I don't know the stats on any bindings, but I would be surprised if the 390s are the lightest. I have them and think they are great, but I don't think they are super light. The are pretty adjustable which means there are more components to the bindings so I would doubt they are the lightest...great binding though and still pretty light. You'd have to check the stats on others to know for sure.
nah my flow NXT FSs are lighter then 390s lol...union contacts are wayyyy lighter then 390s, im not sure but i think nitro zeros are pretty light too. not saying 390s are a bad binding, or if its even possible to distinguish the weight difference between the 390s and contacts.
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