notfred

I have an On-one Il Pompino, which is a single speed cyclocross bike. It has 135mm spacing in the back for mountain bike style hubs. I have a hub on there right now made by the same people who made the frame. The hub is advertised as having a 50mm chainline.

I assume this means that the center of the chain, when mounted on this hub, is offset by 50mm from the center of the bike, correct?

This makes some assumptions - all various fixed-gear cogs and standard freewheels come in some standard spacing. I have no idea what this spacing is, but it looks like it's roughly 1/4" from where the threads on the hub run up against the hub body back to the center of the freewheel. Can Iassume this spacing is standard and taken into account when they say "50mm chainline"? I don't want my chainline changing if I switch from a freewheel to a fixed cog later on.

Ok - here's the meat of this question:
I want to replace the bottom bracket and crankset on the bike with something nicer. How do I go about selecting a bottom bracket and crankset so that I actually get a 50mm chainline to match the hub?

Any help would be appreciated. If I left out any information, please let me know.

peripatetic

Yes.


I'm not an expert on chainline, but I think that you can basically get a different crankset and keep your chainline as long as you make sure that you get something with the same length spindle. Do you want to swap out the whole bb, or just the cranks and chainring?

Sheldon Brown

I have a couple of relevant articles on my Website:

https://sheldonbrown.com/chainline

https://sheldonbrown.com/bbsize

Sheldon "Straight Is Good" Brown

notfred

Ok, so I measured the actual bike.

dropout spacing is 135mm. The center of the teeth on the freewheel is 12mm in from the inside edge of the dropout. That means the actual chainline at the back of the bike is about 55.5mm, which is significantly different than the 50mm that on-one is advertising.

The current crankset has a chainline of about 54mm. Which is probably "close enough" (I'm guessing - how close is close enough?) to the 55.5mm out back.

That's pretty much wider than anything in your list of cranksets, sheldon. What's clsoe to 55.5mm? Should I be using the outside ring on a road double? Also, I want to run a 44 tooth chainring up front. Will a 44 tooth chainring even fit on the outer ring of a road double (I'm not really a road bike expert)?

notfred

No other advice as far as finding a bottom bracket/crankset combination that will give me a 55mm chainline?

Sheldon Brown

I thought I covered it pretty well in the articles I referred you to, but for your specific issue, my recommendation would be to add a Sturmey-Archer sprocket spacer to the right side of the bottom bracket. These are 1/16" thick which is as close to 1.5 mm as anybody needs.

Sheldon "Cheapskate" Brown