yeah, it’s steel. And it is heavy…just under 4kg.😂 with regard to the chain line: I did some research and found that surly makes a 135mm rear single speed hub with a chain line of 50mm. When I use an old campa square taper crank with the campa chorus 102mm spindle bb (which produces a chain line of 48mm) or a JIS 107mm bb and mount the outer chainring it should be near perfect…the real problem is this gnarly fluted seat tube. I think I am going to make some vertical cuts on the seat tube, bend the fluted part outwards and braze a seat post clamp onto the outside, thus getting a seat tube which accepts a 33.9mm seat post. Seat posts with 33.9mm diameter and 600mm length are much easier to source…
Yes, but 33.9mm diameter seatposts, are designed to fit into a slotted aluminum bushing (Dahon calls it a shim) that is only 75-100mm long, that fits into the frame tube. I forget what the outside diameter of the bushing is supposed to be, it's really thin wall, but just enough; There is clearance fit between the seatpost and seat tube, below the bushing. Without the bushing, and with the hugely long seatpost, the inside diameter of the seat tube had better be perfectly round, and dead straight for the whole length, otherwise the seatpost will not go down to fold. I think that is the reason for the bushing, because there are multiple welds to the seat tube on most folders, that can cause the tube to not be perfectly straight or round for its whole length. I had problems with a long seatpost that was still WAY shorter, going into a mountain bike whose seat tube was not kept straight enough from the factory. Most bikes, the seatpost only goes in, what, 6"/150mm?
Take heed, and abandon the siren song luring you to the rocky shore of bike-build heartache. Make a clean break to a reasonably priced but virginal frame that will place you in good stead with the bicycle-building gods. Or, rather than creating your own religion as it were, go for the package deal with a folder that is turn-key fully equipped to go. As it is said.