Hello Lena,

Thanks for the image and detailed explanation. It would be great if you also showed a coordinate system, so I will assume that X is horizontal, Y is vertical and Z is out of the page.

I assume the pedal forces are offset from the plane of the frame to represent the center of each pedal so they create some twisting in the frame.

What constraints are on the Remote Displacement? The only constraint should be Z = 0 and all others free.  You could move the coordinates of the Remote Displacement down by the radius of the wheel since it is the tire on the ground that is the constraint location, but that might not make a difference to the stress in the frame caused by the pedal loads.

I would delete the Fixed Support and put a Frictionless Support on the hole in the front tube. This would be a bit closer to how the frame is supported. 

The seat forces imply a 100 kg (220 lb) rider is sitting on each seat. Have you considered that when a rider wants to put maximum force into a pedal, they lift up from the seat?  One simple scenario for fatigue is that 981 N oscillates from the left pedal to the right pedal with no load on the seat, the opposite pedal or handle bar. If you do that, then a simple fatigue calculation would be two zero-based loads: left pedal and right pedal. You could define a fatigue cycle for this problem as one revolution of the pedal crank arms.

Or did you mean that two 171 kg (377 lb) riders are each putting 100 kg of their weight on their seat while also pushing down on the pedal? That is a scary thought!