Bike fit: Crank arm length, motion capture, and more

Part 2 of this series takes a deeper dive into the world of pro bike fits

Tech
RETUL Fit Bike

Retül offer a fit bike that can mimic the stack and reach of nearly any bike that a customer would want, crank arm length included.

Now that we know the basics of why someone would want a bike fit for their off-road rig (see part 1 of this two part series), it’s time to go deeper into the idea of biomechanics and creating the most efficient lever possible – the rider. Each individual has their own set of limitations and strengths that a professional fitter can diagnose. Just because Jaroslav Kulhavy won Olympic gold with a slammed stem and tilted saddle doesn’t mean you’re going to do the same at your local short track. Here again are fit experts Todd Schoeni and Adam Myerson.

Mtbr: Is crank arm length a bigger deal than we think in MTB?
Todd Schoeni: Crank arm length is a big can of worms in the fitting world, and I’m not sure I want to open it but here goes. My short answer is, no. The studies we have seen do not show a significant change in power output over time when you change crank arm length. The long answer is, somewhat yes. Crank arm length comes down to bike fit, comfort, feel, and preference. For mountain bikes, I would say bike fit and comfort are the two main factors in crank arm choice. Preference would also be relevant since maybe you want to run a shorter crank arm to avoid pedal strikes on rocks and things of that nature. There are even manufacturers running shorter crank arms on larger size bikes for this exact reason.

Adam Myerson: Traditionally, people have just accepted that longer is better in MTB because you need the leverage at slow speeds and quickly changing torque requirements. But I think it’s reasonable to question that and consider how crank length impacts bike fit as much as power production. If you ride long cranks and a low saddle position, that is compounded at the top of the pedal stroke and how high you need to lift your knees. And if you ride somewhere really rocky and technical, pedal clearance with the ground is something to keep in mind.

RETUL Fit

The motion capture experience is a huge selling point.

Mtbr: What gains come from a high end motion capture fit that a more traditional static fit can’t match?
AM: Any fitting tool is only as good as the fitter using it. The new fitting systems and technology that have arisen in the past 15 years are certainly helpful, particularly if you’re a new fitter learning the craft. But the more experienced a fitter is the less they may need to rely on technology for fitting. Well-trained eyes, a tape measure, and a plumb bob are often all a master fitter needs to put a rider in the right spot. The right spot is also always a range. Perfection is impossible and not something to chase.

TS: Good question. I’ll back up a bit. You’ve got tools and you’ve got carpenters. Carpenters being those who have learned and practiced bike fit as a hands-on dynamic process without video motion capture. I say dynamic since the rider is still pedaling and moving on the bicycle the same as a motion capture video fit, so you’ve got two dynamic processes. The tools (motion capture, etc.) add to enhance the fit and perhaps dive deeper into issues that are present. It can uncover a bit more lateral knee movement, help to zero in on back, shoulder, elbow, and hip angles, and sometimes show something that may not be visible to just the human eye. It is also a good consumer tool. With a lot of systems you have the ability to show a customer a before and after fit. That can go a long way to ensuring they are happy with the result. With all of that, it still may not mean the motion capture fit is better than a hands on fit. I think motion capture fitting is a good thing. I actually do have a motion capture system I will implement when I open a studio later this year.

But from a fundamental perspective I want to see bike fitters actually fit and not rely on cameras or software. Use your training, use what you have learned from PTs, doctors, reading, other fitters, and make your best decisions for a bike fit. If a problem arises and it is beyond your scope, refer to a professional. The ability to fit someone hands on is an excellent skill that I hope stays viable for a long time.

Velo Angle Side

Bike fits are a game of millimeters and Velo Angle can help you document those, as well as transfer them from bike to bike.

Mtbr: What is the physiological limiter that some may not know for a mountain bike setup?
AM: A fit that prioritizes handling may limit pedaling and power production. Just imagine racing cross-country on a freestyle BMX bike, to understand the opposite ends of the spectrum. In order to make space to move around on the bike for ideal bike handling, you have to sacrifice an ideal pedaling position for maximum power production and efficiency.

TS: Although this might not be as significant for MTB as it is for road, I would say hamstring flexibility. We as a nation are really tight. We have desk jobs, don’t stretch, and that leads to tightness and decreased mobility. Too long of a reach or too much drop on off-road bikes, or any bike for that matter, can cause knee pain, overuse injuries, and activate old injuries. You don’t always need to slam that stem.

Check out part 1 of this series that discusses the basic benefits of bike fit for mountain bikers.

Photo Thumbnails (click to enlarge)


About the author: Jordan Villella

Jordan comes from the steep streets of Pittsburgh PA, where he learned to dodge cars and rip single track. He has been involved in nearly every aspect of the cycling industry: from turning wrenches, store design, clothing production and bike park creation. Jordan spends his free time racing cross country and cyclocross around North America, though he has been know to enduro every now and then. His love of cycling is only second to his love of his family and punk rock.


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