Only thing extra MX jerseys have usually is a longer tail than the front to stay tucked inside MX pants (downhillers are Bohemians and don't tuck anything in - - slobs) and there is some minimalist padding stitched into the elbows. That's all. Oh, and big-ass graphics that scream the brand, so you can pay $50 to advertise them. I used to hate that until I was getting something to wear those big-ass logos
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