Bicycle Tutor

Hi - another newbie to this forum with an annoying road bike problem... My Trek Madone 5.2 Triple has a Shimano Ultegra rear derailleur.
With the largest front and smallest rear sprocket combo engaged, when free-wheeling the chain flops onto the frame lower bracket - is this normal?
I have tried adjusting the B tension screw and I have even removed a chain link to increase the tension without success. All the gears engage properly and the chain slack problem only applies to the above combo.
Any suggestions would be much appreciated.
Kevin

You say this happens when you "freewheel". Just to confirm, I think you mean that it only happens when you are rolling, but not pedaling (coasting). If so, it sounds like your freehub is not spinning freely. It is getting dragged forward by the wheel slightly which takes the tension off the chain. This could result from wearing freehub or a bent axle which is rubbing on the inside of the freehub body. I'd take off the wheel and cassette and check it out a little.
BTW, you may have made your chain too short now. Not a big issue, but be careful going into the big-big combination, you could tweak your rear derailleur.