My frame won't allow the fitting of a FD, so I'm planning to install a double/triple and change by hand. Do I need to follow any of the usual advice about size differences between chainrings, or am I free to fit any combination that I like - for example 52/22 double or 52/30/22 triple? Also, since there is no FD shifting, is there any likelihood of the chain coming off a chainring while simply cycling along? Finally, are there any other issues which I need to be aware of? Thanks.