ASC 9: Aerodynamics

For both the Challenger class and the Cruiser class, solar car racing is to a large extent about aerodynamic drag. That’s overwhelmingly what the hard-earned solar energy is being wasted on, and therefore it’s what teams need to concentrate on minimising. The drag force on a car is given by the equation:

F = ½ Cd A ρ v2

Breaking that down, v is the speed of the car, ρ is the density of air (about 1.2), A is the frontal area of the car, and Cd is the drag coefficient, a number which indicates how aerodynamic (and therefore, how energy-efficient) the shape of the car is. For Challengers, minimising Cd allows the speed v to be increased, while for Cruisers (for which the average v is essentially given), minimising Cd allows non-solar energy use to be minimised. Of course, minimising frontal area is important too (and that is the motivation behind asymmetric Challenger cars).

To give a feeling for the all-important Cd, here are some vehicles with values ranging from 0.19 to 0.57:


Drag coefficients for a selection of vehicles. Clockwise from top left: 0.57 – Hummer H2 (photo: Thomas Doerfer); 0.30 – Saab 92 (photo: “Liftarn”); 0.26 – BMW i8 (photo: “youkeys”); 0.19 – General Motors EV1 (photo: Rick Rowen)

Because Cd is so all-important, it is the one thing that solar car teams are really secretive about. Challengers generally have values under 0.1. With no need for practicality, they chase their way towards the impossible goal of Cd = 0, trying to come up with the perfect race car, which will slice through air like a hot knife through butter:


Nuon’s 2005 car, Nuna 3, with Cd = 0.07 (photo: Hans-Peter van Velthoven)

Cruisers, on the other hand, have to balance aerodynamics with practicality. Bochum’s early SolarWorld GT had Cd = 0.137:


Bochum’s 2011 car, SolarWorld GT, with Cd = 0.137 (photo: “SolarLabor”)

Eindhoven’s recent Stella Vie, with its sleek aerodynamic shape, does much better than that (but they won’t say how much better):


Eindhoven’s 2017 car, Stella Vie (photo: TU Eindhoven, Bart van Overbeeke)

I understand that Sunswift’s 2013–2015 car eVe had Cd = 0.16. Appalachian State (Sunergy) have stated that their newly-built ROSE has Cd = 0.17. PrISUm’s Penumbra has a higher value (Cd = 0.2), because of the blunt end which they chose for practicality reasons (although they did do a few clever things to reduce the impact of that blunt end). I’m not aware of the Cd values for other ASC cars.


Appalachian State’s beautiful ROSE, with Cd = 0.17 (image credit)


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ASC 8: About Cruiser Practicality

The American Solar Challenge Cruiser class is a contest for multi-person solar vehicles, each powered by 5 square metres of silicon solar cells (or 3.3 m2 of multi-junction cells), with the option of recharging from the grid. The contest is not actually a race – cars must get to the finish line on time, carrying as many people as possible, and drawing as little power from the grid as possible.

Cars are also scored partly on practicality. This can mean different things. Eindhoven’s 2015 car (Stella Lux), for example, was designed as a four-person family car, and the team took photos of it doing family things like shopping, going on holiday, and picking children up from school. A big feature was that, for an average family in the Netherlands, the car would produce more electricity than it used. The car scored 84.5 for practicality at the 2015 World Solar Challenge.


Eindhoven’s Stella Lux (photos: TU Eindhoven, Bart van Overbeeke 1, 2, 3, 4 – click to zoom)

Bochum’s 2015 car (ThyssenKrupp SunRiser), on the other hand, was a luxury two-person sports car, with leather seats and an incredibly beautiful interior. It was an almost perfect example of the car it was trying to be, and scored 80.5 for practicality (far higher than the next car, which scored 63.5).


Bochum’s ThyssenKrupp SunRiser (photo: Anthony Dekker)

One of the highest WSC 2017 Cruiser practicality scores went to PrISUm for their four-seat Penumbra, which was intended as the kind of practical SUV that you might take on a fishing trip. The car has plenty of room for carrying your esky, tackle box, etc. PrISUm deliberately made some aerodynamic compromises in order to achieve their practicality goal, and the car scored 79.8 for practicality at WSC 2017.


PrISUm’s Penumbra (composite image)

This year at ASC, PrISUm’s Penumbra is again a strong contender. Minnesota (UMNSVP), Appalachian State (Sunergy), and Waterloo (Midnight Sun) are entering two-person solar sports cars, while Onda Solare from Italy seems to be inspired by Eindhoven (see my annotated teams list). It promises to be an interesting field.


