Article #4 Carrera UC7 “Serving as a Warning To Others”

Picture of Adam Bahret
Adam Bahret

Ok, next up in the Carrera UC7 series, we are slowly approaching the pinnacle of this exercise which is successfully loading a 12ft catamaran boat on top of a Carrera. This was the benchmark set by our champion, the Honda Crosstour.

The first step: we need to design and build the hitch. For this, I enlisted the help of some friends. The first job I took on when I founded Apex Ridge was in partnership with Acorn Product Development. We joined up to work on Boston Dynamics robotic mule. It’s like the small dog they have now, Spot, except it was six feet tall and looked like a horse. As you can imagine, it was fun to take through the woods and neighborhoods.

Since then, Acorn and I have worked together on many projects and shared a lot of good times along the way. They wanted in on my latest UC7 shenanigans and I needed some good engineering counsel. 

First, I described what I wanted to accomplish. I needed a 2-inch hitch coming out of a sports car’s bumper.  It needed to hold 300lb tongue weight 9.5 inches out and be able to pull 600lbs at an acceleration of 0-60mph in 5 seconds. They threw out the “project concept” at one of their project meetings and a few engineers jumped at it.  Because of his dual credentials as an amazing mechanical engineer and a fellow car nut, we selected Ryan Liu to lead. See, Ryan is a gearhead as well and wanted to make this happen just as much as I did. He runs a Subaru BRZ and a Mazda Miata on and off the track regularly; he gets it!

Ryan and I came up with a design. We performed the stress analysis with finite element analysis, the mathematical modeling of stresses. The design was optimized for low weight and sufficient strength for the UC7.

We thought we would complete this analysis and design in one shot.  We failed. How did we fail? Wait for it… wait for it…  In creating the full set of use cases.  Yup, ironically, the project that was done for the sole purpose of demonstrating how to make use cases did not correctly define use cases.  This was not intentional for the purpose of making this installment of the series more interesting.  


So, how were we taken down a peg or two from full of pride to not so smug? We showed the results of our analysis to the extended Acorn team.  What we got back wasn’t a “Wow great job!”  Instead, what we got in response was “Why didn’t you think of the braking stresses?” “How does the tongue weight change when you hit a pothole?” “Do people stand on the trailer when loading it, adding 200 more pounds to your max weight?” and “Might they stand at the front of the trailer resulting in increased tongue weight?”

We felt deflated, to say the least, and went back to the drawing board. However, in the long run, those questions saved the day because the loading was much higher with all of those factors taken into consideration.

When what had happened finally dawned on me, I could hear something my grandmother used to say to me whenever I got a bad grade or failed in an activity, “Sometimes your purpose in life is not to succeed, but to serve as a warning to others.”  She was from the depression era and not that great with child development. 

Our warning to others? ALWAYS make use case creation a team exercise and an independent step in the product development process.  Don’t let it be just something two guys get done at their desk and then report on. This is actually one of the driving factors for why I created UC7 in the first place.  It forces the team to come together and complete use case creation as a group exercise, not in isolation, and not as a side activity.

Another big value for UC7 is that it makes the development process shorter.  Instead of  finding out in a traditional design process that the base use cases aren’t correct until testing, the team can consider these “extreme” factors early. Even just one added stress element found in a UC7 exercise early can save months of redesign, prototype, and retest. We proved that by having to run our model again with new boundary conditions, redesign to accommodate those stresses, and then run the model again.

So our hitch design is different now because our model has the correct boundary conditions based on a fully fleshed out use case.  We began this exercise to show the value of UC7 and the even greater value of focused use case creation. Although we didn’t succeed in efficiently creating a design based on good use cases and engineering analysis on our first try, we did a great job making a point by serving as a warning to others.


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