Does this put more strain on the forks?
Yes.
How can one tell what the limit is for forks?
The manufacturer will have tested it, there will be a spec somewhere. Seeing as most MTB forks are 36mm+ diameter and only spec'd to 200mm rotors, I'd be surprised if thin spindly steel forks would be rated for anything past 160 rotors
No. Forks and frames are normally tested with the smallest possible rotor size. Levers innit.
So how has this number been ascertained?
@chiroshi started
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Yes.
The manufacturer will have tested it, there will be a spec somewhere. Seeing as most MTB forks are 36mm+ diameter and only spec'd to 200mm rotors, I'd be surprised if thin spindly steel forks would be rated for anything past 160 rotors