Does bottom bracket width matter on a single speed conversion?
 

Assuming the chain line is straight, does the bottom bracket width make much of a difference? I ask because I recently replaced a 118mm with a 122.5mm, since it's what I had on hand. I moved the spacers on the freehub to adjust the chain line accordingly. I was going to post an image to show how much spindle is visible between the BB shell and the crank arms, but the forum won't allow it.

Not really, it sound like you are using a freehub with a single cog and a bunch of spacers. If you were using a single cog freewheel it could be a problem.
Having an arrow straight chainline is not necessarily crucial to have but it will tend to wear out the chain a bit quicker.

To make use of a particular random selection of parts -- Bendix hub and whatever BB and crank were on hand -- I dished the rear wheel by a few mm.

If youwere using a freewheel or track hub, the BB width would matter. Since you are using a freehub with spacers, it doesn't matter.

Yeah I just did this sort of thing last weekend. Undished the rear wheel by roughly 6 mm to change the chainline from running a 7-speed to a single speed. A 5-speed would line up nicely now, too. OLD is still about 126 mm.

Funny thing is that in doing so, a rim that I was somewhat annoyed by is now running truer and rounder again. Plus it’s such a blast running the bike SS. 😊

Otto

The advantages of "doing it wrong." I spread my cheap steel frame from 120mm to 135mm, and run a set of MTB wheels-- many spacers on the hub to set the chainline. Having to set the chainline from the crank seems like a hassle-- obviously doable, but not as easy as having a stack of freehub spacers to choose from.

You are talking BB spindles they are chosen with the crank arms used..

the BB shell width ... 70, (Italy) 68 (most others) 73 (mountain bikes)