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Thread: Overwhelming the Tire Carcass

  1. #1

    Overwhelming the Tire Carcass

    Hello Everyone,

    Since this is my first post I will introduce myself. My name is Jason Roberts, I'm a 5th year student at the University of Cincinnati, primarily responsible for vehicle dynamics.

    The question I would like to pose is one of validating carcass overload due to an excess of FZ. Let me elaborate.

    In the quest to score more points many teams are running more aero, for obvious reasons. However it seems as though even the top level teams which initially were more reserved on the appeared level of downforce are now beginning to up the anti even more. With initial CFD and physical testing complete on wings we have in house we've arrived at a CL of apprx 3.5 for the total package (front and rear wing only, this is the average value)). This is providing very significant levels of downforce at reasonable competition speeds. The question Im asking myself is, how much is too much?

    After diving into our MMM code, which utilizes the TTC data and working with the Pacejka Equations it seems like the 18x6x10x7 R25 B's begin to see negative affects on FY at any vertical load greater than around 2000 newtons per tire (tires at 14psi). This is a rather large assumption as the pajecka equations have to extrapolate to these values however Ill assume accuracy to be around 10%. Either way there will be a normal load in which the carcass will have exceeded its capability and the tire will begin to lose grip.

    So back to the statement of how much is too much? It seems as a novice aero team that many other teams would easily be able to match our levels of FZ generated by our aero devices. Many of whom are running the 10" r25b or lc0 which have much less cornering stiffness and vertical stiffness than the 13" tires. So why is it that we see teams like Michigan ann arbor and others putting together packages which seem to me like they would far exceed these FZ values? Im all for putting together more efficient packages by adding a diffuser and other components in order to try and lower the rear wing and perhaps make the elements lighter overall to reduce mass and inertia. But the all out approach seems a bit overkill. But what do I know, Ann arbor was deathly quick in Lincoln this year.

    Id be interested to see if any other teams have taken this into consideration during your design phases, if you have seen any adverse affects while testing out on track, and if you have found any way of validating these affects?

    As for us where setting this as a basic limit to be safe for now. On to coast down and other testing
    Jason Roberts
    Steeda Autosports
    Head of Product Development & Vehicle Dynamics

    Former
    Bearcat Motorsports, University of Cincinnati
    2015 Chief engineer, Lead Vehicle Dynamicist, Testing Development
    2011-2014 Member

  2. #2
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    In what situation for a current FSAE car do you believe that 2000N/tyre is reasonable? Assuming a reasonably typical 2g lateral, 250kg car/driver, 300mm CG and 1200mm track, we get something like 2500N on the loaded side or 1250N per tyre, unless I am mistaken. To be getting 2000N/tyre with this typical car would mean reaching something like 3.3g. If you're going to achieve that then I imagine you'd be running higher tyre pressures for a start (and therefore different test data)
    Jay

    UoW FSAE '07-'09

  3. #3
    I have seen a few rare blisters, some time huge tire graining but I do not remember having seen carcass rupture in FSAE/ FS competitions. You could have compound issue (temperature too low or too high) but I think you are still far way from what a usual FSAE / FS tire can handle in terns of Fz.

    By the way as an info for you today an LMP1 car has a efficiency of 7. With a drag coefficient of 0.45 that gives also a CL of about 3.5. I know that compared to a Le Mans car your wheels are not covered l but on the other hand the actual rules allow you much bigger wings than a LMP1 car. CL of 3.5 seems quite low to me for FSAE. That is probably because, as most other students, you are most essentially working on wings while the underwing (you mention the extractor/ diffuser) is something that could give you a lot of downforce for a a minimum of drag (again, even more with less restrictive rules than an LMP1)
    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
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    A couple of things that caught my eye. Firstly, if you look at the TTC data for the tire you mentionned, the loads stop at 1200 N ( I believe the max on TTC tests is 1600 but in the case of the 10 in tires the 5th load was reduced due to issues with minimum tire rolling radius). As such, extrapolating your data 800N over that is going to bring about some interesting results. This is due to the fact that Pacejka(BTW you should detail which one) are second order( ie Fy_max ~ dfz0 + dfz1 * (Fz-Fz0) + dfz2(Fz - Fz0)) . If the second order component(dfz2) is negative( even slightly) you'll start to see some interesting things as you get further away from your test matrix which aren't necessarily actually related to the tire. At this point it's all dependent on that second order value for the thing not to go bananas outside the test range. So your error on that component could be a lot higher than the 10% you've stated. I'd also look at the raw data vs model of FyMax vs Fz and take a look at trends. If you don't have as large a range as necessary, the fitting routine may be adding a second order component that's not really in the data. (all depending on how much you filter the data and and what boundaries you impose on dfz2(MATLAB's fitting routines usually allow for those limits although it's a pain to isolate the coefficients with that effect in a pacejka model) and what not.)

