Thursday, June 20, 2013

A virtual ergometer

Long time without posting because I was very busy the last few months. Now I've got some free time and I will try to complete the dynamical model.

Initially, I modified the tire model that I presented some time ago to take into account the whole system but I was finding a "small" error that I couldn't solve. After some days trying to solve it, I decided to step back and build the model step by step to monitor better possible errors. I decided to start with a static bike-rider (something similar to an ergometer) and add the kinematical DOFs of the bike and the wheel later.

Right now, I am developping accurate pedal force profiles for variable cadence and/or power and a tuneable automatic drivetrain (something similar to the SRAM Automix IGH). The upper body and the elastic couplings between the rider and the bike aren't modelised yet but I will add them later.

Assuming RWS, reduced inertia of the wheel and equivalent moments are defined to modelise properly angular velocity fluctuations due to the variations of rider torque. The model involves a 2nd order DAE system of 43 equations that is solved using MAPLE numerical DAE solvers to avoid using index reduction techniques. An animation:

Some possible applications:

- The effect of gear ratio selection on inter-segmental forces, joint moments and muscular fatigue while maintining power
- An accurater loading spectra of drivetrain components and BB bearings
- Analysis of transient behaviour during gear changes

More soon. Thanks for reading!