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PostPosted: April 1, 2015, 11:06 am 
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Hello LocostUSA, Bob here.

I've been designing a mid-engine, Ar-i-el At-om / Mako type car, and I feel that I may finally be at a point in my design where I'm comfortable sharing it here.

I'm already on V2 of my chassis, but I'm much happier with the design this time around. The main design of the chassis is nearly complete, next is designing the suspension pick up points.

Here's the high points of what I plan to do with the car, and the key design constraints.
    - The car should be safe for road use, autocross and track days.
    - Safety is the #1 design purpose, along with chassis strength. Lightweight and aesthetics are secondary, but also important.
    - Although the chassis pulls inspiration from existing cars, I want the design to be my own throughout the entire project.
    - It's not set in stone, but I plan to use the GM LNF Ecotec engine for this chassis.

I've included some images of the design, I'll let them do most of the talking. Thanks in advance for all forms of comments, feedback, and helpful criticism!

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Please let me know what you think!

Cheers, Bob

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PostPosted: April 1, 2015, 11:16 am 
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Very nice CAD work! What program?

Will you do any kind of structural analysis, like Grape or? This could point out where you might want to add some additional triangulation, if necessary.


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PostPosted: April 1, 2015, 11:28 am 
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Bobber wrote:
Very nice CAD work! What program?

Will you do any kind of structural analysis, like Grape or? This could point out where you might want to add some additional triangulation, if necessary.


Thanks for the compliments! I'm using Solidworks 2014, I'm pretty happy with the chassis so far. The engine model and wheels came from Modernkitcar.com and the seats and dummy came from Grabcad.com, but the chassis is all my design.

I'm currently finishing my BA for Mechanical Engineering, and we use Pro-E for finite element analysis, I was planning on testing my model as soon as suspension pickup points are designed. I suspect there's bracing that could be removed, but I'm not sure until I test it.

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PostPosted: April 1, 2015, 11:51 am 
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two comments here,

what is the wheelbase, as it looks long compared to the people and the power plant?

and yes i agree there may be a diet on the cards for the chassis and the curved tubes will be a challenge to make.

oh and a third, looks like you have a pleasant looking vehicle, very tidy.

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PostPosted: April 1, 2015, 12:15 pm 
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Agreeing with john h., wheelbase appears to be at least 2' longer than necessary unless you want it long. Lot of length between engine and seatback and toes and front axle line.

Suspension pickup points will significantly change (add more tubes) the existing design.

Replace some roof tubes with diagonal bracing.

If you will skin the car, design the frame now to accommodate the skin shape/attachments.

What will frame weigh?

A very good start.


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PostPosted: April 1, 2015, 12:37 pm 
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john hennessy wrote:
two comments here,
what is the wheelbase, as it looks long compared to the people and the power plant?
and yes i agree there may be a diet on the cards for the chassis and the curved tubes will be a challenge to make.
oh and a third, looks like you have a pleasant looking vehicle, very tidy.


Thank you! A clean design is very high on the priority list, I don't want this to look like a patchwork quilt when I build it.
I was hoping the curved tubes wouldn't be too hard with the appropriate roller setup, they all follow the same radius. Also, all the 1.5x.063 4130 tubes use either a 4" or 6" bend radius, and all the 1.0X.063 4130 use a 3" bend radius.

Quote:
Agreeing with john h., wheelbase appears to be at least 2' longer than necessary unless you want it long. Lot of length between engine and seatback and toes and front axle line.
Suspension pickup points will significantly change (add more tubes) the existing design.
Replace some roof tubes with diagonal bracing.
If you will skin the car, design the frame now to accommodate the skin shape/attachments.
What will frame weigh?
A very good start.


- I've got the wheelbase at 104" and the track width at 65"... does that seem excessively long? I had read about a wheelbase ratio of 1.6:1, maybe I should revisit this?
- Good call on the diagonal roof tubes, that will help access to the cockpit as well. I'll be persuing this soon.
- When / if I skin the car, I plan to use panels of lexan screwed in place to mounting tabs that will be positioned throughout. This image was my inspiration in that regard, specifically the side panel. Thoughts?
- So far, the frame weighs right at 200 lbs, not counting welds. That seems light, but I'm not sure...

Thanks for the responses folks!

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PostPosted: April 1, 2015, 2:38 pm 
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I'll be up front and say I have a pretty strong bias against "exo" style cars based on the example by Ar-i-el. To my eye the design lands up being about aesthetics first and essentially no considerations about weight or strength.

