The art of bike fitting continues to become more of a science, thanks to the 'space age' technologies available today. Todd Carver and two savvy business partners (Cliff Simms, Franko Vatterott) at Retül have developed the tools and technology to make portable 3-D motion capture a reality for bike fitters around the world.
Reported by Nathan Rand
Like a lot of bike fitters, Todd Carver started out as a bike racer who strived to be as powerful, efficient and comfortable as he could on the bike. So after earning his masters degree from the University of Colorado, Boulder, where he did power based bike fit research in the applied exercise science lab, he worked with Specialized to co-develop the curriculum for the Specialized BG Fit School, before moving on to teach fitting at the Serotta International Cycling Institute (SICI). As a 3-D motion capture expert, Todd joined Cliff Simms & Franko Vatterott to launch Retül in 2007. Retül makes a motion capture technology designed specifically for cycling, and in the rights hands can be a powerful tool in fitting riders to their bikes.
Using the new technology of power meters, Todd's work helped define the biomechanical and physiological parameters associated with road bike racing that we know today. After graduation, Todd continued his study of bike fitting at the Boulder Center for Sports Medicine. Under the direction of Andy Pruitt for 5 years, Todd learned the more medical side of fitting injured athletes on their bikes. During his tutelage, he learned how to fit a rider around an existing injury and how to make a rider more comfortable using a fancy 3-D infrared technology.
Now with Retül, Todd and his business partners developed the tools and technology to make portable 3-D motion capture a reality for bike shops around the world. Retül has developed the technology to three dimensionally capture the movement of 8 LED sensors placed on a cyclist's body. They have also created the software to analyze the power and efficiency of those movements at different positions on the bike - to basically build the rider as a 3D stick figure and record the mechanics of the pedal stroke.
The Retül system uses a 3-D camera, a rider harness of infrared LED lights and a digitizer that continually logs data while the rider is riding. PEZ sat down with Todd Carver at the Retül base in Denver, Colorado to learn more about the system and to see what’s in store for the future.
PEZ: Is Retül a fit system, or a fitting tool? Carver: It is a fitting tool. It is the most comprehensive fitting tool available. There is no such thing as a "fitting system". All good fitters have been to ALL of the fitting programs and hold fitting beliefs based off of the information learned at these fit schools and their own background knowledge in racing, coaching, and academics.
PEZ: Tell us about Retül philosophy. Carver: The one thing that defines Retül philosophy is that bike fitting must be dynamic. To get the most accurate measurements, you need to record the motion of the rider while they are under normal physiologic loads on the bicycle. For example, if you are not capturing knee angle at peak flexion in the pedal stroke dynamically, you are not getting an accurate measurement because it doesn't always occur in the same spot of the pedal stroke. The big benefit of a dynamic fit is that you will end up in a better position at the end of the fit because the system is recording quantitative data that cannot be accurately measured statically.
PEZ: For example, how does Retül measure the best fore and aft seat position dynamically? Carver: A lot of fitters use knee over pedal spindle as the standard for fitting people horizontally on the bike with respect to the bottom bracket. With dynamic fitting, you can actually capture that event as it occurs during the power phase of the stroke and can take into account changes in work loads, changes in terrain, and changes in cadence during a Retül fit. With these variables, as the rider moves their ankle differently, it will move the knee differently fore and aft and allow you to end up in a different fore and aft relationship to the bottom bracket.
PEZ: How do you account for these variables on a fixed indoor trainer? Carver: We use Computrainers and can change the work load from 150 watts, 200 watts, 250 watts and see what the rider does. We can also bring the front tire up and down and record where the center of the knee joint tracks over the ball of the foot. These changes allow us to see if the rider is in the normative range and help us dynamically find an optimal position for the rider.
PEZ: How do you define what is normative or optimal? Carver: I started out referencing normative data from the Boulder Center for Sports Medicine and from years of doing 3-D bike fits. Some of our normative ranges today are a reflection of what I learned from BCSM. However, most of what I consider to be the optimal position ranges have evolved over the past couple years as I learn more about cycling motion and what actually works on the road. With the Retül system, I have the benefit of gathering data from a lot of elite professional cyclists. I have fit the entire Garmin team and a lot of the elite triathletes as well. I am a firm believer that you can learn from the pros of any sport and being able to digitize their positions on these bikes and learn from what they self select as the most powerful and efficient positions is a huge benefit. An elite rider's job is to ride a bicycle really fast and a lot of these riders typically select their positions off of feel. I have learned a lot by seeing how they position themselves on bikes to optimize their power output and, ultimately, their performance.
PEZ: Ok, name drop… Carver: My first fit ever at BCSM was Gunn Rita Dahle, who was the reigning world cross country mountain bike champion at the time. Other names: the whole Garmin team, so Christian Vande Velde, Dave Zabriskie, David Millar, Svein Tuft, all the guys. Triathletes including Christoph Sauser, Craig Alexander, Norman Stadler, Chris Lieto, Sam McGlone, Hunter Kemper and others. Throughout my career as a bike fitter, I probably average at least one or two domestic pro every month.
PEZ. What series of normative data do you collect? Carver: Starting with the lower extremity, we look at a knee angle, hip angle and ankle angle at peak flexion and peak extension. We look at how the knee flexes and extends, how the ankle flexes and extends. We also look at the angles of the upper body, so the back angle, the shoulder angle, the elbow angle, wrist angle, and we collect and analyze normative ranges for all of these variables.
