Duke University Motorsports is a student group that designs and builds open wheel, single seat race cars to compete in the Formula SAE competition sponsored by the Society of Automotive Engineers. The team consists of Duke students from both Pratt and Trinity, in all classes. The purpose of the team is to provide students with a way to gain practical design and manufacturing experience in a fun and challenging setting.

Wednesday, January 18, 2012

"im CAD hat's passt!"

Inscribed on the Porsche 918 RSR, this message was inscribed discreetly under the rear wing.  During the development of the vehicle, there was apparently a lot of back and forth between the design crew and the production crew.  When pieces wouldn't fit together, the engineers would respond: "It fits in CAD!"
image shamelessly stolen from Jalopnik
CAD is an amazing tool, and there is no way we could have made the drastic changes that we did this year without it.  But CAD is merely a representation of reality, only as accurate as what your manufacturing/assembly tolerances are, and only as accurate as your models.

I'll detail the issues we've run into so far after the jump.  Luckily, we haven't run into any showstoppers yet, and the areas that I thought would be the main problem areas are looking good so far.

First, we had a couple of engine mounting issues.  These were small, but almost ended up being showstoppers.  The first is shown in the picture below:

Given the crudeness of our engine model, it's clear that the clearance circled in red there is scary close.  In real life, it turns out there isn't actually that much clearance, and part of the engine hits the frame at that point.  However, it's pretty close, and by tilting the engine very slightly, we were able to fit everything in just fine.

For most of these issues, I had a contingency plan.  For this one, I didn't.  The reason?  Simply that I didn't notice the issue when I was designing the car.  It does fit in CAD (barely - minimum clearance is 0.08"), but not in real life.  Note to next year's team: leave a bit more clearance than I did... especially where your models are not so great.

Another issue that can't really be dealt with in CAD easily is the issue of getting the engine in and out of the frame.  In our case this year, we can slide the engine straight forward, past the main roll hoop, and pop it out through the bottom.  However, with engine mounts in place, that can be tricky or impossible.  See the picture below:
 The engine mount, if left in place, will interfere with the stator cover, and make it extremely difficult to move the engine straight forward, and in our case probably near impossible to get out.  This is a problem, so we made an engineering tradeoff:  We are actually going to use the other set of engine mounts as shown below:
These engine mounts are well out of the way of everything, and will allow the engine to slide in and out freely.  The tradeoff is that the engine mounting is less stiff given the same mounts.

Another stupid mistake on my part:
I added the shifter spline from the engine into the model to make sure that it would fit, but I never thought about how I would get the spline on and off.  Oops.  We have to attach the spline when the engine is not mounted, either out or slid forward a few inches.  Not a deal breaker by any means, but makes assembly more difficult than it needs to be.

And now for the big issue... the rear of the frame.  This was one area where fitment would be critical, and it did not end up matching the CAD at all.  It was a particularly difficult design to eyeball, and next time, I would say provide a welding jig/fixture to place those tubes accurately.
The green represents where the frame actually was when we got it back, an inch off of where it should have been.  This presented problems, discussed here, so we had to chop off the tubes and redo them.  The finished product turned out well enough that the drivetrain actually fits now.


  1. I'm a mechanical engineering undergrad student, looking to start learning autocad in my free time. If you are an engineer, what auto-cad programs do you run into most often? Autodesk? Solid Works?. Thanks!.


    1. From what I've seen, most automotive companies use either UG NX or CATIA. Pro/E is pretty common outside automotive, and solidworks seems to be gaining more traction, though mostly at smaller firms.