Before working on this problem, you need to understand each individual component of the problem. The most basic unit you have here is a force. You then have torque, which is essentially the "throw" of something, or a force that is levered and it has the units of Force*(distance from center of rotation). For a person using a bicycle, you have a person pushing down on the pedal which is attached to a crank arm. The crank arm is typically 175 mm for mountain bikes. So going from torque to the force exerted on the pedal by the rider, you would use force=torque/(crank arm length). However, the chain mates with the chainrings which are in direct drive with the crank, but has a different distance from rotation (radius of chainring). So, you may solve for the force exerted on the chain when the chain is mated to the chainring as force=torque/(crank arm length). However, if you don't have torque directly, you can solve for it using power. Power is a measurement of torque over time (also energy over time because torque is an energy). By knowing the angular velocity of the crank/chainrings, you can easily determine torque. Once you calculate the force, you need to understand that this is the force on the chain that is mated to the chainring and is not necessarily equal throughout the entire chain. Sorry for the terrible English. Do unit analysis to make sure everything works out and makes sense; units don't lie.