Bicycle Mechanics - chainline; two wheels, one fixed, one 9spd.

I just ordered a Surly Karate Monkey frame. I am still deciding how I want to set it up, but I am toying with a couple of things and would like to hear some ideas.

I think I would like to use this bike as both a 1x9 geared bike and a fixed gear. Get two rear wheels, so that I can go back and forth with minimal trouble. But am I going to have trouble setting up the BB/crank so that I get the chainline I want for both? Its a 73mm bb, so already it is going to be sort of dicey getting a track (42mm) chainline. I could always dish the rear wheel farther out than the track standard, but that would mean not being able to flip/flop.

jim

What's the rear dropout spacing? The 73mm bottom bracket says "135mm" to me.

If you visualize the chainline starting from the right dropout, you may find yourself using a wider than 42mm chainline with the fixed gear. I'm thinking that I'd initially set up the bike as a 1X9 and then build the rear wheel to mimic whatever chainline that requires.

Rear spacing on the Monkey is 135. Considerably wider than the 120mm track standard, but that should not make any difference as chainline is measured from the center, and so whatever extra distance there is to the stays (a matter of adding spacers on the hub if the axle is long enough) does not matter. Its not too hard to find track hubs with long enough axles to work with 135. I have done it to a number of bikes.

jim

Rear spacing on the Monkey is 135. Considerably wider than the 120mm track standard, but that should not make any difference as chainline is measured from the center, and so whatever extra distance there is to the stays (a matter of adding spacers on the hub if the axle is long enough) does not matter. Its not too hard to find track hubs with long enough axles to work with 135. I have done it to a number of bikes.

That's fine for the fixed gear but what about the 9-speed?

You are going to want your chainring to line up with the 5th cog to minimize the chain angle to the extreme cogs. You are also going to want the small cog to be as near to the dropout as possible to minimize the amount of wheel dish that you'll need.

If I was building the bike, I'd set it up for the 9-speed first because, as you already mentioned, it's going to be relatively easy to make the fixed gear cog line up.