Denver is all about layers. The days can change drastically this time of year. I have been out in shorts and caught in the snow in the fall. A nice warm and sunny day can turn cold and snowy in a matter of hours. Look West and if the mountains are lost in a cloud, that front could be down here quickly. Most of the time it won't come down though and it will just be mild in Denver.
You will probably want arm warmers and a vest for sweaty descents on cool days, as well as some warmer cycling gloves. In the winter, I sometimes wear tights over my shorts, but it is often warm enough not to.
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