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Thread: torsional stiffness

  1. #11
    Breaking down a 15 Kg monocoque
    FRH ~ 1 kg
    MRH + bracing +fasteners ~ 4 kg
    Hardpoints ~2 kg
    AL AI plate ~1 kg

    That leaves ~ 7 kg for the carbon, core, and glue sheet.

    My rough hand calcs say the minimum surface area for a small monocoque is ~3 m^2. Aerial mass for Toray T800 is ~.325 kg/m^2. Aerial mass for glue sheet is .15 kg/m^2. Aerial mass for 3/4" thick 3.1 lb/ft^3 Al honeycomb core is .95 kg/m^2

    So for a 3 m^2 monocoque
    Core mass = .95 kg^2 * 3m^2 = 2.85 kg
    Glue sheet mass = .15 kg/m^2 * 3m^2 *2 (need glue sheet on top and bottom of core) = .9 kg
    Carbon mass = 7kg - 2.85kg - .9 kg = 3.25 kg

    How many layers of carbon is that? 1/(.325 kg/m^2 * 3 m^2 / 3.25 kg) = 3.33 layers.

    3.33 layers is not possible. That is saying your layup is 1.3c2. This layup would not pass the SES nor give a torsional stiffness of 6000Nm/deg.

    A global 4c4 layup (8 layers total) is on the light side with the current SES (average for different zones). Using the same calcs from above, a 4c4 layup would equal 7.8 kg in carbon and would result in a total chassis mass of 19.55 kg.

    Again, thats assuming a team is using the minimum surface area chassis. Most teams are not (for various reasons), and are probably using heavier core and layups thus leading to the 22-27 kg range.

    Its possible that teams are telling you the bare chassis mass only. IIRC when the design spec sheet asks for chassis mass it does not say if that includes the roll hoops or AI plate (a 6kg difference).
    Last edited by dr. ill; 07-04-2017 at 12:29 PM.

  2. #12
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    Quote Originally Posted by Karam El Hady View Post
    ... i'd like to know how is torsional stiffness is good for chassis and how is it related to suspension force...
    Study this paper, http://papers.sae.org/2002-01-3300/ Then study it some more, and read it several times through. Once you understand the paper you can start to do your own thinking and develop targets for *your* car. Keep notes so you can describe your design process at competition.

  3. #13
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    Quote Originally Posted by dr. ill View Post
    Breaking down a 15 Kg monocoque
    FRH ~ 1 kg
    MRH + bracing +fasteners ~ 4 kg
    ...
    My rough hand calcs say the minimum surface area for a small monocoque is ~3 m^2. Aerial mass for Toray T800 is ~.325 kg/m^2. Aerial mass for glue sheet is .15 kg/m^2. Aerial mass for 3/4" thick 3.1 lb/ft^3 Al honeycomb core is .95 kg/m^2
    ...
    A global 4c4 layup (8 layers total) is on the light side with the current SES (average for different zones). Using the same calcs from above, a 4c4 layup would equal 7.8 kg in carbon and would result in a total chassis mass of 19.55 kg.
    ...
    dr. ill,

    Thank you for your objective analysis. NUMBERS ARE GOOD!!!

    For rough calcs I usually start with a pessimistic/realistic ~7+ kg for the mandatory two-hoops + bracing. I guess you have given optimistic numbers to see if Claude's "15 kg" figure is achievable. I doubt that it is, and such a mass is not really a "logical" goal given that reliability is so important for success. (And note that reliability is more about strength than stiffness.)

    However, one way in which your mass-numbers can be reduced, and/or chassis-strength increased, is by realising that ... "MONOCOQUE" DOES NOT MEAN "HONEYCOMB"!

    The typical "honeycomb-sandwich" construction used by most FSAE teams for their "tubs" is, for the most part, completely UNNECESSARY. This applies to both the AL-AL and CF-AL (or CF-Nomex) honeycomb versions of such tubs. By the very meaning of the word, a "monocoque" (= "one shell") structure carries the majority of its imposed loads via stresses that lie IN THE PLANE of the shell. Such shell-like structures need very little bending-stiffness of the shell-material itself.

    Put simply, the 1.25 kg/m^2 you quoted for the Al-core+2*glue-sheet, is DEAD-WEIGHT. It is an utter waste of mass (and time, money, sticky-fingers++) because it contributes almost nothing to the REAL strength or stiffness of the monocoque.

    So a combined "4c4" CF-honeycomb mass of 3.85 kg/m^2 can be reduced by ~33%, down to 2.6 kg/m^2 of "solid" CF-shell, which would be about 2 to 3 mm thick. So the complete tub (less hoops, etc.) drops from ~12 kg down to ~8 kg. Or the tub stays at ~12 kg (with more plies of CF), but is 50% stronger and stiffer.

    At this stage there are legions of FSAEers crying "The gibbering old-fart has lost it again! IT'LL NEVER WORK" This, despite the fact that they all drive around in just such "monocoque" structures, albeit with skin thicknesses of less than 0.5 mm! Ahhh..., will they ever open their eyes?

