Politecnico di Torino - Corso Duca degli Abruzzi, 24 - 10129 Torino, ITALY

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RaRe – Variable length bicycle cranks

Cyclng cranksetElliptic trajectoryPerformance increasedRecovery radial forcesRiding optimization


The RaRe project is identified by a special crankset able to vary the length of the cranks during the revolution cycle, allowing the biker’s foot to do an elliptical trajectory and to recover, through a particular kinematic system, the radial component of the force (component that is dissipated in traditional cranksets).

Technical features

In traditional cranksets, while riding, the pedal describes a circular trajectory. Moving away from the center of rotation, the torque given by the biker is not constant since it presents two dead-centers. The pedal in the RaRe project (left figure) is bonded to the sliding part of the crack (black component), that is forced to slide into the light blue part, through gears and a conrod-crank system. The combination between the rotational and the sliding motion creates an elliptical trajectory and not circular like in traditional systems. This trajectory allows to exploit in a better way the power given by the biker, and the torque results more homogeneous, during pedaling. Other mechanisms permit to create elliptic trajectory, but they dissipate all of the kinematic system benefits. RaRe instead, thanks to the translation movement created by the meshing, uses a force component given by the biker that would be dissipated in traditional cranksets.

Possible Applications

  • Every type of application in the cycling background, both on traditional and recumbent bike as well as trikes, handbikes and others;
  • Fields in which the maximum possible torque is needed;
  • Fields in which the efficiency is important and less effort along with a smoother and less invasive ride are required.


  • Increase in the useful mean arm of about 8.6% (figure on left);
  • Significant increment in the total efficiency of the cranks, so that the torque curve becomes more homogeneous and the system overall efficiency increases.