Design Guide - Designed Cars
Designed Cars - Build Guide 2020
All Solar Cars are made up of a number of straightforward components: a Body, Wheels, (including Drive Wheel), Guide Rollers, Motor and Motor Mount, Solar Panel and Panel Mount, Electronics, and Ballast. The Chassis is the thing that connects all of those together. Because of this, it is the first thing to think about when designing a vehicle, as the components that put your frame or chassis together will determine your car's eventual performance, as well as your design philosophy and weaknesses. The Chassis is very very important.
A Chassis can take many forms. In its clearest representation, it is a discrete framework of strong materials which form a framework for other components to sit on or attach to. Many high performing solar cars make their chassis out of carbon fibre and aluminium rods, from high density foams and Plywood, or from other materials like Balsawood. The wheels will attach to axles, and guide rollers and a panel mount will attach to your main frame through some way or another. Your body, if you have one, would attach to your frame also. You can get away without the body and panel mount altogether depending on your chassis design. Some people may create a body which is strong enough to be the chassis too. There is no right way to make a chassis, although there are plenty of wrong ways. This page will suggest a couple of strategies.
There are plenty of materials to make a chassis out of, and the best will use a multitude in their strengths. They all have one thing in common: they are strong and they are light. This list is not exhaustive.
A cheap and tough solution, many simple cars use a sheet of this as a base to strap axles and a body to. It is quite flexible on its own so it should be coupled with some form of reinforcement
Plywood's softer cousin. While sheets are a popular body material, it is too soft to form a load bearing chassis like plywood can. Instead, square rods make good alternatives to aluminium axles. Careful with how you attach the wheels
ALUMINIUM SHEET (1-3mm)
While too heavy to make the whole chassis out of, this is a must for load bearing intersections with axles, guide rollers, ballast and panel mounts. It reinforces plywood enough not to crack and break at high pressure joints. It is also appropriate for crafting junctions for bar chassis.
CARBON FIBRE (flat or round rod)
Ever tried to break carbon fibre by bending it? you won't. It will bend but will not break. If reinforced, it won't bend either. It's also light as feathers. In other situations, a strip or sheet of carbon fibre is also useful (think the same applications as Plywood). The downside to carbon fibre is connecting it to things. The fibres will shear apart when there are holes through it (think, a screw for example). So other materials must brace and envelop it for protection.
This is a popular composite material to sheath round rod carbon fibre, which enhances its strength, reduces its flexibility, provides much needed protection from damage to the fibres, and barely adds weight. Be sure to glue the two together.
NUTS AND BOLTS
Nuts and bolts have the advantage of making your chassis modular, accessable and replacable, both in construction and in racing (remember: things can break if and when you crash).
Available through Scorio Technology, these brackets can hold bars to each other, or to something flat, with 4 screws. If they are too loose, leave a couple layers of paper between things to fill the gap.
GOOD OL. TAPE
Often used in a pinch. Sometimes used deliberately. Gaffer and other strong cloth tapes can be ample strong to hold the structural components of a chassis together. They're also a lot easier to apply than other joining methods, and cover a wider range of contexts. However, they are temporary by nature and adhesives will fail. Other good tapes to consider include duct tape, electrical tape and strong double sided tape.
Not for the faint of heart. These things take time, precision, understanding and experience. And a 3d printer or mill to machine from block aluminium. It's a big undertaking but lets the chassis be adjustable but strong.
TWO PART EPOXY
This one is a staple of chassis construction. Known by the brand name "Araldite", this stuff is liquid but sets solid and firm around whatever you scrape it onto. As such, there are any number of applications for the stuff in chassis construction. The enormous caveat is that Epoxy is as brittle as glass, and will break in many situations.
String is excellent in tensile strength, and thus wrapped around things, those things tend not to move. Not only good for holding two things together, but also in certain orientations. The downside to lashings is that the string will eventually come undone. If you lash something, then cover it in Epoxy, the composite of the two materials will be strong enough to not break in almost any circumstance.
POLYURETHANE WOOD GLUE
This stuff is tough, strong, and will "bite" to almost any material, especially woods (like, you know, plywood). Caution: this glue takes many hours to set, and will expand in that time. If you do not clamp your work, it will be a mess when you return to it. The bubbles formed in the setting process (causing the expansion) will also be hard to remove once set, so best to clean up within the first 40 minutes.
