Frame Design - First Car IFS

[Tiago-IFS]

Hello everyone my name is Tiago and I'am the new Frame responsible from IFS Team ( ISEL Formula Student ), Lisbon, Portugal. We are a second year team and participaiting for second time in a formula student competition, the first one was this year in FSUK class 2. This is our first car. I have been reading this forum attentively from some months now, i've looked for various areas and almost every time found most of the answers to my questions already answered, even more, i've seen errors in our design that we would never think about, by reading posts here. Now I would like to request you help with your experience to tell me if you find any triangulations errors or others apart from those, in our frame.
This is the last design of my collegue, I'am now trying to better is work.

One thing i've understood is that our frame is heavily over "reinforced" probably more than necessary, and we are "scoring" very low in terms of mass. The torsional rigidity of the frame is about 3000 Nm/deg ( from a FEA model ) i've read that the values can some times be 50 % lower than the real ones and we have set the target for 2000 Nm/deg, after looking to the total roll stfiness FRT and Rear and also looking to the lateral load transfere. During the design we have paid attention where the forces go and trying iteratively to understand how the system is behaving and which triangulations and tubes offer more or less contribution to the torsional rigidity, and how the energy is dissipating thorugh the strucutre.

Tubes colors vs ODxt are: Green (25.4x1.6) , Yellow (25.4x1.25) and Red (25.4x2.4)

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Thank You, Best Regards

Tiago

[BenMueller]

Hey Tiago, can you upload a frame which includes a sketch of your suspension (A-Arms, Bell Crank)? It should be easier to see if your frame is stiff enough to react the suspension forces.

What kind of engine you will use? What's your weight goal for the frame and the hole car?

Think about the tubes above the SIS - are they all needed? And secondly if the driver is handicaped by them while get out of the car at driver egress?

[Tiago-IFS]

First of all, thank you for answering Ben. Of course here it is, i've made a section view of the frame with suspension lines so it is easier to understand. This is our first car, and we have had some sponsors, our financials are very low, one of the sponsors was KTM that offered us the engine: KTM Duke 390cc . And our overall mass goal was between 230 and 250 kg which is quite high for a small engine. Our Strongest point in terms of knowledge is undoubtedly the engine and we are studying the case to go from naturally aspirated to a turbocharged ( I also must account with a possbile extra power in case we go ahead with this decision ). The Tubes above de SIS is something that has been worring me, about the 5s rule.

Front Suspension Lines

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Rear Suspension Lines

365366

Hope You can clearly see, is that fine to you to understand ?

Tiago

[Tiago-IFS]

Sorry, I haven't got used to the image Shrink of the forum. Here it is, files with greater quality. https://www.dropbox.com/sh/wstnf4fqzxmj438/AADTIld8vF06IQCgrvNKaclUa?dl=0

[Z]

... This is our first car,
... our financials are very low,
... KTM Duke 390cc.
... our overall mass goal ... 230 and 250 kg,
... we are studying ... to go ... turbocharged.

Tiago,

As with all other Teams that have ever entered these competitions, you have the possibility of WINNING OUTRIGHT. (Yes!!!)

But, your posted designs and the selected quotes above suggest that you will likely finish towards the bottom of the field.

YOU ARE MAKING IT TOO COMPLICATED !!! (<- This is intended as helpful advice. :))
~o0o~

Specifically:

1. Your "financials are very low" means that you have an advantage. Your low financials should discourage you from wasting time on silly stuff. Exploit this advantage by building a very simple and efficient car.

2. KTM have given you an adequate engine. So your "engine choice" problems are solved! You can now move on to the more important things.

3. Does your "overall mass goal" include the car AND driver, or just car alone (!)?

I suggest mass targets of:
* Engine+Transmission = 30 kg.
* Front-Axle (ie. 2 x tyres/wheels/hubs/uprights/brakes/suspensions) = 30 kg.
* Rear-Axle (as above) = 30 kg.
* Frame = 30 kg.
* Everything-Else (bodywork/aero/seat/seat-belts/steering-wheel/electronic-junk/other-junk???) = 30 kg.

So, Total-Mass-of-Car = 150 kg.

4. DO NOT "go turbocharged" until you have won at least one competition.

5. Bottom line here, your frame is TOO COMPLICATED. Too big. Too many tubes. Too much work. Too many (Edit: UNNECESSARY!!!) wishbones, and (unnecessary) pushrods, and (unnecessary) rockers... And too much (unnecessary) CADing...
~o0o~

Suggestions:

1. Build an "ergonomic-mock-up" of the car. This is a simple (but REAL!) frame that the Team members can sit in to check if it "feels" right. Fit the engine, steering-wheel, 4 x suspension/wheels/tyres, etc., to this frame to make sure everything fits right. Then check driver sight-lines, driver elbow-room, driver egress-time, etc.

