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  1. #401
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    Poor person's flow-viz. Take your pick.

    Water:


    Dirt (this one is really good):


    Snow (not through the engine bay, but still interesting):


    This year's splitter is helping a lot to keep water away from the inlet area in a watersplash. But as we saw in Monte, Sweden and other gravel rallies, it's also very good at scooping up dirt and snow. I'm looking forward to Spain. If we have gravel conditions anything like last year, it would be very interesting (and demanding).

  2. Likes: abxyz (20th July 2017),NickRally (27th July 2017),pantealex (20th July 2017),seb_sh (28th July 2017)
  3. #402
    Senior Member NickRally's Avatar
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    Watching Rally Finland SS1 and it is incredible to see how flexible the Citroen diffuser streaks are (topic discussed previously), flapping about a lot, good views at 17:17, 26:17 and 33:40.
    Last edited by NickRally; 28th July 2017 at 00:04.

  4. #403
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    Yeah, can't they be changed for different rallies? In a "smooth" gravel rally one could use stiffer ones without fear of breaking them.

  5. #404
    Senior Member NickRally's Avatar
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    Quote Originally Posted by mknight View Post
    Yeah, can't they be changed for different rallies? In a "smooth" gravel rally one could use stiffer ones without fear of breaking them.
    I wouldn't be surprised if the answer to this question is no due to the homologation regulations.

  6. Likes: Mrpengski (28th July 2017)
  7. #405
    Senior Member NickRally's Avatar
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    Hi boys and girls, haven’t been able to post in a very long time, but was able to enjoy Rally Germany and was happy to see that the doom and gloom predictions for a dull 2017 course proved to be very wide of the mark with some very challenging sections in the “country” lanes.
    The topic I would like to pick up now is the discussion about the airborne behavior of the Citroens. First let me say that I am in the camp that thinks the cause of the nose up attitude is in what happens prior and during the jump mechanically rather than aerodynamically, so it will appear as though I am biased in the “logic” below, but I will give it a go and try to be as objective as the facts (or lack of ) in my possession allow.
    What I did was to watch several times few on-boards and also the SS21 re-play around the jump in question (the jump just prior to the finish), while timing the events and noting the velocities and engine rpm involved. My primary objective was Breen’s car and as a result, and taking worst case scenario (worst case scenario from calculations point of view that is, otherwise most optimistic from point of view of supporting the idea that aero imbalance is the culprit for the nose ups), I would say the elapsed time from the start of the jump to the peak is around 0.4s-0.5s, so let’s say 0.5s (remember worst case for calcs).
    Unfortunately car speed and engine rpm data for Breen on SS21 was corrupted and no use, but looking at some of the other cars and taking worst case, I would say his speed at the point of the jump was 140kph. At the same time, re-playing the stage from the outside, I would say the max nose up angle in the air was around 7.5deg – this means that for the aforementioned 0.5s, the car has rotated 7.5deg (I know few assumptions involved, but that is unavoidable) – this would suggest angular acceleration of 60deg/s2 or using more appropriate term ~1.05rad/s2, let’s round this down to 1rad/s2. What I also did was to estimate the polar moment of inertia of the car around the horizontal transverse axis by doing some sketching and assuming 1375kg weight with around 2500mm-2550mm wheelbase – I reckon this number is around 1600kgxm2 (but maybe people with experience of modern rally cars like Mirek can correct me). The combination of angular acceleration and polar moment of inertia above, results in the requirement for 1600Nm of rolling moment. Now, assuming 50/50 weight distribution, that would require for the aero imbalance to be in the order of ~125kg minimum, i.e. at 140kph the rear axle aero load to be 125kg greater than the one acting on the front axle. Such aero imbalance for a roughly 50/50 weight distribution car, would make handling really impossible, but before anybody jumps and says “Citroen’s it obviously is”, it would make it impossible in the opposite way to what the drivers have been commenting (nervous rear). Nevertheless, let’s keep this thread - considering that at 140kph, the total downforce is unlikely to be more than, let’s say 225kg for simplicity’s sake (even that is probably gross overestimate and when running close to the road at that, i.e. not in the air), than the above imbalance requirement would mean front/rear aero load of 50kg/175kg, i.e. understeer nightmare of a rally car, which to my mind suggest that the combination of road profile, suspension settings and driver actions prior to the jump is more at play than aero forces. Also comparing Mikkelsen’s car in the same jump to Breen’s, it did not display any of the nose up characteristics, most likely because he braked heavily prior to the jump, something that Breen does not appear to have done. Also during the jump, Breen’s engine rpm first went significantly up, then he lifted which brought them down, which means engine inertia reaction moments will try to roll the car (people with knowledge can comment which way the crankshaft rotates to enable us resolve which way the car should roll when rpm goes up and down).
    Having said all that, any mechanically/driver induced rolling nose up motion will then result in the flat-ish floor of the car turning into massive lift generating device leading to even more nose up attitude. Of course it is also possible to argue that aero imbalance prior to the jump causes a small nose up roll, which then results in further roll when the car is in the air, but I am of the believe that such aero imbalance will be heavily felt by the driver handling-wise, while also in Citroen’s case, taking into account their issues of nervous rear, I reckon it is very unlikely they have hugely more rear than front downforce, especially considering the flappy diffuser strakes, which probably render the rear diffuser useless. And the other thing is that watching the on-boards from several cars at this jump, the drivers treated the approach differently, which then affects the behaviour in the air significantly.
    Thanks to anybody that had the patience to read this and apologies for any typos as I haven’t got much time to check it properly .

