javascript - Trigger default scroll behavior -

i need trigger default scroll behavior, happens scroll wheel.

however, shouldn't $("body").scrolltop($("body").scrolltop() - 10) because don't know window scroll down 10 pixels. computers, may 20 pixels, others, may 50.

i want trigger built-in default behavior, don't have worry this.

(in case you're wondering, because i'm trying make if scroll inside iframe, parent window scrolls instead. i've managed child window have parent window's function called, need trigger similar scroll wheel event happened in iframe)

ideas? thanks!