I have bent mine 5' radius, this requires pinning down the track.
Also I did not "bend" the track, I just put it on the trackbed, start with a straight section and pin the track, then I had no particular problems just easing the track in place for the next pin, about 15cm between pins. and a pin on both sides of a rail joint.
I also staggered the joints to avoid kinks in the rail where track sections meet.
I do not think I would try to go much lower than 3' radius because the ties will have trouble maintaining a proper gauge. Such sharp bends should ideally be gauge widened as well.