Hello all,

Here are some on-track shots of the new 2009 test car. I started building this one with my younger bro at the start of the year and finally got it to a working stage the night before the state comp.




He didn't end up racing it, so it was run around at the Tasmanian event as a test car to give the official cars something to compete against. I don't think anyone came close. It was still getting over the hill when everyone else was failing to do so. To tell you the truth, I'm actually quite glad the sun didn't come out otherwise there could have been trouble.

The wheels, guides and other components were taken straight off a previous car and as it stands, the car weighs in at a touch over 300g without the egg, panel and ballast.

In a couple of the photos you will notice that there is a blue insert in the drivers cabin. This holds a small USB camcorder that I got off eBay. Unfortunately, we had some problems with it for some reason and we didn't end up getting any footage. Pity. I guess I'll now have to wait until Melbourne. Hopefully we'll get some sun then too.

I hope to be covering the car in solarfilm/profilm before coming over so look out for it in fluoro red.

miseli

Looks like a beautiful car Marc. Your cars are always standouts in terms of quality and workmanship. Your test car last year looked very nice too, and i like the balsa look.

From what i can see, you have very good attention to detail.

I like the airfoil shaped post which holds the trolley wheels.

We had a similar shaped car in the states this year from Our Lady, but it was a fixed wheel design at the front.

I'm thinking of building a test car too (hopefully if time permits), ill bring it down to Melbourne.

Thanks Tony,

I look forward to seeing your car in Melbourne if you get around to it.

The airfoil at the front actually has 2 purposes in mind. Firstly, it adds some additional strength to the car and secondly, it improves the aerodynamics over just a square rod.
It is actually shaped like an upside down plane wing slightly and is supposed to help keep the car on the track at high speeds. It won't make a great deal of difference with the weights that we are carrying, but it's just something else.

The way I see it, a mechanically sound car (providing that it isn't too heavy) is the first step to having a successful car. Just look at all the cars that have ever won the national event. Follow that up with some half decent aerodynamics, the right solar panel, electronics and gear make up and you're pretty much there. To tell you the truth, I can't actually see how anyone could even go wrong.

For everyone out there, you can build a car out of pretty much anything. Carbon fibre rods are a common material and are strong and light, but the school that I started out at didn't have them and so I've been going with balsa more or less ever since. It's just incredibly easy to work with. Anyone can do it. All you need is a scalpel, sand paper and maybe a little hobby shop saw and off you go, you can go and build a car body. If you've ever had a read of John Jeffery's "Model Solar Car Racing for Dummies" then you'll find that he will also recommend the same thing. I suppose its only downfall is not being able to form compound curves very easily.
I've often used the cyanoacrylate glue that bonds materials in seconds, so you could potentially build a new car in a few hours if you don't include the components.

It might not be visible from the photos, but the above test car is basically made up of one 0.8mm thick balsa sheet that comes from the back on one side, goes around the front, and then comes back on the other. Apart from a few ribs and things, that is all that's really connecting the front and back together.

Both this year and last year's test cars and the national winners in 2005 and 2006 all use the same wheel setup. That is, 3 trolley wheels and rear wheel drive. They are attached to the car body so that they form an equilateral triangle for maximum stability and once you have a good set, it's easy just to transfer them from an old car to a new one the next year. The car guides are then offset so the drive wheel doesn't run in the channel.

There are a couple of reason's for trolley wheels rather than fixed:

1. More efficient. There is less/no corner drag (provided that the wheel is in line with the pivot point) since the wheels follow the motion of the car.
2. They're far easier to attach without stuffing something up. A lot of cars have axles with fixed wheels. If these are not aligned properly you'll even have drag on the straights.

You can go with 4 wheels, but you will increase your car's rolling resistance and may also lose drive wheel traction throughout a race.

I will add some more detailed shots of the car once it is completely finished, although I don't think that that will be until shortly before the nationals.

miseli

re: steering
Although I don't have any numerical results on lap times with and without steering, I'm pretty sure any benefits gained by steering are lost through aerodynamics and increased weight. One BHHS team this year (Nigresta) built a car that can run with either, so we will get some numerical results on that soon.

re:front aerofoil
Downforce is barely present in cars due to the low speeds achieved, so you've got some more weight there on the front.

On some more general notes, hows the structural integrity of the car? Is there a chassis underneath made of something stronger than balsa?

Cheers.

PWK,

Because you were at BHHS and probably played around with Ian's simulator I'm sure you would have seen the effects of having or not having steering. The fact of the matter is that steering will theoretically decrease your losses and will do so by greater than the slight increase you get with the increase in weight and/or air drag.
Doing it properly is however another matter all together. I've lost count of how many times I've seen cars with casting systems that fail to align wheels with their casting pivot points.

