I just scored a decent road bike and I'm looking to convert to a single speed. It has what seems to be quality Shimano cranks and bottom bracket. I just need to get wheels and a few other minor things and I'll be set. I've done a fair amount of reading on the web but these are some questions that remain:
1. I currently have 170mm cranks that were set up with two chain rings. The chain line seems to be right at 42mm between the two chain rings. How much play can I have in the chain line? If I install a chain ring on either side of the spider I won't have a "perfect" chain line- it will be off by about 2 or 3mm. Is that the end of the world? Is the anything I can do to perfect it? The BB and cranks are perfectly good so I'd like to use them if possible.
2. I'm wondering if I can use the existing 45 tooth chain ring as well. Seems like everything out there for sale is either 46 or 48 tooth. Is there any reason the 45 tooth wouldn't be compatible? My main concern is that it won't be big enough and offer enough resistance. How different would a 45t chain ring\15t cog be from a 48t CR\16t cog?
Thanks in advance for replies and opinions. I'm a newb so go easy.
2 or 3 mm is fine. The inside ring usually has the better chainline. On my conversion, it actually gave me a 42mm chainline. And you can use the existing 45. 45/15 sounds like a pretty sweet gearing for moderate hills/flat riding, but 45/16 would suit you better most likely.. a little lower and a lot more skid spots. And you will benefit from searching for a gear ratio chart to tell the gear inches of specific chaingring/cog combinations. Most people seem to gear about mid-70's but this depends on your riding style and terrain.
One fallback is to buy a phil wood bottom bracket. These have a bit of play so that you can tune the chainline perfectly. It can be noisy otherwise. Also if you are running a singlespeed, you don't need to worry about skid spots - 45x15t sounds fine.