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Thread: 2015 Formula Student Germany

  1. #51
    Z,

    I'm not going to get to everything in your previous post, we've already covered a lot of it in previous posts.
    I will say that when I said 'I also doubt that [that is possible]', I meant the 2.5g launch, not necessarily the super fast acceleration times. Given the right surface, who knows, although those last 3 tenths will be hard won (and my bet is still on 4WD electrics).
    I'll also say, if it wasn't obvious from context, that the blue A-T curve I posted corresponds to a run of exactly 3.30s. (the red one that's taken from your V-T diagram actually takes a car from 0.3m to 75.3m in 3.15s, so it's not completely apples to apples, the final speeds were very close though, 31 vs 30m/s, taken from Zürichs run and your graph respectively).
    I do think that A-T diagrams are interesting, as they say a lot more about what forces act on the car at any given time, and you can differentiate much clearer between the theoretical runs you and I proposed.
    V-T diagrams tend to look pretty similar pretty quickly.

    We're in agreement on most of the mechanics, and the usefulness of various tools.
    However, one of the most effective Toothless Hillbilly tricks (dramatically increasing mu by putting glue on the track, https://www.youtube.com/watch?v=1VMb0fmalqo, who needs Fz if you have that) is unfortunately not transferable to FSAE.
    I think that means we won't see 2.5g launches, no matter how much dynamic-R% you manage. Electric 4WD's already put 100% of their Fz on the driven wheels. Transferring a larger portion of that to the rear axle shouldn't all of a sudden get you from 1.5g to 2.5g, especially if you take tire load sensitivity into account. I don't we're going to agree on how much of that remaining 1g can be bridged by the dynamic effects of pitch inertia on cars lifting their front wheels.

    I'll be on the lookout for blackberry bushes, thanks for your encouraging words on that topic.

    Thijs


    edit: I just found these strange Brits:
    http://www.fwddragseries.co.uk/index.php
    http://www.fwddragtimes.co.uk/
    https://www.youtube.com/watch?v=kFTZ...ature=youtu.be
    They have lots of fun doing drag runs in their 1100hp FWD cars, averaging 1,7+g over the first 20 meters, suggesting to me that raw mechanical grip has as much to do with why dragsters accelerate faster than FSAE cars, as does how you load your driven axle.
    Last edited by Thijs; 08-08-2015 at 12:57 PM. Reason: found some British Hilbillies
    Alumnus
    Formula Student Team Delft

    2007 - 2008: Powertrain, Suspension
    2009: Technical Lead
    2010 - present: Grumpy Old Fart/Concerned Citizen

  2. #52
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    Thijs,

    It is entirely possible to have a vertical force on the rear tyre that substantially exceeds 100% of the vehicles weight without aerodynamics. Unfortunately if we model a vehicle as a point mass, and exclude certain suspension setups then you are incorrectly lead to believing that a vehicles acceleration must be no greater than the mu of the tyre times gravity.

    This is not true. Part of the peril of not appropriately searching the solution space.

    The only real argument against Z is the likelihood that this transient state can be maintained for a useful amount of time.

    Also the glue on the track at a drag car allows something above 4g. I would expect that drag cars on our surfaces (retuned) could pull 2.5g without the glue. I would also note that our combustion cars have a very underutilised system that is crucial for drag car tuning. A system that once well understood could make sure power delivery is beautifully smooth and no worse than than EV traction control.

    Kev

  3. #53
    Kevin,

    I understand the physics, and Z has talked about this at some length as well. The magnitude of the effect you talk about is however itself still also dependent on mu. On an ice floor, there will hardly be any load transfer, let alone lifting front wheels, and you won't really get to use pitch inertia to your advantage (or only for a very short time, if you put the CoG very high/just in front of the rear axle). It's much easier to make use of on a drag strip that has ridiculous mu to begin with.
    But yes, my main criticism is that with these tiny, short-wheelbased cars, I doubt that this state can be maintained for a long enough period of time, like you yourself indicate might be an issue.

    I'm sure those massive, soft (and pre-burned-out!) drag tires are good for something, so I too have no doubt that an actual drag racer could come a lot closer to 2.5g on an FSAE track than an FSAE car.

    Thijs
    Last edited by Thijs; 08-08-2015 at 12:17 PM.
    Alumnus
    Formula Student Team Delft

    2007 - 2008: Powertrain, Suspension
    2009: Technical Lead
    2010 - present: Grumpy Old Fart/Concerned Citizen

  4. #54
    Quote Originally Posted by Kevin Hayward View Post
    Thijs,

    It is entirely possible to have a vertical force on the rear tyre that substantially exceeds 100% of the vehicles weight without aerodynamics. Unfortunately if we model a vehicle as a point mass, and exclude certain suspension setups then you are incorrectly lead to believing that a vehicles acceleration must be no greater than the mu of the tyre times gravity.

