chainline & cottered cranks
 

Hello all...I'm pretty new to the forums and I'm brand new to fixed gear. I just got my conversion into a rideable condition and I'm trying to sort out a chainline/gearing issue with my old-fashioned cottered cranks. Here's what I'm working with:

http://www.konradprojects.net/temp/cottered.jpg

Right now, I am using the 40 tooth inner chainring with a 16 tooth cog, and my chainline is off by 4mm. I've already ridden it and it doesn't seem to be that big of a deal. However, I would rather use the 48 tooth outer chainring. If I do this, my chainline will be off by 11mm! :eek:

Ultimately, I would like to step up my gearing to 70 inches. To do this I can either run 40x15 and my chainline is only off by 4mm but I have to keep the unused outer chainring, or I can run 48x18 and only have one chainring up front but my chainline is off by an enormous 11mm.

Because the outer chainring is attached directly to the cottered cranks, I can't remove it and use only the inner ring...basically, if I use the inner ring, I'm forced to leave both chainrings on. I plan on swapping the BB, chainring and crankarms later this summer for something that will work better. Since it's temporary, is it OK to run a horrendously bad chainline and quickly wear out my parts until I get a new BB? Or should I just ride on the inside ring and not worry about the stupidity of having two chainrings on a bike with one gear? If I have to stick with the inside ring, I suppose I could saw off the outside ring just past the 3 bolts holding on the inner ring.

why not ebay some used road cranks and bb? the 5 pound weight reduction will be a benefit as well.

Well, i plan on getting new cranks & BB sometime this summer, but I don't know how to make sure that whatever BB I buy will result in a better chainline. And I want to ride in the meantime.

Take a grinder and make the big ring into a nice little chain guard by grinding off all the teeth. That's what I've got going on my Peugeot and it actually looks kind of snazzy.

hey,
if you are prepared to take apart the bb you can try flipping the axle around to see if that will help any..
find a shop with a cotter pin press or maybe the bike co-ops have that sort of stuff ?

Well, I just went for a short test ride with the chain on the outer chainring. Everything worked but since the chainline is really far off, there is some noise. I'm sure it will wear the parts down faster, but I don't see why that would be a big deal since these are trash parts and I'm planning to correct the chainline with new parts.

How far can your chainline be off before you start to run into trouble?

If I can get my hands on a grinder, I think I'll go with kemmer's suggestion and make the outer ring into a chainguard. That'll look good and get my chainline straighter.

Harbor Freight has grinders for like 6 bucks or something silly like that. You could probably get a cheap one elsewhere if you don't have a Harbor Freight near by. It's a VERY handy tool to have, you might be glad to have it when you want to remove those cottered cranks.

how would that fix your chainline?

It will make my chainline straighter because it will mean I am using the inner chainring, not the outer one. The outer chainring would throw my chainline WAY off and since I can't remove it, the best option is to turn it into a chainguard. The inner ring will still be a little bit off, but a hell of a lot straighter.

but yyour using the inner chain ring now and you said it was off, i guess i'm just confused about something, enjoy your bike

Let me clear up the confusion...
I would prefer to use the outer chainring. Even though I posted a picture using the inner ring, I switched to the outer ring just to test it out. Now that I've decided to grind the outer ring into a chain guard and run it on the inner ring, it will improve my chainline compared to using the outer ring. Sorry for making everything so confusing.

End result: I'll use the inner ring and my chainline will still be off by 4mm. That's better than my other option, and I can live with that. I'll straighten it out for good when I eventually swap the BB.

Everyone, thanks for all of the suggestions...I've got my solution now.