Solar car safety & crashes analysis

Twente also crashed in 2009
(photo: Gijs Versteeg)

Solar cars are getting too fast and too unsafe, if you consider the top average speeds and crashes of GGC2009. In this article some safety aspects, causes of crashes and suggested measurements are discussed. My conclusion is that tyre regulations should be greatly reinforced and that tough suspension tests are added to dynamic scrutineering. And there is a list of crashes mentioning about 15 crashes, along with cause, consequences, pictures and links (as far as I have the information).

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Article by Vincent Groenhuis

Table of Contents
1. Introduction
2. Causes of crashes
    2.1 Tyre blowout
    2.2 Suspension/wheel breakdown
    2.3 Adhesive joint getting loose
3. Improving safety
    3.1 Reducing occurrences of technical problems
    3.2 Reducing consequences of technical problems
4. Conclusion
5. List of crashes
    Nuon(2003)
    Blue Sky(2004)
    Nuon(2005)
    Sunswift(2005)
    MIT(2005)
    Michigan(2007)
    Twente(2007)
    Stanford(2007)
    Twente(Jul 2009)
    Nuon(2009)
    Umicore(2009)
    Aurora(2009)
    McMaster(2009)
    Twente(Oct 2009)

1. Introduction
Crashes happen in almost every solar race (just like other car races), but in GGC2009 there were pretty many of them. Luckily no one was injured, but the damage done to some cars was very severe and they involved experienced teams. It is a race driven on public roads, usually by students, so it has to be kept as safe as possible to keep the event enjoyable. And the crashes are not good for the reputation of solar cars in general. We see that many of them are caused by tyre blowouts or broken suspension, so we'll look more closely at these aspects and discuss some suggested measurements to improve the safety.

Also the average speed of the fastest team (Tokai) is exceeding 100 km/h again, which is very close to the 110 km/h limit in South Australia. Because there is still much room for improvement regarding specifications (especially in solar panel efficiency), next generation cars would be even faster if the rules don't change, so rule changes for the 2011 event are necessary anyway.

2. Causes of crashes
Many different causes have led to solar car crashes, but some of them happen more often than others. Most frequently causes are a tyre blowout and a suspension breakdown, and I also mention the adhesive joint getting loose.

2.1 Tyre blowout

Solutra (Twente 2005) blew dozens of tyres,
but none got the car uncontrollable

A small puncture causes a tyre to give out, slowly getting flat and slowing down the car as friction increases, which is usually noticed by the driver (or telemetry team member) so it can stop the car. This is usually no big problem. Much more dangerous is when a tyre loses inflation pressure very rapidly, called a 'blowout', usually after it hit an object which made a sizeable hole in the tyre. This can cause the tyre to lose grip (at instant) at full speed and in some cases the car becomes uncontrollable and crashes (especially when driving at high speeds). In 2009 there were about 3 serious crashes caused by a tyre blowout and considering the total number of flat tyres, you'd easily say that (when driving 90+ km/h) the chance is roughly 20% that a flat tyre results in a serious crash. (The value of 20% is a very rough estimate, as I don't have (complete) statistics of most teams, so it is only an indication.)

Nuna5 blew its rear tyre and crashed (Nuon 2009)

Crashes by a tyre blowout not only happens to solar cars, but also to convoy cars (see Nuon(2003) for example). But it does happen to solar cars much more often than to other convoy cars, because solar cars are equipped with narrow tyres inflated at high pressure, to minimize roll friction. Whenever the teams have the choice which tyre to install under their cars, they tend to use the usable ones with the lowest friction possible in order to drive as efficient (=fast) as possible to maximize winning chances. Replacing a tyre usually takes only a few minutes, while installing more common car tyres would slow down the car by about 10-20 km/h and until 2009 the risk of a serious crash seemed pretty low.

