650a > 650b conversion 🤔

Hey guys. So I have this Davos 603 randonneur frame made by Fukaya in Japan. I’m trying to get it going but there’s a couple issues I’ve run into concerning the brakes.

Now, I have two sets of wheels, a set of 650A wheels which this bike is built around (the tires and tubes for which are extremely popular on shopping bikes within Japan – along with 27” hence why it’s even a thing) and a set of 650b rim brake wheels, which would be my ideal choice to use given there’s just a ton more good tire options out there.

My problem is that I can’t seem to get the canti brakes to align with EITHER size wheel. Like at all. I’ve even tried throwing a 700c wheel in there and that was just too large of diameter. It’s Almost as if the bike wants to be built around wheels with a 600 bead diameter or something? But to my knowledge no such thing really exists.

I’ve been trying a set of dia compe 980 calipers that I have handy.. they’re vintage looking but use more modern brake pads – they’re a simpler version of Paul neo retros. I can’t get the pads low enough to make appropriate contact with the rim even with the 650a wheels. So something is really weird about this setup and I can’t seem to figure it out.

My partner has a Crust Romanceur Canti, and something I noticed between these is that the canti posts are much narrower on this Davos bike – perhaps that’s the issue? I measured the width between the posts on the Davos and it’s like 60mm which is much narrower than on the Crust- which had no issues using these 980 brakes, but we changed them for some low profile cantis in order to get better breaking power. Perhaps the width is the issue here?

A good analogue for what I’m trying to do here with the 650b is a 27” to 700c conversion which requires an even longer, but still pretty meager 4mm drop in pad height (going from 630 to 622 BSD) Perhaps some of you have done that sorta thing in the past, and it should be virtually the same as what I’m trying to do going from 590 to 584 – or even just getting the 590 to work as intended.

If anyone has any experience with this sort of thing or knowhow, I’d really appreciate it. Either to get these dia compe 980s to work, or who has a recommendation for a different set of cantilevers that have the flexibility for this.

Thanks!