Shipping weight, and with light, bulky things, they actually use a volumetric shipping weight equivalent, since the box takes up space and they price per lb.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter