Image forming apparatus

An image forming apparatus comprising:

• a plurality of rotatable image bearing members for bearing toner images;
• a plurality of toner image forming units for forming the toner images on said plurality of image bearing members, respectively;
• a rotatable intermediary transfer member, contacting said plurality of image bearing members at a plurality of transfer portion, respectively, for carrying the toner images transferred from said image bearing members at the plurality of transfer portions and then to be collectively transferred onto a recording material;
• a plurality of first driving sources for rotationally driving said plurality of image bearing members, respectively;
• a second driving source for rotationally driving said intermediary transfer member;
• a detecting portion for detecting information on an output torque of said plurality of first driving sources or said second driving source;
• a changing portion for changing a speed instruction value of each of driving speeds of said plurality of first driving sources and said second driving source;
• an executing portion for executing an operation in a detecting mode in which information on the output torque of said second driving source corresponding to the speed instruction value of a specific first driving source of said plurality of first driving sources is detected while driving said plurality of first driving sources and said second driving source in a period other than during image formation, wherein the detecting mode includes a first detection period in which a state relating to a frictional force between a specific image bearing member corresponding to said specific first driving source and said intermediary transfer member at the transfer portion is set at a first state and then the detection is made, and includes a second detection period in which the state is set at a second state larger in the frictional force than in the first state and then the detection is made, and wherein in each of the first and second detection periods, the speed instruction value of said second driving source is set at a fixed value and the speed instruction value of said specific first driving source is successively changed to a plurality of different values;
• and
• a setting portion for setting the speed instruction value of said specific first driving source used during the image formation on the basis of a detection result in the detecting mode.