    This is not true. Part of the peril of not appropriately searching the solution space.

    The only real argument against Z is the likelihood that this transient state can be maintained for a useful amount of time.

    Also the glue on the track at a drag car allows something above 4g. I would expect that drag cars on our surfaces (retuned) could pull 2.5g without the glue. I would also note that our combustion cars have a very underutilised system that is crucial for drag car tuning. A system that once well understood could make sure power delivery is beautifully smooth and no worse than than EV traction control.

    Kev
    +1 on your notes about getting more than 100% weight on the rear tires and the simulations... Regarding your last sentence, are we to expect something really unique (for FSAE) regarding, ehm "clutching devices"?

  5. #55
    Even I am now convinced you can go faster than 3.30 (maybe place the light beam 5 cm short , wasn't the case at FSG though. It had been remeasured at days start).


    So ... FSA ... Zurich ... please decide this once and for all?
    Last edited by BeunMan; 08-08-2015 at 03:16 PM. Reason: comments
    Tristan
    Delft '09 Team member, '10 - Chief Electronics
    'now' (Hardware) Security Engineer

  6. #56
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    Quote Originally Posted by Z View Post

    Now, Thijs, thank you for that YouTube link, ... because it shows how PATHETICALLY PISS-POOR ALL FS-Teams' cars are! (See, that's how you do Grumpy! )

    Z
    Don't be ridiculous. Those starts are chosen for the Video, because they look more spectacular then a perfect start and this video does by no mean show an average of the FS starter field.


    Quote Originally Posted by Z View Post
    So, first make sure that ALL OF THE CAR'S Fz IS ON THE DRIVING WHEELS
    Z
    Yeah nice idea, but for a rwd car not what you want if the car has to run other dynamic events in similar spec.

  7. #57
    Quote Originally Posted by Kevin Hayward View Post
    It is entirely possible to have a vertical force on the rear tyre that substantially exceeds 100% of the vehicles weight without aerodynamics. Unfortunately if we model a vehicle as a point mass, and exclude certain suspension setups then you are incorrectly lead to believing that a vehicles acceleration must be no greater than the mu of the tyre times gravity.
    Forgive my silly question, I'm having a hard time wrapping my head around how anti-squat works on a car with a-arms, can engine torque in an a-arm car actually compress the suspension (neglecting weight transfer, of course)?

    On a solid axle 3-, 4- or 5-link car the links can be positioned so when the pinion tries to climb the ring gear and the axle housing tries to rotate, the links cause the tires to be shoved into the ground, increasing vertical load. The part about an a-arm car that's tripping me up is the engine torque trying to rotate the differential is captured by the chassis, and the resulting torque at the wheels is delivered in a way that it can neither compress the suspension nor force the tires into the track surface.

    Any thoughts would be helpful, this could be an interesting discussion.

  8. #58
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    [Ominous rumbling...] ... Where to begin this rant... Stupid students? Stupid Officials (and Grumpy Old Farts)? STUPID MODERN WORLD!!!?

    Hmmm... let's start with...
    ~o0o~

    Julian,

    http://www.fsae.com/forums/attachmen...8&d=1439025242

    Thank you for your V-T graph (above) of Zurich's recent 3.300 second FSG-Acceleration run.

    I printed out your graph and started to apply some old-school analysis to it. This started with me making the following ASSUMPTIONS.
    1. The accuracy of the data is of similar order to the thickness of the lines used to depict the data.
    2. The horizontal axis (Time [s]) and vertical axis (Speed [kph]) both vary "linearly".
    3. A Swiss "kilometre" is one thousand metres long.
    4. A Swiss "hour" has 3,600 seconds in it.
    5. Zurich's measured Acceleration run was from the Starting Line at T = 0.220 seconds to a time 3.300 seconds later, namely T = 3.520 s, and presumably in a reasonably straight line.

    From above, I deduced that a Speed on your graph of 36 kph should also equal 10 metres/second, and so on. I also deduced that the "area under" the Speed curve, between a vertical line at the Start Time = 0.220 s and another vertical line at the presumed Finish Time = 3.520 s (and, of course, above the horizontal axis), should equal the Distance travelled during the Acceleration run, because Distance = Speed x Time.

    So I graphically measured that area, and found it to be ... [dramatic drum roll] ...
    ...
    LESS THAN 72 METRES!!!

    (In fact, I have a reasonably accurate 71.6 metres.)

    I decided to make another cup of coffee and move on...
    ~o0o~

    Thijs,

    Very briefly, the ~1.7 G accelerations of Front-WD drag cars, together with the well known 4+ G accelerations of Rear-WD drag cars on the same sticky tracks, CONFIRMS everything I have been saying. But no point me presenting the calculations here (does anyone read them?), although you might try doing them...

