Technical Analysis: WRC Cars 2017

[Mrpengski]

Nick, who do you figure did the best homework as far as optimising for yaw condition, and why? Also, it was mentioned earlier that most of the details for WRC technical regulation is under 400/01 which is not publicly available. I never realised this, and find it rather annoying, because I enjoy reading other regs (e.g. LM P1, LM GT) when I'm not neck-deep in 400/01, which give full description of legality zones (among others), and are openly available on the FIA website. To comment on your earlier post about underbody treatment, check section 902 of article 255a (the public release). Basically, it doesn't say much, and certainly some take a more liberal approach than others with regards to treating it as an aero device.

Re: diffusers. Without disclosing any numbers or commenting on Nick's estimates , I tend to agree that the physical effect of losing the diffuser would be only a small shift in aero balance, but enough to disturb the driver's confidence, which is far more important than a few kilos of downforce. So, as Nick said, some real effects, but mostly what's in the head.

Re: doors. Don't forget that these doors are first gutted out (OEM side impact beam and all) then filled back with energy-absorbing foam. Of course, the OEM beam is replaced by the roll cage, but this is not realistic for a road car, and neither is filling the entire door + outer extension with foam. Among other things, the window won't go down! Incidentally, in my previous job I was involved in a lightweight electric car project with composite monocoque and bodywork. Inside the door was a CFRP side impact beam which housed an outside-firing airbag. I believe it was for pedestrian protection more than occupant protection, but interesting nonetheless. So maybe something like this could be applied to road cars, before we see extra thick rally-style doors.

My earlier comment about the looks of the cars with the side impact is perhaps best appreciated in the context that the regulation of the side impact was finalised rather late for the '17 cars (Article 255a says 10/03/2017 for the official publication date for the side impact ). That's why most "stage 1" test cars didn't have them, including the Polo until its last few runnings. I do like how Toyota took full advantage of the "requirement" and essentially created a continuous aero surface along the top of the extension which runs flat until the rear wing elements. That car is like a giant wedge in that sense, and seems to be making good downforce as well.

What I find also interesting is how the aero concept of the cars seems to have converged so well, first time around. Yes, the individual execution differs, but overall the flow management is much the same. I'm sure we were all thinking that we were being smart and outplaying others within the regulations without going full DTM-mode, but I suppose there is only so much that can be done on a road car base, aerodynamically. Do note that there seems to be two distinct approaches for the rear wing. Had VW been around, it would have been 3 for "fully separate upper and lower element" vs. 2 for "double decker with end plate + side element".

[NickRally]

All very good points Mrpengski.
Unfortunately there isn’t a single technical document that sets the conditions under which all WRC should be designed and build. My understanding is that form 400/01 is simply the homologation document for each car where the actual design features are recorded so that the cars can then be inspected before each rally. I think FIA has left itself a bit of flexibility in deciding the extent of the mods on each car in order to ensure closer competition as the cars are production based and not necessarily designed with competition in mind. I have to admit it is a bit peculiar to me and I would have enjoyed going through all the rules in a single document and seeing what’s ultimately possible and what not, but obviously the concept chosen by FIA works.
To answer your question about the yaw sensitivity of the cars, it is not easy without going to the wind tunnel and testing the various designs, but with this disclaimer in mind, I would say the cars with split upper and lower rear wing elements maybe hold an advantage in that respect, though this might be at the cost of reduced overall straight line downforce.
Having said that, connecting the two elements, does not automatically mean more yaw sensitive car, as shaping and sculpting the rear wing endplates can make the design less so as well as the design of the original car’s rear end.
On a different not, the external airbag concept sounds interesting and hints of one of the concepts chosen for a Martian landing module that was wrapped in a massive airbag at the point of hitting the Martian surface.

[Mirek]

My understanding is that form 400/01 is simply the homologation document for each car where the actual design features are recorded so that the cars can then be inspected before each rally.

Pre-event scrutineering is not being held to check if everything is correct according to the homologation. It's not possible simply due to the time required. What they do in scrutineering is mainly checking papers, safety equipment and sealing of particular parts such as gearboxes, differentials etc. (including spare ones). Of course they can find something against the homologation but more or less accidentally. For Your information scrutineering for Barum rally where I am sometimes present as organizer takes roughly 7 hours even when they do only the basic checking of every single entered car.

