917 talk

Discussion in 'Real Racing Discussion' started by mantasisg, Aug 6, 2021.

  1. mantasisg

    mantasisg Registered

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    Thats right, good observation. But not exact. I am not sure about static rideheights, I know that had not to be lower than 100mm. The teams did cheat on that back in the day, but thats another thing. I want to write about how rideheights changed dynamically and what did it tell about cars physics/setups.

    First of all, I suppose there might have been differences between K and LH rideheights, might have had slightly different rakes, or perhaps it also could have been visual illusion due to bodywork, 917K was neutral, but that white LH does look like it might have had little bit rake, with nose slightly lower. But that is not all.

    First of all, different suspension setup. I think it could be told from this pic that #20 had much softer rear suspension, most likely to maximize rear traction. Well it also obviously accelerates there more aggressively, but it also might do it because it has setup that allows it better comparing to others. The LH looks neutral, but it is hard to tell looking purely from front angle at it. Speed is minimal there, so of course aerodynamics aren't doing anything significant there.
    [​IMG]

    There speed is higher, aerodynamics have some effect. But rideheights looks very similar, with K perhaps having a bit higher front.
    [​IMG]

    There they are at high speed where aerodynamics are very significant. Obviously K has rearwards aero pressure distribution and LH has rearwards aero distribution. While downforce/lift combination can influence this backwards or forwards pitch, the drag also could influence the pitch, especially for 917K. Aero loads distribution combining with soft suspension creates these effects.
    [​IMG]

    This one I included because it gives impression of LH having a bit of rake. Also it shows that #20 and #21 has different amounts of squat, obviously they must have used a bit different stiffness suspension setups. The same can be seen in the movie with Ferraris, one of them had insane amount of rideheight rise in the straight, at least at front end for sure, while others had less. Drivers in the teams simply used slightly different aerodynamic bits and in general - setups. I remember Gurney told in the interview about 1967 GT40MK2, that they would keep on tweaking rear spoiler till they were able to take Muslanne kink flat out. I wish Lemans would have never added chicanes in the straight, it kind of ruined this importance of lowdrag, which has been missign so much in motorsports since. Motorsports are ruled by aerodynamicists.
    [​IMG]

    Anyway, sorry to bother you guys. I think it is just some interesting thing to talk about.
     
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  2. mantasisg

    mantasisg Registered

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    So you work on these cars, and talking about them doesn't fit this topic ? Alright, perhaps I will open a new topic at some time.
     
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  3. mantasisg

    mantasisg Registered

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    I know I am not welcome here, but I am not less fascinated by the subject than you guys. I disagree that assumptions can't be done, but I agree that you can't simply confirm things like that, and arguing about these details is natural and healthy, if I am wrong and I'll realize it that is a win for me so keep challenging me. However, IMO I am not doing anything wrong even if I am incorrect by some chance, closely observing reality is a must if we are here to simulate it. Few pictures are indeed not enough to be certain, but there are many many pictures of these cars racing, most photographed race car ever, there are also motion pictures also known as movies, and they contains lots of information about cars once you start noticing patterns of things they do. It is all like a giant puzzle, pieces can be found in various places.

    As for "specific places". Most of things cars are doing is repeatable in lots of different places, although there are specifics and surely should be taken in account, I don't think there is anything too specific in these photos I posted in last page. Especially when the cars are side by side in same location and in the same moment. Can't think of more obvious opportunity to do a comparison.

    Accelerating, braking, even gear shifts must be taken into account when judging chassis pitch angles, I agree and I think about it when I look at still images. The picture of K having front UP and LH having nose down is not from braking zone though, it is from the straight, same thing can be seen in a movie. It is there an so obvious that it isn't just my assumption or opinion, it is a clearly observable fact.

    Accelerating from Virage De Mulsanne is a bit more tricky, because they could be accelerating differently there, or could make a gear shift at the moment of photography being taken, but I don't think that is the issue, plus I have seen the movie, and I'll look at start sequence once again right now.
     
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  4. mantasisg

    mantasisg Registered

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    Hey thanks for this great effort moderators, I'll move my last post myself then. Obviously this isn't any kind of censoring because Apex team doesn't like any kind of observations and interest about real thing, I wonder why.