2.2 Suspension/wheel breakdown

MIT's wheel broke off in WSC2005's
qualifications

Solar cars are lightweight vehicles, often designed and built by students. The Stuart Highway is long and tough, with many cattle grids, which stresses the suspension to its limits. Also some teams invent a new type of suspension which has not been thoroughly tested before. So it is not surprising that some suspensions fail to stay intact all way down the finish. Some sub-causes for a suspension breakdown are: structure too weak (design or manufacturing errors), joints worn out, not designed for circuit racing, adhesive joint getting loose etc.

2.3 Adhesive joint getting loose
One of the mechanical problems which led to crashes is the adhesive joint getting loose (in these cases metal on composite (e.g. carbon fibre), glued by epoxy or similiar). This not only happens in suspensions, but also in steering systems, and a few times a brake pedal mount got loose (Twente 2005, Nuon 2007; fortunately without a crash as result). Well, some people say 'practice means perfect' and 'haste makes waste', but it also seems that the strength of a metal-on-carbon(composite) adhesive joint is often overestimated. Maybe it is because of the different properties of metal and carbon? Carbon is rather flexible, while metal is rigid. When specific forces (in specific unfavourable directions) act on the joint, forces may accumulate on a single spot in such a way that it causes a crack in a corner of the joint. This then propagates through the whole joint, eventually getting loose, regardless of the total surface area of the joint. Or similiary: the topmost layer of the composite detachs from the other layers (the adhesive joint itself stays intact, but the composite structure gets splitted). Think about peeling off tape. The standard fix (in both cases) is to secure the joint with bolts and nuts, as has been done many times only after the problem occurred.

3. Improving safety
As said before, there were many dangerous crashes in the GGC2009 event, too many for the good name of solar racing. A crash is almost always a result of a technical problem, so in order to make the event safer, the number of technical problems occuring should decrease, or a technical problem should less often result in a crash (or both).

3.1 Reducing occurences of technical problems

Tokai also got a flat tyre in GGC2009

Regarding tyres, the use of stronger, durable tyres will decrease the number of flat tyres. Bicycle tyres and currently-used solar tyres are clearly not doing well. Scooters, motorcycles and small utility cars have tyres which are probably better suitable for solar cars. Because this also increases roll friction, most teams won't do this voluntary and so it has to be reinforced in regulations.

For the 2009 event, the organization indeed specified some minimum profile depth for the tyres and it was expected that it would slow down the cars, but teams have a way of finding the limits to get the maximum out of the regulations and ended up driving with tyres which were approved, but neither considerably increased roll friction nor improved safety.

So in order to really improve safety, the regulations should be reinforced further, e.g. by specifying more physical properties of the tyres (width, thickness, profile area, inflation pressure) and/or to penaltize tyre replacements (e.g. 30 minutes penalty delay for every tyre replacement during the WSC, even at overnight's rest) or to specify existing types of tyres for motorized vehicles, along with some procedure for approving other types. (These suggestions have been stated by other people earlier, so it's not my idea but I just repeat it here.)

Nuon's test-car Mule (with Nuna4 suspension)
jumping over an obstacle (video from 2:40)

Twente 2009's 21Revolution test-driving over
a replica cattlegrid

Regarding suspension, many problems can be prevented by good design and testing, but the teams are not at all consistent with it. Some teams perform rough testing, especially the ones who have crashed in the past. Nuon reported that it does slip&spin tests and their last team built an experimental vehicle (Mule) to do various testing for their cars, incuding some extreme suspension testing with the Nuna4 suspension. And Twente built a replica cattlegrid because of the crashes in 2007.

The dynamic scrutineering involves brake&stability tests and some circuit laps, but these are too easy to pass and do not reflect the required strength to complete the whole Challenge. So I think it is a good idea to make slip&spin tests and driving-over-obstacles part of the dynamic scrutineering too. For example: demonstrate 90 and 180 degree spins and drive at 50 km/h over 15 cm high obstacles (with 45 deg ramp in front) with all wheels. These tests are tough, but my point is that when these tests are specified, teams can prepare for it and do the tests home, in an early stage of testing (i.e. when the car frame is ready to drive). And when a car fails any of these tests home, teams still have enough time to re-design the suspension to resolve the weakness. If any design decisions are doomed to fail, it should fail as early as possible and not remain hidden in the car until the start of the WSC.

