Biking on loose snow and ice requires a different approach than regular biking. (The snow part is kinda like mountain biking through sand.)
- Unlike 99% of the rest of the time, you want to use your rear brake more often than your front. You can lock the front easily and it means you're going to wipeout if you do (bike sliding out). You want to brake much earlier than you normally would, and much more lightly (because hard braking leads to loss of traction which can quickly turn into a fall).
- You want to turn very very delicately. A quick turn tends to make the front wheel dig in on snow (launching you over the bars) and on ice it tends to make the front wheel slide out to a side very quickly.
- Do your damnedest to keep perpendicular to the ground. Leaning on ice is really really bad. The bike will shoot out from under you.
- Lowering your seat a little bit more than you normally would helps, as you can put a foot down more easily to steady the bike when you start to fall.
- Know your route. This part is vitally important. If you know your route you're not going to be surprised by that steep downhill section, you can plan for it and slow yourself down early . . . rather than hurtling and praying.
- It's worth going out on a snowy weekend and just dicking around to get a feel for how your bike will behave. I learned to control slides and how to best apply the brakes by doing this. You don't want to be learning this in the middle of traffic at 6 am.
While tread on your tires doesn't normally make any difference to traction on the road, in snow this is not the case. You want some semi-deep tread on your tire, or you don't get any traction. I prefer a pretty skinny tire in the winter, as I find it punches through the snow and slush to hit pavement . . . wider tires tend to float more and get less traction. Other people feel safer on a very wide tire. Around here the roads get cleared quickly enough that studded tires aren't really a necessity, but you get colder temps and more snow in Ottawa, so maybe that would be something worth looking in to.
When we get a very heavy snowfall the safest place to bike is in the car ruts that form on the road. It has the hardest packed snow, or the barest patch of pavement. The busiest roads get cleared first, so tend to be the safest to ride on when it snows. Side streets can be treacherous.
I run full length fenders through the winter, and love them. You need to set them up so that the fender is closer to the wheel towards the back of the bike, and further away from the wheel towards the front of the bike though . . . if you don't do this they will get jammed up with snow and will make your biking shockingly difficult.
:P
My 2 cents.