    The post:

    @graphicaluserinterface
    I know I am not welcome here, but I am not less fascinated by the subject than you guys. I disagree that assumptions can't be done, but I agree that you can't simply confirm things like that, and arguing about these details is natural and healthy, if I am wrong and I'll realize it that is a win for me so keep challenging me. However, IMO I am not doing anything wrong even if I am incorrect by some chance, closely observing reality is a must if we are here to simulate it. Few pictures are indeed not enough to be certain, but there are many many pictures of these cars racing, most photographed race car ever, there are also motion pictures also known as movies, and they contains lots of information about cars once you start noticing patterns of things they do. It is all like a giant puzzle, pieces can be found in various places.

    As for "specific places". Most of things cars are doing is repeatable in lots of different places, although there are specifics and surely should be taken in account, I don't think there is anything too specific in these photos I posted in last page. Especially when the cars are side by side in same location and in the same moment. Can't think of more obvious opportunity to do a comparison.

    Accelerating, braking, even gear shifts must be taken into account when judging chassis pitch angles, I agree and I think about it when I look at still images. The picture of K having front UP and LH having nose down is not from braking zone though, it is from the straight, same thing can be seen in a movie. It is there an so obvious that it isn't just my assumption or opinion, it is a clearly observable fact.

    Accelerating from Virage De Mulsanne is a bit more tricky, because they could be accelerating differently there, or could make a gear shift at the moment of photography being taken, but I don't think that is the issue, plus I have seen the movie, and I'll look at start sequence once again right now.

    Between us, lets be friendly.
     
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  5. John R Denman

    John R Denman Registered

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    No promises, but I'll see if any new on-board video is produced at Monterey next week. There were some good ones from a few years back at the Rennsport Reunion.
    ROLEX MONTEREY MOTORSPORTS REUNION
     
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  6. mantasisg

    mantasisg Registered

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    Wise words, I chose to be "wrong" in perspective of others unless I'll get proved and argued well enough to becoming certain about being wrong. Otherwise there is no reason to stop believing yourself. You have to become certain that it is right to change. I feel the opposite urge when opposing side doesn't even try to argue, and just try to silence and ignore you. Which is happening.

    To the topic:
    Yes I wish there were onboards from the years it was racing. But there isn't. There are some bits of modern onboards, but they are either not on the limit, or they arguably run a bit better tires than they used to back then. Anyway, even if there were many oboards, I'd still look for as much external footage and I'll look at photos too. They are as valuable, or even more valuable than onboards to be used as a reference for making physics or judging realism of physics. In my opinion denying that and trying to put it as off topic from talking about simulated car itself in its own thread is nothing else than destroying a bridge between reality and simulation, and whoever does this should be ashamed. This is not good.

    Pictures might have lack of information, but on the other hand they are moments frozen in time which effectively leads to closest and most accurate possible observation. Although same could be told about stopped frames of video, if film is good quality. I enjoy so much looking at videos and pictures of this car, because it is such a dynamic car so much is going on with it. Being possibly the most legendary racing car ever, it deserves the best and most honest treatment and attention.
     
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  7. Art_Pereira

    Art_Pereira Registered

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    I think that the rules stated that the 917s should have a minimum static ride height of about 10cm/+-3.5" measured right after the driver´s seat.
    Also in the book "Porsche 917 - The Undercover Story" by Gordon Wingrove are shown the setup sheets for the 1971 race at Spa, for instance, and it says clearly:

    Suspension: Ride Height front 3.75" Rear 4.50" 20200526_182433.jpg 20200526_182447.jpg
     
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  8. mantasisg

    mantasisg Registered

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    Thank you, this is some fantastic material of data. I'll have great time studying it. It will also be interesting to compare with pictures, videos and then with simulation. Just to make a note, I believe classic SPA by ISI is not quite exact, I think it is slightly slower than it should be. At least all cars I have worked who drove there, shows consistently similar results in most tracks available to compare, and then most laptimes for SPA are too slow. Worth mentioning that in 1971 it was probably faster than in 1966, but I believe there might also be some layout inaccuracies, lack of some spacing somewhere, perhaps a bit of camber lacking, maybe some bump is wrong at some place... I don't know, just my opinion...

    Just few illustrations for joy of looking at them:
    [​IMG]
    Plenty of dive, I guess blue springs in the data sheets means soft. Although softness of rear springs also influence amount of dive.