PEZ: A bike shop that uses the Retül system has access to normative data? Carver: Yes, with the purchase of a Retül comes some normative data that we've had the ability to collect over the past 5 or 10 years. That gives the shop some background of fit data that they can use to reference the riders that come in for fits. It also gives them a head start in the fit process and kind of gives them some goals that they're going to want to achieve for their riders. We train the shops to use the Retül system and teach them the parameters of a good bike fit.
PEZ: In my experience, bike fitting is only as good as the fitter, regardless of the tools. What does Retül do to combat that problem? Carver: Retül puts better tools in the hands of experienced fitters. For newer fitters, Retül teaches them how to use the new 3D motion capture technology and an ergometer, like a Computrainer, to give their clients the best fit possible using the latest technology. Fitters who use Retül are a step above those who don't because they realize that there are things that they just can't see when a rider is moving and things that don't always happen when a rider is not moving. From a consumer perspective, going to a fitter with Retül just says this fitter values education, values technology and the perfect fit.
PEZ: Some would consider "the perfect bike fit" to be a recipe of: one part hocus pocus and another part the fitter's authority to pronounce its perfect? Carver: I don't think so. By hocus pocus, I assume you mean the tools- a plumb bob, infrared lights... Retül uses motion capture technology and years of experience to find the optimal balance of power, efficiency and comfort for the given rider being fit. LED technology is used for brain surgery and is accurate less than millimeters. As for being perfect, once a rider clicks into the pedals, I think they realize whether the fit works or not. Once you start riding a lot, you push bigger work loads, and may be even start racing and training upwards of 10 hours a week. Cycling is very, very repetitive and once you're locked into that pedal, if everything is not set perfectly down to the millimeter, you're probably going to get an overuse injury, at least one throughout your career and maybe many.
PEZ: What are some limitations of a Retül fit? Carver:The limitations of any bike fit is that it is indoors. You're clamped into a stationary trainer and you're not able to actually record what's truly happening on the road. We hear this from riders all the time, that they pedal one way on a trainer and they pedal a different way when they're out on the road or the knee hurts when they're out on the road and it doesn't hurt when they're in a trainer. So, I think the main limitations of bike fit are that we need to be able to make it as dynamic as possible and as reliable as possible. When I say reliable, I mean as it would be outside. What we want to do with bike fitting is make it as realistic as possible to a real road analysis. We have been testing a new outdoor tracking system and are starting to quantify movements in the field.
PEZ: An outdoor Retül motion capture system? Carver:Yes. The outdoor tracking system is different from the indoor tracking system in one key area, and that is the wavelength of light that we use. Indoors we use infrared, outdoors we use red. Sunlight is full spectrum so we can't use infrared outdoors. We use very bright red flashing lights and the rider literally lights up like a Christmas tree and the sensor is positioned in a car a few meters away from the rider and we can track that rider as he sprints, as he gets up and down out of the saddle, as he goes uphill, downhill to really see how indoor recording is different than outdoor data. We can compare the fit data, video and wattages and help validate or change our indoor testing protocols.
PEZ. Any other exciting technologies? Carver: The Zin is a hand held 3-D digitizer. In a few minutes you can scan everything from the bottom bracket to the handlebar clamp to the entire length of the saddle and the shifter grips, into the computer and then the software calculates all of the important current bike measurements and fit parameters. Everything from head tube angle, to wheelbase to saddle height, setback, reach and drop is all calculated down to the millimeter.
PEZ: Do you incorporate weight distribution and bike handling in Retül fits? Carver: We don't. Everyone talks about a 60/40 weight distribution on a bike as being proper. No one really knows that. It's something worth investigating, but I ultimately think that weight distribution on the bike is not what you actually want to fit a rider around. You need to fit around the position of the body and that will result in a different weight distribution based on different bikes. I'm not saying it's something you don't want to measure; it's just something you don't necessarily want to fit around.
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With that, we concluded the interview and played around with the cool technology for a while in the Lo-Do Retül studio. Todd gave me a tune up with a fit on my personal bike and found out that I was a few mm's off ideal saddle height.
I would like to thank Todd Carver for taking the time to explain in layman’s terms the science behind the machines; Chris and the crew at the Sports Garage SportsGarage.net for lending us a sweet Serotta for the interview; and Neil Rosenfeld NeilRosenfeldPhoto.com for snapping some great shots of the Retül studio in LoDo.
PEZ Tech Ed Sez: Retul is one of the best examples of tech in cycle fit, well, ever... Custom manufacturers like Serotta (who we've worked with, and who introduced us to Retul and lead the custom industry in Fit) as well as shops that simply have not had the benefit of the detailed data (versus shop standard "eyeball fit") can gather accurate info far faster. As it relates to center of gravity, shops fitting people on stock bikes can't really address weight distribution because a stock bike's geometry is fixed and your hands and butt are fitted relative to a bottom bracket that simply isn't going to move. Custom builders can go one further and easily address weight distribution within custom geometry (wheel base, head angle, fork rake etc) and dial in a ride and handling... Any way you slice it, this level of data gathering is a fantastic step forward it certainly benefits anyone from shops to top custom builders like Serotta.
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