Author Topic: freewheel to sprocket sizes (Read 3242 times)

lk_97 · « **on:** December 07, 2009, 06:39 AM »

I was looking to buy a 25t sprocket, what size would my freewheel need to be? and also if i had 30 what would i need?
Thanks.

HEYWOOD BMX · « **Reply #1 on:** December 07, 2009, 08:20 AM »

25 tooth=9T rear cog.30 tooth=11T rear cog.

madmike-bmx · « **Reply #2 on:** December 07, 2009, 09:07 AM »

you are going o ned to buy a new rear hub ....cause freewheels only go down to 13t .... you will need a cassette or a freecoaster

Stodgy · « **Reply #3 on:** December 07, 2009, 02:48 PM »

Here ya go in full:

44/16
42/15
39/14
37/13
34/12
30/11
28/10
25/9

Hope this helps

madmike-bmx · « **Reply #4 on:** December 07, 2009, 03:26 PM »

and u for got about 23-8

Dr.Robotnik · « **Reply #5 on:** December 07, 2009, 04:42 PM »

Nice of everyone to list the standard sizes. I'll just explain how it works though for anyone who doesn't know.

Sizing of gears e.g. sprocket to freewheel, is your gear ratio. This is worked out by dividng your sprocket by the number of teeth. e.g. 44T sprocket divided by a 16T freewheel would be 44/16 = 2.75:1. This is the golden ratio in BMX originating in racing and all others are derived from this trying to get near to this number with smaller rear freewheels to lighten the drivetrain and a sprocket sized to give somewhere close to 2.75.

44/16 = 2.75
42/15 = 2.8
39/14 = 2.79
37/13 = 2.85
34/12 = 2.83
30/11 = 2.73
28/10 = 2.8
25/9 = 2.78
23/8 = 2.88

Any ratio higher than 2.75 will be slightly harder to pedal than normal, but will have a higher top speed. So slower acceleration but higher top speed. Personally for really motoring about I go for the old 3:1 ratio of 33/11.

Hope that helps.

Dr.Robotnik · « **Reply #6 on:** December 07, 2009, 04:49 PM »

Oh, just a couple more things. With a 25T chain ring you will be after a 9T rear cog. This is only available on cassette hubs and is not available on freewheel hubs. The lowest you can go on a freewheel hub is 13T and this is only if your hub is compatible with the smaller threads on the 14T and 13T freewheels.

Also the smaller the drive train the larger the load per tooth and subsequently the larger the load on the chain. If you are a big lad or a hack or bad at grinds and hit your chain, you will suffer more chains snapping, so keep an eye on your drive train. Just like with moving from 48H hubs to 36H hubs they need a little more care and attention but can run fine. BTW 23/8 is renowned for snapping chains but 25/9 is significant improvement.