    [​IMG]
    Don't know who is more brave drivers, or marshal. Interestingly not much body roll. I suppose rather stiff ARBs, although it could be set towards exiting the curve already. ARBs beams widths are important, but also their attachment points of how much leverage is working at twisting them, which actually seems to be written in the data sheets above.
     
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  9. davehenrie

    davehenrie Registered

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    Ok the last race photo in Manta's collection above shows what I was seeing. It took Art's setup sheets to open my eyes. Look at the photo right above, the 21 clearly has plenty of space below the floor. But what I was seeing in the movie was the white #25 WAS using a chin spoiler.(holy Nascar Batman!) The ride hts were probably very close, but the addition of that aero device blocked the view and my EYES told me the car was running much lower.
     
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  10. Art_Pereira

    Art_Pereira Registered

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  11. John R Denman

    John R Denman Registered

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    Nice find on the post tech sheet.
    The 917 had Bilstein dampers, the ones I've seen on 917's were not adjustable. Of that era Bilstein had not developed tunable valving. That said I would prefer having damper settings on the mod.

    917's were typically softly sprung after learnings from chassis tube cracks (frame gas pressure loss). That may also account for the Bilstein vs Koni (adjustable) as stiff dampers also impact (literally) chassis stresses.

    The 917 suspension had just enough droop to the outboard end of suspension pivots (Control Arms) to have a small level of anti-roll.
    Anti-Rollbars may not look that beefy, but considering the weight, ultra low center of gravity, and some anti-roll built into the suspension the cars could stay pretty flat with high lateral loads. ARB's don't have dampers and the forces are spread out torsionally across the chassis. Some referred to the chassis as the 5th spring.

    The car was designed exclusively to win LeMans. Straight line speed defined aero as most vital asset.

    Aero had some interesting developments. There were various underbody changes made by different teams that were pretty effective but as the cars have the most value in original condition many have been restored to original. You can see the rectangles on either side behind the tires; before being restored these regions had modified upward bulges as raced but restored to flat. Also a number of cars had an "On/Off" mode for the front ARB added for rain setups, and later removed as they were not original. This car still has the holes for the lever mount.
    917-Psych-EngBay1.jpg

    50 some years later the 917 still stands out as perhaps the most remarkable piece of automotive art in history.
     
  12. Art_Pereira

    Art_Pereira Registered

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    From the same book, maps made by Porsche´s computers showing how the 917s should run at both Spa and LeMans. Notice that at LeMans they used the 5L engines and 5th gear, while at Spa 4.9L engines and 4th gear.
    The last picture shows race statistics based on 1970 race I guess Porsche used to plan the 1971 race.
    There are also 2 pages with details of the fuel, brakes and tires wear for the 927 042 driven by Elford/Larrousse and for the 917/20/1 driven by Kauhsen/Jöst. They are in german, but if you think they can be useful, I can upload them too.
     
  13. mantasisg

    mantasisg Registered

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    I agree car should have adjustable dampers. I'd just assume it could be interpreted as literally putting on different dampers, rather than having one that is adjustable. Anyway nice to know details like that.

    As for roll, surely they could stay pretty flat apparently, but obviously they could have been set up to roll a lot. Like can be seen there:
    [​IMG] [​IMG] [​IMG] [​IMG] [​IMG] [​IMG]

    I've just looked up for images with more significant body roll, images like that are less common that images with this car having minimal body roll.

    I have also seen some pics out there where this car had inside front wheel up, typical for many sixties racing cars, so it must have been possible to make rear roll quite a lot relatively to front.
     
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  14. John R Denman

    John R Denman Registered

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    Different setups for different tracks.
    Spa and LeMans tend to favor stiffer bars. Tracks with crucial low gear turn exits favor softer ARB's to reduce inside wheelspin and normally those type of tracks come with brake flat spotting "enhancements" which also favor softer ARB's. The same principals apply to the mod FWIW.

    As I recall when they were set up for the Daytona exhibition race the rear was raised a bit to compensate for the banking compression, otherwise the rear would get really twitchy on the banking. Control Arms are short so as the body rolls the tire camber angle changes much more quickly. Older bias ply tires were much more forgiving with dynamic camber deltas than radials although the original tires were designed specifically to the car. The Avon replacements have reported to have more grip, more reserve at the limit, and far shorter life on the track despite being driven less aggressively.
     
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  15. ThomasJohansen

    ThomasJohansen Registered

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    Even though this is only replica, its quite interesting.
     
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