Dirt Road Blues 

I have a 1985 Trek 830. It's an entry level "mountain bike" using True Temper main tubes, Tange Hi-Ten stays, and a Hi-Ten crowned fork. I'm planning to build it as a low budget, four season beater, city bike, that can handle a bit of light recreation on gravel.

Some pertinent specs:

68 mm bottom bracket shell
Drop outs spaced at 126 mm

Also:

Originally came with a SR triple mounted on a 126 mm JIS bb spindle.

But...

Lets say I stick with a 6 speed rear. And lets say I decide to use a Stronglight 105 double (ISO tapers), but remove both chainrings. Now, let's replace only the outer ring with one that has a proper tooth count to effectively make a single chainring crankset.

What bottom bracket though?

Unfortunately I dont have any bottom brackets to play around with, so I need to ensure that my purchase of one is a safe bet. What would be a target ISO spindle length that might likely provide a nice chainline for such a 1x6 configuration?

nlerner

My WAG is 113m. That’s based on the fact that a Stronglight double typically takes a 118 spindle and likes ISO tapers.

Moe Zhoost

Have you taken chainline measurements with the original spindle/crank. If so, you should be able to calculate the new spindle length pretty closely. Keep in mind that different cranks may seat differently on the spindle tapers. I'm not convinced that a perfect chainline is necessary on a freewheel/cassette equipped bike, but it's good to be close. You can always make some adjustments with either BB shell spacers or FW spacers (I think they are the same).

Having struggled with such issues before, I made it easy on myself and purchased a cheap cartridge BB. I used chainline measurements on the test BB to then determine the proper spindle length.

Dirt Road Blues 

Thanks for chiming in! Pretty much confirms what instinct has has been suggesting. In fact, I was at the community bike shop last week and nearly bought a 113 mm bb. Vintage SKF cartridge and ISO tapers. I put it back in the bin because there was just enough doubt to scare me away and it wouldn't have been a cheep gamble. Think I'll head over when they open to see if it's still available.
Taking a chainline measurement would have been a good idea, but he Trek came to me without a crankset. It'd be nice if I can avoid using spacers, but certainly will if I need to. Thanks for reminding me that that is a practical workaround.

​​​​​​​My understanding is that a 1x system necessitates good chainline because the single chainring is burdened with having to smoothly cover all cogs in back. Ideal position for a single chainring would line up with the center of the rear cluster.

Moe Zhoost

Yes you want to get it close as possible, but remember that with a multi-speed cog it will only be perfect if in the middle cog of an odd-numbered FW. Most chains will have no issue as long as you're less than 5 mm off from perfect. At least that's been my experience. I've converted 2 bikes to 1x, both with 130 mm rear spacing. One is a 10x and the other is 11x. I have narrow-wide chainrings on both to avoid chain-drop. The 11x is a road bike and I initially ran it with a regular chainring without too many issues, but then had a chain drop that really was ugly.

Good luck on your project.

Mad Honk 

DRB,
You might try just flipping the spindle around and use the NDS for the Drive side. Chain line might be OK with just that change. Smiles, MH

Dirt Road Blues 

Picked up a SKF bb just shy of 115 mm long. And while it seems to put the chainring within the zone, oh boy is it a tight fit where the arms pass the chainstays. Maybe too close?

merziac

Shimmit baby, BB, taper or both.

jdawginsc 

Light pedaling would be no issue, but out of the saddle, if the frame flexes it could strike. Though with the position of the crank arm when standing, it actually might be further from the stay.

Others may have crimped the stay successfully, but with the rust, not sure that is a solution.