I have done it, but it was not something that I could imagine anyone would classify as fun. If you are looking for the advantage of not being able to pull the wheel forward, a much easier route would be to simply weld on a few little tabs at the front of the drop outs, like the ones that a lot of forks have.

It's fussy work, but doable for an experienced builder. A beginner could possibly bumble their way through it and maybe end up with a usable frame. Henry James (see sources thread) has some conversion dropouts which are probably a good idea. It would be best if the donor frame doesn't have chrome on the rear triangle.

I vote, not to do it.

It is difficult enough so a beginner should not try and do it without expert instruction and practice. It isn't a place to start. It is easier to go from vertical to horizontal than the other way around however.

I would go the route of a White Industries Eno hub that has eccentric axle to allow for tightening the chain.

You could also just replace the rear derailleur with a spring-loaded chain tensioner. You'll see this mod on a lot of CX and Mt Bikes that are converted to single speed; Surly Singleator, Paul Melvin, etc. Lots of choices.
Brent

The single pulley tensioner is my solution for the next up bike/frame. An IGH with disks and a dynamo ft hub. But I'm not racing or riding at max force over rough stuff on it, or I hope not. Andy

I contemplated making this response but it's not helpful. And with adequate preparation and study, you could do it. It's a significant effort though and more expensive than it looks. I would say the rear dropouts are one of the things that new builders have the most trouble with, and adding the complication of having an existing rear triangle makes it significantly more difficult to do right. And in the end you need a paint job, which is expensive.
Seems like everything that surly makes has horizontal drops and Salsa has a lot of bikes that have adjustable dropouts. For canti, you are talking used or old stock for the most part, I think. OTOH, my LBS has some canti frames hanging from the walls. I suspect they would be willing to discount nowadays.

Salsa/Surly frames have crossed my mind although just a matter of ponying up real $ as opposed to catching lightning in a bottle on a cheap old crmo frame meeting my specs. Waiting to see what falls in my lap now until the weather warms up and urgency to build sets in

I will probably wind up going the tensioner route on a standard vert dropout frame OR I just discovered Origin8 and a bunch of others make derailleur hangers for track type dropouts.

I think my ideal frame at this point is a SS frame and if I decide to convert to touring go IGH or with an add on derailleur hanger

If I can find a frame for a cheap price meeting all of my criteria except horizontal dropouts then I think I will go this route.

Maybe a sign: guy just posted on craigslist bike parts a tensioner for $10. Been resisting that route but maybe it's time to just call it.

If you must take a torch to a frame during this project adding a der mounting hanger/eye is far easier and less paint destructive then replacing drop outs. Andy

On any flavor of race bike that isn't used on a Velodrome fast reliable wheel changes can make the difference between DFL, DNF or a more respectable finish.
Vertical rear dropouts are the easiest, quickest and most unlikely to bodge a rear wheel change on, particularly under the pressure of an in-race wheel change.
Many of us warm up on the "B" wheels and install the race wheels before the start. Screwing that up is about the last thing up need when your class is being called to line up.
Reliable and light In That Order make for a good 'Cross bike in any racing class. Put a tensioner on vertical dropouts and have at it.

-Bandera

That's actually really good advice and something this former NASCAR NW Series pit crew member hadn't considered until now.... no worries about 'centering' the wheel in the frame or anything like that....

On machines w/ mudguards fitted dealing w/ a flats-fix on the roadside having vertical dropouts makes the process much easier, reliable and clean as well.
Nothing like getting your tube changed, inflated and tire properly seated only to have to deflate it because with a horizontal dropout an inflated tire contacts the mudguard at the front of the dropout and will not allow the wheel to be reinstalled.
"Uh, got a CO2 cartridge I can borrow......"

-Bandera

Getting a frame with a broken right dropout and having a set of horizontal dropouts that's what I did..

30 years ago..

I could braze , do metalworking, and had built a frame from scratch with modest access to tools , , assistance and materials.. ..

Yes a spring-loaded chain tension-er is good ... I have 2 IGH bikes doing that..

You aren't, unstated , wanting to do another fixie conversion ?







....

If you look for a bike with sliders, they are out there used. I think that's what I would do, I really dislike the other ways of taking up slack.

My 2 Chain Tensioners are both on folding bikes with a hinge point behind the BB shell ..

No on the fixie - I have no interest in riding fixed gear.

I have two bikes at the present - a Specialized Langster with track forks and drop bars set up as a SS/Freewheel which I use for everyday commuting and an old Nishiki plain-guage/hi tensile bike which was a 10 speed but is now a SS/Freewheel with CX tires, dual pivot brakes and MTB handlebars. This bike is not fast or terribly efficient but a ton of fun to ride with my kids.

I just got rid of a SS folding bike (my college kid needed more reliable transportation than her car so she got it for free, awesome campus bike, actually) and I just sold my CX frame because I determined it would be too tall (who knew the BB shell on a CX frame rides higher than a track frame??) lol

The bike I wish to build is a SS/CX bike which I can easily put back to geared in case I hate racing it, I can just use it for a touring/weekend bike for longer rides. I plan on running a freehub body with a single cog which would be easy to slip on an 8+ speed cassette in the event I wanted to switch back (no re-dishing, etc)

Dave- At least one of the greats in modern building (and is well known for his CX team) has said that the BB drop doesn't need to be less on a CX bike, the taller tier will lift up the height anyway. But with a fixed gear ride (and I know that's not what you plan) the not being able to time one's pedal stroke and terrain stuff suggest less drop is a good idea.

As to folding bikes- mine have all been 3 speeds. I choose the high gear (3rd) to be my flat land steady riding ratio. This way the two lower gears get me up and over the hills (since many folders seem to lack a certain efficiency). Andy