I'm repurposing my single speed commuter for a cross bike, what gearing do you find most versatile--or is it a matter of preference? I've got the parts to run a 39x20 but don't want to die because I've got way too much or way too little.

i think you'd spin too much on that. Depends on where you ride. My commute is relatively flat and I find that 42:16 is fine but a 44 might be better.

On some of my rolling hill training rides, the 42:16 provides just enough challenge where I don't have to stand too much on climbs.

start with 2:1 and work from there

I was getting worked by a kid on a single speed this morning. He was riding a 41x17 and just killing it. (for 'cross racing, not road riding)

That's waayyyyyyyy too small of a gear. Start at about a 39-16 and go from there, depending on your route.

Here's what I get for MPH from Sheldon Brown's gear calculator at 80RPM, with a 700cx32 tire.

cog
----------39-----42 (chainring)
15------16.7--18
16------15.7--16.9
17------14.7--15.9
18------13.9--15
19------13.2--14.2
20------12.5--13.5

So your 39x20 is all of 12.5mph at 80rpm. Extrapolate that to 100rpm and you're barely cracking 15mph. Good luck getting anywhere in a hurry with that. ;-)

I'm repurposing my single speed commuter for a cross bike, what gearing do you find most versatile--or is it a matter of preference? I've got the parts to run a 39x20 but don't want to die because I've got way too much or way too little.
If you need a gear for cross racing I say go with 39/18 fast enough but you can still get up the parts you need to, and it won't wear your legs out as much, just spin faster.
For on the road go for a 39/16, this will allow a good clip on flat roads, good speed down hills, and you can still get up most road climbs.

sounds like a lot of the advice here is given for commutting/road riding. he asking for help with cross racing. are you racing C's, B's, A's?

If its a rolling/hilling course, 39/20 would work great. Heck, my first race this year (Newport, KY) had a terrible run-up that was rideable. It would never be possible with anything bigger then a 39/20, even then your knees might pop, that course was slow and tech in the flat parts and only had one long grass section, you would have been good with 39/20. but last week I don't know how anyone was riding SS (they were). it had long power sections at a slight downhill in grass that I was in 39/15, then gradual uphill climbs that I was in 39/25 for an extended period. It would have been murder on an SS, but they did it. I think the guy I talked to was running a 39/18.

I'd buy 3-4 freewheels and try to get a good pre-ride of the course, because I think it will always change. but anything from a 39/20 to 39/15 might be good depending if your a masher or not, my two cents...

I ride a 42/18 for racing, and I'm just a mediocre 35+ guy. You'll have a high enough gear to gap all the guys running 2:1 on the open stretches, and low enough for the short hills you usually find on a 'cross course. If it's too steep to ride with that gear, you'd be faster running it anyway. The only time I'd go to a 39/18 would be in really muddy conditions when you want to keep your speed up in the bogs.

For commuting I always run a 42/17 fixed gear, but I'm a trackie and spin a high cadence naturally.

sounds like a lot of the advice here is given for commutting/road riding. he asking for help with cross racing. are you racing C's, B's, A's?

hmmmm, I don't see the word racing anywhere in his question.

The advice about getting multiple gear options is solid. It's highly unlikely that a single gear will be suitable for all options. One of our local honches is absolutely slaying everybody in the 35+ A fields, and on his SS. He's running either a 42-17 or 42-18 at most races. And he said he could have used more gear at one of them.

I've been rolling 36:16 in WI/MN and competing near the front of the A's. I can hang but the minute someone attacks I don't have the top end I need so I'm thinking 38:16 next year.

I'm repurposing my single speed commuter for a cross bike, what gearing do you find most versatile--or is it a matter of preference? I've got the parts to run a 39x20 but don't want to die because I've got way too much or way too little.

I've done a few cross races this year on my single speed. I've used a 39x16 and it's a bit though on the longer hills. I find that in soft sections my spin rpms are a bit low and I'm out of the saddle a lot. I'm going to move up to 18t rear for next season, I think that should be about the sweet spot for me.

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I run 44x17 with 37c slicks for the road. Depending on the corse 40x18 to 44x18 on dert.

seem to run around a 40/17 and change in regards to how flat/hilly the course is