ASC 7: Social Media

Having a social media presence is an important part of running a solar team, and generally requires a dedicated social media manager. Social media keeps the fans happy, and it keeps the sponsors happy. Of course, for teams that do crowdfunding, the fans are also sponsors!


Some recent tweets in #ASC2018

A team’s university is generally an important sponsor too. To satisfy the university, a team must achieve both reputation and recruitment goals. For reputation purposes, a team must portray students who are hard-working, talented, and professional in the way that they work – qualities that future employers of graduates would like to see. For recruitment purposes, a team must portray students who are doing fun and interesting work – things that future students would like to see. These two purposes are not necessarily best served by the same social media platforms.


Nuon’s 2015 flightcase being loaded (photo: Jorrit Lousberg)

For industry sponsors, a quid pro quo for sponsorship is generally required. Often, this takes the form of showing how the sponsor’s product has contributed to team success. Classic examples include Twente mentioning their kangaroo-proof carbon fibre shell, Punch extolling the shipping expertise of DHL, Michigan explaining how their semi traverses the planet, and Nuon showing how their flightcase moves around (above). Having a superb photographer on the team helps with this!

Social media platforms being used by ASC teams this year are shown in the chart below. The proportions are quite similar to WSC 2015. However, Picasa is gone, Snapchat has arrived, and the Russian team is using VK.

For the actual team social media links, please visit my annotated teams list and click on the social media icons next to each team name.


ASC 6: Those freaky car shapes

The American Solar Challenge’s Challenger class is a race for single-person solar vehicles, powered by 4 square metres of silicon solar cells (or 2.64 m2 of multi-junction cells). There are three basic shapes for modern Challenger-class cars:

Symmetric cars

Symmetric cars are the most traditional, and the easiest to build. The disadvantage of symmetric cars is that there are three obstacles to airflow – the driver compartment and the left & right wheel fairings. This rear view of Illini’s Argo shows that quite clearly:


Illini’s Argo test-driving on the road (picture credit)

Asymmetric cars

Asymmetric or “catamaran” cars first showed up at the 2013 World Solar Challenge. They have only two obstacles to airflow, because the driver compartment is integrated into either the left or the right wheel fairing. This makes them faster, but substantially more difficult to build. At ASC 2016, only Michigan and Toronto (both fresh from WSC 2015) had asymmetric cars. This year, there are several.

Should asymmetric cars have the driver on the left or on the right? There are arguments both ways. The American Solar Challenge this year runs east to west, so the sun is mostly on the left side of the car (and rising no higher than about 70° in the sky). Therefore cars with the driver on the right (everybody except ETS Quebec) have a slight advantage. On the other hand, seating the driver on the left gives a better view of the road (in the US, at least).


MIT’s Flux test-driving on the road (picture credit)

“Bullet cars”

Last year’s World Solar Challenge saw the introduction of long, narrow “bullet cars,” which were made possible by the reduction in allowable solar cell area. Michigan came 2nd and Tokai 4th in cars like that. Michigan’s Novum is definitely the favourite to win the American Solar Challenge this year. But of course, anything can happen!


Michigan’s Novum coming 2nd at the 2017 World Solar Challenge (picture credit)


ASC 5: Car dimensions

Based on the official 2018 FSGP/ASC Programme, I have updated the dimension and weight information in my annotated teams list. Here are the lengths and widths of most cars (click to zoom):

Cruisers are indicated by diamonds and Challengers by circles. Cruisers are fairly large on the whole, with PrISUm’s Penumbra (team 9) being shorter than the rest. Older Challengers are at the top right, with some newer cars taking advantage of the smaller solar panels this year to be either shorter or skinnier. Here for example, is the compact little car from Poly Montreal (Esteban, team 55):

Western Sydney (team 15), on the other hand, have a longer, skinnier car:

Michigan (team 2), of course, have the longest, skinniest Challenger of all:


ASC 4: Testing

It is critically important that solar-car teams clock up test kilometres before the big race. This is partly because of what engineers call the “bathtub curve.” Failures in any piece of technology are common at the start, but then level out to a low constant failure rate during the object’s lifetime (and of course, once the object starts to wear out, failures increase again).

In the business world, short warranties are used to cover that early failure-prone period. In racing, it’s essential to make sure that the car is out of that early period before the race begins. Therefore, the top teams test, test, and test some more!

Here is a montage of recent solar-car testing, which I have already posted to Twitter: