In the UK, they call it a “winter” bike. In the US, at least in the northern US, we call it a “rain” bikes, because your “winter” bike is normally referred to as a “fat bike”. We don’t ride road bikes in snow and ice – winter cycling is for fat bikes around here (though gravel bikes get a lot of miles when conditions allow).
Most people will simply use their “old” or last road bike as the rain bike as the new steed gets the exalted “good” bike status. At that point, the discussion is elementary. However, there are traits that will help make one bike more desirable over another as the winter or rain bike.
The humble rain bike, or winter bike, will be used in the worst conditions. Parts will have to be replaced fairly often, so what we’re looking for is simplicity or ease of repair, and (if possible) less expensive parts. My rain bike, a re-invigorated Trek 5200, makes a fantastic rough weather bike. The cables are externally routed so they can be changed in minutes. It’s got a simple, inexpensive English threaded bottom bracket (Ultegra installed is between $40 & $50 – do it yourself $20-ish). The crank, a simple Shimano 5-bolt, is easy to remove for cleaning (Shimano cranks are PHENOMENAL in their simplicity and excellence). The chainrings don’t have a lot of surface area on which to collect grime and dirt. I’ve got Chris King headset is known for it’s durability. The wheels have sealed bearings to extend useful life. The component groupset is Shimano 105, known for its reliably useful life expectancy and reasonable replacement cost. 105 is a little heavy, but its Shimano’s workhorse groupset and the parts are reasonably priced.
Another important thing to consider is tires. I go with Specialized’s Turbo Pro 24 mm because they’ve proved to be fairly durable yet surprisingly plush. You won’t want an overly plush, supple tire to use in the rain or before the roads have been thoroughly washed off after a long winter with a few cleansing rainstorms because you’ll have all sorts of gnarly debris on the road. The Turbo Pro offers an excellent blend of comfort and durability. They’re also reasonably priced.
For a chain, I go with a less expensive, SRAM chain, simply because it’s going to be worn out quickly with the deplorable conditions I’ll ride the bike in. No sense in blowing $60 on a chain that’s going to be trashed in a hurry. My pedals are simple Look Keo Classics – nice and durable, while reasonably priced so they don’t sting the wallet when I have to replace them (my last pair lasted six years – I got my miles out of them).
While I did manage to get my 20 pound Trek 5200 down to 18-1/2 pounds when I rebuilt it, I tend to believe the rain bike isn’t necessarily about weight, it’s about durability. If we know anything about lightweight bike parts, it’s that they’re not known for durability. I always strive for a happy medium with my Trek. Reasonably light, but durable first. You won’t be winning a weight weenie pissing contest, but you’ll be riding relatively trouble free when others are choosing to sit it out because of a chance of rain.