  8. Likes: drive (2nd September 2017),itix (8th September 2017),sonnybobiche (3rd September 2017)
  9. #406
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    Quote Originally Posted by NickRally View Post
    Hi boys and girls, haven’t been able to post in a very long time, but was able to enjoy Rally Germany and was happy to see that the doom and gloom predictions for a dull 2017 course proved to be very wide of the mark with some very challenging sections in the “country” lanes.
    The topic I would like to pick up now is the discussion about the airborne behavior of the Citroens. First let me say that I am in the camp that thinks the cause of the nose up attitude is in what happens prior and during the jump mechanically rather than aerodynamically, so it will appear as though I am biased in the “logic” below, but I will give it a go and try to be as objective as the facts (or lack of ) in my possession allow.
    What I did was to watch several times few on-boards and also the SS21 re-play around the jump in question (the jump just prior to the finish), while timing the events and noting the velocities and engine rpm involved. My primary objective was Breen’s car and as a result, and taking worst case scenario (worst case scenario from calculations point of view that is, otherwise most optimistic from point of view of supporting the idea that aero imbalance is the culprit for the nose ups), I would say the elapsed time from the start of the jump to the peak is around 0.4s-0.5s, so let’s say 0.5s (remember worst case for calcs).
    Unfortunately car speed and engine rpm data for Breen on SS21 was corrupted and no use, but looking at some of the other cars and taking worst case, I would say his speed at the point of the jump was 140kph. At the same time, re-playing the stage from the outside, I would say the max nose up angle in the air was around 7.5deg – this means that for the aforementioned 0.5s, the car has rotated 7.5deg (I know few assumptions involved, but that is unavoidable) – this would suggest angular acceleration of 60deg/s2 or using more appropriate term ~1.05rad/s2, let’s round this down to 1rad/s2. What I also did was to estimate the polar moment of inertia of the car around the horizontal transverse axis by doing some sketching and assuming 1375kg weight with around 2500mm-2550mm wheelbase – I reckon this number is around 1600kgxm2 (but maybe people with experience of modern rally cars like Mirek can correct me). The combination of angular acceleration and polar moment of inertia above, results in the requirement for 1600Nm of rolling moment. Now, assuming 50/50 weight distribution, that would require for the aero imbalance to be in the order of ~125kg minimum, i.e. at 140kph the rear axle aero load to be 125kg greater than the one acting on the front axle. Such aero imbalance for a roughly 50/50 weight distribution car, would make handling really impossible, but before anybody jumps and says “Citroen’s it obviously is”, it would make it impossible in the opposite way to what the drivers have been commenting (nervous rear). Nevertheless, let’s keep this thread - considering that at 140kph, the total downforce is unlikely to be more than, let’s say 225kg for simplicity’s sake (even that is probably gross overestimate and when running close to the road at that, i.e. not in the air), than the above imbalance requirement would mean front/rear aero load of 50kg/175kg, i.e. understeer nightmare of a rally car, which to my mind suggest that the combination of road profile, suspension settings and driver actions prior to the jump is more at play than aero forces. Also comparing Mikkelsen’s car in the same jump to Breen’s, it did not display any of the nose up characteristics, most likely because he braked heavily prior to the jump, something that Breen does not appear to have done. Also during the jump, Breen’s engine rpm first went significantly up, then he lifted which brought them down, which means engine inertia reaction moments will try to roll the car (people with knowledge can comment which way the crankshaft rotates to enable us resolve which way the car should roll when rpm goes up and down).
    Having said all that, any mechanically/driver induced rolling nose up motion will then result in the flat-ish floor of the car turning into massive lift generating device leading to even more nose up attitude. Of course it is also possible to argue that aero imbalance prior to the jump causes a small nose up roll, which then results in further roll when the car is in the air, but I am of the believe that such aero imbalance will be heavily felt by the driver handling-wise, while also in Citroen’s case, taking into account their issues of nervous rear, I reckon it is very unlikely they have hugely more rear than front downforce, especially considering the flappy diffuser strakes, which probably render the rear diffuser useless. And the other thing is that watching the on-boards from several cars at this jump, the drivers treated the approach differently, which then affects the behaviour in the air significantly.
    Thanks to anybody that had the patience to read this and apologies for any typos as I haven’t got much time to check it properly .
    Has nothing to do with aero more so with weight ballast placement.....

