Originally Posted by zacster
This is dangerous, not recommended and in many places unlawful. You may think you hear everything but you don't, and that subtle sound that tells you a truck is on your tail a few hundred feet back you won't hear until he's right behind you.
What's wrong with this is that it implies that you're depending on your ears for what's happening behind you ... as if they were a substitute for a mirror. Will sound tell you that another biker, a hybrid car, an electric or someone who's engine has died on the downhill and is drifting is coming up behind you? Will sound give you accurate distance information? I don't think so.
In driver's ed they teach you check your mirrors every 10 seconds or oftener depending on circumstances. If this is true for cars, how much more so for bikes? There's a variety of mirror solutions for bikes available these days.