Solar Challenge Supplies
This section has been designed to provide you with an overview on where to go for your solar car bits and pieces. Further down the page you’ll also find details on how to build a super easy, low cost car chassis. You can literally put one of these together in your lunch break at school.
Suppliers
Local hobby shops can be a good source of basic construction materials but you will need to turn elsewhere for specialised solar car components. You cannot just use any wheels, motors, gears, etc and expect to compete at the highest level. Some of these can be manufactured by teams themselves but not everyone has the skills or equipment to do so. The two main suppliers of commercially available solar car parts are:
These companies are both based in Victoria and offer a comprehensive range of products that have been designed specifically for the solar challenge. It’s highly recommended that you check out their websites for a full list of inventory. To help you get started we’ve included Scorpio’s main solar catalogue below but be sure to also check out a few of their other catalogues.
R & I do not list prices on their webpage so you will need to contact them for a quote. Both sell ready-to-assemble solar car components and Scorpio now even offer a CHALLENGER SOLAR CAR kit that includes a basic carbon fibre chassis.
(Scorpio Technology’s CHALLENGER SOLAR CAR kit)
Building a car to race in the 2018 solar challenge can be as simple as putting together this kit and adding a Faulhaber 2232 motor, egg cabin and space for the 200ml Juice Box. As an example, the following car finished in the top 4 at the 2017 Victorian event using one of these kits.
R & I components are typically better quality but a full kit is considerably more expensive. Two or possibly even three Scorpio kits can be purchased for the same price so schools may like to take this into consideration. These two are the only places that offer parts to build a complete car without spending time in a workshop.
Teams can reduce costs by making their own wheels or guide rollers. This means using workshop machinery, namely a Lathe. Sometimes modifying an existing wheel might be easier than starting from scratch and some potential candidates for this are the 60mm and 37mm wheels from the Technology Education Centre in South Australia. These are a much cheaper option if teams are able to turn them out to fit a bearing.
Another alternative now available in more and more schools is 3D printing. Be aware that wheels made by 3D printers may still need to be trued up in a Lathe or drill for optimal performance. We’re currently investigating a few printing options and will discuss our findings here once some more testing has been carried out.
Supplier Breakdown
Here’s a quick breakdown on where to go for your main car components.
| Scorpio | R & I | TMSC | Ebay | Tech Ed | HobbyKing |
|
Wheels Gears Bearings Motor 6mm collars 6mm carbon fibre tube Axle brackets Electronics unit Motor mount Competition solar panel Micro Deans plugs |
Wheels Gears Bearings Guide rollers 6mm collars Motor mount 18mm standoff |
Wheels Gears Bearings Guide rollers 1V solar module for testing 4mm male banana plugs |
Bearings 6mm carbon fibre tube Low cost solar modules for testing 4mm male banana plugs Micro Deans plugs M3 washers, nuts and bolts M3 spacers and standoffs |
Low cost wheel blanks |
6mm carbon fibre tube 4mm male banana plugs M3 washers, nuts and bolts |
Please keep reading down the page for more details on these items. This is then followed by an example of how to build a simple, low cost car chassis for the 2018 competition. Use this information in combination with everything covered on the Challenge Help page to assist you with your full car design. Unfortunately there’s no way of getting around the expense of a Faulhaber 2232 6V motor but an attempt has been made to keep all other costs to a minimum. Keep tuned as more info and images are added over time.
Solar Panel
The solar panels used at competitions are based on Scorpio Technology’s SOLAR26 panel. Event organisers hand these out to teams before each race. You don’t need to buy your own panel unless you want it for testing purposes.
(Scorpio SOLAR26 aluminium backing design)
The official panels are reinforced with an aluminium backing that is made according to the design above. 4mm Banana sockets are used for quick and easy connection with car wiring.
SOLAR26 panels are approximately $90 each but you can also do some basic testing using a much smaller and cheaper solar module. A Faulhaber motor will have no problem running off a single 1V 500mA module and this will be enough to power a solar car on a sunny day, albeit quite slowly. Solar car wiring, motor direction and the on/off switch can all be tested using such a module. Only the Scorpio and Automax electronics units cannot be tested at such low voltages.
As it’s likely that you’re already sourcing parts from Scorpio, it may be easiest to just combine on postage and grab a module from there. A full list of hobby panels can be found in Scorpio’s 2018 Technology Catalogue and quite a few of them would be suitable. Of these the SOLAR7 and SOLAR8 panels offer the highest power per dollar value but a lower cost option like the SOLAR14 would probably also be sufficient. Or you could try the $14 SOLAR13 panel which consists of four separate 1.5V sections and allows for several different wiring configurations. Connecting these up in series should give you enough voltage to test an electronics unit (4 x 1.5V = 6V).
If you don’t mind waiting a few weeks on free shipping from China or Hong Kong then you can find even cheaper solar modules on Ebay. Just don’t go too low on the milliamps otherwise you may not get the required motor torque to get the car moving, especially when it’s overcast. We have seen 1V 0.5W modules on there for around $2 or slightly larger 6V 2W panels, similar to Scorpio’s SOLAR13, for under $5. These 6V panels would again be suitable for testing your electronics system.
Teams can also email us at the TMSC for a 1V 500mA test module. We have a number of these ready to be handed out at no cost.
(L-R: Side by side comparison of a Scorpio SOLAR26 panel with backing, 6V 2W Ebay module, Scorpio SOLAR8, 1V 500mA test module available from the TMSC)
Motor
Please see the Challenge Help page for more information on solar car motors but you pretty much want to use a Faulhaber 2232 6V to power your solar car. These can be purchased from Scorpio Technology for close to $100 each and are the most expensive part of a model solar car. If you look after them they will last for many years though. Some schools have used the same motor for 10+ years of competition.
Provided it can be added and removed in a short amount of time, clever design might even allow a single motor to be shared across multiple cars. This is not ideal but would be acceptable during earlier round robin rounds of the Tasmanian competition.
