So I have a 2013 viper x. I wanted to put a new sprocket on it so I did. I bought a 25t and put it on. My stock setup was 44/16 so now I'm running a 25/16 when I should be running a 25/9 correct? What do smaller sprockets do? What are the benefits? If I need to get a 9t rear how should I do that, would I have to buy a whole new cassette?
Yup! Now that you've upgraded your front sprocket to a smaller size, you now need to get a smaller back driver to accommodate that change. The mechanical details are a little hard to explain, but just know that that smaller your front sprocket is, the smaller your rear cog/driver needs to be, otherwise you'll be pedaling hard and going nowhere, looking like a hamster in a wheel-cage.
The benefits of a smaller sprocket include a little weight loss, as well as more clearance when doing various grinds or lip tricks (so your sprocket doesn't hit the coping, ledge or rail).
To my knowledge, if you don't already have a cassette, then you're going to have to buy an entire new rear hub (cassette) in order to have the 9-tooth driver. I believe the smallest cog you can get for a non-cassette hub would be about a 12 or 13 tooth, allowing you to run a 33 or 36 tooth sprocket up front.
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Just click where it says "How To Choose a BMX Sprocket" to get more detailed answers to your questions.