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Doing the FEA modeling does not require drawing. You just put in the position of the tube nodes and it draws a stick figure for you. Then you tell it places where you want to push or pull on the frame and where you want to lock it down. So it's as though you bolt some pieces to the floor and then lean on it and it will tell you how it moves and how much you stress it.

FEA stands for "Finite Element Analysis", which basically means breaking a problem down into little pieces, in this case individual tubes. Then by doing the math on all those little pieces the computer can give you an idea of how the whole thing works. You can get yourself in trouble if you are not constantly asking yourself wether the results you are getting make sense.

It has been a great learning tool for me. I still remember how excited I was when late one night I changed a tube in the very back of the car - and the tubes in the front bullhead that make a "V" shape changed color, indicating a big change in their stress. Everything affects everything else in these frames and the better the job you do, the more that is true.

The program I use is called "Grape" and is available free for non-commercial use. It only deals with tubing, so we can not look at monocoque or stressed panels or upright design. Those are much more difficult problems and even though there is free software to handle these things, I feel more comfortable with the basic stuff and knowing that I can check the results in the real world with a dial gauge and some weights.

When we get a little further along in the design we will get to this.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

What are we assuming ride height and tire size to be? I have been measuring based on the OEM tiresize for the WRX of 205/55R16 (24.848in diameter) and 5.5in ride height. I figure the OEM height is good enough because 225/50R16 tires are available as a street tire and a race tire and happen to be almost exactly the same height (24.858) and 225/50R16 fits well on OEM 16x6.5 rims.

Also, I flipped the front suspension around so that the parallel arm would be in the front and then I imported the WRX front spindle, set it at 59 inches from hub to hub and then set it back 8mm for where the face of the rotor would be and the control arm you've already built is nearly spot on for this hub! haha.

Set up this way in the screen shots with the trans/motor where it is, it brings it to a ~99in wheel base and what looks like plenty of room for the wheels to steer.

So here's an idea for the rear frame. It looks like it can be tucked in a little closer to the trans but, this creates enough clearance to build a downpipe that turns in towards the center of the car and out the center of the rear under the frame rail. I didn't draw in the bottom frame yet, trying to leave it open for ideas how to drop the whole lower frame out of the bottom. Maybe make that entire lower frame a removable subframe and the engine/trans and frame can come out as one piece?

I flipped the rear control arm around and moved the shock as well, seems to fit quite well.

https://3dwarehouse.sketchup.com/model. ... de9ea3b0ca

Quick update to the model. I had the front spindle facing the wrong way, the WRX is front steer. I also closed the rear tubes in closer to the transmission, moved the front members back into place instead of floating and adjusted the arm length a little bit. Looks to me like there is plenty of room in the front this way. The screen shot is with 225mm wide tires assuming they were mounted on a 16x6.5 55mm offset rim. Obviously a lower offset would give even more room.

There is room to move the motor/trans forward a bit to shorten the wheelbase an inch or two...

Inboard mounted shocks and pushrods might be something to think about considering there is so much extra room in the front.

https://3dwarehouse.sketchup.com/model. ... 3a68ae2b27

If I'm not mistaken you can swap the front spindles from left to right. So where ever is the best placement for the rack and pinion. Weather it be front steer or rear steer. Don't quote me on this, I could be wrong. haha

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30 years old, not sure what I want to be when I grow up…

I would suggest that you read up about Ackermann before swaping RH/LH spindles. Dave W

Finally back in wireless range, we had a fantastic time in the mountains and made it a point to stop by the Tail of the Dragon and get a couple runs in with the WRX. All I could think about after the fact was "what if the tube chassis was finished" haha


So, I downloaded the Grape software, how do you get it working?

When you start up Grape, it will put up a screen for "License Registration", do't fill out the fields and just click "OK". Then a little box pops up to say invalid and you have the option to cancel or retry. We are non-commercial so we just hit cancel.

At this point you should have two windows that open up. The actual tool window and another window which let's you browse thru the help information. The help information has lots of good stuff, basically a user manual and some tutorial material. It's worth spending some time there.

We have a thread here on LocostUSA that has within it some models members have made. There is a book chassis, an early version of Car9 and also a chassis by Andrew Moore, maybe also a Kurtiss chassis. There is a table of contents in the first post that points at some models done in Grape already.

http://www.locostusa.com/forums/viewtopic.php?f=39&t=2224

Basically what you need to do to use Grape is to define the tubes that are involved. What their material is ( steel in american customary units ) and what their outside and inside dimensions are. Then you place the nodes and connect them with the tubes you just defined.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

For a first effort you could define a 1" square tube, Home Depot sells that if you haven't bought your tubing yet. Then in both Grape and the real world make a tube several feet long, prop it up on a brick and stnd on it and measure how much it bends. The numbers should match well.

Wait, didn't you just get married? You should be practicing, marriage is hard, practice makes perfect!

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

I went to go thru the thread I pointed you to and found that there has already been a table of contents added to the first post. It has pointers to several models already done in Grape. The files that Grape uses are kept in simple text which you can look at.

You may be able to make a copy of a model and then delete the nodes, but keep all the definitions of tubing etc. Help you get up to speed a bit. The case I mention above for putting a tube on two bricks and standing on it is useful because it gives a perspective on putting loads into the middle of a tube or at least not into a node. This commonly happens with suspension brackets and engine mounts for example, especially if the ride height has changed or a different motor is used.

Taking measurements from a SketchUp model can be a bit of a pain, but you will get the hang of it and I can help. Many of the basic measurements you can sort of just determine, for instance the car is or was 44" wide from outside to outside and some of the bulkheads are on even dimensions like 36" or 56" ( for the front engine car ).

When getting the dimensions for Grape it wants to know where the center of the tube is. For a 1" tube frame you could say all the bottom tubes are .5" above the x-y axis. Oh, in my models I made X be fore and aft, Y be width and Z be height. Sometimes people do otherwise.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.