On older bikes anyway, I've found there are at least two different size eyelet diameters on rear dropouts. Some pannier racks have a mounting hole smaller than the one on the dropout which is aggrevating.

Some dropouts have one or two mounting eyelets (anyone ever seen three?) and some none. It also seems there is no standard spacing between the eyelet CtoC distance on the dropouts. Some dropouts afford quite a bit of distance between themselves, others not. Is there a "common" or universal dropout eyelet configuration out there on the newer bikes? On the older frames it doesn't seem to be the case.

Nope, no standard. One hole for fender, one hole for rack, or one for both, or none. 5mm, 6mm, some older ones have some fractional inch size.

The only "standard" is 5mmX.8 coarse threads. Anyone that ignores that is being willfully awful. Another new trick manufacturers came up with is bolt-on eyelets. I think they are mostly good for fenders, but I haven't tried a rack. I feel like there is usually an opportunity to hide a 5mm hole somewhere near the dropout, but apparently the manufacturers disagree.

Ok, thanks fellas. I'll just have to do some workarounds. Perhaps in the future the consortium of bike manufacturers will meet in a cigar-smoke filled dimly lit room and discuss some across the board standards for these things.

I think the current state of bike design is a standard of sorts. Do you know about Shimano's guide to frame manufacturers? It lists all kinds of dimensional/angular relationships that make using Shimano parts better. So one might claim that the tail is wagging the dog (the parts companies are dictating what the frame designers can do and still sell a bike with their parts on them). Andy