Your car needs a solar panel. It also needs wheels, and it needs four guide rollers (see the little things below the blue car [above]. Between them runs the guide rail of the track). Guide rollers are much smaller than wheels are vary greatly in size. However, the space between the guide rollers is of critical performance, as too wide apart will cause "fishtailing" (bouncing down the track along the guide rail, wasting a lot of energy) and too close together will slow the car down in the corners. In most cases, 27mm between the tips of the guide rollers is best, although this will change from car to car. It is better to be a tiny bit wider on the back than the front. Of course, we're talking about the distance between the tips of the rollers, so factor in roller size when designing where their mounting holes will go (they do spin). That spot should probably be reinforced as it experiences a lot of energy in a corner.
There are a number of "features" that can play into your advantage. common ones are presented here. It is worth pointing out that the common car will have none of these, and it is very easy to win with none of these. A well made car will always beat a well designed car. However, if all cars are made perfectly, the cars with these features will be marginally faster. These are the areas where annual incremental improvements occur.
This is a passive "trolley" that the wheel is mounted on, which can pivot on an axle to allow the wheel always to point in the direction of travel. This is the common form of "steering: found on many cars that reduces the wasted energy of a wheel skidding around a corner. The difference in time between a car with well implemented steering and car without is a few seconds. The key word there is well implemented, for if it wastes more energy than it saves, or it is so stiff it won't pivot under normal circumstances, it does more harm than good. Common ways to waste energy are to have the pivot flex, or the arm itself to be flexible or excessively heavy.
Why run with four wheels? Why not run with three? It saves one quarter of your rolling resistance by having one quarter fewer wheels. it also saves weight: however much the wheel weighs, plus the axle to connect to it. The obvious is reduced stability from these wheels, especially at higher speeds. Proceed with care, but 3 wheeled cars are often incredibly fast. Sometimes too fast..
A normal Solar Car has four wheels, but unlike conventional road cars, these wheels spin on stationary axles. The bearing is between the wheel and the axle, rather than between the axle and the chassis. This means that you can mount things to the axles (like a body). It does, however, mean that all of your power is coming from the one wheel your motor is directly attached to (called your drive wheel). This imbalance of power will push your car to one side, towards the guide rail (eg a car with the drive wheel on the front left will try and steer to the right). This is called Motor Torque, and it wastes energy.
A driven axle is similar to a road car, in that the motor drives the whole axle, thus both wheels attached to it. The axle must be held in place to the rest of the chassis through blocks and bearings, and can be hard to design for. The wheels also need a way of spinning free against the axle, as the distance traveled by the two are different at a corner (the outside travels further). Look up how differentials work and why they're needed - same problem. If you can manufacture a differential that is under 20g, it will be worth it. The popular solution is to use a one-way ratchet bearing, such that wheels can spin faster than the axle, but not slower. The poor man's method is to anchor one wheel firmly to the axle, and have the other one on a normal bearing. This does greatly reduce the benefit of a driven axle, but is a lot simpler to pull off.
The benefits of a driven axle should be clear: no Motor Torque, no wheel spin, full power in both corners, and the ability to hide the motor inside the body of the car, (better aerodynamics) The flipside to better torque off the line (and less wheelspin) is unfortunately less power and more losses and complication, but this can be largely accommodated by gear ratios.
Motors, Panels & Electronics
There are many small, low voltage, direct current motors available, but not all are suitable for use in the model solar competition. The dilemma is which motors are suitable and which is the best for your vehicle. The data below will give a starting point.
Student designed cars, in this event there is no limitation placed on the motor used it is entirely up to the team.
Within the competition regulations you can choose from a limited number of motors. There are some basic motor features that limits your choice.
- Motor size and weight. As in all motor sports power to weight ratio of the vehicle is critical to performance. This is particularly so in the boats.
- Motor power. The motor must be capable of converting the electrical power available from the solar array into mechanical power. It is no use having 6 watts available from the solar array and a motor that can only produce 1 watt. Things are just as bad in the opposite direction if you have a 30 watt motor a significant portion of the solar array output will be used in just running the motor. The additional weight of this higher powered motor will adversely influence the power to weight ratio.
- Motor rated voltage. This rating must be compatible with the output from the solar array. But do note that in many cases the motors used in the student designed car competition are rated at 6 volts but are being operated at up to 20 volts. Operation at over voltage increases motor RPM and the power it can deliver but will shorten the motor life.
- Motor efficiency. Obviously the highest efficiency possible is desirable. Generally high efficiency is only available in proper industrial type motors not the typical toy motors available at the hobby shops. High efficiency usually comes with a high price.
