I never saw this said from the manufacturer, just from some reviews i read. While researching them i read a few reviews that said they had problems with the sensor reading the magnet when they positioned it near the rim. But in hindsight that may be because at that point it's too far from the sensor, while at the hub it would be closer because the spokes are closer there. It may have been that which caused thier problems and not the speed at which the magnet passes at the rim. Who knows. But in any case thats where i heard it. I read a mountain of reviews.