    Anyway, local reinforcement of point loads applied normal to the shell (eg. wishbone mounts) can be done with CF-"top hat" sections, either incorporated in the middle of the ~8 plies, or glued inside or outside the shell. The sides of the cockpit (ie. the "SIS") can be similarly reinforced. I would do a cockpit-upper-side "P"-section that is designed (and tested!) to carry all side-impact load. As such, this strengthening of the cockpit-rim would greatly improve overall torsional stiffness and strength.

    As I have suggested many times before, similar "true monocoques" can be done in Aluminium (using 1 to 3 mm thick Al-sheet, as used on many small boats), Steel (I would use mostly the super-cheap 0.6 mm thick "zincalume" used for roof-flashing here in Oz), or Plywood (2 to 10 mm thick marine or aircraft-ply). All these would be much quicker and cheaper to do than anything with a honeycomb-core, and have at least as good performance.

    To repeat, the shell-material of a true monocoque DOES NOT NEED A SQUASHY CORE! Using such is blindly following the flock. It surely ain't engineering!
    ~o0o~

    Last teeny-weeny rant. Having a target torsional stiffness of ~8 kN.m/deg (or more?) is highly ILLOGICAL. I am not sure if Doug's linked ~15+ year old paper covers it, but this issue has been discussed on this forum for ages.

    In short, aim for,
    Chassis-torsional-stiffness = N x Suspension-torsional-stiffness.

    Choose "N" such that small tuning adjustments to the suspension (ie. to TLLTD) are NOT swamped by chassis "floppiness". N = 10 is a good enough round number to start with. Note that the softer the suspension, then the more torsionally flexible the chassis can be, while still being "tuneable".

    Z
    Last edited by Z; 07-05-2017 at 10:07 PM. Reason: Added more shouting. :)

  4. #14
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    Quote Originally Posted by Z View Post
    ... I am not sure if Doug's linked ~15+ year old paper covers it, but this issue has been discussed on this forum for ages.
    Yep, the paper I referenced talks about that and more. Here's another one that's a bit older, http://papers.sae.org/2000-01-3554/
    Follow the links to read the abstracts. If you really like reading posts, search this forum with the paper numbers, plenty of hits.

  5. #15
    Quote Originally Posted by Z View Post
    dr. ill,
    The typical "honeycomb-sandwich" construction used by most FSAE teams for their "tubs" is, for the most part, completely UNNECESSARY. This applies to both the AL-AL and CF-AL (or CF-Nomex) honeycomb versions of such tubs. By the very meaning of the word, a "monocoque" (= "one shell") structure carries the majority of its imposed loads via stresses that lie IN THE PLANE of the shell. Such shell-like structures need very little bending-stiffness of the shell-material itself.
    I fully agree. The monocoques are full of stuff not adding anything.
    The SES must take a lot of blame for poor creativity in this area. You very quickly run into problems if you try to be clever.
    Your design is perfectly reasonable, and you could maybe reach the targets set by Claude, but I think the hardest part would be to make it compliant with the SES.
    Everything is based on out of plane bending tests, and the only recognized failure mode is tensile. Which means an automatic advantage to sandwich structure with relatively thin and dense cores.
    Alumnus Revolve NTNU

  6. #16
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    Quote Originally Posted by Z View Post
    The typical "honeycomb-sandwich" construction used by most FSAE teams for their "tubs" is, for the most part, completely UNNECESSARY. This applies to both the AL-AL and CF-AL (or CF-Nomex) honeycomb versions of such tubs. By the very meaning of the word, a "monocoque" (= "one shell") structure carries the majority of its imposed loads via stresses that lie IN THE PLANE of the shell. Such shell-like structures need very little bending-stiffness of the shell-material itself.
    {snip}
    Anyway, local reinforcement of point loads applied normal to the shell (eg. wishbone mounts) can be done with CF-"top hat" sections, either incorporated in the middle of the ~8 plies, or glued inside or outside the shell. The sides of the cockpit (ie. the "SIS") can be similarly reinforced. I would do a cockpit-upper-side "P"-section that is designed (and tested!) to carry all side-impact load. As such, this strengthening of the cockpit-rim would greatly improve overall torsional stiffness and strength.

    As I have suggested many times before, similar "true monocoques" can be done in Aluminium (using 1 to 3 mm thick Al-sheet, as used on many small boats), Steel (I would use mostly the super-cheap 0.6 mm thick "zincalume" used for roof-flashing here in Oz), or Plywood (2 to 10 mm thick marine or aircraft-ply). All these would be much quicker and cheaper to do than anything with a honeycomb-core, and have at least as good performance.

    To repeat, the shell-material of a true monocoque DOES NOT NEED A SQUASHY CORE! Using such is blindly following the flock. It surely ain't engineering!
    ~o0o~

    Z
    You mean build a racecar monocoque the same way they have been building airplane monocoques for decades? BLASPHEME!
    Last edited by Marshall Grice; 07-07-2017 at 11:31 AM.
    '01-'06 Cal Poly Pomona

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