2. Spend a long time looking at that mock-up frame, then SIMPLIFICATE it and ADD MORE LIGHTNESS.

Z

[Bemo]

This happens not too often but in this case I have to agree with Z ;-)

What I read from your post is that you are going to make the same mistake most rookie teams are making. You're building a way too complicated car. As Z already said, just don't turbocharge that engine. The only modifications you should do at the engine are what is necessary to make it rules compliant. Basically that will mean to build a new intake with a restrictor and you will propably want to build a new exhaust system. Leave it with that. Take care that the engine is running reliably and that throttle response is adequate (hint: a turbo doesn't help here). But in general don't make the mistake to think that the engine is the most important thing in a Formula Student car. IT IS NOT.

If your weight target is already 250kg you will propably end up with a car above 300kg. It is always heavier than expected, so set a low target. Skip all parts which aren't necessary to make the car running. Get the whole thing running as early as possible. And then test, test, test!

[Tiago-IFS]

Thank You, Bemo and Z I think i've heard what i really needed from you two. Is just that for first time, we maybe aren't ballancing complexity and simplicity in a good way. We are now taking your advice and left the turbocharged engine apart. I will simplify this frame in order to reduce weight and keeping a reasonable torsional stifness from our stifness objective. Thank you for your time again.

[MCoach]

Tiago,
Thanks for listing some of your targets.
What is your frame torsional stiffness target? My thinking is that your stiffness target may be quite high for the weight compromise you're taking.

[Tiago-IFS]

MCoach, the target set is between 1700 to 2000 Nm/deg, and those values were taken after computing the lateral load transfere for a two mass model, the references that i've used for calculations were RCVD and the SAE paper ( 2000-02-3554) The Effect of Chassis Stifness on Race Car Handling Balance. The point is that we've read that the frame torsional stifness determined by a FEA can be about 30% - 50% higher that the real value (Here in the forum, in a post that I don't remember now). We have played safe in this area, I know from the calculations and plots that the effect of the chassis stifness in lateral load transefer and therefore other consequences like vertical load on tires and therefore it will change the tire grip in quasi-static and transiente behviour and also other dynamic effects. But the "real feeling" and numerical results of those consequences... those ones we don't have them yet, even of previous experiences, because it is our first car or even with a model of our vehicle which we are still devlopment. Some questions like, which performance cost will a X times chassis stfiness lower of what it should have. Those are still to be answered.

I hope we are in the somehow in the right way, thats why the feedback of you and the others like Z and Bemo is important to us.

Maybe this "safety factor" is to high!

[MCoach]

Tiago, I can say that compared to several successful FSAE cars you might be in the range of what I've seen. You might also see that things such as roll centers play a bigger difference than your chassis stiffness from where you're at, assuming you hit your targets. Unless you have a derpy chassis that handles like a wet noodle, don't worry about the transient tire loads, cause baby, we ain't there yet.

Depending on how you did your FEA, pulling in all of the tube properties or assuming a wireframe beam model (latter is easier and faster to iterate), you may see that your real values may deviate quite far from the paper you referenced, in fact the actual chassis may be only 1/3rd the stiffness of the simulation!

A side note, should only the tubes be simulated? I ask because I got our team to run stressed body panels for the whole frame rather than the large nosecone that covers the whole leg area that is typical of steel tube chassis teams. The panels, although minor in weight, added 30% stiffness to the chassis, putting us in the 2700N-m/deg range. What we learned from the whole ordeal is that the panels carried more than the expected 15% and that tube weight can be reduced to the bare minimum for this year. Just throwing that out there.


Basically, iterate it to the minimum tube sizes with respect to the rules, pull all of those thousands of grams out of that frame and see where that puts you.
It should still be right around where you are in terms of stiffness and a hell of a lot lighter. If you can get some thinner tubes for all of the non required tubes, do so. I like to use 0.035" for anything I can get my hands and 0.049" if the sim says it would fail.

Good luck. It's better to be wrong and finish early than to be right and never finish. "Speed over accuracy", It's ok to clip a few cones if you're still on the podium :)

[Z]

Tiago,

Have a look at the photo on this post (http://www.fsae.com/forums/showthread.php?11848-CR14-Frame-Design&p=121292&viewfull=1#post121292), from another thread asking for frame advice. There are many things on that car that make me feel rather ill, but let's start with these three.

