Wednesday, September 9, 2015

WSC 2015 Challenger Summary

Let's do a quick summary of the Challenger class designs.

Arrow qualifying in 2013
(image source)
First, a very quick recap from 2013:

The regs were changed to require 4-wheeled cars in 2013. Most teams made some simple design changes from the previous 2007-2011 style cars: They kept the thin airfoil, tall front wheel fairings, and tall driver fairing - and moved the rear wheels out into their own tall fairings. Some people referred to these as "five fairing" cars. Team Arrow's car and Toronto's B-7 are canonical examples.

Stanford's Luminos in 2013
(image source)
Other teams built thicker cars that blended the driver compartment into the main body; these cars performed better. Twente's The Red Engine and Stanford's Luminos finishing the race in 3rd and 4th, respectively, ahead of all of the five-fairing cars.

However, four teams attempted something a little more radical: JU, UM, Tokai, and Nuon all built asymmetric catamaran cars, with the driver pod placed between the front and rear wheels on one side of the car. This worked out great for Nuon and Tokai (who finished in 1st and 2nd), and not so great for UM and JU (who both crashed; JU's crash being race-ending)

Nuna7 qualifying in 2013
(image source)
Their cars had some different features. JU, UM, and Tokai all built cars with three fairings - a long driver-side fairing and two individual wheel fairings on the other side - while Nuon's car had long monolithic fairings on both the driver and non-driver-side. Nuon and JU had the driver on the right, while Tokai and UM had the driver on the left.

This year, of course, everyone is following the champ's lead and building asymmetric catamarans. Let's count up the cars:

Out of 30 Challenger cars, 19 are asymmetric catamarans this year:
  • 15 of those have monolithic non-driver fairings. Only 4 have split wheel fairings opposite the driver.
  • 12 have the driver on the right, and 7 have the driver on the left. 
  • 10 of the cars have precisely the same arrangement as Nuon's winning car from 2013: a monolithic non-driver fairing and driver on the right.
Red One's separate wheels fairings
are a little unique in the 2015 field.
(image source)
So, why do so most of the cars feature a monolithic non-driver faring? I think it's simply because the car is easier to design this way. With the exception of Michigan's car (and presumably Nuon's, if Nuna8 is designed like Nuna7), all of these cars look like they have a completely symmetric aerobody - the canopy is just off-center. Doing the car like this takes much less time to CAD - there are half as many details to model and fix (and one third as many underbody fairings to blend in). The suspension and chassis design is correspondingly easier - all of the hardpoints for the suspension can be the same on either side, etc.

There are also aerodynamic considerations - in a 4-wheeled car, it's difficult to keep the wake from the front fairing from spoiling the airflow over the rear faring. Teams with split-fairing catamaran cars can't play the trick that Twente did in 2013 by staggering the rear wheels inward; the wheels on the driver side of the catamaran need to be nearly in-line, and regs require that the wheels be symmetric about the centerline of the car.

The Red Engine with radically staggered
fairings in 2013 - but they can't stagger
the fairings like this on a catamaran car.
(image source)
It's interesting to note that all three teams that did 3-fairing asymmetric cars in 2013 shifted to dual monolithic fairings in 2015. Twente, NWU, Durham, and Beijing are the only four teams doing 3-fairing catamaran cars this year, and Twente is the only one that's really a credible competitor for the overall victory.

Which side of the car the driver goes on is a little fuzzier. Since the race is slightly west-to-east in the southern hemisphere, the left side of the car should get more sun overall. Placing the driver on the right should shade the array less, and it also seems safer to put the driver on the "normal" side for a country where people drive on the left side of the road (not that safety always matters when it comes to solar car performance).

In 2013, some of Michigan's team members claimed that because the race day is biased towards more racing hours after solar noon than before solar noon, placing the driver on the *left* actually resulted in less shading on the array over the day. I never had time to do a full solar simulation, but it's worth noting that Michigan switched their driver from the left side to the right side for 2015, hmmmmm...

Some more comparisons on driver side:
  • Of the 11 teams from countries where normal cars have the driver on the left, 3 teams built solar cars that kept the driver on the left, and 8 teams switched to the right side.
  • Of the 8 teams from countries where normal cars have the driver on the right, 4 teams built solar cars that kept the driver on the right, and 4 teams switched to the left.
Although our sample size is fairly low, there definitely seems to be a firm trend to put the driver on the right side. Whether this is a performance-analysis-based choice, a "that's what Nuon did last time" choice, or simply "it's the correct side for the driver in Australian cars" is unclear. The last group mentioned in the bullet points above is the particularly interesting group - they're teams that switched away from their home country's conventional side, as well as away from the conventional Australian side - The University of Adelaide, Tokai, EcoPhoton from Malaysia, and Durham from the UK. I'm curious to pick the brains of these teams and see what explanations they have.