The TMSC is currently in possession of several Faulhaber motors and we are looking to lend these out to new schools in 2018. Please email us if you’re interested in receiving one. It’s first in, best dressed and we are limiting them to one per school so they get shared around.
Although it’s tempting, try not to use batteries or a power supply to test your car or expensive Faulhaber. This can result in high current flow, accelerated brush wear and possible motor damage so it’s best to play it safe and avoid the risk.
Take special care of how you attach your wiring to a Faulhaber motor. Use a cable tie or tape to fix the wiring to the body of the motor to help relieve stress on the terminals. Some teams have even been seen to apply hot glue to each terminal once soldering has taken place. Accidentally pulling on loose wiring and bending the terminal back and forth can otherwise lead to eventual metal fatigue and cause it to be ripped from the motor.
(Faulhaber 2232 6V motor wired up with a Micro Deans plug and cable ties)
Carbon Fibre Frame and Axle Brackets
Countless teams have used carbon fibre tubes over the years due to their high strength to weight properties. They are a simple and effective material to use for solar car axles or even a complete chassis. Both the Scorpio and R & I kits are designed to be used with 6mm axles. The main issue with these is that they are round and difficult to connect or attach anything to. Past teams have machined special blocks for connecting them or wrapped twine around joints and then epoxied these for a permanent fix. These are still valid options but Scorpio Technology now have a much simpler solution in the humble axle bracket. These are arguably Scorpio’s single best product and highly recommended by the TMSC.
(Scorpio Technology Axle Brackets)
Scorpio sell a set of 8 brackets in their Axle Bracket Kit (AXBKTK) for approximately $6 or you can buy the Axle & Frame Kit (AXFRK) for $17.50 which also includes enough 6mm carbon fibre tube for one car. This will give you the same basic chassis as the CHALLENGER SOLAR CAR kit but without wheels. Couple this with some free wheels from the TMSC and you can put together a basic car at very little cost.
You can also use these brackets in other ways as they are a great way to attach carbon fibre tubes to flat surfaces. They are made from approximately 1mm thick steel and are robust enough to use even when cut in half. An example of this can be seen below on a car from Rosny College in 2017. Not only does this save on weight but also reduces car cost as a set of 8 brackets can now be spread across multiple vehicles.
(A 2017 Rosny College team used axle bracket halves to attach 6mm tubes to flat carbon fibre plate)
The example here uses just 2 whole brackets to attach carbon fibre tubes to flat surfaces at both the front and rear of the car. More brackets and some custom connection plates are needed in designs that still use a tube-to-tube connection as can be seen below.
(Using axle bracket halves with a custom connection plate, top and bottom)
This type of design requires half an Axle Bracket Kit and will weigh less than a full bracket version if the connection plates are made from a lightweight material like carbon fibre plate or aluminium sheet metal. One of the advantages with this arrangement is that there’s a flat surface to ensure perpendicular guide roller standoffs. Designs using only Scorpio brackets like the CHALLENGER CAR kit, for example, need to be careful that bracket curvature doesn’t affect guide roller angles.
Scorpio’s 6mm carbon fibre tubes are a very reasonable and convenient option but you may be able to find some even cheaper options if you have a bit of a look around on Ebay or Hobbyking. Also be aware that there are two types of tubes with different manufacturing processes. Most tubes, including Scorpio’s, are pultruded and have all of their fibres running length-ways down the tube. This makes them very stiff if you try to bend them but weak in other directions and susceptible to splitting if you’re not careful. The other type are roll-wrapped tubes that have carbon fibre wrapped at various angles around a rod. Due to a more complicated manufacturing process these tend to be a little more expensive but are much less likely to split.
(Roll-wrapped and Pultruded carbon fibre tubes)
Feel free to check out your local hobby or archery store to see what carbon fibre they have on offer. One of the best Ebay stores we have seen based here in Australia is Rippa Racing. When in stock this supplier sells both pultruded and roll-wrapped 6mm OD x 4mm ID x 1000mm tubes. Hobby king also offers an even lower cost 6mm OD x 5mm ID x 750mm option. These tubes are the lightest of the lot but you’ll need to be careful not to split them due to their reduced wall thickness. These tubes also tend to be a little over 6mm in diameter so you’ll almost certainly need to sand down sections where the wheels, collars and axle brackets go.
Depending on frame design, and the number of cars being built, buying multiple tubes from Ebay or HobbyKing and combining on postage may be more cost effective than buying from Scorpio. Teams and schools will need to crunch the numbers themselves for their particular situation.
Wheels and Guide Rollers
Scorpio’s CHALLENGER CAR kit includes a set of 4 wheels. These are all 70mm in diameter and one of these, the drive wheel, comes with a groove for an O-ring tyre to increase traction. R & I offer better quality wheels which are 63mm in diameter. Be aware that this means the two types cannot be interchanged directly without affecting the ride height of the car. Wheels from the Technology Education Centre are 60mm and those provided by the TMSC are smaller again at around 52mm due to a smaller gear ratio. Smaller wheels will lower car heights on a standard carbon fibre chassis and help improve cornering stability.
(L-R: 70mm Scorpio drive wheel, 63mm R&I drive wheel, 60mm Tech Ed wheel blank, 52mm TMSC wheel)
Scorpio sell packs of 10 wheels for a bit over $30 so you’re looking at around $3 per wheel. This is a relatively low cost option for schools, especially for those that are intending on entering multiple cars. You will need to contact Scorpio to find out whether they will sell smaller quantities for a single car. R & I’s wheels are machined to a higher quality but cost approximately $15 each. Tech Ed are by far the cheapest option and sell packs of 100 wheels for around $50 but these then need to be machined out to fit a bearing. Tech Ed are happy to sell smaller quantities but you will need to email them a request rather than order online.
