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I don't know the name of it but there is a part on single speed bikes. The gear? I think right where the crank is connected. Well on bmx bikes and especially dirt jumping bikes it is really small and if it is small then that means you have to pedal a lot more to go faster. Well can that gear part or whatever is connected to the crank be changed to a bigger single speed to pedal less? Sorry for not knowing the name guys please help though it's important before I buy this bike.

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It would help if you could post a photo. It sounds as though you're describing the chainring, in which case the answer is yes. – Móż Mar 28 '14 at 2:07
As @Mσᶎ says, it sounds like you're describing a chainring. Chainrings have teeth, the chain wraps around them, and they look like this. Is that right? – jimirings Mar 28 '14 at 12:40

It's called a chain ring and, yes, it can be changed to whatever you want it.

The dirt bike above has cranks which seem to use chainrings of type 104mm bcd 4-bolt. So any 104 bcd chainring will do the job. E.g:

if it is small then that means you have to pedal a lot more to go faster

That is correct, but usually those bikes have a very small cog at the rear. So you really don't have to pedal like crazy to go fast. Note though that the main purpose of those bikes is not to go fast but to jump.

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So the larger the sprocket the less I will have to pedal? – john towers Mar 28 '14 at 6:25
large chainring = less rotations, more power, ends up really fast. small chainring = more rotations, less power, ends up slow – cherouvim Mar 28 '14 at 6:29
Yeah, reading up on it, you can get 8 tooth cogs for BMX bikes, which pretty much makes up for the difference in tire size between a road bike (where the smallest cog is 11 tooth) and a BMX. However it's important to note that using large gears is more efficient than using small gears, even if they end up at the same ratio. – Kibbee Mar 28 '14 at 12:50
@Kibbee: Nice link. But smaller gears mean less weight and also more clearance. – cherouvim Mar 28 '14 at 13:32
I'll give you the clearance factor, which is important on a BMX, but according to the article, the weight savings is negligible, and completely overshadowed by the savings in chain friction. – Kibbee Mar 28 '14 at 13:39

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