Bottom Bracket upgrade cottered to cotterless
 

I commute on an old Czech Rapido folding bike. It's a Raleigh Twenty inspired model circa 1970's-80's. I need to replace the BB which is cottered. I figured I'd take this opportunity I'd upgrade to a cotterless system. I'd like to purchase the cheapest all-weather BB that would be reliable for daily commuting in wet weather. The bottom bracket shell is 68mm in width. I believe the threading is English. I really don't know many other details. I'd like to buy the bb and SS crank set for under $100. If it's cheaper to purchase double crank set then I'll go that route and put a bash guard on. Please help. Thanks.

check the spindle length too. I have a raleigh supercourse (road bike) and the dimensions were 68x113. just check with the old spindle from the bb and get one according. check craigslist ebay and lbs for help.

68 x ??

Sure it is not a bigger BB house than normal? I did this on an old polish folder and did a DIY upgrade annd put a second hand (free) cranc from an oldroad bike.

I'm not really sure what the standard way to measure a bb is but here's what I got (these are not exact since I don't own a caliper):

BB shell width: 68mm
BB shell circum: approx. 133mm
cottered spindle length: approx. 141 mm
chainline (c-c): approx. 44mm

46t chain ring on 170mm cottered cranks.

I wouldn't mind getting a larger chain ring up front. I like the feel of the 170mm crank length but if someone can convince me to get a shorter length I may consider it.

Any suggestions for specific models of BB and crank sets?

This would be very nice and just a little over your $100 budget:

http://www.velo-orange.com/suxdtrcrhipo.html
http://www.velo-orange.com/su10bobr.html

Would that bb definitely fit? How do I know what the chainline measurement will be for a a particular spindle length. For example, with a 103 BB, I subtract 68 and divide by two to get 17.5mm for the length of each crank mount but how do I know where the chainring will fall? I'd like to preserve the current chainline as much as possible.

I also currently run 1/8 chain. Would this be less than ideal with a 3/32 chainring?

I think I'm asking the wrong questions and probably don't have the most accurate measurements but I'll take another stab at this:

I guess what I should be asking is for an old Shimano 3 spd hub, what chainline will I need upfront? Once that is settled, what length spindle should I be looking for to maintain that chainlilne? Based on some threads I've been looking at in the fixed ss forum, sheldonbrown's bb chart is a good start. But after looking at this I couldn't pinpoint the most useful information since most charts were based on specific cranksets which all happen to be out of my budget. I'd like to try a cheaper crankset off of ebay which unfortunately won't make sheldon brown's chart. So can anyone help me pinpoint a sure way to pinpoint the right size parts for any no name part?

Since you have the rear hub, you need to determine the chainline from that.

Measure your rear dropouts from inside to inside. Let's assume it's 120mm. Divide that by 2 = 60mm. Measure from the inside of your right dropout to the center of the rear cog. Subtract that figure from the 60mm. That's the chainline you need to shoot for up front.

Thanks for the instructions. Once I figure out the exact chain-line how do I know what crank-spindle length combination will fit that chain-line. Is there a general rule of thumb? Sheldon Brown's charts only list certain cranks which all seem to be out of my budget. Is there a way of knowing without actually buying the parts first or am I just stuck with trial and error?

This is why I like my Miche and Edco bottom brackets. If you calculations are a little off, you have a few millimeters of adjustability one way or the other. The Miche is a beautifully made piece that has an incredibly low price. They're made for track use, so you can't get a spindle loger than 115mm.

Thanks for the suggestion. Right now I'm looking at the Pake JIS crankset and a shimano UN26 bb but I'm still not sure what spindle length to buy. Anyone know which spindle length will give me a 45mm chainline with this combination?