Scorpio do not currently offer a guide roller and the CHALLENGER CAR kit uses plain bearings to guide the car around the track. This is not ideal for negotiating track misalignments so teams like the 2017 Victorian entry seen up the page use Scorpio wheels that have been machined down on a lathe. R & I guide rollers are a good option here and are relatively cheap at $2 each. Tech Ed sell a cheap 37mm wheel that could be suitable but again needs to be turned out to fit a bearing. Or you can use some from the TMSC. We have designed these to have the same 25mm diameter as the R & I roller so the two can be exchanged at any time.
(L-R: 37mm Tech Ed wheel blank, 25mm R&I and TMSC guide rollers)
(Attaching R&I and TMSC guide rollers using M3 socket head bolts)
If you take a closer look at these you’ll notice that the head of the M3 fastener is recessed up within the roller. This let’s you get the roller as low as possible without catching or scraping the bolt head on the track. R & I rollers are sandwiched between two flanged bearings while those from the TMSC use only one which has been press fit into place.
Bearings
The two most common types of bearings used by cars in the solar challenge are the F623zz and MF106zz. The outside diameter of these is 10mm and aimed to fit inside Scorpio or R & I wheels and guide rollers. The “F” signifies that these are flanged, an important feature that helps with alignment or locking a wheel between two bearings. The “zz” indicates metal shields as opposed to the bearing being open or having rubbers seals. This offers the lowest friction option while still giving good protection from dirt and dust ingress.
The main difference between the two bearing types is the internal diameter. This is 3mm for the F623zz and 6mm for MF106zz. 3mm is a particular useful size as it big enough to slip over a threaded M3 bolt and allows for wheels and guide rollers to be securely fastened to a model solar car. The larger 6mm version makes it easy to slip wheels onto a 6mm carbon fibre axle, as is intended by both the Scorpio and R & I kits.
All of the bearings supplied by R & I are the MF106zz type. These are suitable for fitting wheels onto a 6mm axle but also allow guide rollers to be attached by an M3 fastener using their flanged aluminium inserts.
(L-R: F623zz, MF106zz, MF106zz with R&I flanged insert, standalone flanged insert)
Bearings from R & I are good quality but cost around $5 each. One of their full car kits requires 16 of these so you’re looking at $80 just for bearings! Scorpio are a little less expensive and offer packs of 10 for around $30. There is however another option and that is Ebay. There are lots of Chinese sellers on there offering packs for much much less. We have recently seen sets of 10 F623zz and MF106zz listed for as low as $3.80 and $5.20 respectively!
These are shipped free from China so keep in mind that postage will take several weeks. There may also be the odd bearing that isn’t up to standard but performance tends to be acceptable in general. Low cost Ebay bearings were used by the winning car at the 2016 national event from Rosny College. Any wheels or guide rollers provided by the TMSC will come supplied with these bearings.
M3 Bolts, Nuts and Washers
M3 fasteners are typically used to attach wheels, guide rollers, motor brackets, etc. to a solar car frame and are readily available in a wide range of types and lengths. You can get zinc plated, stainless or high tensile versions and several different head types. Probably the most common of these is the phillips pan head and these are supplied with Scorpio’s Axle Bracket Kit.
(Common fastener head types – image courtesy of boltdepot.com)
If you’re not careful there’s sometimes a danger of stripping a phillips bolt head, particularly when the screwdriver isn’t an exact fit. One way to overcome this is to instead use a hex socket head which will require a 2.5mm hex wrench or allen key. High tensile versions of these also have a higher resistance to bending which sometimes occurs to longer guide roller bolts when there’s a crash.
You don’t always need to use washers but they’re recommended wherever possible. They help spread the bolt head or nut load and prevent either from digging into the surfaces that are being tightened against. Regular M3 nuts are okay to use but you will need to check them from time to time as they may loosen from vibrations when racing. Perhaps a better option is the nyloc or lock nut which are much more resistant to this. These take a little longer to thread onto a bolt but definitely worth considering.
A set of pliers can be used to tighten an M3 nut but may leave it marked and rounded off. It’s instead recommended that teams use a small 5.5mm spanner/wrench which fits the flat to flat distance of an M3 nut. A basic spanner is supplied with Scorpio’s Axle Bracket kit or you can purchase a miniature spanner set from places like Super Cheap Auto or Jaycar. Your local hobby shop will also have a few options including a mini cross-wrench. A number of teams like to use these as they slip over the nuts and can make holding onto them easier. Ebay will be your cheapest avenue for individual 5.5mm spanners or cross-wrenches. The same goes for 2.5mm hex wrenches or allen keys.
(5.5mm spanner, Scorpio’s axle bracket kit spanner, mini cross wrench, 2.5mm hex/allen key, 1.5mm hex/allen key)
If you don’t want to wait on shipping, the best place to go for large or small quantities of nuts, bolts and washers is your local fastener shop. One example of this here in Tasmania is Nuts & Bolts which are based in most major centres around the state. Bunnings and Mitre 10 will also supply some M3 sizes but have a much smaller range.
Ebay is otherwise a great place to go for anything nuts and bolts related. Here you’ll find some incredibly cheap Chinese listings that also offer free postage. You can get specific types and lengths or a set with an assortment of sizes, it’s up to you. If you don’t mind waiting a few weeks on postage then definitely consider this as your lowest cost option.
(A typical socket head M3 nut and bolt set from Ebay)
Electronics
Everyone will carry the same amount of ballast at the 2018 Tasmanian event so getting an electronics system for your car is a no brainer. It will make your car faster and also easier to race across different weather conditions.
At just $11 the TMSC recommends all teams purchase a low voltage SPPCL kit from Scorpio. Please note that the solar catalogue lists this as being useful for the solar boat challenge but it is now also the unit to use for the car event. The higher voltage version was previously used when car panels had much higher voltages.
Even if you decide to use the much more expensive Automax the Scorpio kit will provide teams with a great learning exercise in PCB assembly, soldering and unit setup. It’s advised that you use Micro Deans connectors so it can be easily interchanged with an Automax, just make sure you maintain plug polarities across the two units.
