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Julie Dibens Aero Testing

Julie Dibens is on of the best cyclist in triathlon. On May 17, Julie will be testing herself against some of the world’s best cyclist during the Tour of California, San Jose Time Trial. The TT is 32 kilometers (19.9 miles), which is a big contrast to the 90-180k races Julie has trained for over the past several years...not to mention having to leave something in the tank to run off those bike efforts. This event will be much shorter and at a higher intensity than she's trained her body to perform at when racing triathlon.

The demands for a professional cyclist and triathlete are quite a bit different and challenging in their own right. Ask a cyclist to time trial 180k as the best triathletes will and they'll soon respect their talent, especially since the triathlete will then lift the intensity once more to run a marathon. In contrast, ask a triathlete to hold 6.0-6.5 w/kg up a climb and you'll see the differences in their physiological profiles. I'd have more faith in a cyclist being able to train to TT 180k than I would a triathlete being able to produc the high end power of a cyclist. What I don't have faith in, is most cyclist to ever be able to run a solid marathon!


Julie has always had one of the best bike positions in triathlon. In order to comply with UCI regulations, some pretty significant changes had to be made to her fit coordinates. The biggest change was to her saddle fore/aft position. Julie normally ride an ISM podium positioned 12 mm in front of the bottom bracket. UCI rules, state the saddle has to be be positioned 50 mm BEHIND the bb. To work within the rules, we decided to switch ISM models to the ISM Time Trial saddle which has a shorter, more usable nose compared to the podium, allowing us to get Julie as far forward “in space” as possible.

I only moved her arm pads back by 15 mm even though her saddle came back 60 mm to encourage Julie to stay forward by chasing the bars. This is essentially what most pro tour riders do. Looking at the pro tour riders with great TT positions, you’ll notice that it looks very much the same as an aggressive triathlon position because the principles of rotating everything forward are the same.

After the initial fitting, we received the UCI legal Trek seatpost which has a large rear offset so it’s pro tour teams can actually get their saddles 50 mm behind the bb. With the Time Trial saddle that she had been using, we could only get as far forward as 60 mm. It would be pointless to use another saddle with a longer nose unless it could actually be used to get the body further forward. Julie had brought pretty much every ISM saddle she had and after looking at them, decided to try the Attack because it appeared to have the most usable nose. This saddle worked even better for her than the Time Trial and because it’s overall profile looks flat, it won’t be as big of a worry with the UCI rule that a saddle must be level.

Aero Testing Once the position was set, we had the opportunity to go to the LA Velodrome to test out a new aerodynamic analysis technology run by ero-sports.com. Any time you are able to make decisions based on data is a good thing. You can always make an educated guess, but if you have the chance to make a decision about a position or product without guessing...I'd take it.

Before we started, I was asked which helmet I though would be the fastest. I said that the Giro Advantage II, but not the one we brought. We had a size medium Advantage, but asked the guys at EroSport if they had a size small, which they did...that will be the fastest helmet. The other 2 helmets being tested were a small/med Giro Selector, and a one size fits all Kask Bambino. I’ve been to the wind tunnel with Julie in the past where she was able to test most helmets on the market, giving us a fair idea which works best for her.

The key areas that I wanted to test were...

1. Helmets in multiple configurations...visor, no visor, sunglasses, covering the underside of the tail, what to do with Julie's ponytail (cutting was not an option...we asked)

2. Posture: head, shoulders, back and how each helmet interacts w/ those changes in rider posture

3. Hand height and elbow width

In the end, all the helmets were comparable, but were the Giro Advantage II tested best for all the potential head and body postures Julie will use during the TT.

Alphamantis Technology

Aero testing on the velodrome has been around for quite some time, but this was my first experience testing on the track. I was also interested in using the new Alphamantis aerodynamic analysis technology.

What I liked about the velodrome was the fact that the rider is “riding” the bike and you can obtain valuable feedback from the rider on how sustainable a position posture is. I also liked the real time measurements as you could see when the rider made an movements or were slowly coming out of the test position. As with anything, data is better than no data, but I would like to see a complete yaw angle analysis, which from what I've been told is in process. Hopefully, in the future, we can take a rider out of a controlled environment and into the real world while still capturing aerodynamic data.

I'm not an aerodynamic engineer nor do I have any sort of expertise in making sure the data we aquire is right other than what I'm told. Having said that, from my past experience with Julie in the wind tunnel, I wasn't surprised by anything.

Aero Testing For You

With any of this testing, time is money and you need to go in with a clear idea of what you plan to test. Once you start, your plan will often change once you’re armed with information, but stick to your process even when making changes on the fly...don’t waste time scratching your head. If you don’t know what to do next, get more data on your baseline or initial runs while you’re thinking. When I leave a testing session, I don’t often wish I had tried more options, but wish I had more test/data between my 2-3 major decision points.

I'm happy that people are blending fit with aerodynamics. In the past, a wind tunnel operator or aerodynamicist would be looking for the lowest drag number in the tunnel with little regard to a rider's biomechanics. Now, the "false choice" of aero vs fit is diminishing. I think it’s best to have your fit and possible changes complete before arriving for testing. EroSports has fit expert, Jim Manton, on board for fitting and testing, but most testing facilities don't have a fitter on staff. So, you should either come prepared, request a fitter, or bring your own. Knowing what fit moves you're going to make also prevents you from playing around with unrealistic positions.

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