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The gap should open up a bit so that anything that gets between the guard and tyre has an easy path to exit.
I guess you could argue that if you make it even then anything that can get in should make it out but it might have to grind and rattle it's way around. Or the gap could actually close ever so slightly and the offending article would then jam.
Make it open up a bit and it'll definitely get out.
I think there's a book with the 'mudguard rules' in it, probably written by a French guy or something.
I don't really get what you're saying. Why don't you make the gap at the back the same as towards the front? Or the one towards the front the same as at the back, however you want to see it. Surely it would be just as functional, as well as better looking?
My point is that you can't achieve this with the stays being so far up the mudguard; you have to settle for the curve of the guard at that point, decided by the manufacturer.