Gearing and derailleur question.
 

I am a crit racer that took up cross this year. Raced in 5-6 races and fared well in the flat ones and not so well in yesterdays hilly race(surprise). After the race I was talking to the guys that are in 1/2(I am tied for 2nd) in the points. They are using 34/27 and 39/34 for their tallest gearing. This seemed huge to me, as I thought I was going tall with a 39/25. But riding some of the "run ups" and even some of the steeper grades, I was chugging along while they were sitting and spinning easily. I lost contact on the second lap after watching them spin away, while I could barely turn the pedals.
So, I use Q-rings on all my bikes and was getting ready to order a 40T Q-ring for my cross bike to run a single front ring. I was contemplating going to a 12/27 cassette to help with the climbing for the next few races. My question is does a 27 give THAT much more than a 25, or should I maybe find a 28 or 29? And off of that, I figure a 28-29 would need a long cage RD, but do you guys run 27's with a short cage RD's okay?

Thanks for any help!

Welcome to 'cross racing. It's fun isn't it?

I'm pretty much of a crit guy myself. For 'cross I run a 46/38 up front and a 12-27 in the back and find it works well for the courses in my area. Basically, if I can't get up a hill on a 38-27, then I should be running. A couple of my teammates that are better climbers do the same courses on a 42T single ring and a 12-27 in the back.

A 27 is noticeably easier then a 25 in my experience and a regular short cage RD is fine up to a 28T cog (though they are sort of hard to find.) A 40-27 will be easier to push then a 39-25, though you may not notice it. Why not use the 39 ring for your single? A 39 with a 12-27 should be enough for most courses I would think...

May depend on the specific course, but most guys I know running single up front are running 42T. 12-27 on the back should do the trick nicely, and as mentioned a regular road RD can handle that easily. If you need larger than about 28-30T, you are probably into MTB derailleur territory, depending upon the bike.

I'm currently running 46/36 front with 12-28 rear (8s). MOST of the time (in races) I'm in the small ring and in the middle of the cassette somewhere. For that reason I'm strongly considering the single front ring, with the same cassette. As bitterken mentions, if it's too hard to pedal 36x28 it's probably time to be off the bike anyway. Only one course I can think of that had a climb where I was in that lowest gear for more than 30s and it is well-known as a course with a lot of climbing. Most never need that much.

HOWEVER: I think the consideration is different for rec. riding versus a race. If you're planning to hit your local MTB trails, you are probably going to want a wider gear range. That's the main thing that's holding me back from switching to single ring...I do a lot of non-race CX riding. Maybe when I get my dedicated CX race bike :)

A sora rear der will handle a 42 frt with a 11 32 moutian bike cassette in back.

Cool, thanks guys. Looks like I'll try the 40 Q ring and 12/27 rear.

I run 42 in front and 12-27 in back, but if I were starting from scratch I might run a 40 or 38 in front. I don't think I've ever used the 12t cog during a race.

BTW I think you mean "shortest" when you write "tallest" gear?

(FWIW I run a Salsa Crossing Guard and a Jump Stop, and I shortened the chain to where it looks about like this
http://www.parktool.com/images_inc/r.../chain-OK1.gif
in the biggest cog. No dropped chains so far this season (knock on wood). A couple of times the chain's been knocked off on a rocky section or while pushing the bike, and the jump-stop and crossing guard put it right back on in a turn or two.)

. . . but don't fall into the trap of thinking the smallest gear (ie. biggest cog) is always the best. If you spin too fast, you can easily end up slipping in moist grass or leaves or just plain dirt. Sometimes better to use a slightly harder gear with slower tire rotation and just power up the climbs. The lightweight guys are going to have an advantage on the climbs no matter what you do. Also, consider running some of them.

That is true. Sometimes it's better to power through stuff in a harder gear.

Also a good idea, especially if it's short and/or there are turns at the top/bottom that would require slowing on the bike. I've successfully out-run people on bikes in several cases like this. Unfortunately they passed me back on other technical sections, but that's another story :o But, don't carry *too* much or you'll wear yourself out.

Thanks guys. The longer hills in question are probably 250m or more, so I don't think running is an option.
Good point on the tire slipping though.