- Motor RPM. This does not matter much in the student designed car event as a gear reduction is required between the motor and drive wheel. The gear ratio is chosen to suit the motor RPM. It is very important in the boats as matching motor RPM to propeller characteristics is critical in obtaining maximum power transfer from solar array to propeller. In the advanced boats a gear reduction is permitted so it can be selected to obtain maximum power transfer. Many advanced boats use a direct drive, junior boats must use direct drive which makes motor RPM critical.
Motor type is another variable, typically permanent magnet DC motors are used. There have been many suggestions that a brush-less DC motor such as those used in model aircraft and drones would be better. So far no one has come up with such a motor that is suitable for the model solar competition. The main issues seem to be RPM and efficiency and possibly more importantly the characteristics of the solar array being so different to the batteries normally used to power these motors.
Keep looking. Just because no one has found a suitable motor yet does not mean there is not one out there somewhere! You never know what will appear in the future.
What motors do most competitors use? This is a good place to start.
We scrutineer the student designed cars each year, so have a very good idea of what motors are being used.
In over 90% of cars we see the Faulhaber 2232 6 volt motor. Maxon motors make up most of the remainder.
The Faulhaber and Maxon motors are both proper industrial motors with efficiencies around 86%. From dynamometer tests conducted on both these brands there is nothing in their performance that would suggest one is better than the other. They both perform to their advertised specifications. The range of motors available from Maxon is wider than from Faulhaber. It is most likely that the main reason for the Faulhaber being so prevalent is its increased availability for competitors to acquire with relative ease.
Tests have been performed on motors from two other manufacturers with claimed performance equivalent to the Faulhaber and Maxon. Neither of these motors performed to the quoted specifications one was over 20% below the claimed performance, while the other was quoted to have a no load current the same as the Faulhaber but when tests actually had four times this no load current and was so poorly constructed that it exhibited serious vibration at high speed along with low efficiency. Be careful when selecting motors from relatively unknown manufacturers.
In order to get the best out of your vehicle it is important to know what power your panel is delivering. For the student designed cars there is strict limits, the regulations stipulate that the panel power produced at standardised AIM 1.5 sun level must not exceed 10 Watts. And when the car is presented for race scrutineering the panel is checked using calibrated equipment.
While this may seem a complicated process your can perform a Power Test using some relatively simple equipment and still estimate the power your solar panel is generating to within less than 10% of what a calibrated system would measure.
If you also use a Calibrated solar panel at the same time you test you can adjust your measure values to correct the reading to the AIM 1.5 standard to generate a more accurate maximum panel power value.
What is a Solar Panel?
A solar panel is one or more solar cells, or photovoltaic (PV) cells, that coverts the energy in light waves into electrical power.
These cells all share a same trait, they generate a voltage and current flow when a photon of energy in light is absorbed by the cell. This powers devices.
There are MANY different types of construction technique for creating a PV cell and they range from the relatively cheap and common Silicon based unit all the way to the very expensive and uncommon Gallium arsenide devices that are used to power satellites and even the International Space Station.
Practice Solar Panel to AIMSC & VMSVC 2018 Regulations
The 2017 regulations changed the rules around solar panels significantly from all previous regulations. See current regulations for more precise details on these panels' construction, properties and performance.
Under the current regulations the car must race with a panel provided by the organisers, however for practice the competitors must provide their own solar panel.
This should not present any problems to most previous competitors as the panel to be provided by the organisers is a single Scorpio No. 26 panel (14 cells) calibrated to output 5.5 watts in full Sun. The majority of competitors have been using two such panels in past competitions, a simple reconfiguring of their existing panels will give them a practice panel.
To make a practice panel that exactly matches the panel to be provided by the organisers two actions are necessary, firstly mount the single Scorpio panel to an aluminium backing identical to the backing the organisers will be providing and secondly, calibrate the panel to produce 5.5 watts at full Sun.
The aluminium panel backing
Details of the wiring and connectors is provided in the regulations, the aluminium backing will be manufactured to the following sketch.
Calibrating the panel
For more information, see our guide to solar panel testing.
Without a calibrated light box, it is difficult to achieve exact calibration, however the simple process described below detailing how calibration can be performed in Sunlight will give reasonable accuracy.
At a known Sun level you can measure the panel’s open circuit voltage (OCV) and short circuit current in amps (ISC). Make these measurements with the solar panel at 25 Degrees C and at a Sun level greater than 70% for best accuracy.
Approximate power can be calculated by multiplying voltage and current together then multiplying by 0.7 which is an average fill factor for these panels.
