Is it safe to write an "encrypted" PIN on a card?

I have a number of bank cards and I keep forgetting the PIN-codes, in particular because most of them I use rarely. I came up with an idea – for each card: add a secret PIN that I remember to the PIN of the card and write it on the card. Whenever I need it, I can just subtract the ‘secret’ pin.
Does this approach have any significant security vulnerabilities?