Romancing Gear Ratio
An open secret to most experienced riders involves the knowledge of staying away from certain gear ratio when riding a road bicycle. The definition of a low gear typically imply the use of the biggest chainring in front and the smallest-diameter gear at the back.
This configuration gives rider minimal torque but fastest, achievable speed on the road.  In this same position, you would notice that the chain actually remains parallel to the frame front to back. But if a rider is to proceed to flick his back gear to the biggest gear set (with the chain in front remaining on the biggest chainring), he would have now placed the chain in a stretched setup, all while actualizing that the chain is now connected in a slanted (or diagonal) axis between the front and back gears!
This inevitably put the chain and gears in an "unnatural" position, introducing frictions and rubbing and noise when you pedal. Though its still possible to peddle in this position, it is not a recommended position.
Hence, when using a low front gear (biggest chainring), it is highly recommended to only use the first six smallest back gears (out of a ten gear cassette), while on a high front gear (smallest chainring), the first six biggest back gears.