(L-R: Automax and modified Scorpio SPPCL unit to include Micro Deans plugs)
An example of a Scorpio unit that has been modified to include Micro Deans plugs is pictured above alongside the Automax. Details on how to do this are on the way or you can contact us for some further assistance.
There’s a 3 position DPDT switch included with the Scorpio SPPCL kit. This isn’t needed when assembling the unit so you can use it as your regular ON/OFF switch if you like. Or you can get a smaller SPDT switch from Jaycar.
Full car wiring examples and some more information on the two units can be found on the Challenge Help page. The main difference between the two is that the Automax automatically tracks the maximum power point of the solar panel using a microcontroller. The Scorpio unit requires you to set this point manually by adjusting the trimpot with a screwdriver.
The Automax is a little more efficient at lower load voltages due to its superior circuit design. This can be seen from the following plot and will result in slightly better car acceleration at the start of a race. Top speeds will be near identical.
(Electronics unit efficiencies vs load voltage)
Retaining Collars and Spacers
Car designs like the CHALLENGER CAR kit have wheels that are fitted onto 6mm carbon fibre axles. A set of retaining collars are then used to stop them from sliding along the axles. These are fixed in place on either side of each wheel by a grub screw and have a small rim that presses up against the inner race of the bearings. Scorpio Technology sells packs of 10 collars for $4.45 and 2 are needed per wheel. R & I also supply a similar collar.
(6mm ID retaining collars)
Teams sometimes make their own retaining sleeves from a 6mm ID tube and glue these onto the axle. If you do this make sure the sleeve end that faces the bearing is square and only glue these to the inside side of each wheel. You then still use a Scorpio collar on the outside of each wheel so they can be removed but only need half the number per car.
Nuts and washers are good for packing out small distances or making small adjustments to guide roller heights. For larger spaces it may however be easier to use an M3 spacer or standoff instead. In some cases a plastic/nylon spacer will do the job but otherwise you can seek out steel, brass or aluminium ones. Just be mindful of the weight they’ll add to the car.
R & I stock an 18mm aluminium standoff which is suitable for a lot of cars using their 63mm diameter wheels. These are used to space the guide rollers away from the underside of the car so they are low enough engage with the guide channel. For designs with smaller wheels or lower car heights these are however too long and teams have been seen to shorten them in a Lathe to suit.
(L-R: 6mm long M3 plastic spacer, 18mm long R&I standoff)
Alternatively, have a bit of a look around online and you’ll find a big range of M3 spacers at different lengths. Your car design will dictate how long your spacer needs to be or you can just get a set of various lengths. A lot of these are meant for separating PCB boards so you’ll also find a selection at your local electronics supplier like Jaycar.
Drive Train and Gears
The efficiency at which a Faulhaber motor is able to power your car is down to the design and build quality of the drive train. This is the single most important area of a model solar car and encompasses how you go about using the spinning motor shaft to propel your car into motion. Making your car super light or having good aerodynamics will count for very little if the power isn’t getting to the wheels in the first place.
Without going into too much detail here, a Faulhaber motor that is connected to a SOLAR26 panel will be most efficient when it spins at around 7000rpm. That’s about 117 revolutions per second. In full sun a top solar car can reach speeds of over 7m/s. Using these two figures we can then go ahead and approximate a drive ratio.
First, consider attaching a wheel directly onto the motor shaft. 7m divided by 117 revolutions gives you around 6cm per revolution. This means that the wheel diameter needs be about 6cm / 3.14 = 1.9cm (using circumference = pi x diameter). This is smaller than the diameter of the motor itself and makes a successful setup more difficult. The problem gets even worse at lower sun levels where the wheel would need to be even smaller.
An example of a direct drive setup can be seen below. This team used the larger and much more expensive maxon motor mentioned on the Challenge Help page. This motor is more suitable for this kind of application but the wheel here is still too large. The only way a smaller wheel could be used is if you angled the motor or designed a drive roller to run along the top of the guide channel.
(St Paul’s direct drive setup in 2010)
Note that attaching a wheel directly onto the shaft will result in accelerated bearing wear and likely lead to motor damage. Supporting a car like this will result in excessive radial loading, especially when there’s a bad mismatch in the track. A safer option would be to design a system that supports the wheel with a separate set of bearings.
Also note that track misalignments will have a greater affect on smaller wheels. Ever gone skateboarding and had a small rock wedge itself in front of a wheel? Ride a bike over that same rock and you’d barely notice it was there. It’s the same kind of thing when your solar car wheels run over an edge in the racetrack.
Instead of dealing with these issues it’s much easier to gear down the motor speed. This gives more torque and so a larger drive wheel can be used. The simplest and most efficient way of doing this is by using a single-stage spur gear reduction as seen in the following examples. A direct drive setup is theoretically more efficient but the minimal losses seen in good quality, well-aligned spur gears make them a great option for a model solar car racing.
(Single-stage spur gear reduction examples)
You can choose any wheel size really. Just keep in mind that a small wheel will find it harder to roll over track misalignments while a bigger wheel will add more weight to the car. Scorpio’s wheels are 7cm in diameter which have a circumference of 7 x 3.14 = 22cm. At a speed of 7m/s this means the wheel needs to spin at 700cm / 22cm = 32 revolutions per second. If the motor spins at 117 rps then the gear ratio needed is about 117 / 32 = 3.7:1.
Usually a slightly higher gear ratio is used which makes for a more efficient system during acceleration. Scorpio offers a 20-tooth brass pinion to match up with the 80-tooth gear on their wheel to give a ratio of 4:1. If you check out the Scorpio Solar Catalogue then you’ll see that they also sell several other brass gears with fewer teeth. These cost somewhere between $12 and $15 each and are needed to fully optimise car performance at lower sunlights. The Scorpio drive wheel plus 80-tooth gear and adapter for attaching them together come in at around $15, so around $30 including the motor pinion.
