Building a low cost car in 2018
The following information outlines how to build a simple car chassis for the 2018 competition. Please use this in combination with everything covered on the website’s Challenge Help page to assist you with your car. Unfortunately there’s no way of getting around the expense of a Faulhaber 2232 6V solar car motor but an attempt has been made to keep all other costs to a minimum.
Carbon Fibre Frame
Countless teams have used carbon fibre tubes over the years due to their high strength and low 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 Bracket kit – pic coming shortly)
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.
(2017 Rosny College car – pic coming shortly)
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 require a tube-to-tube connection as can be seen below.
(Axle Brackets with custom connection plates – pic coming shortly)
This type of design requires half an Axle Bracket Kit and will weigh less than a full bracket version if connection plates are made from a light weight material. The example above uses carbon fibre plate but could have also been made from a similar thickness aluminium. One of the advantages with this arrangement is that there is a large flat surface to ensure perpendicular guide roller stand offs. Designs using full brackets like the CHALLENGER CAR kit need to be careful here due to hole position and curvature of the bracket.
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.
(Pultruded vs Roll-wrapped carbon fibre tube – pic coming shortly)
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. Hobbyking appear to offer an even lower cost 6mm OD x 5mm ID x 750mm option but the TMSC is yet to investigate the thinner wall thickness and suitability of this product. Depending on frame design, and the number of cars being built, buying multiples of these tubes 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 50mm due to a smaller gear ratio. Smaller wheels will lower car heights on a standard carbon fibre chassis and help improve cornering stability.
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’s guide rollers are a good option here and are relatively cheap at $2 each. Tech Ed sell a cheap 37mm wheel that would be suitable but again needs to be turned out to fit a bearing. Or you can use some from the TMSC. These have been made to have the same 25mm diameter as the R & I roller so the two can be exchanged at any time.
If you have a closer look at these you will notice that the head of the M3 fastener is recessed up within the roller, a design characteristic which prevents it from catching or scraping on the track. R & I rollers are sandwiched between two flanged bearings while others like those from the TMSC use only one which has been press fit into place.
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.
(F623zz and MF106zz bearings plus R&I flanged insert – pic coming shortly)
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 also 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.
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 high tensile hex socket head bolt which will require a 2.5mm hex wrench or allen key. These types 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, especially in locations where bolts do not need constant adjustment.
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.
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. They are based in major centres around the state. Bunnings and Mitre 10 will also supply some M3 sizes but have a much smaller range. Otherwise, Ebay is a great place to go for anything nuts and bolts related and you will find plenty of Chinese suppliers offering free postage. If you don’t mind waiting a few weeks for things to arrive then definitely consider this as your lowest cost option.
Everyone will carry the same 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 SPPC kit from Scorpio. Even if you decide to use the much more expensive and advanced 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 plugs so it can be easily interchanged with an Automax, just make sure you maintain plug polarities across the two units. Some basic wiring examples are given on the Challenge Help page or contact us for some further assistance.
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 this same collar.
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 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, but for larger distances it may 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 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 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.
Alternatively, have a bit of a look around online and you will 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 set of various lengths. A lot of these spacers are meant for separating PCB boards so you will find a number of types at your local electronics supplier like Jaycar.