If I have to wear underwear while running - for instance if I'm wearing my multi-use thermal tights, or this one particular pair of shorts that's looser in the legs but has no built-in brief - the Ibex briefs are the ones I wear, and I adore them. I want to say the longest I've run in them is 18-20 miles, and not a bit of chafing. That's if I have to.

But on the bicycle is something else. You're sitting directly on whatever you're wearing. The chamois is made to protect your sensitive parts from chafing.

The whole reason for underwear is (1) under a skirt or loose shorts, so you have something between your parts and the fresh air and prying eyes , and (2) under pants, so you can wear them more than once before washing. Neither of those is true for cycling shorts!