The Technical Questions Thread

[johnnynumfiv]

does an aircon unit sap any power from the engine when it's turned off?

Yea, it still has to turn the pulley and belt.

[drakkie]

Anyone here happens to have NASTRAN ?

I'd like to have a good FEA done, but Inventor gives some problems because of the size. School is not where i can be found next months anymore, so.. Hope someone can do me a favor. Im not sure it passes, but adaptations can always be made. Attached a picture of the design involved. This is the 2nd mock-up for the finished design, so it needs to be good after all the final version will be based on it, including all parts. PM me for the drawing files if you can help me out.

[clutch-monkey]

Yea, it still has to turn the pulley and belt.

ah ok. well, i threw it out anyway.

[johnnynumfiv]

Anyone here happens to have NASTRAN ?

I'd like to have a good FEA done, but Inventor gives some problems because of the size. School is not where i can be found next months anymore, so.. Hope someone can do me a favor. Im not sure it passes, but adaptations can always be made. Attached a picture of the design involved. This is the 2nd mock-up for the finished design, so it needs to be good after all the final version will be based on it, including all parts. PM me for the drawing files if you can help me out.

What kind of vehicle is that chassis for?

Anywho, the middle(red area) looks very weak. In a frontal or rear impact, the area in green will give way very easily because there is really no strength there. The joints on the floor will probably act as pivot points so the car will fold like \___ or ___/ .

[drakkie]

What kind of vehicle is that chassis for?

Anywho, the middle(red area) looks very weak. In a frontal or rear impact, the area in green will give way very easily because there is really no strength there. The joints on the floor will probably act as pivot points so the car will fold like \___ or ___/ .

Exactly my thoughts. But its pretty difficult to make it as light weight, strong and cheap as possible and still fit some passengers. I was thinking of manufacturing in in alloy with min. yield strength of 690MPa. First see how this comes through the analysis, then update, modify and perfect it.

[RacingManiac]

have you tried other software? I personally find Pro/Engineer is easier to learn and use than nastran, and as a team we've had decent success with analysing the tubular structure such as that with good accuracy if you input the parameter correctly. Our previous FE results from the FSAE car come within 9% of the actual torsion test results...

[culver]

Anyone here happens to have NASTRAN ?

I'd like to have a good FEA done, but Inventor gives some problems because of the size. School is not where i can be found next months anymore, so.. Hope someone can do me a favor. Im not sure it passes, but adaptations can always be made. Attached a picture of the design involved. This is the 2nd mock-up for the finished design, so it needs to be good after all the final version will be based on it, including all parts. PM me for the drawing files if you can help me out.

For a tube structure there are a number of good FEA modeling techniques available. There was an SAE paper published at least 10 years back that talked about alternative modeling methods for tube frames. We are talking about methods which were developed to allow modeling with 486 based computers.

One method is a truss style model. In that model every tube is modeled as a straight beam. All tube intersections are considered 3DOF joints. The tubes can freely bend but not rotate at any joint.

An alternative method is to break the fame up into triangular surface panels. At that point the assembly of panels can be modeled as a surface rather than a 3D shape.

The SAE paper I mentioned above looks at those methods. IIRC it also compares them to FEAs of the same frame conducted by Ford using their advanced solid modeling systems. I believe they found the triangle panel method returned more accurate results. However they also found that the truss element method returned accurate trends. In other words if the truss model said a tube was a week point or adding an additional tube would help, it would help in real life. The truss model always underestimated frame strength because it assumed the joints could bend freely. That means anything that was rectangular in section had no strength even though that wouldn't be true in real life.

As for your particular frame, I think it could use quite a bit of stiffening. I'm not sure what your design intent is so I can't just say where to add tubes but I would point out the following:

Look at the frame from the side. The passenger area is basically un triangulated. The top and bottom are almost acting like 2D structures. At least add a boxed in central tunnel or door sills so that you have some larger 3D sections running front to back. Consider that if possible every joint should be supported in 3D space. That means when 2 or more tubes join together they are out of plane with each other. When trying to triangulate it is very helpful to think of every joint as a flexible joint. You want the strength of the chassis to be inherent in the triangulation, not the rigidity of the tubes them selves. If the triangulation is good then the natural rigidity of the tubes is almost a bonus.

If you can post a few pictures showing some additional views and things like suspension pickup points I might be able to offer some additional advice.

[culver]

have you tried other software? I personally find Pro/Engineer is easier to learn and use than nastran, and as a team we've had decent success with analysing the tubular structure such as that with good accuracy if you input the parameter correctly. Our previous FE results from the FSAE car come within 9% of the actual torsion test results...

Keep in mind that often it is more important that the software gives you a good idea what trend you are seeing vs absolute numbers. It definitely is worth looking into a truss model (which requires you create a separate wire frame model). The truss fame model does an excellent job of showing just how well triangulated the chassis happens to be as it doesn't let you get away with poor triangulation and stiff joints. Of course most of the FEA packages included with solid modeling packages can only do 3D models, not alternatives which work for tube frames.

[RacingManiac]

Our chassis frame model was done that way, in that you input the properties of the tube you are using to the dots. It was abit more complicated in that we have bonded panels in sections. But AFAIK(I am not chassis btw) thats how it was handled in Pro/E.

Doing it that way also saves a whole lot of computing time, as it is alot easier to do calc for truss than the full 3D FE....

If the structure is simple enough(and it looks to be), you might even consider doing it in Ansys. Its not as user friendly but for that work it has not much legwork involved to learn to do it that way....

[culver]

OK, nice to see that method is still used. I believe you can handle stress panels by giving them an equivalent tube bracing. However, I don't know the details of how you select the appropriate equivalent tubes.

[NSXType-R]

Didn't want to make a new thread, so yeah.

Just curious, in an indoor go kart track, is it necessary to beef up the ventilation system because of all the exhaust gasses from the two stroke engines?

[clutch-monkey]

for a small displacement engine, say a 4cyl of some kind, if you fit a supercharger - i'm gonna say because it's easier than fitting a turbo in the example i'm thinking of, plus no dicking around with exhaust manifolds and exhaust note isn't strangled - should you opt for a free-er flowing exhaust? do you even have to change the exhaust, or is it just recommended to get the most out of the supercharger?

[fpv_gtho]

I'd say there'd be some pretty good gains to be made with the exhaust. Afterall theres more pressure/air going in, and that has to get out as well.

[Kitdy]

What is the torque split on Lambo's AWD system and can it be varied a la some other systems?

[Kitdy]

Another one, I see that the Phantom used to use the 6.75L V8 but now uses a 6.75L V12. Other than smoothness, engine size, balance, and I assume revability, what sort of other characteristics will change about the engine? Higher redline? What about peak torque and hp? Keep in mind I am speaking in generalities.

Thanks in advance.