I have a BD windsor the hour and I want a lower gear ratio. I understand how chainring sizing works with road bikes (compact has 110mmBCD etc.), but how does it work with my single ring crank?

My question is: What do I need to buy to easily replace my 48t front ring with a 42?

Simple, buy a new 42t chain ring.
-measure the bolt circle diameter (BCD) (https://www.hostelshoppe.com/tech_bol...e_diameter.php)
than after you obtain such info, buy a 42t with the same amount of bolts (4 or 5) and is the same BCD as your crank arm, install it, but you may need a new chain.

Wouldn't the simpler/cheaper alternative be to buy a new Cog?

Maybe he already got the cog and needs to go even lower. If you go from 48 to 42 you will have to shorten the chain not get another one.

there are advantages/disadvantages of both (replacing chainring vs. cog)

if you downsize chainring:
- less weight
- won't need extra chain links
- costs more than replacing the cog

if you increase cog size:
- slight weight increase
- you have to use a lockring tool
- if it's a big jump, may need another chain link
- costs less than replacing the chainring

You're forgeting that a smaller chainring or cog will decrease drivetrain longevity. Not that it really matters. And chainring/combos means more skid patches if the fraction is not reducible. e.g 49/16 = 16 skid patches.

buy a new chainring, shorten the chain, and enjoy







got my 40t chainring gear installed and picked up some bmx grips. i know the bars are close together, and the bike rides as twitchy as a crack*****, but i LOVE it! lol

40x15 gearing FTW!!