Maybe some of these tests can also be officially performed home and be videotaped (especially the spin tests) if teams don't like to do these at the Hidden Valley track. In the DARPA Grand Challenge, a video demonstration is also part of the complete scrutineering process (see regulations).

Solar cars are getting lighter; the lightest ones only weigh about 150 kgs (excluding driver), which is advantageous for efficiency, but it makes the car more susceptible for winds (because gravity and friction forces decrease proportionally with mass, but wind force not). Sudden strong winds (like a willy willy) have attacked Nuon in 2007 and Umicore in 2009; some people suggest that there should be a minimum weight (200 kgs for example), but I doubt whether this is really necessary or not. After all, the Nuna4 stayed on the road and the Umicar (2007) also had steering problems. Specifying a minimum mass also takes away some of the technology innovations, while other measurements (like better steering or aerodynamics) also improve handling in extreme weather.

3.2 Reducing consequences of technical problems
When a tyre blows (or suspension breaks off), sometimes the car can keep track on the road (with the other wheels) and safely slows down, sometimes it slips and becomes uncontrollable, sometimes it flips upside down or worse. Some cars are more stable than others (and so more easily stay on the road), because of the number and configuration of wheels, the location of the center-of-mass, aerodynamical stability and so on.

Four-wheeled cars are generally more stable than three-wheeled ones and some people say that four wheels should be obligatory, but this has some important drawbacks: it limits the teams in their creativity, and some existing in-wheel motors need to be supported on both sides of the wheel and is therefore difficult to be installed as a side wheel. Also there are other important factors to consider with, so a four-wheeler is not necessary more stable than a three-wheeler.

Important for stability is the location of the center-of-gravity: the lower it is (or more precisely, a lower height/distance to side ratio is better ("side" refers to the relevant edge of the polygon formed by the contact points of the tyres on the road)), the less likely it is that a car flips over a side. Also a three-wheeler with two front wheels (and one rear) is more stable than the reverse configuration, because in an emergency situation the car has to slow down quickly. When braking, weight transfer shifts to the front wheel(s); if there is only one wheel in front then it can (diagonally) flip over more easily.

A motor usually drives only one wheel, so a four-wheeled car (with motor in a side wheel) has an unbalanced driving system which is disadvantageous for stability (see Stanford(2007)).

The stability of a car can be tested by performing extreme manouvers, like slipping at certain speed and spin over a specific angle (90 degrees or so). Cars with a too high center-of-gravity (with respect to the distance to the sides) will probably flip over; so including these tests in dynamic scrutineering will force the teams to design cars with good stability.

Stability can also be measured in static judging though, e.g. by lifting one or two wheels and measuring the weight change on each separate wheels. From this data the location of the center-of-gravity can be calculated and also the distances to the sides of the car (or wheels).

Speed is also important: at high speeds, a tyre blowout is more likely to make the car uncontrollable (and therefore more dangerous), and whether it gets uncontrollable or not, at higher speeds a car requires a larger distance to stop.

If a car gets uncontrollable, the driver's safety is most important. There are already a number of regulations to protect the driver: helmet, safety belt, roll bar and such are obligatory. I think most of these regulations are good enough: no one has ever been seriously injured before/during WSC's despite numerous serious crashes. The only time ever a solar driver was killed was in Canada in a frontal collision (see Blue Sky(2004)), but little can be done to protect the driver in a full speed collision right into an opposing vehicle (especially if that vehicle is a road train).

The rollbar of Umicar Inspire was sleek and strong
(and survived the big crash)

Nuna5 initially did not pass scrutineering
because of an incorrect seating angle. Next day
it was adjusted and approved.

