I need to bring my bars closer, and it feels like 4-5 cm would be perfect. But I only have an 80 mm stem right now. How short a stem can I get without handling going all wacky? I guess 60 mm would be fine, but any shorter?

I have an itsy-bitsy tiny stem lying aroudn I should try, but I'm kind of scared of it... When I put it on my mtb last year I crashed spectacularly after clearing a small jump I've done hundreds of times, and I'm pretty sure it's because the ultra-short stem messed up my steering.

OTOH road bike handlebars already have a built in "stem" compared to flat bars, unless you ride with your hands on the top of the bars which I rarely do, so it might be just fine.