (but REAL!) frame that the Team members can sit in to check if it "feels" right. Fit the engine, steering-wheel, 4 x suspension/wheels/tyres, etc., to this frame to make sure everything fits right. Then check driver sight-lines, driver elbow-room, driver egress-time, etc.

    2. Spend a long time looking at that mock-up frame, then SIMPLIFICATE it and ADD MORE LIGHTNESS.

    Z
    Last edited by Z; 09-30-2014 at 12:28 AM. Reason: Added unnecessaries...

  6. #6
    Senior Member
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    This happens not too often but in this case I have to agree with Z ;-)

    What I read from your post is that you are going to make the same mistake most rookie teams are making. You're building a way too complicated car. As Z already said, just don't turbocharge that engine. The only modifications you should do at the engine are what is necessary to make it rules compliant. Basically that will mean to build a new intake with a restrictor and you will propably want to build a new exhaust system. Leave it with that. Take care that the engine is running reliably and that throttle response is adequate (hint: a turbo doesn't help here). But in general don't make the mistake to think that the engine is the most important thing in a Formula Student car. IT IS NOT.

    If your weight target is already 250kg you will propably end up with a car above 300kg. It is always heavier than expected, so set a low target. Skip all parts which aren't necessary to make the car running. Get the whole thing running as early as possible. And then test, test, test!
    Rennteam Uni Stuttgart
    2008: Seat and Bodywork
    2009: Team captain

    GreenTeam Uni Stuttgart
    2010: Seat and Bodywork / Lamination whore

    Formula Student Austria
    2012: Operative Team

  7. #7
    Thank You, Bemo and Z I think i've heard what i really needed from you two. Is just that for first time, we maybe aren't ballancing complexity and simplicity in a good way. We are now taking your advice and left the turbocharged engine apart. I will simplify this frame in order to reduce weight and keeping a reasonable torsional stifness from our stifness objective. Thank you for your time again.
    2013 - Chassis/Body Group ISEL FS
    2015 - Dynamics Head

    - Lisbon Higher Institute of Engineering

  8. #8
    Tiago,
    Thanks for listing some of your targets.
    What is your frame torsional stiffness target? My thinking is that your stiffness target may be quite high for the weight compromise you're taking.
    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

  9. #9
    MCoach, the target set is between 1700 to 2000 Nm/deg, and those values were taken after computing the lateral load transfere for a two mass model, the references that i've used for calculations were RCVD and the SAE paper ( 2000-02-3554) The Effect of Chassis Stifness on Race Car Handling Balance. The point is that we've read that the frame torsional stifness determined by a FEA can be about 30% - 50% higher that the real value (Here in the forum, in a post that I don't remember now). We have played safe in this area, I know from the calculations and plots that the effect of the chassis stifness in lateral load transefer and therefore other consequences like vertical load on tires and therefore it will change the tire grip in quasi-static and transiente behviour and also other dynamic effects. But the "real feeling" and numerical results of those consequences... those ones we don't have them yet, even of previous experiences, because it is our first car or even with a model of our vehicle which we are still devlopment. Some questions like, which performance cost will a X times chassis stfiness lower of what it should have. Those are still to be answered.

    I hope we are in the somehow in the right way, thats why the feedback of you and the others like Z and Bemo is important to us.

    Maybe this "safety factor" is to high!
    2013 - Chassis/Body Group ISEL FS
    2015 - Dynamics Head

    - Lisbon Higher Institute of Engineering

  10. #10
    Tiago, I can say that compared to several successful FSAE cars you might be in the range of what I've seen. You might also see that things such as roll centers play a bigger difference than your chassis stiffness from where you're at, assuming you hit your targets. Unless you have a derpy chassis that handles like a wet noodle, don't worry about the transient tire loads, cause baby, we ain't there yet.

    Depending on how you did your FEA, pulling in all of the tube properties or assuming a wireframe beam model (latter is easier and faster to iterate), you may see that your real values may deviate quite far from the paper you referenced, in fact the actual chassis may be only 1/3rd the stiffness of the simulation!

    A side note, should only the tubes be simulated? I ask because I got our team to run stressed body panels for the whole frame rather than the large nosecone that covers the whole leg area that is typical of steel tube chassis teams. The panels, although minor in weight, added 30% stiffness to the chassis, putting us in the 2700N-m/deg range. What we learned from the whole ordeal is that the panels carried more than the expected 15% and that tube weight can be reduced to the bare minimum for this year. Just throwing that out there.


    Basically, iterate it to the minimum tube sizes with respect to the rules, pull all of those thousands of grams out of that frame and see where that puts you.
    It should still be right around where you are in terms of stiffness and a hell of a lot lighter. If you can get some thinner tubes for all of the non required tubes, do so. I like to use 0.035" for anything I can get my hands and 0.049" if the sim says it would fail.

    Good luck. It's better to be wrong and finish early than to be right and never finish. "Speed over accuracy", It's ok to clip a few cones if you're still on the podium
    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

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