  10. #407
    Senior Member SubaruNorway's Avatar
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    Quote Originally Posted by logic View Post
    Has nothing to do with aero more so with weight ballast placement.....
    If it was that easy we wouldn't see it with three rounds left, it has everything to do with aero. And why don't see people so much on the rev limiter on big jumps because they need to compensate, it's the force of the wheels that controls the angel in the air not the crank really, same in Motocross. I can even feel it on my downhill bike if i look the rear brake mid air it tips forward.
    "Die with memories, not with dreams" Scott McIsaac
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  11. Likes: pantealex (3rd September 2017)
  12. #408
    Senior Member Sulland's Avatar
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    If only weight/ballast distribution, why not fix it?
    I think it is deeper than that.
    www.Rallye-info.com

  13. Likes: pantealex (3rd September 2017)
  14. #409
    Senior Member itix's Avatar
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    Quote Originally Posted by NickRally View Post
    Also during the jump, Breen’s engine rpm first went significantly up, then he lifted which brought them down, which means engine inertia reaction moments will try to roll the car (people with knowledge can comment which way the crankshaft rotates to enable us resolve which way the car should roll when rpm goes up and down).
    Having said all that, any mechanically/driver induced rolling nose up motion will then result in the flat-ish floor of the car turning into massive lift generating device leading to even more nose up attitude. Of course it is also possible to argue that aero imbalance prior to the jump causes a small nose up roll, which then results in further roll when the car is in the air, but I am of the believe that such aero imbalance will be heavily felt by the driver handling-wise, while also in Citroen’s case, taking into account their issues of nervous rear, I reckon it is very unlikely they have hugely more rear than front downforce, especially considering the flappy diffuser strakes, which probably render the rear diffuser useless. And the other thing is that watching the on-boards from several cars at this jump, the drivers treated the approach differently, which then affects the behaviour in the air significantly.
    Thanks to anybody that had the patience to read this and apologies for any typos as I haven’t got much time to check it properly .
    I agree with your analysis regarding aero, but I wonder about the engine inertia rotating the car (it's probably a factor but the rotating assembly doesn't weigh a lot). In my head it makes more sense that the rear wheels which broke traction from the ground the last was what caused the rotation. If he had lifted before the jump it would likely have had a very different outcome.

    We've had famous backwards rotations before, like Novikov in finland a few years back and that was with the old C4 which obviously had good balanced downforce.

    If you brake at the crest, the nose should want to come down, if you accelerate, the nose should want to go up as I imagine it.

  15. Likes: NickRally (8th September 2017)
  16. #410
    Senior Member itix's Avatar
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    I have a question about suspension setup. Does anyone know whether there are regulations concerning the suspension setup you use? Does it have to be McPhearson like the road car or would the regs allow you to change it to a double wishbone configuration? If I remember well, some group B cars had double wishbones.

    I am not sure what would be ideal for performance but in my mind the increased unsprung weight of two wishbones would likely have a negative impact and the neutral camber can't in my mind offset that... anyway, just curious how liberal the regs are.

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