OCV × ISC × 0.7 = Power in watts
Having calculated the power at a specific Sun level as described above, ratio the power obtained up to the power expected at full Sun.
To calibrate the panel to produce 5.5 watts at full Sun, mask a portion of the panel with a light excluding covering to reduce the power produced to 5.5 watts. Note this covering should cover the same proportion of every cell in the panel. The approximate size of the mask can be calculated using the calculated full Sun power and the desired 5.5 watt maximum and using the ratio of these powers to reduce the panel surface area by the same ratio.
Always recheck the panel power after masking to ensure the correct size of masking has been applied.
(Special note: Not all Scorpio No.26 panels will produce 5.5 watts, some panels may only produce 5.2 watts. In past years Scorpio provided “standard panels” with a power output of from 5.2 to 5.8 watts at a lower cost than their “premium panels” which had power output above 5.8 watts.)
This stuff is really important. More of your speed and performance will come from granular improvement of what you've already built, rather than trying to build something better. This is testing, and improving the quality and reliability of construction. You can do this in many ways: you can run localised experiments down a corridor or in your hand, you can test on the real track on the Saturday of the State Event, you can visit Box Hill High School during any Wednesday of the school year (including school holidays) and make use of their facilities and test track. Below are a number of features to observe and improve, each of which will shave seconds off your time if addressed properly. Of course, all this takes time and should be factored in to the build time at the very beginning.
TIME AND PROJECT MANAGEMENT
There's a reason why this is at the top of the list. It really is something you have to address before you even begin, and won't save you if you begin construction two weeks before the event. If well managed, you won't be in such a position. Before you begin, sit down and figure out if you have what you need to build this thing: experience, time, resources, equipment, people. If you don't, you need to find them (such as contacting us). You also need to plan your project from day one to day 200, and what you will do when. This will save you from only breaking ground on construction on the 16th September. The more granular your schedule is, the easier it is to tell if you're on track. Once you've made it, try and stick to it.
The things to consider when making your timeline or schedule are all the stages of building the car: planning, design (drawing the whole thing up), sourcing components, building the vehicle, and testing it. You also need time to make a poster (as prescribed by the regulations) and all good schedules will leave a few weeks of contingency, such that when things go wrong, you still have spare time. If things don't go wrong, take a week off to congratulate yourself.
POWER TO WEIGHT RATIO
A good car will have a power to weight ratio of about 160g/w. The best way to improve this is to reduce the weight, which largely comes down to the materials choices and their applications. Often, an easy way to shave weight is to remove the excess, like making a foam body thinner, cutting off excess metal from the chassis, or identifying whole components that could be removed. There are always ways of shaving weight. Another benchmark to aim for is to get the car weight (minus the ballast and panel) to less than 400g.
This refers to the accuracy of the construction of your car - your axle alignment (how parallel they are), and the tolerances of your gears and guide rollers. A poor quality of build can cost a car five seconds. Put another way, at max sun, a car with axles 3mm out of alignment (93 or 87 degrees compared to the other axle) does the same thing to a car as adding 700g of weight. Do I have your attention now? If pushed down a smooth corridor, a car with a good axle alignment should make 20-30 metres before being clearly off course.
Another victim of build quality is your gear mesh on your motor. Gears should require very little effort to spin, should never stick and should feel smooth to turn, and should not make noises when running.
If your axle alignment, steering or guide roller distancing are off, your car may "crab" down the track, bouncing between the guide rail down the straight. This is a symptom of wasted energy from one of these systems.
These will slow your car by 4-6 seconds if not addressed. Making ball bearings run with less effort has both an exact science to it, and no particular procedure at all. Your bearings should be clean and oiled, but not over oiled. They should always be run in - never run a fresh bearing for a competition.
An easy benchmark for validating the effectiveness of bearings is to give them on a wheel a good fast spin in midair, and wait to see how long before they stop. You'll find that most new bearings will spin from 8-15 seconds. Old or dirty bearings may only make 3 seconds. Once you've done the above, they should last for more than 30. If you run them in enough, they can last over a minute before finally stopping. Then, you are wasting close to no energy, which is exactly what you want.
Designed Car race simulation
This Simulator is a mathematical model of a student designed car on the figure of 8 track. It is not a 100% accurate simulation as it approximates a generic car configuration.
If you supply the model with accurate input data the resulting predictions are typically within 5% of results obtained from track testing.
Use this mathematical simulator to aid your testing and investigating for your designed model solar car.