Chassis Design

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.

Guide Rollers

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.

Optional Extras

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.

(TROLLEY) STEERING

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.

THREE-WHEELED CAR

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..

DRIVEN AXLE

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.

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.)

TESTING

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.

BUILD ACCURACY

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.

BEARINGS

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.