Posted by: drracing | May 30, 2010

Let’s Start!

It’s done! We have started to design a new car!

I finally get to involve in the project another friend of mine, who can help me a lot during the design phase since he knows quite well CAD softwares and he has worked during the last year on CFD simulation. This means we are going to make an aero car, or at least we are going to try! If Virgin did a car only using CFD why we should not be able to do the same? Maybe we will have to find some computational power somewhere, but we have some good friends…

We will start designing a car complying with Formula Free rules. I looked around a bit, above all after i found Formula 1000 in the USA, and i recognised that FIA Formula Free rules for cars built after 1st of january 2010 are a lot more demanding in term of driver safety than, for example, Formula 1000 rules. But since we want at least to dream to be able to build someday the car we are going to design, we have to respect these rules. It’s not so bad, we will have a lot of FEM experience after we will have done it! Since testing and FIA omolgations are very expensive, we will have to calculate nearly everything before to build the first piece so that we could at least hope to pass al safety test at the first attempt.

But let’s start talking something more interesting. How will this car look like?

We will use a steel space frame, with alluminium panels fixed to the tubes with glue and rivets. I still don’t know if we are going to use square or round tubes. Square are easy to manage when building, they make it easier also to add panels as well. On the other hand, round tubes are normally stiffer and have more strength keeping weight costant.

Rules of the champioship where the car could race one day ask to use Formula Ford 1600 AVON ACB10 Tyres for cars of group 4, with bike engines of 600 cc. Since we decided to start in this group (but we would like to build something easily adaptable to a 1000 engine if we need), we took a look to some BritishFormula Ford 1600 data to have an idea of which wheelbases, track and overall dimension they use.

I have to say, i believe that, at least from a vehicle dynamic point of view, bigger is better, so i like car with long wheelbase and wide tracks. Anyway, how much is too much? Formula Fords are often around 2500%2550 mm for wheelbase and 1600 mm for front track. To be honest i would like to make our car with a little longer wheelbase, but i though about that a little and i found that:

  • a longer wheelbase reduces wheight transfer in braking and should help to stabilize the car.
  • A longer wheelbase, if yaw moment of inertia of the car is not increasing when you increase wheelbase, will make the car more reactive, with less corner entry understeer and less corner exit oversteer. It’s J/L (being J yaw inertia and L the wheelbase) the term that rule in transient phase.
  • On the other hand a longer wheelbase means often a longer car, so more weight and, if also chassis is longer probably less torsional stiffness.

Anyway, a too long wheelbase maybe could also be a limit in small corners or in track with tight corners. And i don’t think we will ever race in Indianapolis with this car.

So, to find a compromise, we decided to stay around 2550 mm.

What about tracks?

Again, i would like to have them very very wide, also because rules don’t specify a maximum track. Have a wider track means

  • you have less lateral weight trasnfer and, normally, more grip, above all when you have high centripetal accelerations.
  • you have a better control on suspension movements, because arms are longer.
  • on the other hand, longer arms means more weight and also more problems with buclikng, and this ask for more material again.

So, to have a compromise, we should stay around 1600 mm at the front, since tyres we are using are not so wide. Something less than this at the rear, something around 1550 mm should be ok. We decide to have rear track smaller to help the car to follow easily a better line in corners and also to create a basic understeer tendency. Moreover, since rear tyres are wider, we will probably have an overall rear width very similar to front one.

Next step will be to define a mass distribution target, based on tyre data. I look on Avon site to find something about that tyres, but the data they give doesn’t seem very consistent and moreover they don’t give Pacejka coefficients. So i cannot use them to make some basic simulations since the code i wrote use pacejka 96 model. This is not nice, but i will see how to solve this problem. Maybe i will use some other tyres data (maybe Formula Student with similar size) just to have an idea and to have an exercise also on the work on tire data.  We shall see!

In the mean time i tested my code with some Formula Student Tyres and it works quite fine! Graphic Interphace is quite poor now, and i should make it better. But the results seems resonable. And this is far more important to me! And if i decide to use formula student tires i have quite a lot of data on them…

Anyway what is important is that we started! Now let’s go on! I hope soon we can start to design something!

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