Fiddling with magic gears...
 

So I'm this close to getting a specialized crossroads frame fully built up as a fixed gear, problem is the vertical dropouts. I found a 48-19 gearing to be very very close to right, the chain is just slightly slack. It seems fine riding around, but after a few bumps today the chain jumped off the cog.

Now, while the chain is slack, I don't see it being slack enough to accept a half link chain. I'd like to know, would going up one size in the cog (20t) make less of a difference in chain tension than a half link chain?

And I'm dead set on getting this thing to work. The frame has such a nice shape to it, it'd be a shame to give up on it.

I'd appreciate it if anyone could help! Thanks!

Waste of time.

Rebuild the rear wheel with a eno eccentric hub or **** it. The reason why it's called magic gear is because it's magically crappy after 2 weeks of riding.

hmmm yes I'd sure love to spend $130 on a hub plus the cost of wheel building on a budget build. It's just going to be for cruising/messing around on, not something I'm going to log some serious miles on.

Does the chain seriously significantly stretch that quickly?

Have you checked out the magic gear calculator? half-links give you a bunch more options.

Thanks for that, according to that, the ideal area is just in between 48-19 and 48-20, with 48-20 on the left, and it's a bit closer than 48-19 on the right. Then again, the range is so small on that graph, and it's hard to tell exactly what the length is down to 0.01" when I have my cranks and rear wheel in. This is all without half-links.

I guess I'll just have to go down to the lbs and give it a shot!

rofl my ****ing lollerskates.

What's wrong? Have I offended your senses with having fun building a bike?

Nevermind, just read your post history and figured you were the local bully. haha

Magic gears... will only lead to tears. :(

Johnny Nemo is just causing fears,
magic gears been around for years,
get a magic gear into your rear,
like many of us that hang out here,
then quit worrying about your rear
and just concentrate on where you steer.

:D

Quoted from Sheldon "Adding or subtracting a link in the chain will move the axle 1/2". Changing either sprocket size by one tooth is the equivalent of moving the axle 1/8" (4 mm)."

1/8" = 3.175 mm. A lot closer to 3 than 4.

I filed out my dropouts enough to allow about 1/8" of adjustment. When the chain stretches that much I put on a new one.

I filed files flats on the threads on my axle, which gives me about 2mm adjustment, which was plenty. As long as you don't go near the axle itself (just the threads) it seems to be fine. And if it does go wrong, you can replace the axle for about £5, instead of buying a new frame :) .