I have an '07 Specialized Roubaix Elite (triple) and I've been thinking of some credit card touring in the mountains and deserts of eastern SoCal. I'd planned on 20 - 25 pounds in two small panniers.

I took the bike to a Specialized dealer and to another shop that sells lots of other carbon fiber bikes. I know both shops fairly well and have done a lot of business with each. They have always been pretty straight and honest with me.

I asked them the the identical question you posed

Specialized dealer said NO - NO - NO
- rear triangle is too light for the weight of rack and 20 pounds
- do not attach any rack mount of any kind to the carbon fiber seat post
- they doubted the clearance of any 28mm tire UNLESS the wheel is in and stays in perfect trueness
- mounting a rack to the QR or rear axle hub would still put too much load on the rear triangle
- pulling a trailer would put too much load on the rear triangle and would probably be very detrimental to handling

PS - A Specialized corporate employee was in the store installing a new computer system. He was an experienced long distance tourer and knew the Roubaix bike - he was very skeptical of the idea of touring with panniers on the back. He thought it was a bad idea.

The other bike shop looked closely at the rear triangle and seat post on the Roubaix. They (mechanic and store manager) felt there was not enough structure back there to support 20 or 25 pounds no matter how it was mounted. They recommendation was go with traditional steel frame and a purpose build touring bike. They do NOT sell any such bike so had nothing to gain from such a recommendation.

Another consideration - new lower gearing would be at least $100 for the chain rings and another $80 for cassette. The rack and mounts would be at least $50. That is $230 to convert my Roubaix endurance bike to a not so good touring bike - see the next paragraph.

I did find several very nice use touring bikes with full luggage setups for less than $1,000 here in the Pacific NW - I think I will purchase a used - medium quality touring bike and try that for a year or so.

OR - convert my 2000 Stumpjumper MX MTB to a touring bike. I already have good street tires and racks/panniers for it since it is my grocery hauler when I am out cruising on my sailboat.