But some teams seem to take the technical regulations rather sloppy. Many are meant to improve safety, but sometimes a team makes the car such that it obeys the regulation without actually contributing to safety. For example: technical regulations Section B.5.1 says: "The vehicle must have a balanced, dual braking system so that if one system should fail, the vehicle can still be stopped.". But in some cars, a seperate braking system itself is not balanced. So if one system fails while the driver brakes, the car is likely to spin. A similiar story for the rollbars: some teams construct very strong ones, while other make it as thin as possible (within the regulations) to minimize weight and improve aerodynamics.

It is of course very difficult for the organization to actually take a car out of competition (at scrutineering or qualification) after a problem with regulations. For many students it is an once-in-a-lifetime experience and you simply don't want to refuse permission to start if it obeys all but one detail (or specific interpretation) of the rules. But if teams know this in advance, they may even get more sloppy about the rules. And what about the cars which crashed during qualification (or in WSC itself) and subsequently repaired (but not redesigned) and went on? If such a half-wrecked, overnight-repaired, ducktaped car with (apparently) structural weaknesses eventually manages to complete the challenge, people call it great and call the team members heroes, but if a half-wrecked, overnight-repaired car with (apparently) structural weaknesses crashes again, destroying the car, the same people call it irresponsible that the organization ever allowed the team to go on. Luckily, most part of the Stuart Highway is deserted, but in some parts there are other cars driving too. Is this the reason why the Adelaide government didn't allow most teams to enter Adelaide itself?

4. Conclusion
Too many solar cars crash; mostly because of a blown tyre or a suspension breakdown. To reduce the number of these problems, I propose the following two measurements:

1) Reinforce tyre regulations such that the approved tyres are as strong and durable as the tyres used in commonday motorized vehicles of comparable speed and weight. E.g. specify tyre dimensions and physical properties, or list a few reference tyres and an approvement procedure, or penaltize tyre changes (even at overnight's rest).

2) Add tough suspension tests to dynamic scrutineering. Teams should demonstrate 90 deg spins and driving over obstacles, for example. These tests (which maybe can be officially performed home and videotaped) should test the suspension and stability thoroughly, and be exactly specified, so that a) cars which pass these tests are likely to complete the challenge, and b) cars with structural weakness(es) are likely to fail, in an early stage of construction/development, allowing the team to improve the mechanics in time.

The first measurement also reduces the average speed of the car, which improves safety and creates room for further improvements without hitting the speed limits.

5. List of crashes
Here is a list of crashes, mainly solar car crashes during/before the last few World Solar Challenges. I've tried my best to be as accurate as possible; if you see errors or have additional images/comments/other information, then please let me know (w@wsctube.com, or post a comment)!

Nuon(2003)
Blue Sky(2004)
Nuon(2005)
Sunswift(2005)
MIT(2005)
Michigan(2007)
Twente(2007)
Stanford(2007)
Twente(Jul 2009)
Nuon(2009)
Umicore(2009)
Aurora(2009)
McMaster(2009)
Twente(Oct 2009)

Nuon(2003)
A convoy car of Nuna II blew a tyre, flipped upside down and moderately injured its passengers; one of them broke an arm. This is not a solar car crash, but in terms of injuries, it is the most serious accident during WSC I have heard of, so far.

Blue Sky(2004)
During a promotional tour in Canada, the solar car of Blue Sky from Toronto swerved to the wrong side of the road and frontally hit a minivan. The solar car driver was killed, the driver of the minivan was unhurt. It is unclear why the solar car swerved; maybe because of the winds, or a driver's misjudgement, or a mechanical problem. This is the only time ever that a solar car's driver got killed in a crash, as far as I have heard of. News post about this crash.

Nuon(2005)
During a test drive (driving 100+ km/h) a few weeks before the WSC2005, the right front suspension of the Nuna3 broke off, then it swerved, got off road and came at rest in the shrubs. The driver was unhurt, but there was a lot of damage around the right front suspension and at the nose. It could all be replaced/repaired and the Nuna3 managed to win the race.

The suspension broke off because an adhesive joint got loose. The A-arms of the suspension were made of carbon, with aluminium joints at the ends which were glued on the carbon arms. In the repairing process, the team replaced all A-arms with fully aluminium ones.

Sunswift(2005)
The two-seater Sunswift III got an accident in the week before the race. The suspension broke off, none was injuried.

The repairs lasted several days and the team had to skip the scrutineering events. In WSC2005 it unofficially finished 9th.

MIT(2005)

MIT's Tesseract after the crash

During qualification, one wheel broke off and then the solar car flipped upside down. The driver was almost unhurt, but the solar panel was heavily damaged. It could still compete in WSC2005 with a half-working panel and finished 6th. (I heard that the car did hit the edge of the circuit at a bend which caused the wheel to break off, but I can't find references). The team later explained that the car was not designed for circuit racing.

Michigan(2007)

Damaged topshell of Michigan's Continuum

About one hour after start of WSC2007, Michigan had to stop in front of a congestion, which occurred when Stanford overtook Michigan and then stopped. Then Michigan's solar car Continuum crashed right into its own lead escort vehicle. Michigan then trailered back to Darwin, repaired the car and continued the race on the following day, eventually finishing 7th.

Apparently the stopping distance of the Continuum was larger than the available space. The Continuum was equipped with one front wheel (with motor) and two rear wheels; in this configuration (with regulations of that time) the mechanical brakes usually are in the rear wheels only. But during braking, weight transfer shifts to the front wheel(s), so cars with this wheel configuration have a relatively large stopping distance (important when considering driving style).

Michigan's car showing seat position
(photo: Aurora)

And the driver's position is a crouch position; Aurora was wondering how long the drivers would last in their seating position. (I also noted that when the driver's legs are sleeping, it is more difficult to kick the brake pedal at sudden, but it is said that the brakes are hand-operated.)

From 2009 on, braking regulations have been reinforced, after which usually all wheels have mechanical brakes now.

Twente(2007)

Twente One with broken suspension
(see video from 0:50)

The suspension showing
the spring and lever

Twente's car crashed three times on three consecutive days in WSC2007. Each time, a part of the front suspension broke off after which the bottom of the car dropped to the ground. The broken part could be replaced each time and the total delay was only about two hours. A team member explained that the forces occurring in the suspension had not been re-calculated after an important design adjustment was made (which involved placing the wheels 10cm further from the body).

The suspension is similiar to the one used in F1 cars. The up-down motion of the wheels is transferred to a horizontal motion within the body itself, by means of a pushrod and a lever. So the spring itself fully lies within the body, and the outerior part of the suspension is all aerodynamically clean. The joint between the lever and the frame was the one which broke three times.

Stanford(2007)

Equinox after it crashed (and set back
upright and panel removed by the team)

Damaged topshell of Stanford's Equinox
(more photos)

During WSC2007, the left rear tyre of Stanford's four-wheeled solar car gave out after which it blew. The car then spun and drove off road, hitted a concrete ditch and flipped upside down, landing on the rollbar. The driver was unharmed (also because the rollbar was many times stronger than required by regulations). The car's frame was seriously damaged, it could be repaired (and the car could drive again), but it was impossible to restore the original strength. So the team decided to trailer the remainder of the race.

The left rear tyre probably got the puncture at the last campsite, which was full of thorns. When it got flat, it effectively braked the left side of the car, while the motor (in the right rear wheel) pushed the right side forward. The resulting torque was apparently enough to make the car spin, just when the driver decided to pull over anyway.

Twente(July 2009)
During a test drive of their solar car 21Revolution, the steering system broke off. It drove straight forward in a curve, hitting a small pillar at low speed (30 km/h). The pillar hit the rear-right side of the car, seriously cracking the frame. The team built a new frame and repaired the broken one.

