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18 is amazing. never been at that level, but when i was a bit... ahem... lighter, and did a lot of pull ups once I could do 8 I did a mix of higher rep bodyweight sets and lower rep weighted sets.
There's also the russian fighter pull up program - https://www.strongfirst.com/the-fighter-pullup-program-revisited/ - you could do the weighted variation.
But yeah, add weight at this point I reckon. if you don't fancy the program above you can find your 2rep max (2 rep max cause easy to accidentally cheat a bit on rep 1 of a chin/pull up) and try to do 3-5 sets of 5 at 85-95% of that weight (where 100% is your bodyweight plus whatever you hang off you) you'll be in a good spot.
Any tips for overcoming plateaus doing chin ups? I've been stuck at 18 for some time now.
Time for some weighted ones at lower reps?