Re:Update December 3, 2007 - 2007/12/13 03:42Finally, after countless attempts, I was able to run a successful FEA on the basic cockpit frame. I am very new to FEA, and I have a lot to learn. I struggled getting this very simple analysis to run for several days. I thought the problem was related to the way I was contraining the model. I tried all different combinations of loads and constraints and got the same errors. Tonight, I realized the problem was related to how the part was meshed. I'm no expert, but I think I was using elements that were too small for the tubing walls. Of course, when I started using smaller elements, I started running into problems with memory. Running the FEA takes HUGE amounts of memory. I am going to have to figure out how to reduce the number of processes running in the background of the computer to free up some memory. Oh well, at least I was able to make some pretty pics. My goal is to start with the most basic frame shape, and then start adding the rest of the structure and see how that changes the stiffness and weight. I've had it for tonight, but I'm feeling good. Here are the results:
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JScottGT
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Re:Update December 3, 2007 - 2007/12/14 17:58Instead of modelling everything as a solid tubular structure, you should be able to model everything as a stick model. Then you just apply the cross-sectional moments of inertia to each "stick." This will greatly reduce computation time as well as how much memory is used up by the model. Just remember every time you reduce the mesh size, the file size (memory usage and computation time) goes up exponentially. If you are still having difficulty you can model everything as solid tubes. That will give you a relative chassis stiffness (torsional). Then as you make changes you can evaluate is as a % increase/decrease. I'm not familiar with the FEA package with UG at all, so I couldnt even begin to tell you where to start... now if you were using CatiaV5 I could help.
I created a simple model using shell elements to demonstrate the theory...
I found the commands to set up a stick model, but could not figure out how to apply the cross-sectional properties into the model. Once I figure that out, this becomes a much simpler model and will calculate very fast.
Post edited by: JScottGT, at: 2007/12/14 19:15
Post edited by: JScottGT, at: 2007/12/14 19:18Jason 2000 Mustang GT 5.4L Power
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Head Ball
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Re:Update December 3, 2007 - 2007/12/14 23:37Jason, Thanks for the tips. I will give them a shot. What kind of constraints did you use in your example?
Thanks for the advice, and keep it coming. I know I have a lot to learn in this area.
Ken
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JScottGT
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Re:Update December 3, 2007 - 2007/12/15 00:17I clamped the far end and put an up force on the right front, and a down force on the left front.Jason 2000 Mustang GT 5.4L Power
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JScottGT
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Re:Update December 3, 2007 - 2007/12/17 20:02Okay, so I did some playing around with modelling and solving 1-D beam elements for structural analysis and finally figured it out (for CATIA anyway). I created a series of lines to create a box. Each line was meshed using 1-D beam elements. Each beam was given a tubular cross section with OD & ID specified. I then used a clamp restraint on one plane, then added a couple-load to the other plane. See the amination below for the final result. For a quick comparison, the analysis I showed above was a simple "H" structure using shell (2-D) elements with a given thickness, and each joint had a seam weld applied. The above model took about 5 minutes to solve on my UNIX machine. The wireframe model with 1-D elements was somewhat more complex (structure-wise) and it took 4 seconds to solve. If you can get that figured out on UG, that would be the way to go. Jason 2000 Mustang GT 5.4L Power
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scooter70
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Re:Update December 3, 2007 - 2007/12/18 01:03Hey Jay - How about we start a Catia consulting company and Ken can be our first customer? I already have the C5/C6 suspension modeled parametrically so that I can move pickup points (and/or change arm lengths/angles) and view the changes in the kinematics module. Maybe I'll have some time to touch it up some more this week.
Ken - What's it worth to you? Free track time? 2005 Pontiac GTO LS2/T56 For Sale 1970 Corvette LS1/T56
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Head Ball
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Re:Update December 3, 2007 - 2008/01/02 13:54scooter70 wrote: Hey Jay - How about we start a Catia consulting company and Ken can be our first customer? I already have the C5/C6 suspension modeled parametrically so that I can move pickup points (and/or change arm lengths/angles) and view the changes in the kinematics module. Maybe I'll have some time to touch it up some more this week.
Ken - What's it worth to you? Free track time?
