Article #9 Boat on top of the car Carrera UC7

Adam Bahret
Adam Bahret
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Well, we’ve reached the finale, the final task of the Carrera UC7 challenge has been completed. There is a boat on a Carrera and, to be honest, it seems natural. The car handles really well. Other than the sailboat mast float hanging out in front of you as you look down the road, you really could forget the entire boat is there.

The only obstacle stopping this setup from getting anywhere is the people that want to stop you to take pictures. The most common quote I heard was “I have a buddy who is a Porsche guy and he is going to freak out when he sees this.” I’m expecting some hate mail.

For those who haven’t followed along since the beginning, this “exercise” was done to explore the principles of the Use Case 7 method.
Use Case 7 is a technique where a design team explores their products capabilities using extreme use cases. There are three benefits to doing this, design robustness, finding new product markets, protection against litigation. In this Carrera UC7 initiative I found that improving other reliability testing methods should be considered a fourth.
What do I mean by extreme? How about, Can we pickup the lawn mower and use it to trim the hedges? Can a food blender be used to shred plastic soda bottles? Are golf clubs good for knocking frisbees out of trees?
Those seem silly but you will be amazed at what you can learn by answering questions like that.
The UC7 question that led us to this situation was “Can a Sports car be made to do utility jobs like an SUV…?” To see the full set of challenges you will have to go back to the beginning of this series.

So here are my observations on this final “modification.”
It is very easy to load and unload the boat. With the car being so low you can just load it from the back, lift it up to the first T-bar, and just slide it forward until it contacts the front crossbar. Then you throw on the sail along with four straps and it’s off to the beach. It was easier than the four wood palettes I loaded on the roof in an earlier step.

Installing the setup is easy. The back section of the frame simply goes into the 2” hitch receiver and the balance crossbar just tightens to the car crossbars with four bolts.

Stress margins: From everything I weighed and calculated for force on acceleration, braking, and turning there are plenty of margins. Nothing is working too hard and it all will, on paper, have an infinite life (subtract some margin for “garage” build quality).
In fact, it was shocking how stable it felt. I guess when you start with a center of gravity (CG) that is 15” off the ground on a 3,200lb car, 300lbs on top doesn’t change the CG all that much.
So what problems did I find with this modification? Truthfully none, it was kind of boring. It seemed no different than putting a kayak on an SUV. The Porsche is so low, so stable, and so powerful that it was like it had been an original design requirement. Maybe if that had actually been a feature when they originally designed the car, the design control document would have read, “vehicle top speed of 190mph, frunk can fit one golf bag, can carry two adults, two half adults and a 5-meter boat on the roof.”

Despite my surprise, doing the math, the great handling makes sense. The Carrera’s actual design requirements are pretty strenuous. Strain comes from force, high force comes from either high mass or high acceleration; you only have to increase one. Force=mass*acceleration.

The Porsche was designed with high acceleration in all directions in mind. So, the increase in mass along with driving like a normal law-abiding citizen is still well within the race track driven design specs, without a boat on the roof.

UC7 is about inspiring new ideas. In addition to new ideas about the product it also inspires new ideas about the reliability test program. In this case it was interesting to see that by adding the high mass load that we could possibly demonstrate base use cases without high acceleration.

Let’s say race track time for testing is expensive or difficult to access, could adding additional mass and driving on regular roads be a quick and cost-effective way to test certain aspects of the design? I would say so. For a major car manufacturer, this may not mean big savings in the grand scope of their program, but there are many companies and products where minimizing full use case testing could result in substantial savings in both dollars and time.
To clarify, these savings are not just for small companies. In fact, there is a fascinating study (“Reliability as an Independent Variable Applied to Liquid Rocket Engine Test Plans”, Strunz, Herrmann, Oct 2011) involving optimization of test time on liquid fuel rocket motors using similar methods. Firing a rocket motor is very expensive and time consuming, plus samples are not easy to come by. So any simplification of savings is enthusiastically pursued.

So, did my extreme adaptation of a sports car in a utility function yield any other important insights? We got a few good UC7 design observations, but one really stood out. Strangely enough, the most valuable observation was the broken hood switch in the door sill. Unexpected, but that’s kind of the point of UC7, to find the unexpected.

If you recall, this was a failure that occurred within 3,000 miles of the rooftop carrier modification. What occurred is that the electronic hood release switch filled with debris and eventually broke. It had filled with debris because of its location in the bottom of the door sill, which doubles as a ladder step when you load items on a car roof rack. Sports car designers don’t think of door sills being used as steps.

The scenario where that would be tremendously valuable would be if a company that only designed sports cars was thinking of expanding to vehicles with more utility, ones that might have roof racks. Coincidentally this was actually the position Porsche was in when that hood latch design was first released (1999).; Porsche came out with their first SUV just a few years later (2002).

What a valuable lesson, a lesson that could have saved a great deal of expense. The cost of discovering this new failure mode in the field would have added to warranty cost, resource drain in root causing and redesign, and brand damage. But discovering this failure early in design due to an exercise like UC7 would have transformed that experience to simply being a new line in the design requirements, “Door sill to function as ladder step.”

All in all, the Carrera did amazing in the Use Case 7 exercise. I knew it was a well-engineered and robust car that had a lot more to offer than was expected. If you recall, one of my inspirations for this exercise was the Porsche 959 that hung on my wall as a kid. It was the result of an exercise Porsche did in the late 1980s to see what the true potential of a 911 is if no expense is spared. They made a one-of-a-kind supercar that not only set records on the racetrack but, surprisingly, off the track as well. Like way off the track, in the desert, mountains, snowy countryside, and even mud.

They surprised the world by entering their street race car into some of the world’s toughest off-road races with nothing more than a suspension lift and mud tires as modifications and won. This was accomplished racing in the sand, dirt, mud, and snow against purpose-built off-road race vehicles. Robust is robust, plain and simple. That technology they developed in the 959 project found it’s way into their production cars for decades to come. The 959 wasn’t a UC7 but I would say the spirit is the same. Maybe I’m even plagiarizing by saying UC7 was my idea!

The big takeaway: If you take the time to create a robust product for one application, it will likely also serve well in use cases never intended. So why is this important? Because “never intended” is your definition, not the customers. They can define it however they like. What you share with the customer, in exact terms, are your “failures.” Regardless of the conditions they were created under, they are still your failures to own and address.

So enjoy developing your products and make the time to explore their limits in your product development program. Don’t just hope there will be time, allocate time upfront. Document your wins with reliability testing and exercises like Use Case 7. They will serve as your air-tight case that investment in reliability testing and analysis is always a smart move.

-Adam

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