Originally Posted by pacificcyclist

Unfortunately tarwheel, based on Bob Jackson touring specs on the older 531ST, the oversized top tube is only 1" whereas the non ST version is 0.8". 1" is a classic tubing of the 1990s design and if the 631 OS is any similar to that, then that is what is happening here. You need an oversized top tube of at least 1 1/8" or 1 1/4" is better on some modern heavier steel frames to reduce the fish tailing effect you are experiencing by increasing the torsional rigidity to resist frame twisting by heavy rear panniers. The now deceased Cannondale T series touring has the BEST torsional rigidity and is as stiff as it gets due to the oversized aluminum top and downtubes and what made it stiff made the ride a bit stiffer, though you can compensate this by running fatter tires than the 28mm contis. I used to ride a clone of the Cannondale and loved it as the answer to my problems with my older Miyata 1000GT. Unfortunately due to the success of the steel for real movement, the remaining AL touring bikes are pretty much killed off and now we're back to steel again.

How to solve this? It's very easy and it had been practised by many tourers in the early 80s and 90s by placing 60% of the load on the front and 40% of the load to the rear. Some people like the soft steel ride (just like my former Miyata 1000GT), but you can't have both a soft ride and a really stiff frame at the same time.