A sensor for pedalling force and/or chain tension is mounted on a bicycle frame bottom bracket axle. An intermediate element that can pivot relative to the bottom bracket axle can pivot relative to the frame about a ring offset relative to the bottom bracket axle. A deformable retainer counters pivoting of the element. The element signals a stiffness controller with an indication of pedaling force and/or chain tension in response to the element being rotated in opposition to the holder.