Depends.
Usually there's some leeway for adaptation, but it's possible that your bike was delivered with the smallest sprocket available for that hub.
Small sprockets reduce the weight of your bike so that's always a plus. Also a smaller sprocket doesnt get in the way and they look cooler :)
You might have to either lengthen or shorten the chain, depending on if you're getting a bigger or smaller sprocket.
First you need to be clear about what you're talking about.In regular Bike-speak, sprockets go at the rear, and chainrings/chainwheels go at the front.In BMX-speak, a sprocket goes at the front, and a driver goes at the rear.If you're speaking BMX, the most obvious difference is the size, the tooth count.For the same size driver, a bigger sprocket will make the bike slower off the start, but give it a higher top speed.For the same size driver, a smaller sprocket will make the bike quicker off the start, but give it a lower top speed.If you change the driver to match, start and top speed will remain the same, even with a smaller/bigger sprocket. A small sprocket will increase the ground clearance. Better if you're riding ramps as there's less risk of the sprocket/chain hitting the lip as you drop in. If you're not riding ramps, well, a smaller sprocket is still lighter, which is a kinda-sorta advantage. The downside is that the smaller they get, the faster they wear. And the chain wears too.
jcs or any bike store
it would depend on whither you are riding on a mountain bike or something that wont be doing stunts or a bmx or dmr. for mountain bike it probably will not matter whither it is big or not however for bmx or dmr it would be a good idea to have a smaller one as when at skate parks eg. you where going to drop into a bowl that is quite steep if you have a big on then the chances are it is going to hit the coping (top of edge) and maybe fall off. also if you are thinking of getting a smaller sprocket you will need to get a smaller cassette but don't guess how big the cassette should be for a sprocket just take it to a bike shop and they should be able to tell you. for a smaller sprocket i would recommend a 25t to 30t,, the 25t will have to have about an 8 or 9 tooth cassette. hope all of this helps took me a while to type out anyway :P!!
First you need to be clear about what you're talking about.In regularBike-speak,sprockets go at the rear, andchainrings/chainwheelsgo at thefront.In BMX-speak, a sprocket goes at the front, and a driver goes at the rear.If you're speaking BMX, the most obvious difference is the size, the tooth count.For the same size driver, a bigger sprocket will make the bike slower off the start, but give it a higher top speed.For the same size driver, a smaller sprocket will make the bike quicker off the start, but give it a lower top speed.If you change the driver to match, start and top speed will remain the same, even with a smaller/bigger sprocket.A small sprocket will increase the ground clearance. Better if you're riding ramps as there's less risk of the sprocket/chain hitting the lip as you drop in.If you're not riding ramps, well, a smaller sprocket is still lighter, which is a kinda-sorta advantage. The downside is that the smaller they get, the faster they wear. And the chain wears too.Changing to a smaller sprocket on the rear will make your bike heavier to pedal. If you're strong enough to keep cranking at the same speed as before, then your bike will be faster.
First of all, the rest of the cycling community calls the thing by the rear wheel for sprocket, and the thing by the pedals for chain ring or chain wheel - but I understand what you mean. And only you can answer your original question, as it depends on how you want your bike to behave. If you want more top speed - go for a smaller sprocket. If you want quicker starts - go for a bigger sprocket. If you don't have one - read up on the specs for a bike for the type of riding you want to do - then get the same for your bike.
The bike doesn't really care what sprocket it has. "Best" sprocket depends on the rider's preferences and ability. If you're spinning out and want more top speed - get a smaller one. If you're bogging down and slow off the start - get a bigger one.
The chainring by the pedals and the sprocket(s) by the rear wheel.
First you need to be clear about what you're talking about.In regular Bike-speak, sprockets go at the rear, and chainrings/chainwheels go at the front.In BMX-speak, a sprocket goes at the front, and a driver goes at the rear.If you're speaking BMX, a small sprocket will increase the ground clearance.Better if you're riding ramps as there's less risk of the sprocket/chain hitting the lip as you drop in.For speed it doesn't matter, as long as you pick a driver with a suitable tooth count.If you're not riding ramps, well, a smaller sprocket is still lighter, which is a kinda-sorta advantage.The downside is that with a smaller sprocket, you will have to use a smaller driver. And the smaller they get, the faster they wear. And the chain wears too.
25t means that there is 25 of the little spikey teeth on the outside of the sprocket :]
No. The two chainrings are linked together with one chain. The chainring of the second person actually drives the rear sprocket, but since there's a chain that links up that person's sprocket with the one in front, both parties must pedal at the same speed. I suppose it would be possible to use a larger or smaller sprocket in the front to accomplish this. one could also use a 2 speed sprocket with a tensioner to change the ratio of the front sprocket with respect to the rear.