thenomad

I'm currently running a 12-27 SRAM cassette with 39/53 chainrings.
I know that most front chainrings for CX are in the range of 38/48, so I suppose I'm fine with a 39 as I doubt I'll ever need to shift up as a beginner.
So that brings me back to the rear.
I just finished a custom set of gears to maximize my wife's new bike for road use as a beginner.
13-30 is what I created with 2 tooth steps the first set of gears and wider for the last set.

So I'm now thinking of using the remaining cogs to make up a cassette for some CX use.

Would going to 30 tooth be worthwhile? Or does dismounting negate the need for granny gearing?

knobster

Yeah, I've found that any run ups don't require the bike, but unfortunately you have to bring it along anyways. I'd stick with what you have currently. Do some races, then decide. Most likely won't need anything lower geared. If you use too low a gear, you'll be getting passed like you're sitting still and irritating everyone behind you because you'll be holding up the race.

Andy_K 

I don't think I agree with Knobster. There were a couple of races around here last year (Sherwood & Astoria) where being able to ride a hill that some other people were dismounting for got me a couple of extra places in the standings. I suspect that's mostly a beginner thing, because nobody in the upper cats were dismounting for those hills. For me, having a 30-25 gear at my disposal made all the difference.

Obviously, the course will dictate whether or not this matters for you. This year, I plan to use 50-39-30 front/12-27 rear gearing for a few races but 46-36 front/13-25 rear for most others.

FWIW, a 27-tooth cog with a 39-tooth chainring has slightly higher gearing than a 25-tooth cog with a 36-tooth chainring, while a 30-tooth cog with you 39T ring, would have slightly lower gearing than a 36T ring with a 27 -tooth cog.

knobster

Completely depends on the course and the person riding. For me, this is what worked on the courses I raced. The hills were seriously steep and rough. Anyone trying to ride it ended up loosing some serious momentum so based off that, you never seen people using triples or low gearing because you'd want higher gearing for the straights. But like Andy said, for his courses the triple would be beneficial. Triple may be a good idea for someone getting into racing though since it would give you the range for a variety of courses and wouldn't require you to have a bunch of extra parts like you'll probably need if you stick with it.