Now that I have access to V5, we might have to make a deal. I didn't do much modeling or FEA work over the last two weeks, but I will get started again. It was nice to take a little break and work on some other things. I may very well give you guys a call about V5.
Thanks Ken
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Head Ball
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Update January 2, 2008 - 2008/01/02 14:11Happy New Year to everyone. I used the new year as motivation to make some progress on the car. The two weeks previous to that were spend relaxing with family and friends. Now, it’s time to get back on track. Yesterday, I got two big things done. First, I moved the engine back about an inch on the motor mounts. The engine was just too close to the seat. I could have simply moved the seat further forward, but every time I move the seat forward, I reduce the legroom and the amount of room available to climb into the seat. Below, you can see the revised mounts. It wasn’t that much work, but I tossed the idea around in my head for a long time.
The other major accomplishment was connecting the rear frame rail sections with the rest of the car. Once I made the decision to move the engine back, I figured I was all set to start connecting the pieces. I had a lot of plans to tie down the rear suspension and make sure it was all square to within millimeters. Then, I realized that GM doesn’t get it that close, and the main goal of this project is to actually get something on the track. So, I checked the rear frame rails to make sure they were straight and level, and started welding. I still have to attach the frame rails in a few more places, but at least now they don’t move too much. I will continue to check the position of the rear suspension as I add the other connections.
A problem I was facing was how to attach the rear down bars to the frame rails. My chop saw would not cut the sharp angle easily. I had some cutoff pieces of rectangular tubing available, so I used them to mount the down bars. I still need to add some reinforcement, but I think it came out well.
The Harbor Freight tubing notcher continues to do good work.
With this done, I am now out of tubing. It’s time to head for the store and order some more so I can finish the rear section and start working on mounting the front suspension. While I am waiting on the tubing, I will try to get some more work done on the CAD model and FEA. I need to update the CAD model to reflect these down tubes as they are in a slightly different position.
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Head Ball
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Update January 6, 2008 - 2008/01/06 16:16One of the design elements I want to figure out using FEA is how to reinforce the top hoop. My first thought was an X because it makes sense. However, when I was reading through the paper on strengthening the NASCAR chassis, they used a V formation. The point of the V attached where the middle windshield bar came up, and went back to the corners. Below are the two different scenarios. I will have to run the second one with the windshield bar, because I do intend to run the bar either way.
The actual diplacement numbers are pretty much useless because they don't really represent the chassis. I am still strugling to learn how to set up the FEA. It hasn't been going very well. As a result, I have been using solid tubes for the model. However, I am keeping the constraints and loads the same, so the comparisons should be accurate. The rear of the model is being held fixed at the two rear corners. Each front corner has a load of 500 lbs, but is opposite directions to create twisting.
The animations are fun to look at, but they make it a little harder to see how the different geometry effects the areas of interest. In the above cases, I am most interested in the upper front corners of the main cage. That isn't where the max deflection is, so I made the following still pics to compare the areas. The V shape actually shows less deflection, which is opposite of how I thought it would go. My gut tells me that the windshield bar is a large part of the difference, so I will rerun the X with the bar.
EDIT: I have now added the X with the windshield bar. The deflection at the front upper corner is slightly reduced, but the deflection at the rear upper corner is more than the V. The overall max deflection is also reduced. The difference between the deflections is pretty small, but remember I am using solid tubes. When I finally figure out how to run hollow tubes, I expect the difference will be greater. For now, I am going to plan on using the X.
The pictures above show a model that is not finished. I will rerun the same analysis when I get all the door bars, front end structure, etc. built into the model. Just for comparison, here is the basic cage structure without any bracing.
Post edited by: Head Ball, at: 2008/01/06 16:18
Post edited by: Head Ball, at: 2008/01/06 16:36
Post edited by: Head Ball, at: 2008/01/06 17:09
Post edited by: Head Ball, at: 2008/01/06 17:29
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Head Ball
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Update January 6, 2008 Con't - 2008/01/06 17:27Here are some more good comparisons. The last pic in the reply above shows the basic chassis structure. The animation below shows the same chassis but with the addition of two tubes (on each side) between the rear subframe and the roll hoop.