The part which broke off was held by an adhesive joint onto the composite (carbon fibre&epoxy) frame. At the location of the joint, the top layer of the composite detached from the other layers (the adhesive joint itself actually stayed intact). In the repairing progress, some parts were redesigned and some adhesive joints were reinforced with bolts and nuts.

Nuon(2009)

Nuna5 just after the crash (source, video)

Three weeks before GGC2009, the Nuna5 blew its rear tyre when driving 110 km/h. Then it lost track on the road, spun and slid sideways off the road. The car hit the embankment, the suspensions of all wheels broke off and the topshell (with the solar panel) flung off the car. Both parts then came at rest, severely damaged, but luckily the driver was uninjured. The repairs lasted one to two weeks and the repaired car managed to drive the race and finished 2nd. Crash investigation by the team revealed that the tyre blowout was the single cause of the crash, the suspension and other mechanical parts were not malfunctioning at time of the crash.

Umicore(2009)

Umicar Inspire just after the crash (more)

Umicore crashed twice during qualification and once during GGC2009. In the first qualification attempt, it was hindered by a slower car after which it had to brake firmly to avoid an accident. Because of this, it got a second chance, in which the right front suspension broke off. The team repaired (or rather replaced) the right front suspension and got a third chance. In this run, the left front suspension broke off, seriously damaging the body. The team repaired the car again and got a fourth chance and managed to qualify for the race.

On the first day of GGC2009, it started at 9th position and managed to reach the first checkpoint at 2nd position. Somewhere after the first checkpoint, when the car was driving 100+ km/h, the car got uncontrollable, slipped, got off road and curled around a tree along the road. The driver was unhurt, but the car was destroyed.

Team Umicore filmed
their own fate
(video after 0:30)

The cause of the qualification crashes were in the ball joints of the front suspension; they were worn out and got loose (message).

About the third crash the team reported that it got caught by sudden strong winds (also called a 'willy willy'). Initially the team also wrote that the car was difficult steerable because of the qualification crashes. Some people indeed saw the car swerving a bit, which can also be seen in this video. There is no doubt about that the strong winds, which attacked the car, are indeed very dangerous (Nuon also got caught by a 'willy willy' in 2007, which sucked the Nuna4 sideways by one metre or so during a test drive), but I'd say that when a car's steering system is smooth and perfectly responsive, it is better able to do the necessary actions to retain grip on the road in extreme circumstances.

In 2007, Umicore also had steering difficulties on the second day of WSC2007, because some parts of the steering system were worn out. In this video the car almost knocked over a cameraman.

The media crew of the team published some video fragments about the crashes of 2009 (Dutch spoken; not subtitled but the images speak for themselves).

Aurora(2009)
Aurora 101 blew a tyre on the first day of GGC2009 when driving 90-100 km/h. The driver then tried to keep the car on the road, but the front suspension locator broke off (news post). No injuries for the driver and only the front suspension was damaged. The repairs lasted all day and on the following day the team continued the race, eventually finishing 6th in Challenge class.

McMaster(2009)
The right front wheel buckled and broke off on the second day of GGC2009 (news post Oct 29) after which the car fish-tailed. The driver was uninjured, but the team decided to trailer the remainder of the race.

Twente(Oct 2009)

Crashed 21Revolution
(photo: Gijs Versteeg; more)

On the third day of GGC2009, the car 21Revolution blew its right rear tyre after which the car got uncontrollable. It got off road, flipped upside down in which the wing (with solar panel) flung off. The driver was unharmed, it only got a scratch in getting out of the car. The wing landed on the road upside-down, but most of the array was undamaged. The team managed to repair the car and continue the race on the next day.

On the day before the crash, the team also got three flat tyres, reportedly because of debris on the road, but these three flat tyres did not result in a crash.

Maybe one reason why the car flipped upside down is that it has a high center-of-gravity (because of the tiltable wing mechanism) and has only one front wheel; as described earlier in this article, such cars are less stable.