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Crank Arm Update From Interbike

  • By Aaron Hersh
  • Published Sep 18, 2013

Some component and bike manufacturers are embracing a shift to shorter crank arms.

Short crank arms are a potential solution to fit discomfort caused from riding a low aerobar position. The appeal of dropping the aerobars is obvious: A low position is often faster than a high one. But lowering the aerobars impinges the hips and can result in less power once the drop from saddle to aerobar becomes too extreme.

Hip angle—the angle between the rider’s torso and leg—is often measured with the pedal at the six o’clock position, but hip angle is actually most important at the top of the pedal stroke. Most cyclists intuitively know that the hip becomes impinged in this position because it feels obvious when riding in a scrunched position.

Shorter crank arms do not affect hip angle at the bottom of the stroke, but they do impact the angle at the top. A shorter arm keeps the rider’s foot from lifting as high at the top of the pedal stroke, meaning hip angle at the position that really matters is effectively more open. And unlike elevating the bars to achieve an efficient hip angle, short crank arms don’t increase front surface area. A case study conducted by Mat Steinmetz, a triathlon coach and bike fitter, for Triathlete magazine found that shortening crank arms by 1cm had the same affect on hip angle as raising the aerobars by 2cm.

Like many new ideas that originate in triathlon, the cycling world has been slow to adopt the concept and triathletes are still largely beholden to companies focused on road cycling when it comes to gear. A few companies such as Rotor and Quintana Roo were early adopters and now more of the industry is catching up.

Rotor was the first mainstream crank manufacturer to offer cranks that were dramatically shorter than the old standard lengths of 170mm, 17.25mm and 175mm, and they are still helping to advance the trend. In 2014, the 3D+ crank will be available in a 150mm length, making it the shortest option we are aware of other than extreme solutions such as custom or adapted BMX cranks. Rotor’s Flow aero crank arm will only be available in 170mm and longer, however.

Vision is creating three short versions of the Metron aero crank in 155mm, 160mm and 165mm lengths. Vision is a subsidiary of FSA, a larger road component company that creates many of the cranks that come standard on bikes priced less than $3,500. While FSA cranks that are spec’d on new bikes will not go as short at 155mm, FSA is making affordable 165mm cranks that give manufacturers the option to build affordable bikes with semi-short crank arms.

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SRAM’s new Force 22 and Red 22 cranks are also being produced as short at 165mm. The Elsa crank-based power meter produced by SRAM subsidiary Quarq will come in 162.5mm.

Shimano is matching SRAM’s length with 165mm Ultegra 6800 and Dura-Ace 9000 cranks that are compatible with both mechanical and electronic groupsets.

Stages Cycling is producing power meters as short at 165mm and the SRM Dura-Ace power meter comes in the same length.

These cranks are all available as aftermarket upgrades, but buying a new crank is expensive and most people choose to ride the equipment that came stock with their bike. As a result, the crank lengths chosen by bike manufacturers are just as or even more important than the full span of short aftermarket options. A few triathlon bike makers have embraced the trend to shorter crank arms, and others are sticking with longer, more traditional crank arms in the range of 170mm to 175mm.

Quintana Roo only makes triathlon bikes and they have outpaced other aero bike makers in adopting tri-specific equipment changes in the past, including spec’ing the ISM Adamo saddle. QR is again setting the example when it comes to shortening crank lengths. Size XS, S and M QR tri bikes are coming with 165mm cranks in 2014 and sizes ML and up will come with 170mm cranks. “We’ll get them as short as they’ll sell them to us,” says QR engineer Brad Devaney.

Specialized hasn’t yet decided where they fall on the issue of crank length. While they acknowledge the biomechanical impact that comes from shortening crank arms, Triathlon Manager Mark Cote says they haven’t yet seen compelling evidence that the biomechanical change translates into aerodynamic or positional benefit. As a result, Specialized is “neither endorsing nor bucking the trend,” Cote says. Specialized is using slightly shorter cranks on their Shiv tri bikes than would be spec’d on road bike of equivalent size. The XS has a 165mm, S has 170mm, M and L are built with 172.5 and XL frames come with 175mm crank arms.

Felt and Scott are sticking with traditional crank arm lengths, and Cervélo is going slightly shorter in 2014. The P5 and higher-end P3 models are coming with 167mm cranks for bikes 51cm and below. The 54 and 56 frame sizes are coming with 170s and 58 and 61 are spec’d with 172.5mm cranks. The P3 Ultegra comes with 165mm cranks for sizes 51 and under, and all other crank lengths are the same.

More from Interbike.

FILED UNDER: Bike / Gear & Tech TAGS: /

Aaron Hersh

Aaron Hersh

Aaron Hersh is the Senior Tech Editor of Triathlete magazine. To submit a question, write Aaron at Ahersh@competitorgroup.com.

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