If you’re buying from R & I then you’ll find that their drive wheels are a little smaller and a slightly different gear ratio is needed. They are 6.3cm in diameter and have a black acetal spur gear that press fits onto them. Note that their default spur gear is the 100-tooth version. This was used in previous years when panel voltages were much higher. It’s important that you don’t get this and now ask for the 80-tooth gear instead. Otherwise you won’t be able to make the required gear ratio. You’re after about a 3.4:1 so a 23 or 24-tooth pinion gear will suit here. This R & I drive wheel, spur gear and motor pinion combination will cost you around $40 without bearings.
At the TMSC we’ve sourced a set of low cost 56 and 20 tooth gears to give a ratio of 2.8:1. This sees the wheel spinning at 117 / 2.8 = 42 rps and requires its diameter to be about ( 700cm / 42 ) / 3.14 = 5.3cm. Reducing wheel size has the same affect as increasing the gear ratio so we’ve taken our drive wheels down to 52mm and are offering these to Tasmanian teams at no cost. Just head on over to our contact page and send us a message if you’d like to get hold of one.
The gear ratios above should give good performance in bright sunlight. If you’re running with Scorpio SPPCL or Automax electronics then the same ratio will work ok in cloudy conditions too. You can choose to leave it at that but top teams will change gears to make their car a little faster in lower sunlights. Two gear ratios will cover you across most sun levels so the TMSC is currently looking to also supply teams with a second gearing option, a 72 tooth spur gear. If you’re using a Scorpio wheel then you might like to try the 16-tooth pinion and perhaps an 18-tooth with the R & I wheel.
(L-R: 56 tooth TMSC gear, 72 tooth TMSC gear, 90 tooth R&I black acetal spur gear)
Motor Mount
Ok, so say you have your drive wheel fully fitted with a spur gear. You also have the small pinion gear for the motor. To minimise your drive train losses you now need to get them to mesh together as smoothly as possible.
The front face of the Faulhaber motor has six M2 threaded mounting holes. It therefore makes sense to use these to fix your motor to the car. You don’t need to use every hole, 3 are more than sufficient and what most teams use.
If you have a few different gear ratios then the distance between the motor and the wheel centres is going to change. This means you’ll need to adjust the distance by using a motor mounting plate. Scorpio and R & I both supply one of these as seen in the pic below.
(Scorpio and R&I adjustable motor mounting systems)
These both allow the motor to be fixed in place and then manoeuvred to and from the wheel using more robust M3 fasteners. Either setup will work well for the wheels and gears they’ve been designed for.
It’s expected that you purchase Scorpio’s $5 motor mounting kit (FAUMMK) if you’re going to be using TMSC gears. Get the FAUMOTK if you’re also buying the motor as it will save you a couple of dollars. These kits get you an aluminium mounting plate plus three M2 screws for the motor. The steel bracket that’s also included isn’t suitable for TMSC gearing but don’t get rid of this, it makes a great template for hole spacings.
(Scorpio’s aluminium motor mounting plate with attached motor and 20 tooth TMSC pinion gear)
For your new bracket you’ll need to grab some aluminium angle from somewhere like Bunnings. A 1m length of 25x20x1.6mm will cost you about $7 and you’ll then need to cut and drill it as follows.
(New angle bracket for TMSC gears. Details on hole spacings will be included shortly)
A metal bandsaw would be handy here but a hacksaw and clamp (or vice) will also do the job. Further shaping as well as cleaning up any edges and burs is then done with a file. The final step is to drill the 3mm holes and this is where Scorpio’s existing bracket becomes useful. Once the first hole has been drilled, tighten the two brackets together before drilling the rest.
It’s important that you use a countersink or large drill bit to remove any lips or burs after drilling. You want all surfaces to be flush when tightening things together. Aluminium is a soft material so twisting the drill bit by hand will usually take care of any excess.
Putting It All Together
Now that we’ve gone over most of the components needed to build a solar car let’s have a look at putting it all together. You can literally assemble the following example in your lunch break at school, one of your STEM classes or after school at home before dinner.
Parts List
The following table outlines the minimum parts list to get you up and running. Materials for a car body and egg cabin have not been included and are being left up to your imagination. Anything from free recyclables to an expensive carbon fibre shell can be used.
| Part | Supplier | Cost |
|
Axle & Frame Kit (AXFRK) SOLAR PANEL POWER CONTROLLER (SPPCL) FAULHABER MOUNTING KIT (FAUMMK) Collar/Axle retainer (COLLAR) Deans Micro Plugs Wheels x 4 (bearings included) Guide Rollers x 4 (bearings included) Gears 1V 500mA Test Solar Module 4mm male banana plug x 4 |
Scorpio Scorpio Scorpio Scorpio Scorpio TMSC TMSC TMSC TMSC TMSC |
$17.50 $11.40 $5.15 $4.45 $2.85 No cost No cost No cost No cost No cost |
| Total: | $36.20 |
We’ve limited this list to only Scorpio and TMSC parts to make ordering as easy as possible. As was seen up the page, you’ll find even lower cost micro deans plugs and 6mm carbon fibre tube on Ebay and HobbyKing. Car wiring and an On/Off switch have not been included as they can be easily sourced from your local electronics store for a couple of dollars. Please use the contact page to arrange getting your TMSC components with us.
It’s important to note that the total here does not include the cost of the Faulhaber motor. You’ll need one of these to be competitive and Scorpio sells them for $99.75 each. We understand that the upfront investment on such a motor can be quite significant so we’re lending out a number of these to help schools get involved. Just email us if you’d like to get hold of one but note that we’d like to share these around and are currently limiting them to one per school.
Chassis Assembly
Once you’ve ordered and received your parts you can go ahead and start assembling your chassis.
First you’ll need to cut your carbon fibre tubes to size. This can be done with nothing more than a fine-bladed hacksaw but pay SPECIAL ATTENTION to your safety when doing so. Carbon fibre dust particles can be very bad for your lungs if you inhale them so make sure you’re taking the right precautions to prevent this from happening.
