The most difficult part of your body to keep warm on cold rides are your hands. You can layer up every other part of the body for warmth, however the hands need to be free enough to operated the clutch, front brake and throttle controls so layering will be limited. Hand guards or a fairing will help, but for long rides nature will eventually suck the heat out of your hands and make them stiff and giving them the same feeling as if hundreds of pins are being stuck in them. It can make a ride very unpleasant.
To keep your hands warm the best addition you can make to your motorcycle is heated grips.
The beauty of heated grips is they are always on your motorcycle and they can be used with any pair of gloves you're using. Some riders chose to go with heated gloves, which is great provided you have your heated gloves with you. Heated grips, on the other hand can be used at anytime with any glove. This has some advantages besides those cold riding days. I use my heated grips on the low setting for evening rides or for those rides where it starts out warm and then gets cold into the ride, for example when you're riding into higher elevations. I can keep my hands warm without changing gloves.
For those really cold days you can mix heated grips and heated gloves and this will keep your hands very warm, however I've found heated grips alone to be more then enough.
If your motorcycle has stock grips, add some heat! You'll be very glad you did.
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