The extra 10 to 20 grams that the casting arrangement adds is minor in the scheme of things and the benefits, especially under low sunlight conditions, can be significant.

Because wheels and guides themselves significantly increase the overall aerodynamics of a car, there is some element of truth in what you say about increasing air drag because of having a casting wheel and not just a fixed wheel at the end of your axel. The difference however will again only be minor.

It is true that the fixed Nylon wheels that a lot of cars are using have low friction but they are still not optimal.

Another thing about having fixed wheels and axels is that (as I mentioned in a earlier post) often they are not aligned properly and this then results in not only corner drag, but also drag on the straights. This problem is eliminated if trolley wheels are used.

Steering is however just one piece of the jigsaw puzzle (and a smaller one at that). The reason why your car did so well last year was because you had so much of the other stuff right. For starters it was well built and mechanically sound. In combination with its low weight, aerodynamics and panel to gear ratio configuration it made it an excellent car and I congratulate you on your win.

The competition might of been a different story if the sun level would have stayed down though. The top 4 seeds from the time trials were all from WA.

You are correct in saying that the airfoil downforce at the front of the demo car above will be next to nothing at the speeds that cars are running at. The main reason it is there is to improve the aerodynamics of the front axel over a square rod. The extra 5g or so that it adds to the weight of the car is negligible.

On the structural side of things there is not a whole lot to it. There is a square rod going through the front of the car for the front axel. This is just an ordinary pine rod that one can purchase from any model shop. The front guides are also attached to this. At the back there is another 100mm or so long section of this for attaching the the rear guides and wheel. The front and the back are then linked up with balsa. I might include a photo or two.

miseli

I was very happy to get a look at both your test cars Marc.

Very well built indeed, probably the best I have ever seen. I can see now why you were able to win nationals in 2005 and 2006.

Shame we didn't have time to give you a race, for what it would have been worth

Great car, an inspiration for many!

Just wanted to ask a few questions on the car.

-What is the wheelbase ?
-How many watts is your panel ?
-Did you offset the guides or the drive wheel ?
-Do you have a certain gear ratio that you have put on that car ?

Just some curious questions i faced while designing my new car.

Thanks!

Karo,

The test car has 3 trolley wheels and is rear wheel driven. The guides are offset to give maximum stability.

The car is pictured above with a 10 segment Dicksmith 6.5 Watt panel with a maximum power voltage of about 14V. In Melbourne, it was tested with a panel made up from 10 of John Jeffery's new Engelec modules. This panel produced around 8W with a max power voltage of around 14.5V to 15V.

If anyone out there is currently looking for a new car panel, I highly recommend making up your own from the modules that John is selling. For more details, visit the Tas website's homepage.

The car runs a 6:1 gear ratio with a wheel size of around 55mm and this will suit most conditions.

miseli

Thanks heaps for that!

Hey miseli,

I am having troubles at the moment on how to mount my guide pegs. How did you mount yours on the test car? Do you have any pictures underneath the test car?

Thanks,

KARO

Hello Karo,

Like many of the Tasmanian cars these days, the 2009 test car uses a set of aluminium brackets for mounting the front and rear guides. The guide wheels are attached to the bracket and the bracket is then mounted to the frame of the car. The folded brackets prevent long and thin bolts from extending down with bearings at the end.

If you have seen the post with the CAD images of the 2008 test car on this forum then this should give you some idea how things are done. Below is the image of the frame of the 2008 car and also a couple of close ups of the front and rear brackets with the guide wheels attached. Remember the guides are offset on the car.





From memory the aluminium sheet used was around 2mm thick.

As John mentions in his help file for dummies, the guides are not the area to save weight at the expense of strength. This is important to keep in mind.

There is a little thing called centripetal acceleration (and force) which is present when a body moves in a curved path. It can be identified as the rate of change of tangential velocity and, without it, a body would otherwise simply keep moving in a straight line. This centripetal acceleration is related to the velocity of the body and radius of the curve and is given as v^2 / r.

If this is applied to a model solar car (where top speeds of around 9 m/s are possible and the curves of the track have a radius of around 5m), this results in a centripetal acceleration of 9^2 / 5 = 16.2 m/s/s. In other words, this is beginning to approach twice the acceleration due to gravity or 2 G's.
This means that the two little inside guides keeping the car on the track around every curve must be capable of withstanding much more than the 3 or 4 wheels on which the car is driven around on.

miseli

So i understand now. I remember John telling me before that you should be able to pick up the car, wiggle it around by one guide and nothing come apart. I wasn't really worried about weight, it was more the concept i was looking for and you greatly helped in both ways so thanks heaps miseli!

Karo

No problems. That is what the forum is for after all. Good luck at this year's NSW event.