Wrap the cut-point in some masking tape to help stop it from splitting and do your cutting outside or in a well-ventilated area with dust extraction. Also wear a dust mask for added security. Roll the tube so you cut all the way around it a little bit at a time. This is better than just going straight through from the top as it will help reduce spitting around the outside. Following the cut and still wearing your mask, clean the ends up with some sand paper and run some water over it. That’s it, you’re done.
For stability reasons you should aim to make your front axle as wide as possible, so the maximum 32cm. You can go with a variety of widths for the back axle or even make a 3-wheel car. Scorpio tubes are 65cm long so this will pretty much give you two 32cm lengths plus a small amount extra. It’s up to you if you want to make the frame any shorter than this from front to back.
You can now look at joining your frame together using the axle brackets and supplied nuts & bolts. Don’t fully tighten everything up yet though as you may still need to make some small adjustments later on. Bracket spacing can be made much easier if you make some connection plates with holes already measured and drilled out at the right distances.
(Bracket connection plates)
If you make these plates a little longer then you can also use them to provide a flat surface for attaching your guide rollers. Teams willing to go a bit further can even cut their axle brackets in half to save around 30 grams in car weight. Probably the easiest way of doing this is with an angle grinder and a thin cutoff disc. Just remember to clean up any burs with a file.
Below are two frame examples, one for 3 wheels and the other for 4. All else being equal, the 3-wheel design will be faster. It weighs less, has less rolling resistance, more drive wheel traction and drives the car forwards rather than in an arc. About the only advantage of the 4-wheel design is that it provides greater stability and is less likely to roll over while cornering.
(3 and 4 wheel chassis arrangements)
Here, on the 3 wheel design, the back drive wheel has been centred between the front two to help maximise car stability. This means that the guide rollers need to be pushed off to the side slightly. Such offsetting has been perfectly acceptable for years, just make sure that all parts of the car remain within 190mm of the guide rail centre (as per section 8.3 in the regulations).
The 3 wheel design needs a short (approximately 5cm long) carbon fibre tube section for attaching the drive wheel. Because of this we would probably recommend cutting one of your 65cm carbon tubes into 3 sections, a 32cm and 5cm bit with a remainder of a bit under 28cm (due to the material lost when cutting). This way you use the 32cm and 5cm pieces on the 3 wheel setup, the 32cm and 28cm (or shorter) pieces for the 4 wheel arrangement.
The new motor mount bracket has been designed in such a way that allows it to be used in either a 3 or 4 wheel configuration. You can choose the arrangement you want to go with. Both setups use the same aluminium motor mounting plate from the Scorpio’s FAUMMK kit. Once you’ve securely wired up your motor you can fix it to this plate using the three M2 screws that are also included.
Now carefully press the small 20 tooth pinion gear onto the motor shaft. You want as much of the gear on the shaft as possible but not have it rubbing against the mounting plate. A small gap of around 1mm should do the trick.
(Small gap between motor mount plate and 20 tooth TMSC pinion gear)
Follow this up by using the kit’s M3 nuts, washers and bolts to attach the mounting plate to the new bracket. You may like to swap out the two regular nuts for lock nuts here so there’s less chance of having things come loose when racing.
(Attaching the motor plate to the new bracket)
Now, move the motor out of the way and slide the first collar, drive wheel and second collar onto the rear axle and fix them in place using a 1.5mm allen/hex key to tighten the grubscrews. Note that a lot of carbon fibre tubes don’t have the same wall thickness all the way around. You should therefore try and spin the tube or collars around and tighten the grubscrews on the thickest part to help reduce the risk of splitting your axles.
(Fitting the TMSC drive wheel and outside collar into place)
As there’s a separate bearing at both the centre of the wheel and the gear you need to be careful not to compress the inner races together too much. Once the wheel is locked in place you can adjust the motor plate until you’re happy with the gear mesh.
Attaching the front wheels is done the same way, slide on the first collar, wheel and then second collar. Because these only have a single bearing there’s no danger of damaging the races, so press the two collars tightly against the inner race while tightening the grubscrews.
(A non-drive wheel held in place with two collars)
The final step to putting your chassis together is to attach your guide rollers. We supply these with bearings, washers, nuts, M3 sockets head bolts and some plastic spacers. You want maybe 3-4mm between them and the track so make your adjustments accordingly. Don’t forget to take into account any sag in the frame under full race weight. The solar panel will weigh roughly 240 grams and the two eggs another 100 grams. Add a full 200ml juice box and you’re looking at having to carry +500 grams.
(Final 3 and 4 wheel chassis configurations)
That’s pretty much it for your chassis. There’s nothing to it really. You can even put one together during your lunch break on a rainy day. Once you’re done you just need to sort out your electronics & wiring, construct a body and you’re ready to race!
Soldering Up Your Scorpio SPPCL Electronics
The Automax is the best solar panel controller this competition has seen to date. Scorpio sell these units for close to $100 each and they come ready to go straight out of the box.
We’ve instead chosen Scorpio’s $11 SPPCL kit in our parts list above. This still gives very good performance but at a much much lower cost. It’s a great little unit for teams that are just starting out.
You’ll need to solder this kit up yourself. It includes the printed circuit board, components and instructions on how to put it all together. If you’ve never done any PCB soldering before then you might like to check out a YouTube video or two before getting started.
You don’t have to do this but it’s recommended that you modify your kit to include some Micro Deans plugs. This will make it easy to swap out and replace with an Automax if you ever to decide on an upgrade. Organisers of the Tasmanian event will also have some emergency Automaxes on standby in case teams are having issues with their electronics.
Let’s now have a look at one way of modifying your SPPCL kit to include some Micro Deans plugs. This isn’t overly difficult but you’ll need to be fairly precise. Tools you’ll need include a 1-1.2mm drill bit, cordless drill or drill press, a scalpel, centre punch and hammer.
The addition you need to make to your PCB is circled in red below.
