Steel chassis in formula student

[megathorx]

Hello to the community,

My name is Javier I'm founder and chassis responsible for the Formula UC3M Team from Universidad Carlos III of Madrid (Spain) (www.formulauc3m.com). As this is our first year with enough support to go to the competition with a full car (Or at least we are trying hard on it) we have some problems regarding the rules. So I'll explain my doubts in this post and see if some of you more veterans in this matter can help us. Forgot to mention that we want to go to Formula Student Germany and Formula Student Spain.

So as a new team we are aiming to make a steel frame and a combustion engine. When I read the rules it says in T3.4.1 "Baseline Steel Material.The Primary Structure of the car must be constructed of: Either: Round, mild or alloy, steel tubing (minimum 0.1% carbon) of the minimum dimensions specified in the following table,
Or: Approved alternatives per Rules T3.5, T3.6 and T3.7."

By this point I understand that carbon steel is the main material and that 0.1% of carbon is the minimum and only requirement. As we don't want to use the AF rules so we can pass on and go for minimum dimensions which is very clear what to do with that.

Then if go far the dimensions comes "NOTE 4" that specifies "Baseline steel properties used for calculations to be submitted in an SES may not be lower than the following:
Bending and buckling strength calculations:
Young’s Modulus (E) = 200 GPa (29,000 ksi)
Yield Strength (Sy) = 305 MPa (44.2 ksi)
Ultimate Strength (Su) = 365 MPa (52.9 ksi)"

Yes and if you go to T3.9 it specifies the steel grade to reffer when doing the SES. So at this moment I understand that the minimum properties ar those shown on T3.4.1 and the steel grade of those properties is AISI 1010 (which is normally CDS according to matweb (http://www.matweb.com/search/DataSheet.aspx?MatGUID=4b64586941754ec2a4a8ef0fde2 299db&ckck=1) ).

So what is the problem? The thing is in Spain usually in Europe is difficult to found this steel. Ok lets find an equivalent!!!! Read and done, now we found this equivalent steel E355 according to EN 10305-1 which is CDS and have superior properties than AISI 1010. Now comes again another problem, This steel in the minimum dimensions specifieds in the rules is quite imposible to find in stock or someone that produce it(small quantities not 5 tons of minimum order). So looking for alternatives a lot of people told us "go for ChroMolly" "4130" rocks and it was ok, we can buy that.

But again more research of 4130 on this forum and the internet says that we need to normalize the weldings and that steel can only be weld by TIG (correct me if I'm wrong but that was the info I found :-D) and also a nice post form Pat Clarke on FSG website. https://www.formulastudent.de/academy/pats-corner/advice-details/article/pats-column-space-frame-chassis/ giving some nice recomendations on Alloy vs mild steel.

So we don't want to use alloy steel we want to keep simple as possible, and here comes another doubt: In Formula Student Germany if you download their magazine with some info on the participant teams (https://www.formulastudent.de/fsg/media/) you realize that a lot of teams use a baseline steel with less yield strenght than the specified in the rules, like the E235 that is CDS also but with a yield strenght of just 235MPa when the minimum specified is 305MPa (AISI 1010).

So how is this possible? Any advice for us? We are low on budget and we want a local sourcing or at least a european distributor, buy 6 meters long tubes from the states or asia maybe is not the best pricing solution.

Guys hope you can give us some advice on this.(Sorry for the length of the post)

Greetings from Spain

Javier

[Menisk]

The only reason people in the know use 4130 for space frames is availability. We can't get 2.4mm wall roll hoop material in 1040 so we choose to use 4130 despite the price difference in order to save some weight from the heaviest tube in the car. We TIG our car but do not normalise welds. Don't forget that your frame should be no where near the limits of the tube material if you've designed it right and most SAE cars would be lucky to do 1000km at best. Normalising 4130 welds shouldn't be of great concern to you.

[jd74914]

The only reason people in the know use 4130 for space frames is availability. We can't get 2.4mm wall roll hoop material in 1040 so we choose to use 4130 despite the price difference in order to save some weight from the heaviest tube in the car. We TIG our car but do not normalise welds. Don't forget that your frame should be no where near the limits of the tube material if you've designed it right and most SAE cars would be lucky to do 1000km at best. Normalising 4130 welds shouldn't be of great concern to you.

As Menisk said, typically 4130 use is just for availability since there are a wide range of wall thickness available. There are potentially some ultimate strength advantages for strange areas (tabs with a lot of shear load, etc.) but in general this can be mitigated with good frame design. The other advantage to using 4130 is that it is generally of higher quality (less impurities) than the cold-roll mild steels which makes it easier to weld.

If you are using 4130 and TIG welding your car you really should normalize the welds. The likelihood of cracking a brittle, un-normalized weld joint is relatively high after a lot of driving. We had problems with suspension tabs cracking at the heat affected zone before we started normalizing*; normalizing all joints completely solved that problem. There is an easy normalization process highlighted in Carroll Smith's Engineer to Win, basically you just heat the joint to a dull red and let it cool as slowly as possible.

I'm not sure if it is still allowed by the rules, but you used to be able to put in an SES to allow a deviation in main roll hoop size. Our first years we went with a 1.125" OD x 0.083" wall (IIRC) mild steel tubes because it was easier to get and a little lighter than the 1" x 0.095" tube. If allowable the only caveat is that not many people can bend odd size tubing.


*Note that the cracking was generally fatigue-based since it took months to show up.

[megathorx]

Hello guys!

Sorry for the time to answer but I was busy with a lot of work to do. First of all thank you very much for your answers I really apreciate that. The thing is that we don't have access to al TIG welding equipment, I think our partner only have MIG and other equipment but we need to check. And again We find the process to normalize tyhe weldings too risky for a one year car, and laso that we want to keep things simple.

I was reading again and agian the rules, and seems confusing the part of alternative steel tubing. The key thing here are NOTES 2 and 3 that says that you should mantain EI (increasing OD if you decrease wall thickness) and to mantain the equivalent yield strenght and ultimate tensile strength you should mantain the same cross sectional area.

And from here what I understands is, if I go on thinner walls than T3.4 we need to test it with the H pattern to see if it complies with the original joints (As it is said in the SES). So that mean that with a steel of lower yield strength than the AISI 1010 if we maintain wall thickness and increase OD, to surpass or mantain the values on the SES we are complying with the rules as we are mantaining equivalent yield strength and having a greater EI.

How do you see this reasoning?

Thanks a lot for your answers guys!

[Charles Kaneb]

ThyssenKrupp sells 1026 steel in FSAE sizes now. It's good stuff (the last order said it tested out at 72500 psi yield) and cuts like butter. Their site is onlinemetals.com in the USA, but I'm sure they've got a way to sell it in Spain.