How does the sprocket affect bike speed etc.?

Question

ive noticed that nicer bmx bikes have a small sprocket,, whys that?

Answer 1

More expensive bikes look to be lighter so smaller parts were developed where possible. If you change the amount of teeth on the sprocket your gearing will change and it will become harder/easier to pedal. However it is possible to downsize both sprockets and keep the same gearing. I've put a detailed explanation below on gearing.

If you change the back cog you'll probably need to change the front cog (chainring/sprocket)

or you're gearing will change. When it comes to gearing it's all a matter of taste really and you'll need to do a little math to work it out. Here we go:

Divide the number of teeth on the front cog (chainring) by the number of teeth on the back cog (freewheel or cassette) and see what you get.

eg 33T on the front 12T on the back equals 2.75

If you take 2.75 as the "average" value as a lot of stock bikes have this. It basically means that it's not too hard to pedal and you get a reasonable top speed.

Flatlanders like to pedal very easily and don't need a high top speed so they would reduce the number of teeth on the front cog whilst keeping the back cog the same (eg 23T front 10T back). Then if you do your sums you will have a value of less than 2.75.

If you're doing jumps and carving then you may just need a little pedal to get going then your momentum will do the rest (gear ratio less than 2.75), or you may like to get even more speed after a push off down a ramp with extra pedaling (gear ratio more than 2.75 but not more than 3.2 as you'll really struggle higher than that)

I personally like to go fast and currently have 36T on the front and 12T on the back equaling a value of 3. To save weight I'm changing my cogs down to 30T and 10T, I'll have smaller cogs but the sums still make 3 so my bike will have the same acceleration and top speed. A sum of 3 is a less popular but although I found it difficult to pedal at first, my legs were used to it in a week and now anything lower that 3 (I have a 2.75 bike too) doesn't feel fast enough!

See if you can go down a BMX shop (with a calculator) and have a test ride on some different gearing setups.

Sorry for the long post but I remember the stress I had trying to work out gearing for the first time! It also seems you're not alone as I've copy paste and modified my gearing post quite a few times.

I hope this helps you but if you're unsure before you change anything just go to a probike shop that sells good BMX parts.

Good Luck

Answer 2

smaller sprocket in the rear = faster

Answer 3

As has been said, the smaller the rear sprocket, the faster the bike will travel.

That's the simple bit. However, the size of sprocket used will also depend on the type of riding that you do, the strength of the rider and the terrain on which you ride. Although a smaller sprocket will allow (in theory) you to ride faster, you will have to be able to put in more effort and your cadance will have to be maintained. If you are a high cadance rider, the smaller sprocket will perhaps not help.

The smaller sprocket will also reduce your acceleration (unless you are really strong) and if you are riding flat land, may make some tricks harder to perform.

If you are a racer with a low cadance and lots of strength, a smaller sprocket (or a larger chain ring) would probably help.