I am putting together a Cannondale frameset with the box of 'modern' parts I have accumulated from other builds. The I am running a Shimano 105 short cage 5701 RD and matching FD with the same line of brifters. Crank is an FSA Gossamer 50/34 and the cassette is a 11-25 10spd hyperglide. Everything seems ok until I shift to the big ring and start to shift up on the cassette, once I get to about the 3rd largest cog (21t) the RD is pulled almost parallel to the ground and the chain tension is so high that shifting and turning the cranks become very difficult.

The drive train was recently removed from another bike which was set up with 52/42 crank, other than that, there have been no changes. My question is, is this a chain length or a cage size issue, and what might be the remedy. I am operating under the following information:

For Shimano:
SS - Short Cage Road Double - Maximum Cassette Cog is 27 and Total Capacity is 29
GS - Medium Cage MTB/Road Triple - Maximum Cassette Cog is 34(MTB)/27(Road) and Total Capacity is 33(MTB)/37(Road)
SGS - Long MTB - Maximum Cassette Cog is 34 and Total Capacity is 45

By my math I am 1 over on the total capacity, but I have heard that these numbers are often a bit conservative, and it seems that the tension is far to great for such a small overage.

Thanks in advance for any and all assistance!