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Thread: A Arm Load Calculations

  1. #31

    Is there such a thing as too much compliance for a race car?

    Sid,

    You are already "polluted" by the passenger car industry where compliance is a necessity for ride, comfort, noise decrease and simply reliability: car manufacturers could not warranty a car with no bushing and just rod ends for 100,000 miles. Uprights and chassis would simply fall apart after just a few hundreds miles.
    On a race car if you notice a bit of play in a pushrod rod end you just change. Or if yo have a good car race car like a Dallara there will be a user manual telling you after how many km what rod end / bearing etc... has to be replaced.

    You need to put my comments in the context of race cars and/or this forum and FSAE / FS where I have been judging for many years and where I see every year real s**t boxes where compliance is awful, make the car undriveable and wasn't clearly a part of the design process.

    Before even deciding if they have too much or not enough compliance (if there is such a thing as "too much compliance" for a race car), students should at least give themselves (and later on design judges) basic numbers in simulation and in workshop measurements like the ones in the attached spreadsheet.
    The reality is that 95 % of them don't even know what I am speaking about!
    And I am not even speaking about combined efforts (like Fy and Fx and Fz and Mz and Mx all together) or non linear numbers; just basic numbers.

    I have been in racing for 40 years and I never heard any race car designer or race car engineer complaining about too much compliance! In fact every year, n very new car, they try to reduce it and most of the time they succeed.
    Of course there are compromise to make with cost, weight etc... but there isn't such a thing as too much compliance!
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    Claude Rouelle
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    Vehicle Dynamics & Race Car Engineering
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    [url]www.optimumg.com[/u

  2. #32
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    Claude,

    I never said 'increase' or 'decrease' compliance. All I said was compliance must first be understood before labeling it as good or bad (increase or decrease?).
    Which is why in my post I offered literature material that'll help students understand the why and how of compliance before they implement the 'what' of compliance (Simon Sinek anyone?)

    I think I did put my comment in the context of this forum and Formula Student in general, which is a student engineering competition.
    Sid Attravanam
    ----------------------------------
    Susension/Vehicle Dynamics, UTA FSAE Alum (2009-2012)
    Vehicle Dynamics, Cooper Tire & Rubber Company

  3. #33
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    Claude, I think the table you showed is a great way to visualize and communicate these factors efficiently. I know you just made it very quickly as an example, but I am wondering if it is more useful to present them in a more non-dimensional (or normalized) manner.
    For instance, some hypothetical amount of toe compliance presented in deg./kN could be a very good number for a sedan, but very poor for a Formula SAE car. Would be better to show it as, for instance, deg./G, or deg./percent of maximum capability?

    Sid, I think your comments came across very clearly, or at least I understood them as a way to encourage thinking about "why" before "what," as you (and Simon) say.
    Mountain Lion Motorsports

  4. #34
    Mikey,

    Of course it would be good to show the influence of compliance on (as I wrote earlier) on grip, balance, stability and control (on entry and at the limit), all other things (tires, springs weight distribution, etc...) being the same.
    Only that kind of analysis, validated by good on-track test drivers opinions will tell you what is a "low enough compliance" . That is what, as a design judge, I expect the students to do.

    Sid, we are on the same wave length.

    Worth to direct people to Simon Sinek again: whether it is engineering or non-engineering issues start with WHY. https://www.youtube.com/watch?v=u4ZoJKF_VuA
    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

  5. #35
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    Maxhouck,

    One question I still am not sure of an answer to is how much deflection/compliance is significant...
    Part of me says “well I know we can’t measure toe much more accurately than 1/16 of an inch, so maybe half that much compliance would be no big deal”. But ... we get this tolerance stack-up affect that could make things much worse...
    ...how much is acceptable?
    Here is one of my posts from 2005 giving a rough guide to "...how much is acceptable".

    http://www.fsae.com/forums/showthrea...ll=1#post92714

    Half way down this next post I give a rough guide on how to determine "the significance of things". (You can skip over the Gyro stuff.)

    http://www.fsae.com/forums/showthrea...l=1#post127273

    The reason I give "10%" as the starting point for "significant" is simply that ... we have ten fingers!

    Imagine you are helping tune the engine on the dyno, and you are in a rush, and there are spinning metal bits, and you accidentally get ALL your fingers chopped off. Bummer! This will have a major effect on your future lifestyle. You won't be able to brush your teeth, or wipe your backside, or do a whole lot of other everyday things. So this 100% loss of fingers is an OVERWHELMINGLY BAD influence on your performance.

    At the other extreme, imagine you just lose a bit of skin on the tip of one finger. A quick application of some antiseptic alcohol, taken internally of course, and a bandaid, and you are good to go. In a few weeks you will have forgotten what finger it was, and will have to look closely for the teeny little scar in order to tell the newbies about your heroic engine tuning efforts.

    The middle ground is when you lose one finger. Not a major hindrance to your performance, but annoying when, say, you want to count past nine.
    ~o0o~

    In VD-performance terms you might choose as "SIGNIFICANT", a change in toe-angle that generates 10% of the tyre's peak Fy force. This delta-toe-angle is different for different tyres, and varies according to where you are on the tyre-curve. But as round numbers I would suggest that 0.1 degrees is "starting to become significant", and 0.5 degrees is already "quite significant".

    Compare these numbers with, say, a sudden change in toe-angle of 5 to 10 degrees, which would reasonably be described as "overwhelming", because the sudden increase of Fy to its maximum would very likely spit the car off the track. At the other extreme a toe change of 0.01 degrees would probably have an immeasurable effect on Fy, given all the variables of grip coming from the track itself, such as dust, coarser or smoother gravel, and so on.

    So your suggested figure of a bit less than 1/16 inch, say 1 mm measured at rim (= ~0.2 degree), of total toe-compliance is a reasonable limit to aim for. A little bit more won't cripple you, in the same way that you can still function reasonably well with two fingers missing. But a lot more is generally BAD.

    And at the other extreme there is no point spending vast resources chasing 0.1 mm total toe-compliance (= ~0.02 degree), because that will NOT make your car "significantly" faster.
    ~~~o0o~~~

    Claude,

    I never heard any race car designer or race car engineer complaining about too much compliance!
    ... there isn't such a thing as too much compliance!
    For the sake of the students' understanding of this issue, did you not mean to say "...there isn't such a thing as too much stiffness"???

    Z
    Last edited by Z; 11-05-2017 at 05:29 PM. Reason: Clarity...

  6. #36
    Z, Thank you for correcting me: of course I meant "I never heard any race or race car car design engineer complaining of too much stiffness (or too little compliance". Must have been jet lag! Claude

  7. #37
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    Claude,

    I get them mixed up too.

    Z

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