1. Look at how close the steering-wheel is to the driver's stomach. Imagine a slightly fatter driver! These cars should be COMFORTABLE to drive. The driver should be able to QUICKLY rotate the wheel 90 degrees left, then 180 right, then 180 left, ... again and again, almost non-stop for the ~15 minutes of the Enduro.

This is why building an ergonomic mock-up is so important. Many newbie Teams (and also many not-so-new Teams) make the mistake of ignoring ergonomics ("Hey, it's all about power, and turbos...!!!"). Bad ergonomics is NOT something you can see in CAD. You must be able to sit in the car, push the foot pedals, turn the steering-wheel, change gears, and really simulate yourself driving fast. (A racecar video-game in front of you can help here.) More on mock-up frames below.
~o0o~

2. The overall "mass-distribution" numbers of the car are very important. These are perhaps even more important than power, or frame torsional-stiffness, etc. The main numbers you want to get right are;
2.1 Total mass. For a lot of reasons LOWER is better (obvious, especially when you have a smallish engine).
2.2 CG-height. Again LOWER is better (obvious).
2.3 Yaw-Moment-of-Inertia. Again LOWER is better (this one is very often ignored).

The car in the photo has at least a foot (0.3 m) of fresh-air between the driver's feet and the Front-Bulkhead! That is an extra 9 x ~0.3 m tubes (plus floor-sheeting and bodywork) adding to total mass.

Furthermore, the Rules mandate quite heavy tubes for the FB, and the area inside these tubes must be filled with a heavy plate. Then the IA must be mounted in front of the FB. All this adds greatly to Yaw-MoI!

There is no Rule saying that the Brake-Master-Cylinders must be in front of the pedals. Think about this, and then maybe find a way to put the Brake-MCs under the driver's knees. This will shorten the car by 0.3 m, reduce its total mass, and greatly reduce its Yaw-MoI.

Similar thinking can be used at the back of the car. Try to limit the rear frame to the mandated Main-Roll-Hoop-Bracing, and just enough to hold the engine and diff in place. Rear-suspension can attach to the MRH and the engine-diff supports.
~o0o~

3. Look at the position of the front Spring-Dampers in the photo. Read 2.2 above, and ask if these SDs (and their pushrods&rockers!) could be in a WORSE position!!!??? Also consider the SD's effect on the driver's sight-lines (ie. driver should see the cones).

My suggestion is low-mounted Direct-Acting-SDs, with these mounted to the frame at a pre-existing node (eg. the top wishbone's rear frame mount, possibly on the FRH). Result is better visibility, less total mass, lower CG, lower Yaw-MoI, less cost, less friction, etc...
~~~~~o0o~~~~~

ERGO MOCK-UP FRAME - There have been many suggestions for how to do these posted over the years. Eg. plywood, PVC piping, etc. Here is another "quick-and-easy" way.

Buy several lengths of 12 mm (or 1/2") diameter mild-steel bar. These are dirt cheap, typically used in reinforced concrete, etc. Buy a small "stick welder", again dirt cheap. Do multiple quick sketches of "frame concepts". Pick the one that looks best. Cut up the MS bar with a hacksaw, or BIG bolt-cutters, or angle-grinder (easiest but noisiest), and stick-weld together to make the frame. Bends are easy to do, NO fish-mouthing is needed, and big gaps are easily filled.

Bolt in the engine, and weld on whatever suspension configuration you have in mind, with the wheels at the ends. Do some quick torsional-stiffness tests on the frame. These are very easy to do, and you only need +/-10% accuracy. Fit a seat, steering-wheel, and pedals (just bits of the 12mm bar). Jump in and start "driving".

All this should only take a day. Ok, for students, maybe a week. At the very least your Team members acquire some fabrication skills.

Now the REALLY IMPORTANT BIT. After all Team members have done some "driving", you all sit around the car, perhaps with some refreshing beverages, look long and hard at the car, and think of ways to MAKE IT BETTER. This means making it more comfortable, making the mass-distribution more like above notes, perhaps adding more torsional-stiffness, but mainly making it SIMPLER to build.

Tomorrow get the angle-grinder out and re-work the frame. Then repeat above paragraph. NO CAD is needed, and you should have a really good frame after a few weeks.

Z

(PS. Here is a link to an old post regarding frames and torsional-stiffness -> 44 gallon drum chassis (http://www.fsae.com/forums/showthread.php?7732-Steel-Frame-Design&p=113807&viewfull=1#post113807). The rest of that thread may also be of interest to you.)