The deep check is being done in post-event scrutineering for which only several cars are being chosen (again due to the time required). Usually top-placed ones in their categories.

[NickRally]

Thanks for the correction Mirek. In that case I will rephrase what I said – “My understanding is that form 400/01 is simply the homologation document for each car where the actual design features are recorded so that the cars can then be inspected when required.”

[Mrpengski]

Nick, indeed the 400/01 form refers to the homologation dossier specific to each car. But in the working copy of the reg, this column is where you will find all the technical details. I know, because I work with it.

It's funny because if you see the 2017 LM GTE tech reg available on the FIA website, it has the exact same format as the WRC tech reg, but they left all the details intended "for manufacturers only" completely intact for public release. I don't know if it's an oversight by the FIA but the inconsistency is striking, to say the least.

Just saw the article on Race Tech magazine about the Fiesta. Interestingly, it also talks about the convergence of aero design, and how it could be considered as validation of each team's effort, where no one really has something coming out of the left field, so to speak. That's exactly how I felt when I saw the Polo with the final kit. In fact, the similarities were almost shocking, be it the treatment of the door extension or the rear wing. As reigning champions, I thought it was still fair to consider them as the yardstick for the new cars.

The fact of the matter is, though, I don't believe any rally will be won or lost purely on aero, at least not yet. And I hope we don't get to that point, ever.

Earlier I saw some confusion and misinformation about the central diff, but sadly I'm not at liberty to discuss anything interesting. For me it's just another black box, anyway

[Mirek]

It was same at least with S2000 regs - all the details were not shown in the public documents.

[NickRally]

Very intriguing, thanks for sharing all this info Mrpengski. For me this is certainly very amusing, my initial impressions were that there wasn’t much hidden behind the fields with the text “400/01” and a lot was left to negotiation. How silly of me… I fully agree, it is a very puzzling attitude considering that even the F1 rules are publically available.
To be fair this is the first time I see someone directly stating in writing that there are two versions of the tech regs with some fields blanked off (or 400/01ed off), rather than implying the rest of the rules are in form 400/01 and it is genuinely appreciated.
Once again for someone like me coming from circuit racing this is highly unusual.

[OldF]

Here you can find the S2000 homologation regulations for 2010. An old one but I think it was the last one because any new homologations of S2000 cars was not allowed from 2011 and onward (if I recall correctly).

http://www.sarallying.co.za/nrc-info/regulations/ (FIA Homologation regulations, word document)

I think these specific regulations are for the ones who maintains the car and telling which part are free, which must be homologated etc.

Earlier I saw some confusion and misinformation about the central diff, but sadly I'm not at liberty to discuss anything interesting. For me it's just another black box, anyway

Are you allowed to say what the confusion and misunderstanding was about without revealing any secrets?

[NickRally]

To add to OldF’s post, although maybe pushing you a bit Mrpengski (please don’t do anything that will put you in trouble), after you said there was a misunderstanding in the differential discussion earlier in the thread, it prompted me to summarise the potential options below, though of course I don’t know what restrictions are placed on the type of diffs and hidden in the differential filed of the tech regs under 400/01 expression, so here it is:
1. Spool – previous generation of cars, currently unlikely to be utilised by anybody other than when central diff is locked turning it into a spool
2. Symmetric epicyclic central differential, normally opened and requiring hydraulic pressure to lock - doubt its use on the current cars due to being symmetric and reducing the options available in varying the torque split front to back
3. Symmetric epicyclic central differential, normally locked and requiring hydraulic pressure to un-lock - doubt its use on the current cars due to being symmetric and reducing the options available in varying the torque split front to back
4. Asymmetric epicyclic central differential, normally opened and requiring hydraulic pressure to lock – one of the two options I would choose if I was designing the cars, possibly my first choice as it will allow precise control of the lock
5. Asymmetric epicyclic central differential, normally locked and requiring hydraulic pressure to un-lock – one of the two options I would choose if I was designing the cars, less precise control of the lock, but more robust in case of hydraulic failure
6. Independent two clutch packs type diff – very unlikely anybody was brave enough to embark on this and I wouldn’t be surprised if it is actually banned
7. Any other type of central diff – probably banned by the regs

[jparker]

So, we can ask Ogier "Is your diff locked or unlocked by hydrolics?". Hope he doesn't get the question wrong.