I have traced and expanded a tire that I now want to cut straight in the middle so I can decrease the opacity of the left half. That group of paths the expansion created I can't cut now - even with a line where I want to cut an select cut objects below command. Any suggestions?
Don't actually 'cut' anything. Just use clipping masks:
Draw a rectangle over half of your artwork, up to where you want the 'cut'
Object > Clipping Mask > Make
Duplicate your masked artwork
Edit your clipping mask on your duplicate to reveal the opposite half: Simply select the two outer anchor points of the mask and drag them to the opposite side of your masked artwork.