So that's my bias. You are doing very good work on your modeling and I know it's a lot of work. Nothing wrong with that. I think you should incorporate use of FEA right away. Without that underpinning your work it's very difficult to understand the tradeoffs in your design. What is the cost or benefit of the any tube, you just don't know.

So here's one thing I think you'll find in FEA. In the floor of the car there is an opening under where the driveshaft tunnel would be in a front engine car. This will allow the car's right and left sides to twist compared to each other. If the bottom of the car is a plane and the upper rail area is a plane, neither of these has any diagonal bracing. Both should have this bracing in at least one bay. That will make a tremendous improvement, I think. It doesn't need diagonal bracing everywhere, but one place is a big step forward.

Doing it in the roof over the passengers will help, but a really good place for this is in front of the dash.

Have fun!

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PostPosted: April 6, 2015, 3:06 am 
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This looks beautiful. My only suggestion is to connect the tubes into "nodes", a lot of your tubes are attached exactly between other connections, this makes the tube rely on its bending strength instead of its tension and compression strength which is significantly better. Think of it like bending a wooden dowel on your knee versus trying to crush it or pull it in appart from the ends

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PostPosted: April 6, 2015, 10:24 am 
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I notice that you are using pre-bent tubes for a lot of your connections. Can you buy these pre-bent like el welding fittings for pipe? This will require full penetration butt welds for the tubes. This is sort of how we design railing for industrial facilities.

I would love to see how Pro E works on this.


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PostPosted: April 7, 2015, 8:13 am 
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I'll echo what these guys have said. It looks cool, but it has some potential structural deficiencies.

Triangles are alot stronger than squares and rectangles. Use them where ever possible. As mentioned, that will include reworking the floor, and having tubes land come together to form nodes. Strength through geometry not material properties should be your first choice.

I know curved tubes look cool, but they're also weaker. They're effectively pre-bent. Would you build an engine with several pre-bent connecting rods? (Probably not.) Now, I understand the design intent, so I'm not going to say don't do it, just make sure you understand the trade-offs, and have dealt with them (maybe using larger diameter or heavier wall tube). The curved tubes overhead (halo bars - a fairly important location) would not be my first or second choice. Also, those halo bars should be closer to the bend.

The passenger compartment is the area you want to be the most structurally secure, since the squidgy bits (that'd be you) are the hardest to replace. And from a structures standpoint, it's the hardest to do, since the passenger compartment is the biggest area, with the longest spans, and the least amount of opportunity for diagonals (those squidgy bits get in the way, again) and triangulation.

One last piece of advice would be to locate a sanctioning body rule book (SCCA, NASA, NHRA) and consult with it. I'm not sure that the track is important to you, but if it is, you need to figure out whose rules you will play by, and go with that. If track time is not important to you, you can learn alot from the various rule books about what is accepted roll cage design. Those rulebooks are based on decades of data from lessons learned, typically through crashes, of what works well and not so much. Use them, they're (typically free) expert advice.

Again, I'm not trying to say don't do it, but know all of the compromises that you are making (and why), and make them by choice, not by accident.

JustDreamin


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PostPosted: April 7, 2015, 8:38 am 
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First of all, thanks everyone for the very helpful replies! This is the kind of response I was hoping to garner, I know my design needs work.

horizenjob, I can understand your bias towards one type or another, no doubt. I agree that the mid-engine exo style car may be heavier and not quite as strong, but aesthetics are somewhat important to me (count me ashamed, but honest) and I just love the shape of the exo type car. I think with proper design it can be close to as strong, and hopefully as light as a 442.

I have access to Solidworks 2014 at work and at home, as well as AutoCAD 2013 and ProE Mechanica. I'm having a great deal of difficulty getting any useful FEA out of any of these, what do you all use?

In reference to what horizenjob, Aetro, and JustDreamin mentioned about node connections and chassis stiffness, that will be my next refinement. It's funny, I thought I did a decent job of sticking with node connections, but when I look at it objectively, I do see many areas in need of improvement. The floor and front firewall area will be my first areas to attack.

In reference to the curved tubes, I agree that it's a weak point in the design, but I found that keeping them straight made making space for occupants very difficult (i'm 6'-4", 210lbs). The tubes are only bent slightly, the center of the long tubes is only 4" off, but I know that is splitting hairs. I'm definitely going to be rethinking my chassis and trying for more triangulation to increase strength and rigidity. JustDreamin, when you are refering to halo bars, are you suggesting that I put one over each occupant within the main roll hoop? Safety is my primary concern for this car, since it will indeed be seeing track and autocross use on a pretty regular basis.

I have removed the curved overhead tubes in favor of a crossbracing design over the cockpit, as well as shortened the rear overhang. Thoughts?