Of the 11 cars that still have the driver on the center:
  • MIT and Principia are both American teams that presumably want to race in the American Solar Challenge next summer, which has side-impact regulations that make asymmetric catamarans difficult.
  • Cambridge has their little bullet car.
  • Siam and RVCE are both brand new teams with no experience, and Solaris seems pretty inexperienced as well.
  • Nagoya, Kanazawa, and Goko are all Japanese teams and are bringing pre-existing Suzuka cars, designed to handle well on the track.
  • Arrow is bringing back their old car, and Anadolu's car may have been under construction before WSC 2013.
  • Beijing is kind of ???, maybe they're bringing back their old 2013 car, maybe they have something new that I just can't find? EDIT 9/21/15: They're doing a new asymmetric catamaran, and the information in the post above has been updated.
Basically, almost everyone who COULD bring an asymmetric car, DID. I hope the WSC officials expected this - pretty much everyone else in the wider solar car community expected this when WSC didn't rule these cars out after Nuon's big win two years ago. I'm personally a little worried about all of the rookie teams with asymmetric cars - solar cars are hard enough already without adding extra complexity to the mechanical and aerodynamic stability of the cars, and half of the asymmetric cars at the last WSC ended up in the ditch (my scrupulous honesty forces me to point out that both South African rookies that ran asymmetric cars at SASC 2014 seem to have done fine).

Finally, how many cars should we expect to see in the Challenger class this year? I don't think we're going to see Antakari, Durham, or RVCE. Some teams like Beijing and Siam are big question marks, with no particularly good or bad news available. I think we'll see at least 25 Challenger cars this year, but no more than 28.

EDIT: Looks like Durham's car actually shipped last week, although perhaps in an unfinished state. I'm going to leave those numbers un-edited, and just eat my words if I was wrong.

EDIT 9/25/15: Updated counts above with new information about Goko's entry (see comments)


  1. Hello! My name is Cara Gonzalez, and I'm going to be traveling to Australia with the U-M solar car team as part of their media team. I'm working on a blog post for the U-M #WSC15 digital gateway about the top cars, and I'd love to get your input. My email is and shoot me an email if you're interested!

  2. Twente has posted a lengthy justification for 3 fairings... but in Dutch.

    1. Basically, "to reduce frontal area and whetted area". There are some inherent issues with closely couple wheel fairings, but I think that blog post talks about the team taking them into account. From the google-translate english version:

      "...since there are end up at two wheel hoods air behind the other vertebrae of the front wheel pant on the rear wheel pant. That komtde resistance is not good. To reduce this as much as possible we have a lot of night hours to draw."

      What I get out of that garbled translation is "since there are two wheel fairings, air behind the trailing edge of the front wheel fairing ends up impinging on the rear wheel fairing. This resistance is not good. Many late nights were spent to reduce this drag as much as possible." This is an inherent problem with their fairing layout, and a hard one to solve - especially at a variety of crosswind angles! I'm very interested to see how well they address it, and how the car performs.

    2. They promised a translation... I've been tempted to do one myself.

      Basically, I understand that they've used 3 fairings to reduce frontal area, and then put a lot of effort into fine-tuning the aerodynamic downside -- in particular to reduce the problem of vortices from the first fairing hitting the second. I presume that the "many night hours" refers to CFD modelling.

      In other words, it's a very conscious trade-off. As you say, we'll see how it pays off.

    3. They posted their own translation here.

  3. My translation of the key paragraph from the Twente post:

    “So what exactly is the idea behind having two fairings on one side? ‘When you have two fairings instead of one long one, you can make the fairings smaller,’ says Lars. ‘Smaller and shorter fairings mean a reduction in frontal area and in the area of the boundary layer. That’s a big aerodynamic positive, because the fairings are not “in the way” of the airflow as much. Because we built an asymmetric car, the driver sits in the fairing on the other side. That fairing is consequently wider and less aerodynamic. On the other hand, the single driver-side fairing avoids a problem with having one fairing behind the other, which is that vortices coming off the first fairing collide with the second, increasing drag. To avoid this as much as possible on the non-driver side, we spent many late-night hours fine-tuning the design.’”

    1. Aha, a much better translation, thank you! I had the gist of it, but had to guess at some of the details.

    2. Having an all wheel steering also helped Twente make their fairings smaller

  4. Twente just announced that they too are running a 4 wheel steering system, something that Tokai is doing as well. Interestingly Twentes Farings do actually look smaller compared to Tokai, I see no other reason to have a 4 wheel steering system.

    1. Thanks for the tip! Agreed, I don't think there's a major reason to do 4-wheel steering except for aerodynamic reasons.

  5. That's good and impressive thought.