Computational Modelling Group

13th November 2009 4 p.m.  B54/10B

Human pseudo-reciprocating force to rotational motion transfer

Alexander Forrester
School of Engineering Sciences

Human environment interaction, Value-driven design
Hans Fangohr

The drive train of a bicycle is remarkably efficient (~98%), however, its interface with the rider is the source of considerable losses. The 'pseudo-reciprocating' forces from the legs and feet are transferred to rotational motion via the cranks and chain ring. Inefficiency is due to forces which are not aligned with the circular path of the pedal. This results in large variations in torque throughout the pedal revolution.

We are considering what is potentially the simplest remedy to the problem of inefficient pedalling: altering the shape of the chain ring. It is hoped that by controlling local pedalling speed variation through chain ring shape change, an improved torque profile can be obtained. Although non-circular chain rings are not new, off-the-shelf designs have been unsuccessful and circular rings have been the status quo since 1885. Previous non-circular chain ring designs have assumed idealized torque characteristics, in particular that both legs exert the same forces, but 180 degrees out of phase. This is an oversimplification, considering the asymmetry of torque profiles measured in the laboratory. The ongoing program of research will embed a computational chain ring optimization (based on musculoskeletal simulations) within an athlete specific iterative experimentally based validation scheme (based on motion capture, torque and metabolic data).

We hope to extend the work to applications in rehabilitation. While we will initially be minimizing maximum muscle activity (i.e. minimizing the activity of the muscle group which is working hardest), we could re-formulate our optimization to maximize, or at least increase, the activity of certain muscle groups (muscle activity here is defined as the ratio of the force exerted by a muscle to its maximal force capacity). Such a scheme could be used to produce a chain ring to aid the rehabilitation of muscle groups where strength has been lost, motor control has been affected, muscle imbalance has resulted, etc., following injury.