    As for checking these things out in real life, I'm fairly sure different sized skid pads would be able to give you an idea of the point at which the returns start to diminish a lot. (ie when does Ay max stop growing with speed). As Jay mentioned, different tire pressures could account for the size of those aero packages living up to expectation.

    Finally another thing to keep in mind is that most of these tires are made to run on F500s which have a minimum weight of 750lbs or such and most probably higher CGs than in FSAE. As such they probably see more max load than in formula SAE (despite the aero).

    All of this is also dependant on the speed at which you run your simulation. If you're running your sim at 100kph. you might get some weird results just because you've got more downforce than at your corner average 50kph.

    Food for thought rather than a definite answer but really cool first post on your part. Hope this helped to some extent.

    Now off to bed
    Paul Charbonneau
    Formule Polytechnique Montreal
    2009 - 2012 - Suspension Lead
    2012 - 2013 - Team Captain
    2013 - 2014 - Suspension and Electronics Lead

  5. #5
    Adding to these good replies: I wouldn't expect an FSAE tire to be overloaded at 2000N. Tire model extrapolation that far above the highest test load, however, can give inaccurate results. Assuming 10% accuracy in this range is very optimistic.
    Dr. Edward M. Kasprzak
    President: EMK Vehicle Dynamics, LLC
    Associate: Milliken Research Associates, Inc.
    Co-Director: FSAE Tire Test Consortium
    Lecturer: SAE Industrial Lecture Program
    FSAE Design Judge

  6. #6
    Thank you for the good input so far.

    Jay
    In what situation for a current FSAE car do you believe that 2000N/tyre is reasonable?
    This all came about in the "worst case secenario" situations that we have simulated to try and achieve reasonable maximum forces for our uprights to be able to withstand. (fx,fy,fx,mx,my,mz).

    I would agree with you that in a 2 G corner with 100KG of DF we similarly see approx 1250 &1400 N of normal force on the FR & RR Tires.

    However the case simulated was max comp speed and max G. So that being 320 KG of DF and 2.6G Cornering at about 100 kph( basically the max cornering capability of the vehicle on a skidpad where we can travel at max speed). This produces FZ's of 1900 and 2100 N on the FR and RR tires. Which are well within the trouble area for the carcass that I was curious about.

    Is that situation realistic. Meh, probably not as most of us slow down for corners , Average or max corner speed gathered from our MOTEC data would be more representative of the situation.

    After going back and iterating through the weight transfer spreadsheets we have, the max FZ during a competition setting a top team should see is 1800N on one tire even with a very aggressive aero package. This is mostly do to what would be considered "achievable" speeds during cornering on a competition course.

    This is simply something I wanted to try and take into account as we havent delved into the realm of this magnitude of force on our tires previously. Figured It would be good to cover all the bases. If Edward isnt worried about Carcass failure at 2000 N and we see a max of 1800 than it looks like we have one less item on our list to be concerned about. Not to mention I've never seen a failure of that type in Fsae previously.

    Still for curiosities sake it would be interesting to see what the actual FZ needed for carcass failure would be , but I digress.
    Jason Roberts
    Steeda Autosports
    Head of Product Development & Vehicle Dynamics

    Former
    Bearcat Motorsports, University of Cincinnati
    2015 Chief engineer, Lead Vehicle Dynamicist, Testing Development
    2011-2014 Member

  7. #7
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    > FZ needed for carcass failure

    In my (limited) experience, carcass failures are either from foreign object damage, or from long term overload which over heats the tire material (or some local part of the tire, for example, one shoulder). Short term overload does not cause problems unless it is so severe that the rim bottoms out or pinches the sidewall. Tires are tough, try sectioning (cutting up) a tire sometime...this is a worthwhile experience.

  8. #8
    Doug, I still have a busted wheel and tire I got from a 24 of Daytona race one year. The plan was to cut the tire off and keep the rim. Nope, belts are quite hardy things.
    20 minutes of fussing around with hand tools was enough for me to learn my lesson.
    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

  9. #9
    Given that these tires are basically SCCA tires I'd say you'd be hard pressed to come anywhere close to "overwhelming" one.

  10. #10
    Cool thread! I'm interested to hear from some more developed teams, especially those running 10's.

    My approach to solving the problem would be to look at a higher level question - how many points do you stand to lose by having uprights that are too beefy? Go with your worst case scenario, and remember that your driver is the biggest variable in the equation. Finishing endurance, and potentially recycling big ticket items year to year (like uprights) might be things that make this question easier to answer.

    From a VD standpoint, you'd be gaining unsprung weight, but at least it's down low. Not a huge show stopper IMO, as it's probably under a pound or two per corner.

    If I were you I'd play it safe and do as much testing as possible before comps once the car is done. Then if you've got the resources (time, money, know-how) make a new set of uprights! If you were really clever you might even design uprights that can be machined down to minimize cost..

    If you couldn't tell, I don't know a whole lot about aero

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