I dont think its against the rules as such, the problem is how do you take it off whilst essentially racing when you need full concentration and hands on the handlebars, and where do you put it safely so that it doesnt get caught when you mount/dismount/carry your bike etc, as you arent going to chucking €100 jerseys into the crowd as a CX racer, even if you can afford a Lambo to drive to the race.
CX stuff is all fleecy backed anyway, you run more risk of overheating than being too cold ime, unless the weather is freezing rain and you are getting soaked, as its not waterproof gear, but the efforts you are putting in should keep you warm enough, obviously I guess sometimes riders might get that balance wrong,, choose a baselayer thats not upto the temp and if youve sat on the start line too long losing heat,there is a tipping point where its very difficult to get body temp up again regardless of how much effort you put in.
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