Cyclocross - gearing question

hi!
still trying to make it up the hills with my poprad.
can i use a mtn. bike cassette with my shimano 105 rear der.?
i already switched out the cranks with a truvativ double in the front so i'd rather not switch to a triple in the front.
thanks!

It depends on wath the size of the rear dér.'s cage is: small (27 max) long (more but I don't know the exact number)

hi!
still trying to make it up the hills with my poprad.
can i use a mtn. bike cassette with my shimano 105 rear der.?
i already switched out the cranks with a truvativ double in the front so i'd rather not switch to a triple in the front.
thanks!

there 2 limitations to consider: derailleur capacity and max cog size.
the largest rated sprocket for the 105 is 27 teeth.

capacity is 29 teeth, based on how much extra chain the derailleur can accomodate. subtract max/min chainring difference and max/min rear cog difference and add these numbers together.

it is said these limits are conservative, but you'd have to be aware you might run into problems ( in this case the derailleur hitting the cogs. without the specifics of your set-up , i can't comment on the capacity.

if you have a compact front crank you could run something like a 46/34 front and 12-27 rear
a 34 x 27 combo is about equivalent to a 39 x 32 set-up

marc

I have a Fuji cross with a double up front and it had an ultegra rear derailleur (probably the same specs and capacity as the 105 der). I also wanted to get some lower gears.

I did some research and according to Sheldon Brown, the Shimano rear derailleurs (for double) can handle up to 30T cassette cogs. I considered getting one of his century cassettes 12-30 but ended up just getting a Shimano Deore (mountain derailleur) and an 11-32 cassette because it was much cheaper. I got a new chain, deore derailleur and 105 cassette for a total of $50 from nashbar (stuff was on sale).

I am pretty happy with the setup but now I want to do some touring and part of me wished I had gone and gotten a triple (but I hate the feel of triples and the way it's easy to drop the chain). In any case, I am glad I got the mtn derailleur because having a 32 vs a 30 and the potential to get a 11-34 is nice.

I installed it myself and it works great. Yes, shifting is a little clunkier than before (because there are larger jumps between gears), but the shifting is still precise and responsive.

Good luck...

practice, practice practice.

I was struggling when I first got my CX w/ a double. I did the same trails as I had done w/ my MTB. Some places I had to walk. In time, you'll build your legs.

thanks for the responses!
oldskool, i've been working on the hills around SoCal on my road bikes and the CX but some of the stuff i'm still feeling i'd be better off with higher (easier) gearing because there are fire roads and trails i know i want to hit but am sort of putting off because it'll be such slow going...SO...i basically did the same as as borderline and i'm having an XT casseette (11-34, i think) and a deore rear derailleur installed. and a new chain i'm told will be necessary as well. sadly i am NOT much a bike mechanic so i was sort of cringing at the cost of installation, and HUGE bike shop markups...
the upside is i'm able to use the 105 cassette from the poprad for my new set of ksyerium wheels on my road bike.
should get the bike back to me on wednesday and i'll report on weather or not the gearing is now TOO easy!!