Am looking to switch cranksets on my jamis nova(steel). Currently have a 105 with 53/39 rings. The LBS has two FSA gossamers. One is a 175mm with 46/36 slightly used. The other a new 172.5mm with 50/34.
First race in November, thinking the 46/36 but how big a deal is the arm length?
Crank sizing tends to be a function of leg length (which less specifically relates to height). What are the 105 cranks you are replacing? Are you content with their size?
Personally at 5'9" with a 31" inseam riding a 52 I run 172.5, have changed back and forth between 175 and 172.5 with out much of a noticeable difference. For cross the specifc frame and BB height for that bike with have an effect in the areas of toe/front wheel overlap and hitting the pedals on obstacles (not that 2.5mm is that big of a difference).
I am guessing at 6' the 175 would be closer to ideal.
Just as a data point, I'm 5' 10" with a 30" inseam and ride 172.5 on the road. My first cross bike had 175 cranks and I was always digging pedals into off-cambers. One night before a race I noticed that my BB was in really bad shape, so I swapped cranks & BBs out for an old set of ultegra 9sp I had in my parts box--that happened to be 170 (carried the chainrings from the 175s over to the 170s). I didn't notice a difference in leg length or position issues, but I did notice that I could pedal through off-cambers. YMMV.
Got the new 2010 FSA gossamer, 172.5 and put new 46/36 fsa rings on it. Just installed and test riden. Wow, torque just like that!
Btw, i got a 54cm frame with a longer top tube, being 6' i only have @ 2" inseam clear.
Also a phobia of big bikes due to my first road bike at 15 years old that I rode till I was 30 and never grew into. My dad said it would fit in a couple years!