denaffen

I'm restoring a vintage German folding bike and am a bit stumped by my bottom bracket options. Originally it used a bearing cup + cotter axle setup. I would like to put square taper cranks on it. Originally I figured I just need a commonly available square taper conversion kit. But what I'm seeing is that the bearing cups on those appear to be 50mm, as normally found on cruisers and other bikes with one-piece cranks. I'm measuring the ID of my BB at 40mm. So... what are my options?

I'm not seeing anything with 40mm bearing cups. The bb isn't threaded, and I'd prefer to avoid the expense of getting it threaded (if anything would even fit then). I could theoretically maybe marry up 50mm conversion kit with re-used 40mm cups and bearings, but even if everything fit that seems sub-optimal.

So I throw it to you... what might work?

Iride01 

It's pretty much all in the BB shell that will tell you. What is the width of the shell? The inside diameter of the shell? If threaded, then what is the threading?

Telling us what year, make and model vintage German folding bike might help too for some that might know where to find its specs.

There was something named Chater-Lea that was a 1.450" x 26 TPI.

denaffen

Well, like I said above, it's a 40mm ID shell, unthreaded. I can't get a width measure of the shell right now (wet paint) but I can tell you the cottered spindle measures at ~140mm, so I think something around ~120mm should work well.

Edit: paint dried. Shell measures at 71mm wide

Iride01 

Well you could have someone cut a BSA threaded BB shell off an old bike and braze it on in place of the current. But there goes your new paint job. BSA will probably give you your best selection of stuff to choose from.

Note the edit I made above your reply.

denaffen

Yeah, I'm not going to cut in a new shell. Definitely not worth it on this bike. I do wonder about something like this https://www.sunrace.com/en/products/detail/bbs08 although I do wonder how well it'd stay in place

Camilo

If it's a good interference press fit, it would be worth a try. Maybe with some mild-ish locktite?

I wonder if this one would work? Velo Orange Grand Cru Threadless Bottom Bracket (velo-orange.com)

They might have some ideas in the Classic and Vintage forum.

dedhed

Try "Thun" Thompson bottom bracket
https://hollandbikeshop.com/en-gb/bi...son-press-fit/

https://www.bikeforums.net/classic-v...ompson-bb.html
https://www.bikeforums.net/bicycle-m...placement.html

Russ Roth

If the shell is 40mm unthreaded you could try a pressfit adapter which is 41mm and might fit. There's also an old school bmx standard called mid which is 40mm. Usually uses pressed in cartridge bearings and then sleeves and lockrings to keep things in place. Worth a look.

denaffen

dedhed that Thun looks like it should work, as does the Sunrace. Problem is I can't find a non-European supplier for either, and I'm not too wild about $50 shipping on an $18 part. I may have to, though


Edit: And I now know I have what's called a Thompson BB. So at least I know what I'm looking for. I still don't see any US suppliers, but there are several in Europe. Now I need to figure out the width I'm going to want. I was thinking I'd just get something a bit shorter than the cottered spindle, but really, i need to be thinking about the cranks. So maybe 123. Maybe 127. Maybe shorter? Dunno. I'm looking at putting some vintage Sugino cranks on it and I *think* they require a higher offset, right?