Chain & Sprocket Sizes

[sweeper]

This topic comes up a lot.
For those of you who aren't all that familiar with gearing the bike, some criteria.
What works for someone else,in most cases is not going to work for you.

First and most important,skill level.Be humble or realistic when evaluating your riding ability.The more skilled the rider,the more gearing that rider can manage.And are you getting better,think kids clothes,if you are improving as a rider,gear a tooth higher,youll get there.
Where do you ride most of the time.If you are the kind of rider that rides a lot of diferent terrain,stay up a tooth.Better to have to use some clutch or force yourself to carry a little more speed in the tight stuff,then run out of gears in transfer sections or open terrain.
What do you ride,more cc the higher gearing you can manage.A 300 can run 1-2 teeth higher in the same terrain as a small bore 125-200.The 250 maybe a tooth higher.Keep in mind all things being relative.Same rider testing gearing in the same terrain on the same bike.
Rider weight.Be realistic here.
Riding style.
Power characteristics of the bike.Aside from cc.If you spent a bunch of money to build a bike with high torque characteristics,you will negate most of those gains by gearing the bike too low.

13-49 is a good middle bench mark.A tooth up or down is a fine adjustment.Anything beyond that you will begin to change the characteristics of the motorcycle exponentially.The power characteristics need to be in relative relation to the gear box gearing and ultimately the final gearing.
Tire size will effect what gearing you choose.I run IRC M5B 120/80 almost exclusively on everything except the 300.Its a low profile tire,which means the gearing is lower and it spins easier,so 1 tooth smaller on the rear sprocket would make up most of that difference.A trials tire would be just the opposite and then some due to lack of wheel spin.

12 tooth sprockets are risky,hard on the chain,they throw chains,break teeth,wear faster,and I swear just don't seem to get the power to the back wheel as well.Large rear sprockets are hard on your chain guide,will in many cases actually create drag.So gotta gear a little lower now to compensate for increased drag due to gearing lower,scenario.Hard on rollers and more chain equals more stretch.Large rear sprockets get in ruts,glance off rocks,pickup sticks and run them through the sprocket.Not kidding.
The tighter the loop between sprockets the better in general.

There are just too many variables for one rider to suggest to another rider what gearing he should run unless they ride together.

Hope this helps