Cyclocross - long cage rear der on double

is there any harm in running a long cage rear der on a cross bike with a double ring up front? Or is it just unecessary? I bought one by mistake and raced with it last Sun. I run a double 39/46 up front and 12-25 in the back. I noticed when I am in the small ring and shift to a small cog, the chain skips when I really crank hard. Is this due to the long cage or do I need a new cassette or chain (chain is new, cassette shows no wear)? I don't think the long cage keeps as much tension on the chain as a short cage. However, the shift from the 39 to the 46 is sweet! Comments, suggestions?

Not necessary to run long cage.

it's not needed. but i run a similar combo (38/48, 12-25 with a long-cage XTR M952), and it works fine. almost sounds like your chain is too long.

I ran a long cage on a 1x10 setup last year since I had it in my extra parts bin. I didn't have any issues running a long cage on the back.

As far as skipping, have you tried adjusting the barrel adjuster on the rear? Besides checking the chain length as mentioned above, it sounds like the cable just needs a little tightening/loosening if your parts (chain/cassette) are new.

I suppose one might want to do this if they were running a mountain cassette in back since road deraileurs won't work with that large of a range.

But it should work just fine, I've seen it done on a road bike (probably another case of using whatever was lying around).

I took two more links out and fiddled with the barrel adjustment. Now the cage is at about a 4 o'clock position in the 46/ 15 while before it was in the 6 o'clock, but shifting seems tighter. No skipping this morning, although didn't really crank it yet. Is there any rule of thumb for chain adjustment? I think cx setups would run a tighter chain to avoid drops. ?

I took two more links out and fiddled with the barrel adjustment. Now the cage is at about a 4 o'clock position in the 46/ 15 while before it was in the 6 o'clock, but shifting seems tighter. No skipping this morning, although didn't really crank it yet. Is there any rule of thumb for chain adjustment? I think cx setups would run a tighter chain to avoid drops. ?

I'd probably leave it about two links longer than the Largest Chainring to Largest Cog method:
http://www.parktool.com/repair/readhowto.asp?id=26
Although it now sounds as though you're now about there, you can carefully (never force it) shift into the large-large combo and estimate how many links you have to spare by pulling the chain tight and the RD cage nearly horizontal.

If you were running a road double RD which shifts a 12-25 better, I might look at the 6 o'clock when squarely in the middle guideline. (39+46+12+25)/2=61 total teeth which is the 46-15 combo you've been using. However, the long cage can handle 44-22+34-11=45 total teeth and you're only using 21 so this method won't even be close.