Originally Posted by
lukerh
Since really the goal here is to increase axle-to-crown, what about just brazing on another crown race seat right on top of the existing crown race seat (assuming I can find someone locally with a crown race cutter)? The kingpin, like the R20 and a lot of other folders has an extra long steer tube to accommodate the (unnecessary to me) quick-release stem clamp. Presumably one could shift some of that extra steer tube length below and still have plenty left over for the headset.
Yes this is the way to go! You don't necessarily need to braze on a new crown race seat though. If you have access to a lathe or a minilathe you could just make a "lower-stack-extender", that sits on top of the existing crown race seat, and then presents a new one 20mm higher up. The whole thing could be made out of aluminium which is much easier to lathe (but I do make metal crown race seats on my minilathe, so it can be done).
If pedal strike is the issue you could also try shorter cranks. Those are trending these days anyway.
No reason why the original idea of clamp-on extenders couldn't work but it would be tricky to make them so they got a good purchase and they would be a bit ugly.