what is the coldest weather you will ride outside in?

About 40 degrees for riding.  I won't ride if there's a good chance of ice on bridges, under piles of leaves, under overpasses, etc.  I've seen too many crashes, too many broken collarbones, too many injuries.

Just the other day, a coworker crashed on his way into work.  Here's his Facebook post and it's a great example of why I won't do ride much colder than 40.

"Well, I got a serious surprise this morning when I bit the dust on a patch of ice while bicycling to work. My right hip is bruised and sore, I cracked my helmet, and it looks like my rear derailleur is bent. When I left the house it was around 35-37 degrees but as I got to Redmond the temperature dropped. "

In the UK the cold is always accompanied by rain so cold weather is treacherous for road biking. I have loads of stories of people coming off the bike and breaking wrists, shoulder blades etc...

Around 30-35 degrees is where I hit the gym for running (or 1.5 degrees in old money)

Just a little cooler than the coldest water I'll swim in.  Maybe 37 degrees F riding, probably about 40-42 degrees F water temp swimming.

A +1 here. I've ridden well into the low 20'sF, but the biggest issue is whether it's been dry long enough that there isn't danger of ice on the roads. Off-road isn't so much of a big deal (though that often involves road to get there).

It depends on how desperate I am. In the Fall I balk at anything under 50.  In the spring, there's no lower limit for temperature.  Mostly I will ride my roadie outside, and do real training rides, as long as it's not icy.  We know where the favorite paths are likely to thaw last, and when it's down to the last small ice puddles we just go and walk the bike as needed.  Well, that's what I do - plenty around here ride through it, often with studded tires.

I actually ride outside year round and have a fat bike - some things you just have to do to survive in Minnesota.  It's super fun but has opened up a whole new area of need when it comes to clothing.  Fortunately there's some crossover between skiing and winter biking (and running).  If you're covered, you can really do anything.  So far I've done about 15 degrees.  My husband does get his kicks from doing crazy things, so he rode on Sunday at around 0.  If you watch football, we live near the stadium where the Vikings lost to the Seahawks.

Boasting aside - I have tons of advice, clothing-wise, for anyone interested in biking in colder weather.  A wimpy little toe cover or arm warmer is just the beginning!