These two tubes reduce the max deflection by 21% but with only a 5% weight increase.
Here is another animation with the addition of an X brace between the rear down bars. This brace reduces the deflection another 29% over the two tubes, and 44% over the base. The increase in weight over the base chassis is about 9%. Over the two tube scenario, it only adds 4% more weight. That is the kind of efficiency I am looking for.
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Head Ball
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Re:Update January 6, 2008 Con't - 2008/02/19 02:17Finally, some more progress. It has been a busy month doing anything except work on the car. I did make a little progress a few weeks ago, but nothing I thought worth updating. I simply welded on some plating that I needed for the work I did tonight. Using some 1.5" square .120 wall tubing, I connected the rear frame section to the main roll cage at two more spots. I used the square tubing because I plan to make a panel that is bonded or welded between the two pieces to make a vertical sheer panel. It didn't hurt that the square tubing was less than half what the DOM tubing cost. Here are some pics:
A pic of the plate I welded in and the two tubes coming into it:
Looking backwards from the main roll hoop:
A side view:
Top of the notch:
Bottom of the notch:
The Harbor Freight notcher continues to do good work. I think the key is to go slow and use plenty of cutting fluid (or WD40 if that's what you have).
Besides the work I did here, I purchased all the front suspension pieces on ebay. I can't wait for the delivery man. A friend of mine has agreed to machine up the front suspension mounting brackets as well. Finally, I bought a Wilwood brake pedal on ebay. Nobody else bid on it, so I got a good deal. It seems the deals on ebay are fewer and fewer every day, so I am happy to find one.
I expect to make a lot more progress in the next month, so stay tuned.
Ken
Post edited by: Head Ball, at: 2008/02/19 02:19
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Head Ball
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Update March 6, 2008 - 2008/03/07 00:25Time sure seems to fly between these updates. I haven't made much progress on actually building the car, but I have been buying parts like mad. As I said in the previous update, I ordered all the front suspension pieces. Well, they showed up in good shape and are waiting for some mounting brackets to be made. Here is a list of some of the other things I have purchased:
Bilstein coilovers Transmission Gauges Steering wheel and quick disconnect Rear half shafts Rear toe control rods Right rear upper control arm All the rear suspension mounting hardware Z06 exhaust manifolds Differential Driver's seat cover Rear brakes
Below are pictures of some of these items. You will notice that the toe control rods and half shafts are not in the best shape. I wasn't happy when those showed up, because they were advertised as in good condition. The seller said they came from a car with 40k miles. After some back and forth, he agreed to refund me some of the purchase price. I will use the half shaft with the torn boot as a core for a rebuilt one. That still puts me ahead of buying a new one. As for the toe control rods, I am going to see about getting some new ends.
Post edited by: Head Ball, at: 2008/03/07 00:27
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Head Ball
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Update March 8, 2008 - 2008/03/08 15:40Big milestone: I picked up an engine last night. It's a 5.3L truck engine with about 61k miles. The engine I went to look at originally had fewer miles, but when we went to look at it, the water pump would not turn. This engine looks like it has a few oil leaks around the valve covers, but some new gaskets and a good cleaning should solve that. I now need to start stripping off the parts I am not going to use, and track down the replacements. The intake manifold won't clear the cage, so it has to go. The exhaust manifolds are also going. Here are some pictures of the engine as I pulled it out of the back of the truck:
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Head Ball
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Update March 10, 2008 - 2008/03/10 12:16A couple more updates from the weekend. I picked up some nice wheels (from a 3 Balls participant) on Saturday, and am now looking for some tires to go with them. On Sunday, I fitted four more tubes on the cage. Getting the front dash tube on is important because it allows me to position the brake pedal and steering column. That is what I'm hoping to get done next weekend. Here are some pictures:
Post edited by: Head Ball, at: 2008/03/10 12:32
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Head Ball
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Re:Update March 10, 2008 - 2008/03/25 01:58Here are the latest pics of the construction. I got the side impact bars in this weekend which was a PITA. I also got the much of the front subframe put together. A friend is working on the lower control arm mounts for me. When those are done, I will finish up the subframe. In the meantime, I think I am going to start finish welding the frame.
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