function set_fraction( f, tm ) {
    f += fractionOffset;
    while ( f > 1.0 )
        f -= 1.0;
    angle = f * 6.28 * cycles + phase;
    deltaHeight = endHeight - startHeight;

    position_changed[0] = radius * Math.sin( angle );
    position_changed[1] = f*deltaHeight + startHeight;
    position_changed[2] = radius * Math.sin( angle+1.571 );
}