Is it better to use an inner or outer idler pulley?

It generally pays to use an inner idler pulley for belts. Outer idler pulleys always cause stress to the belt in the opposite direction of the bend. If an exterior is required, e.g. due to an increase in angle of wrap or if it is simply essential (regarding spatial or technical arrangement), it is necessary to comply with the basic rules for its size and location. Informatively, it can be said that the diameter of the inner idler pulley must be at least a minimum diameter for the given belt type, and a minimum diameter inner pulley must be at least 1.4x minimum diameter pulley for the given belt type.

It is recommended to position the device with the slack side closest to the drive pulley. More detailed information can be found in the guides for calculating individual belt types.