I'm pretty sure this isn't legal per 2014 rules (I don't know if it was supposed to be, besides "change is coming") given that the implied definition of "end" of a bumper is any corner.
4.6.8 R26
BUMPERS must be supported by the structure/frame of the ROBOT (see Figure 4-10). To be considered supported, a
minimum of ½ in. at each end of the BUMPER must be backed by the FRAME PERIMETER. Additionally, any gap
between the backing material and the frame
A. must not be greater than ¼ in. deep, or
B. not more than 8 in. wide.
This would mean that your side bumpers aren't properly supported. The definition may be grey enough to pass at some regionals but definitely not everywhere. Be careful.
http://www.usfirst.org/sites/default...ame_Manual.pdf
Aside from that, you don't need to use shaft collars for attaching encoders. Just drill a hole in any driven axle and shove the shaft of the encoder in there. (
http://puu.sh/bSXmC/4694259cf2.jpg) Check the discussion here for more alternatives/experiences:
http://www.chiefdelphi.com/media/photos/40864