(A Scorpio PCB that’s been modified to include Micro Deans plugs)
The first step is to take your scalpel and start scraping away at the protective layer of the rail. This will expose the bare copper you’ll be needing for soldering. Then mark out the four holes and lightly centre punch them to help locate the drill bit. The distances between the holes will be approximately 2mm, 3.5mm and 2mm but check your plugs with a ruler just in case.
It’s advised that you push the drill bit right up into the chuck so only the very end sticks out. This will help reduce the chance of snapping it off it when drilling. You may find that a 1mm drill bit is slightly too small for your Micro Deans plugs. If this is the case then use the drill to continue working out the hole a bit at a time until you can fit both plugs into the holes side by side.
With the drill still in hand, you might also like to angle the holes for capacitor C1. These are spaced too far apart so need modifying to allow the capacitor to sit flush on the board.
Leaving the inductor until last, solder all the other components in place and use some side cutters to trim the leads. Pay special attention to the orientation of the two Deans plugs as you’ll want them to match up with the Automax. The male and female sides need to be the right way around. Please see the final assembly pics for details on this.
You can now place the inductor across a few different holes. We’ve gone with the two holes circled below but moving the nearest lead to the MOT- hole may give you a bit more space if you need it.
On the underside you want to bend the outside inductor leg inwards and solder the it to the tab circled in red below. Use a short piece of insulated wiring to connect the middle two Deans plug pins with the PANEL+ solder tab.
Your finished product should then look something like this.
In this example we’ve used a red plug for the input from the panel and a black plug for the output to the motor. This isn’t essential but colour coding can often make things a little easier to follow. As long as you know which plug is which, or have them labelled, using two red or black plugs won’t be an issue.
Wiring Up The Rest Of Your Car
The solar panels used at the Tasmanian event have four 4mm banana socket terminals as per Section 8.4 in the regulations. You’ll need to connect your wiring to these using some 4mm banana plugs or bullet connectors. The TMSC can supply you with 4 of these or you can get some from Jaycar, your local RC hobby shop or online.
You get a switch with your Scorpio SPPCL kit. Since it isn’t needed to complete the kit you can use it as an ON/OFF switch if you like. The switch is pictured below with four 4mm bullet connectors and a micro Deans plug. This is basically everything you need to wire up your car.
(L-R: 4mm bullet connectors, Scorpio switch, micro Deans plug)
Now you just need to solder everything up. Again, check out a few videos online if you need some hints or pointers with your soldering. Sometimes it just takes a bit of practice until you get the feel of it.
Your panel has to be in SERIES to run your electronics unit so let’s have a look at that. First solder up a short, maybe 5cm, single-wire piece with a 4mm connector on both ends. Then do a longer two-wire section with two 4mm connectors at one end and the Micro Deans plug at the other. The male side of the Deans plug should be soldered to the red positive wire.
Follow this up by cutting either the black or red wire (not both) somewhere between the two ends and insert the ON/OFF switch. The Scorpio switch has 6 terminals in two rows of 3. You want to attach your first wire to the middle terminal and the second to one of the side terminals of that same row. Once you’re done your two bits of wiring should look something like this:
(Longer switch wiring and short connector piece)
You can now go ahead and connect everything up as follows. Make sure that you’re plugging the red and back wires into the red and black sockets on the solar panel. This will ensure correct polarity into your electronics unit.
(Full car wiring when connected to a competition panel)
Be sure to insulate the underside of your Scorpio unit with some gaffa or duct tape. You don’t want it accidentally shorting out and damaging the circuitry. Try and use shrink tubing to help insulate the soldered switch terminals and all connectors. You can get lengths of various diameters for a few dollars at Jaycar. Or use a bit of electrical tape.
You may not have noticed but the chassis examples seen up the page all have their drive wheels set up to face the left hand side. Faulhaber motors are intended to spin in a clockwise direction, viewed from the front face, so they’re generally positioned like this to meet this recommendation.
Provided you’ve got the plugs on your electronics unit the right way around you’ll want the male side of the Micro Deans plug to connect with the negative motor terminal like so:
(Male side of the Micro Deans plug connected to the negative black lead of the motor)
Testing Your Wiring and Electronics
To test your wiring out you can use the small 1V 500mA solar module supplied from the TMSC. You don’t need to use a competition solar panel.
To do this you’ll need to solder some 4mm banana sockets to the 1V module. Then simply plug in your wiring from above. You won’t need the short connector piece here. Bypass the electronics unit by connecting the switch straight to your motor.
(Testing your wiring using a 1V 500mA solar module)
To test your electronics unit you’ll need to use a slightly bigger solar panel, one with a bit more voltage. Something like the low cost 6V 2W solar panels seen on Ebay will do the trick. There’s a few Australian suppliers selling these for about $10 or you can wait a bit longer and pay less than $5 for the same one from China.
After soldering on some 4mm sockets you can hook everything up again. This time you can include your electronics unit as part of the circuit.
(Testing your wiring using a 6V 2W solar panel)
The final step is to set the maximum power point. You do this by using a screwdriver to turn the trimpot circled in red below. You’ll also need a multimeter to help tell you where the maximum power point is.
First, go outside. You’ll need reasonably sunny conditions for this panel to work. You’ll also need the sun level to remain fairly constant otherwise it will affect your results.
The simplest way to set up your unit is to stop the motor (or drive wheel) from spinning with your hand and monitor the voltage across it using the multimeter. Then adjust the trimpot until you get the largest voltage. You may need a friend to hold the multimeter probes for you as doing it all by yourself can be a bit of a handful.
Although you’re measuring the voltage across the motor you’re actually setting up the unit to maximise your power output. Power equals voltage squared divided by resistance (P=V^2 / R) so maximising your voltage will also give you peak power.
When you turn the trimpot you’ll find that the multimeter reading drops off a lot faster on one side of the peak than the other. Have a look at a solar panel PV curve and you’ll see why.
(A typical PV curve of the Scorpio SOLAR26 competition panel at 100% sun)
Bring the trimpot setting back a touch on the side where the drop is more gradual. This will ensure that the set point remains near peak power if the panel warms up or the sun level drops.
