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15 May 2015


In the last two days there have been two high speed accidents at Indianapolis, involving cars using the Chevy speedway kits.  Both Helio Castraneves and Josef Newgarden hit the wall at turn one and flipped their cars. Firestone have since spoken out over Josef's incident citing a cut in the tyre which deflated it ahead of the accident.  However, Helio's accident remains a source of some mystery, the footage I have seen suggests he was struggling to rebalance the car as he entered the turn.  We must remember that these speedway aero kits are new for 2015 and will undoubtedly force the teams and drivers to think differently.

I immediately questioned Chevy's endplate-less rear wing configuration when I saw the render, as it will clearly cut drag but what about the requirement of downforce?  The underfloor is a carry over from the original DW12 design and so everything has been designed to complement it and compliment it must.  The relationship between the rear wing and floor is often forgotten about but it's symbiotic, both in terms of drag and downforce.  I suspect without the endplates the rear wing 'stalls' at a certain speed (ie the boundary layer on the surface is so large the flow detaches) ordinarily the endplates would control this 'stall' retaining its connection to the under floors airflow structure, keeping a semblance of downforce.  When changing direction there is a fraction of time that passes as the car decelerates for which the airflow needs to 're-attach', stability can become a massive issue if it doesn't happen quickly enough.  Paramount to this is ride height, with any change disturbing the consistency of downforce that is generated by the underfloor and rear wing.  Therefore, I wonder in the moments leading to Helio's accident did the car lose downforce sufficiently to start the chain of events that led to the car ending up on its roof?

At the end of the day practice is for finding these sort of limits and with the kits being so new the teams will be pushing the envelope, to find the perfect setup.

The other thing that has irked me about the incident is a piece that I read about why the flip happened after the accident, especially as the information is a little light on facts but comes from a source from 'within' the sport: http://www.racer.com/more/viewpoints/item/116730-pruett-why-helio-castroneves-car-flipped

Pruett claims that ordinarily the floor 'functions like an inverted wing' creating downforce, what he fails to explain is that a racecar is in 'ground effect' when it generates downforce, meaning the relationship between the cars tunnels and the ground needs to stay relatively stable for the given speed.  He goes onto make the following statement: 

"Turn an Indy car around at 200mph and feed that air backward through the underwing, and it will behave like a normal airplane wing and generate lift. With enough air speed, and pressure build-up in the  tunnels--which Castroneves obviously achieved, lift turns into liftoff".  

I get the whole "dumbing down" of technical content, it's my niche... and although it makes for easy reading I'm sorry but in my opinion it isn't wholly accurate, the lift in my opinion came from another source of aerodynamic complication we have seen far too often in the past, in LMP racing (see below) and is the reason their fenders now have cut outs in the top.




You may argue that the Indy wheels are exposed on the top, however, they weren't designed to work at extreme yaw angles, perhaps that's why we see this as the car goes sideways (yes that is air between the tyre or tire for my American friends and the track):  


Granted being in reverse led to a stall in the floor tunnels but for me the lift came as a consequence of the bumper pod design.  Racecars aren't meant to work at such extreme yaw angles and at that speed and certainly not in reverse. Accidents will happen and you can't mitigate for every circumstance, I think the most important question is what led to the corrections that Helio had to make?  Here's the video for you to draw your own conclusions....


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7 comments:

  1. Well written and well put. In reality, this isn't anything different than has been going on at Indianapolis for the last 98 editions of this great race. Mechanics and teams make changes, issues arise. Issues are corrected. Sometimes the issues are fixed before someone gets hurt. Sometimes they are not. I can't blame IndyCar or the teams and I'm sure that people with more smarts than any of us will figure it out.

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  2. I wonder if a flat spin would result in a similar flip. All three recent Indy flips seemed to get that little extra initial bounce into the air from the contact with the wall. After that it's less about lift in the sense of airplane wing lift and more about two bumper pods catching the air and acting as sails. I vote bumper pods for the greatest contributing factor.

    Ed Carpenter's left rear pod is knocked off during his crash and the right rear of the car lifts higher than then the left rear as it turns over. Same with Josef Newgarden's crash and flip. Castroneves' car, however, goes over more or less evenly. In fact, Castroneves' left rear dips initially but then rises to roughly the same height as the right rear before turning over. It lands more or less less equally on the left and right rear.

    I'd argue that is evidence for the role the bumper pods are playing in turning the cars over.

    These cats are earning every cent. Scary stuff.

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  3. Like Matt said, it's not why the car took off like a plane, but what is the cause of him to crash in the wall in the first place. These cars are obviously not designed to go drifting at extreme angles and even less doing flat spins at 200 mph. If you look at the last video (crash of Castroneves) you can see at 3 seconds that he spins the rear wheels. At that moment he over corrects the steering input making him loose the rear. Is it the rear wing that lost it's downforce because of the boundary layer detaching during the turn? Could it be that the rear touching the ground such as Senna's crash which broke the air flow between the ground and the floor and losing downforce instantly? Tyre degradation and he lost grip during the turn?

    Anyway, Formula 1, Indy, Nascar, LMP, etc. Take a car going at 180 mph, break a wing, hit a bump, get interlocking wheels, rear end a follow driver....cars will flip, it's part of the game! And as long as no one dies or get badly hurt, it will always be a Wow! factor that keeps us spectators to the edge of our seats.

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    Replies
    1. If you believed the Indycar blurb put out at the introduction of the DW12, these things could NOT fly.....?

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    2. If you believed the Indycar blurb put out at the introduction of the DW12, these things could NOT fly.....?

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  4. Two words, roof flaps (but on the wheel covers)... see Nascar roof flaps.

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  5. The worst are the WEC LMP1 flips. The Lotus 88 twin-chassis concept solves this problem and is much nicer to the drivers spine also. The dynamic stability of a twin-chassis is far superior to single-chassis. WEC should go this route to avoid flips, F1 for the drivers sake and to facilitate adaptive front and rear DRS down force to allow close pursuit and passing.

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Whilst I'm trying to keep atop of the blog you may have noticed of late that there is less content appearing. For those of you that haven't realised, most of my work has now been moved over to Motorsport.com where I'm working with Giorgio Piola.

I'm still doing the technical image gallery for each GP with the continued support of friend of the site Sutton Images. However, as always my time is limited and so this might not be updated as quickly as it once was, so keep checking back.

As some of you may have found out already I'm also working with the Missed Apex crew on their podcast from time-to-time, either doing race reviews or dedicated 'Tech Time' shows.

I've embedded the latest version of the podcast below and will update this a frequently as I appear. However, please head over to Itunes if you want it to appear in your player when episodes are available. The show is great to work on and has a great lineup of 'regulars' but has also enticed some bigger names recently too, with Will Buxton and Bradley Philpot on shows during the summer break.


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