acape

OK, I have a single speed that I built up a while back on a spare road frame I had (with vertical dropouts). I was using a 9-speed freehub rear wheel and an old 17 tooth cog that I lined up with spacers for a good chainline. I was also using a Surly Singleator for chain tension. Keep in mind that I am not riding fixed, just single speed.

Well yesterday I got thinking that I'd like to ditch the tensioner for a cleaner look, so I started thinking about the magic gear and trying to get that work. First I figured I might be better off getting a few BMX cogs (for taller teeth) just to minimize the risk of chain slippage.

Well to make a long story short, I ended up getting it to work (I think) with the 45 tooth chainring I was using and a new 17 tooth BMX cog. I have some concerns, though: my chain is tight enough such that when pedaling in the workstand, I can feel some slight grinding, and the crankset barely wants to bind at a couple spots. Yet, when I check the play in the chain my pushing it up and down, there's slightly more than the 1/2" of movement that I have read to be optimal. I'd say it's about 3/4" total vertical change. So how can it be that my chain is looser than recommended when checking the play, but too tight to cause a little resistance?

Also, according to the online magic gear calculator, this combination shouldn't work for my frame, if the frame maker's chainstay spec of 41cm is accurate. Also of note is that with my old 17t cog from an old cassette, the chain was way too loose, but it's tighter with a new cog. Could the old cog have been that worn?

So I just want to make sure that I'm ok on safety with this setup. Also, I assume I'm ok using a regular 9-speed chain with a couple Powerlinks to get the length right.

Thanks, sorry for the long-windedness.

acape

Bump, thanks.

1fluffhead

Sounds like your chainring is not centered. https://www.sheldonbrown.com/singlespeed.html#tension

acape

Yes, I actually did that, but the ring fits rather snugly on the spider such that it doesn't move much even with the bolts loosened.

What I'm wondering is whether I'm risking anything other than increased drivetrain wear by running this setup. I'm not putting a lot of miles on the bike, and I plan to get an ENO hub soon and go fixed with it anyways. So I don't mind going through a chain or two, cogs, and rings, I just don't want to suffer a broken chain while grinding up a hill.

Revolution Smmr

you seem to know what you're doing, but just in case... is the chainring biopace/ovaltech/some other form of eliptical chainring?

acape

Not that I know of, just a simple Rocket Ring as far as I can tell.

Revolution Smmr

if it were, there would be a visible eliptical shape to it

radical_edward

Few chainrings are actually truly round. Road bike rings are generally the best, bmx rings are often built to incredibly poor tolerance. CNC is usually better than stamped, but your mileage may vary.

A regular track or bmx bike can dial in enough slack for it not to be an issue, but you get this all the time when magic gearing.

Cynikal 

Rocket rings are really bad about quality. My Salsa 45 is very round.