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(pay no mind to the front suspension pickups, they are a very early draft right now.)

Last question for today: the material I've chosen for the main roll structure is 4130 1.5"x.063" tube. On my first pass through the SCCA rulebook, this seemed to surpass the requirements for cars 1750lbs and under. Should I be using heavier tube, or larger diameter tube? I'm having a difficult time getting a baseline idea for what the typical size is. All other smaller tube is 4130 1.0"x.063 tube, is that overkill?

Thanks in advance for all the assistance and feedback guys, I really appreciate it.

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PostPosted: April 7, 2015, 9:57 am 
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I wouldn't worry too much about using curved tubes. It appears they have adequate supports along their length which will inhibit their "Mc/I" stresses. I have designed arch bridges and they work.

For structural analysis, I have used STAAD, SAP, MicroSafe, Strudl, and Risa. I'm currently using Risa on a plate monocoque design since it handles plates well (all 3500 of them). I also use AutoCAD but it is difficult to download their files on this site.

I'm looking forward to your analysis. Does your program support the AISI codes?


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PostPosted: April 7, 2015, 10:21 am 
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My comments about halo bars were indeed directed to the bars that are overhead. Typically, those bars run from bend to bend (remember, the intent is to create nodes where ever possible). There is usually a diagonal (maybe two) that run from corner to corner. Most sanctioning bodies are very specific as to the configurations that they will accept.

Also note that the main roll hoop structure should really have at least one diagonal from bottom corner to opposite top corner. Multiple diagonals are ok, as long as they're notched into the primary diagonal. Again, see what your sanctioning body says is acceptable.

You also need to consider rear braces, and where they join the chassis. You want some portion of the rear frame to serve as a crush structure, to absorb some energy in the event of a collision or other "excitement". You don't want your rear braces to pick up this structure and try to feed crash loads directly into the main hoop (trying to fold it over). You probably need the rear braces to land about where the rear suspension pickup points are located (they are likely inboard of the crushable structure).

Same thoughts go for the front structure, the front portion should be weaker on purpose, allowing you to absorb some energy in that portion, prior to the main structure (where the people are) beginning to see big loads and deformation.

As far as your tube sizes go, I would suggest you consider thicker wall tubing. 0.063" wall tube is certainly as thin as I'd go, and I personally would go thicker, especially for the main tubes. Big diameter thin wall tube is more sensitive to physical damage (denting a tube) which could compromise it's strength (and appearance).

Whatever you design, you'll need to pick a sanctioning body to play with, and make sure your design falls within their rules. Some sanctioning bodies will play together (NASA & SCCA seem mostly compatible) some will not (for example NHRA rules are not directly compatible with SCCA rules, they require different stuff). SCCA rulebook will give you plent of guidance. You will also want to get to know your local scrutineer, so that once you have a design that you think is good enough, you can get feedback from the folks who will be deciding if it really is good enough.

JustDreamin


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PostPosted: April 7, 2015, 10:44 am 
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Bobber wrote:
I wouldn't worry too much about using curved tubes. It appears they have adequate supports along their length which will inhibit their "Mc/I" stresses. I have designed arch bridges and they work.

For structural analysis, I have used STAAD, SAP, MicroSafe, Strudl, and Risa. I'm currently using Risa on a plate monocoque design since it handles plates well (all 3500 of them). I also use AutoCAD but it is difficult to download their files on this site.

I'm looking forward to your analysis. Does your program support the AISI codes?


Interesting, so AutoCAD can do 3D structural analysis? What suite do you use, Bobber? I'm very new at the analysis side of things, what is AISI code?

JustDreamin, thanks for the helpful insight regarding the crash structure. I have a lot more research in front of me! I like the idea of isolating the main passenger compartment from the front and rear crash forces. I'm currently redesigning the main roll structure based on your info, and trying to do more triangulation. Also, I'm thinking of stepping my tube size up to 2.0x.073, or should I just increase the thickness of a smaller diameter tube? I expect that i'd gain more from a larger diameter tube with a thinner wall vs. thicker wall small diameter tube, but as you said, it's more sensitive to damage that way.

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PostPosted: April 7, 2015, 2:31 pm 
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Clarification: AutoCAD just does drawings, although there are structural add-ons, they are not widely used.
AISI (American Institute of Steel and Iron) is the code for light gage steel design . It's primarily for the design of building structures but the theory applies to the steel member sizes used in locosts (steel less than 3/16" thick).

If you have a cad program that you can use to input the geometry info directly into your analysis program, it will be a huge help. I'm up to "Car85" in my current analysis and have to redraw each time.


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