Fixed gear conversion: BB length? chainrings?
 

Hi!

I hope there is a generous person out there that doesn't mind explaining a few simple things to an amateur :)

I'm currently converting an old 10-speed bike to a fixed gear and am kindof learning as I go. I've never worked on bikes before but I'm slowly getting the gist of it, but I apologize if my questions seem clueless!

I would like to get a new, cheap crankset with only one chainring and I've been reading up on it online (there's a bike co-op near me where I could definitely find one). I am having trouble understanding what BB length I should look for and how it relates to the chainring, as well as, how to choose what length crank arms I want? If anyones willing to explain this to me, I'd really appreciate it!

Thank you.

To pre-empt the less than helpful types who are going to rush in here and say "google sheldon brown", there's a couple basic things that might help you on your way.

Different length BB's give you different chainlines. You need to find the right combination of BB length, crank, and rear wheel/cog to get a straight chainline. Read about chainlines.

Crank length is a personal preference. Longer cranks give you more torque, but are a little bit more difficult to spin with (meaning pedal as quickly as you can at very high RPM) and can be susceptible to pedal strike. The longer the pedal, the closer to the ground it will be. If you lean far enough into a turn, the pedal can hit the ground, and you will be sad.
Shorter cranks are the opposite; easier to spin, less fear pedal strike, but less leverage.
I use 165mm, which is one of the more common "short" crank lengths.

Thank you!! I have actually googled sheldon brown already, I just wanted a somewhat more "layman" explanation. That makes sense.

A few more :)
How do you adjust BB length? Adjusting the crank requires spacers or "stack bolts" correct? Can I buy any crank as long as it has the crank arm length I want and work with that?

I'm glad my thoughts were right about the crank arm length. It makes sense to have a shorter length, especially for more reckless riding ;)

Thanks again!

You don't adjust BB length, it's a specification of the BB when you buy it. If you need a 103mm bottom bracket for the chainline you're after, you buy a 103mm bottom bracket. If you need 107mm, you buy 107mm.

You can get any crank you want as long as it matches the bottom bracket spindle type. You'll probably looking at something in a square taper. That's the most common.

That makes sense. I don't have the bike in my hands yet, my friend bought it. I'm hoping I can just remove the extra chainring and use the outer one along with the crank I have now... at least for awhile. That should save me some money. I found some nice square taper cranks at the bike co-op for $21, but since I don't have the bike in my possession, I can't check for what BB spindle type the bike has.

Thanks for all your advice!