Spinning will help some, but I wouldnt rely on it for big improvements. The best way to get better on the bike is to ride more. If it's too cold and/ or dark for you to get one long ride in during the winter, look into trainers. They're not really that expensive if you don't go with a fluid trainer. While fuilds are nice, the magnetic trainers work pretty darn well, and the newer ones arent that loud at all. I picked up my Minoura 850 w/ remote for around $100- new.

I've only used a couple. The Cyclops Fluid2 is considered one of the best there is, but it's $300. I can feel a difference between it and my lower end Magnetic trainer, but not $200 worth of difference. ;)
Just hunt around online untill you find a trainer in your price range- whatever that may be. As I said, I found my Minoura 850D prety cheap w/ free shipping.

I pretty much used spin classes as my "bike training" my very first season because I found it difficult to get actual road time on the bike with my young kids. I have to say I did pretty well considering.. but cycling is the one and only leg that comes kind of naturally to me.

Now well into my third season, I do train more outside but rely on my cycleops fluid trainer heavily. I am a certified spin teacher up here in Maine, so I design my profiles and test drive them on my trainer a few times before delivering them to a class.

If your going to use spin class for training puposes I suggest finding an instructor who has outdoor riding/racing experience and who creates cycling profiles with a "purpose". It's not about riding to the beat of the music and bike "choreography." The classes should include a variety of different profiles which include strength, interval training, endurance, recovery and race day. If you find yourself in classes which continuously appear to be "all terrain"...meaning including a little of everything at an intense effort, then your probably doing yourself more harm than good. These types of classes over time just wear your muscles and joints down and you end up getting very little benefit. Bottom line ...you have to get a good instructor who knows and cares about what they are doing.

I agree with the others that you should invest in a trainer. I had my first magnetic trainer for 20 years. It just recently rusted out and I bought the cycleops to replace it. It's worth the investment and you'll never have an excuse not to ride in the winter.

Good luck!

Amy

I did a training class this winter-it kicked my butt!

I'm not used to keeping a high HR or pedaling constantly when I bike so it was a wakeup call! Do one and see how you like it.

I just wanted to chime in on Spinning too. I found it very helpful in the winter just for overall cardio conditioning. I have great instructors who are all cyclists, so they definitely made the class feel more like a ride than an aerobics class. But, it was still night and day to actually riding.

I think it's a great thing to do, just another thing to keep your lungs working hard for 60-90 minutes.