Optimal Ski Rack Aerodynamics Pt 2: “What about a box?”

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The last few years have been different, to say the least. Regrettably, I haven’t skied at all for the past two seasons, and my gear has just been gathering dust. However, a confluence of events has me both very excited for this ski season and also revisiting my blog post from nearly 4 years ago. I’ll start with a bit, or more than a bit, of backstory.

Why I Care About Ski Rack Aerodynamics with a Box

I have helped Atomic at Colorado-area Labor Day ski sales since around 2008, only missing 2020 and 2021 dues to in-person events not occurring. As I was about to reach out to see what help may be needed this year, I received the text, “Yo dude, long time! Labor Day is happening….do you want to earn some Bents the hard way??” The stoke for the season officially started to build. Even though Labor Day sales have changed drastically from good old days of SNIAGRAB at the Sports Castle in downtown Denver, they are always a blast.

It was great to be back and chatting with people about the season to come and the ‘need’ for the new 120mm underfoot Atomic Bent Chetler with fresh graphics designed as always by Atomic athlete Chris Benchetler. With the ski pass paid for and fresh gear ready to be mounted up, I really started to get excited for the season. No preparation for a ski season is complete without a key ingredient… a ski movie premiere.

Not long after Labor Day sales had wrapped, I got another message, “Yo dude, just circling up to confirm if you can work the TGR events at the Oriental Theater?” This was a surprise, and rightfully so because the message was not actually intended for me, but it turns out help was needed, and I’m always happy to help out. The film this year from Teton Gravity Research is called Magic Hour and was a really fun production with a mix of skiing, snowboarding, and clearly some time for a couple of athletes in a makeup chair. Anyway, I promise I’m getting to the point!

Ski Rack Aerodynamics: The Oriental Theater filling up for the Friday showings of Teton Gravity's Magic Hour
The Oriental Theater filling up for the Friday showings of Teton Gravity’s Magic Hour

With the Atomic Star banner deployed and a setup of 100mm, 110mm, and 120mm models of the Bent on display, Atomic athlete Sage Cattabriga-Alosa was ready for autographs and some giveaways. While chatting with Sage, it turns out we have wildly different careers (who would have guessed?!) and I know from experience that when “Computational Fluid Dynamics” comes up in conversation outside of meetings at PADT, most people just stare back at me.

However, I recalled part one of this blog and was able to pull it up on my phone and show what I had previously done, which was a perfect connection from the engineering world to a ski movie premiere. One of Sage’s first questions was, “What about a box?” Immediately the wheels start turning in my mind of more analyses while Sage and I chat back and forth on what we think might be better or worse. A box is much larger but has a smoother form factor than a rack and skis. Would that make it better aerodynamically? This article started writing itself.

Ski Rack Aerodynamics: Sage Cattabriga-Alosa and I getting ready for the show.
Sage Cattabriga-Alosa and I getting ready for the show.

Use CFD to Look at Ski Rack Aerodynamics

With this backstory complete, I found myself at the computer finding all my old models and, luckily, finding quite a detailed CAD of a roof box on GrabCad. A few minutes later, I had all my versions of CAD for a comparative analysis. This time I’m using the Ansys Fluent Fault-Tolerant Workflow for meshing and Ansys Fluent for my analysis. The Fault-Tolerant Workflow lets me start with my old faceted vehicle and Ansys SpaceClaim-designed rack and ski configurations and create an external flow domain directly in the meshing workflow. The skis are designed to mimic the Atomic Backland 117 from a few years ago, with a good amount of tip-rocker and not much in the tails. All analyses use a flow velocity of 65mph.

The cases I have at the start are:

  • Forester Only
  • Forester with Rack
  • Forester with Rack and Tips Forward
  • Forester with Rack and Tails Forward
  • Forester with Rack and Bases Up
  • Forester with Rack and Box
Ski Rack Aerodynamics: Depiction of the external flow domain created within the Fluent Fault-Tolerant Meshing Workflow
Depiction of the external flow domain created within the Fluent Fault-Tolerant Meshing Workflow

During the solution, while monitoring residuals, I also set up report definitions for the drag force experienced on the car as well as the rack. With this data in hand, I summarized my results in Excel and created some contour plots using CFD-Post.

Ski Rack Aerodynamics: Results, No Rack (Top) and Rack Only (Bottom)
No Rack (Top) and Rack Only (Bottom)
Ski Rack Aerodynamics: Results, Tips Forward (Top) and Tails Forward (Bottom)
Tips Forward (Top) and Tails Forward (Bottom)
Ski Rack Aerodynamics: Results, Bases Up (Top) and Roof Box (Bottom)
Bases Up (Top) and Roof Box (Bottom)

I also added one case, which a keen eye would notice is depicted in the fault-tolerant meshing image previously shown, which is a set of 4 pairs of skis with tails forward on the rack. The results summary can be found in the image below:

PADT Fluent SkiRackAero data summary

The data shows that simply installing a rack increases the total drag force by over 9%, and the car with a rack can be used as the new baseline. The most aerodynamic way of having one set of skis would be to split them and put the bases up, but please do not do this, as UV damages the base material and wouldn’t be good for your wax either.

The first advisable way to mount skis of this shape would be tails forward with a 2.5% increase in drag force compared to rack only, followed by tips forward with a 3.7% increase. With the analyses complete for the single pair of skis, the box stands alone with an increase in drag force of nearly 9%. This was when I decided to add the additional case with 4 pairs of skis in a tails forward orientation, as comparing a roof box to a single pair of skis is a case massively underutilizing the box. 4 pairs of skis with the tails forward have a resultant drag force increase of 8.4%.

The conclusions I make from this data are:

  1. Installing a ski rack is detrimental to the aerodynamics of the vehicle.
  2. Tails forward is slightly more aerodynamic for this shape of ski, so if you can do this and not impact the functionality of your vehicle, mainly opening the hatch on this Forester, go for it!
  3. If you are skiing by yourself, put the skis inside the car and don’t use a rack at all.
  4. The roof of a car is not the best storage location for an empty cargo box, so I should probably find a better location for the one we keep on my wife’s car…

As far as next steps for more blog posts, Sage and I also talked about bikes and the potential differences between roof racks and hitch racks. There are also plenty of ridiculous scenarios that could be analyzed with things I could easily find on GrabCAD.

Hopefully, this article shows a practical use for Computational Fluid Dynamics, how easy it is to use the new meshing tools in Ansys Fluent, and that you, as an engineer, do have something to talk about with a world-famous skier. If you want to learn more about Ansys CFD or how the consulting team I’m part of can help you answer your questions about performance, contact us.

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