    Also, "pitch inertia" is NOT IMPORTANT for good launches (as I explained last year). When I say that RWD-cars should lift their noses off the ground, that is just to indicate that all weight is indeed transferred to the rear-wheels. For really good launches, the tail of RWD-cars should also lift upwards, preferably even higher than the nose!
    ~o0o~

    RenM,

    I am quite sure that video was made to show just how "awesomely fast" all the FS cars are. And that very same thinking is why 30 year's worth of students have been designing and building their cars to do exactly that same thing. Namely, spend ages spinning their rear-wheels, ... while going NOWHERE fast. (And did you see the wheel-wobble at 1:08!!!?)

    Yeah nice idea, but for a rwd car not what you want if the car has to run other dynamic events in similar spec.
    As I have explained endlessly now, an FS car only needs about 60%R (or maybe 66%R if it has extremely low CG), PLUS some simple set-up adjustments, to do really fast Acceleration times. The modern-era Bowlby DeltaWing runs ~72%R as standard, and is as fast as most other contemporary racecars that have twice the horsepower. Many ~1970-80s F1, F5000, etc., cars ran ~70%R, firstly because that was allowed back then, and secondly because it made them MUCH FASTER than the ~60%R cars! (Note that most modern Formulae very tightly constrain tyre sizes, thus preventing high R% and good acceleration, because that keeps the top speeds down.)

    You kids should stop making excuses for being so slow...
    ~o0o~

    Tromoly,

    Again, I explained this all in the similar discussion last year, and many other places. Look up "longitudinal n-lines" and "jacking".

    Specifically, raise the wishbone-to-chassis front attachment points, and/or lower the rear ones, so that in side-view the wishbones pivot about an axis (on chassis) that slopes up-to-front. A lot! Start the car with very low ride-height, and then during launch let the wheels go a long way into droop (ie. so car lifts a lot). (Julian, you might advise AMZ to do this on all four wheels, but especially so on the rears.)

    It is also well worth fitting a large area undertray, preferably with front-splitter and side-skirts mounted to the uprights so they are always close to the ground, but with the majority of the undertray fixed to chassis so it rises up with it. Any sudden upward movement of the undertray will cause a large "suction" below it, for large "unsteady" aero-downforce. (It is the "dPhi/dt" term, which most of your Colourful Flow Diagrams DO NOT calculate!)
    ~o0o~

    I wonder if any young person will EVER do anything interesting...

    Z
    Last edited by Z; 08-10-2015 at 12:53 AM.

  9. #59
    Quote Originally Posted by tromoly View Post
    Forgive my silly question, I'm having a hard time wrapping my head around how anti-squat works on a car with a-arms, can engine torque in an a-arm car actually compress the suspension (neglecting weight transfer, of course)?

    On a solid axle 3-, 4- or 5-link car the links can be positioned so when the pinion tries to climb the ring gear and the axle housing tries to rotate, the links cause the tires to be shoved into the ground, increasing vertical load. The part about an a-arm car that's tripping me up is the engine torque trying to rotate the differential is captured by the chassis, and the resulting torque at the wheels is delivered in a way that it can neither compress the suspension nor force the tires into the track surface.

    Any thoughts would be helpful, this could be an interesting discussion.
    Z gave a deeper analysis of this in the past, I'll be brief...think how anti-dive works and now think this backwards on the rear of the car. It is all about reaction of tire force and n-lines.

    Quote Originally Posted by Z View Post
    It is also well worth fitting a large area undertray, preferably with front-splitter and side-skirts mounted to the uprights so they are always close to the ground, but with the majority of the undertray fixed to chassis so it rises up with it. Any sudden upward movement of the undertray will cause a large "suction" below it, for large "unsteady" aero-downforce. (It is the "dPhi/dt" term, which most of your Colourful Flow Diagrams DO NOT calculate!)
    Z
    I wonder how this would work under braking or in any other dynamic event except acceleration...
    Last edited by mech5496; 08-10-2015 at 03:56 AM.

  10. #60
    After an absence of some years I went to this forum to read up on some FSG comments.

    To my uttermost delight I find here exactly the same conversation as there has been for many years with the same Z trolling about, and the same people, yes that's you Thijs & Julian, wasting their time to argue with him . I wonder where Bemo is at, normally he also would be contributing...

    I also wonder where Z would be in life if he would dedicate his energy on actually doing this instead of talking about it; there is a guiness book of world records you know... With the (E-)acceleration times getting increasingly difficult to beat (or so we think), surely your expertise in this field would be highly valued. I cannot imagine how busy you must be in enlightning racing teams all over the world, your phone must be red-hot.

    I think the only thing we really get out of this discussion in the longterm will be the actual proof that the Godwin principle does not apply for ALL internet discussions

    ---
    Stef de Jong
    Delft
    Last edited by Stef de Jong; 08-10-2015 at 05:35 AM.

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