Locking up the drive wheel isn’t exactly the best method of tuning your electronics. It causes high currents to flow through the motor for a prolonged period of time. Have a bit of think about using a dummy load instead. A stalled Faulhaber has a resistance of about 1Ohm so head down to Jaycar and grab a 1Ohm 10W resistor for about $2. Wire this up to a Deans plug and connect it in place of the motor. Now you can measure the voltage across the resistor, set the electronics and then plug the motor back in when you’re done. That’s all there is to it.
(Using a 1.2Ohm 5W resistor as a dummy load and connecting it to a multimeter)
Car Body and Final Design Considerations
We’ve only constructed a basic chassis here. If executed well this will give you the basis of a competitive car. You’ll now need to design a body to meet this year’s regulations. Not only try and keep it as light and streamlined as possible but also think about your centre of gravity and weight distribution. Solar panel, juice box and egg placement will all affect car stability and drive wheel traction. Please see the 2018 regulations for all your design requirements as well as details on the size and weight of the solar panels we use for racing.
Teams competing in the big World Solar Challenge from Darwin to Adelaide use monocoque designs. This integrates the chassis and body together as one to form a stiffer and lighter composite structure. This generally involves a difficult construction process and so most model solar cars, like the example we’ve been following above, have a design where the chassis and body can be separated quite easily. Here, the body may still be used to stiffen up the car but it’s the chassis where your most of your strength will come from. This means you can make an extremely lightweight body that doesn’t actually need to be all that strong. It just needs to be able to hold its shape for racing.
Common materials to use for the body include Balsa wood, various types of Styrofoam or vacuum moulded plastic. You can also use recycled goods such as plastic packaging, cardboard, corflute, polystyrene, etc. Just keep in mind that the car body should be made water resistant if possible. You don’t really want it soaking up water if you have to race in the rain. It doesn’t happen very often but we’ve had the odd event where racing has taken place during a passing shower. You’d be surprised at how little sunlight some cars need to make it around the track.
Head on over to the Challenge Help or Photos and Videos pages for some more information or examples of previous years’ car bodies. We haven’t seen egg drivers since 2010 so be sure to check out some of the older pics too.
As can be seen by the Drag Equation below, the aerodynamics of your car will be directly proportional its cross-sectional area:
This means that the smaller the body, the faster your car will go. You don’t need a massive body for your 200ml juice box. Rules from previous years required cars to carry different objects so that’s why some of them are quite big while others much smaller. For example, the rules in 2003 required cars to have a minimum cross-sectional area of 200 square cm. The next year, in 2004, the rules required cars to have space for a 375ml soft-drink can. End-on this only has a cross-sectional area of about 30 square cm so cars were obviously going to be completely different and quite a bit faster.
Hint: The 200mL juice box and eggs are quite small so a body that fits snuggly around them will have sides that are less than the 150mm x 80mm needed to comply with the regulations. To overcome this problem here’s a few examples where teams have added fins to their cars. You don’t have to do this but if you go down this path just watch that they don’t shade the solar panel.
Not only try and keep down your frontal area but also look at body shape and streamlining. In fact, body shape and the resulting Coefficient of Drag (CoD) is even more important as can be seen by the following diagram.
In this example there’s three Drag Coefficients listed alongside a flat plate, circle and symmetric airfoil, all with the same width. This shows that the drag force of the flat plate (CD = 2.0) will be almost 17 times greater than that of the airfoil (CD = 0.12).
Because model solar cars have the solar panel, wheels, guide rollers, motor, etc. all affecting air flow you’ll find that the difference between a square box or airfoil design car is reduced somewhat. To give you a better idea, wind tunnel testing was done on the following two cars.
These were both constructed to have a 200 square cm transverse cross-section, a requirement of the 2002 regulations. Testing found the CoD for the box design on the left to be 3 times greater. This makes the car around 2m/s slower at top speed and over 2 seconds slower in a 1-lap race when sunny. All else being equal, that’s a difference of 15+ metres at the finish line!
Not many schools are going to have access to a wind tunnel so teams might like to evaluate their designs on computer before starting construction. Perhaps one of the simpler and more accessible software packages for this is Autodesk’s Flow Design. This is free to use for students and educators and something schools involved with the F1 in Schools competition are no doubt already familiar with. The program allows you to model airflow around your design and then analyse surface pressures, flow lines and turbulence. It even provides you with drag figures.
Balsa wood is probably the one of the easiest materials to work with. It has a very high strength to weight ratio, is easily cut with a scalpel and shaped with sandpaper. Glueing can take a matter of seconds with an appropriate CA glue. Your local hobby shop and even hardware outlets like Mitre 10 and Bunnings will stock sheets of various sizes and thicknesses. 1mm thick sheets are quite flexible and often used for the main shell while thicker 3 or 5mm sections make good cross-members or bulkheads. You can also use thin strutts, chamfer or fillet pieces along edges to increase glueing surface area while adding very little weight. Waterproofing is usually done by applying two or three layers of model aircraft dope.
Another material used by some teams is Depron. This is a type of extruded polystyrene that’s generally used for insulation but also popular with model aircraft enthusiasts. It’s ultra lightweight and can be purchased in different thickness sheets much like Balsa. As far as we’re aware there’s no one supplying this here in Tasmania so you’ll need to take your shopping online. Cutting is again done with a sharp scalpel and you’ll need to use a special bonding adhesive like UHU POR for glueing it together. Depron is water resistant so can be left as it is unless you want to paint it or apply some other covering.
Vacuum moulded plastic offers a neat solution for designs with 3D compound curves. Just be weary of the weight. Even a thin shell can end up weighing quite a bit more than something of a similar size in Balsa or Depron. Again, this is water resistant and doesn’t require a finish.
Further Assistance
If you need any other help or have any questions then please contact us for further assistance. Also let us know if there’s anything else you’d like to see added to the website to assist you.