Thomastik Superflexible Double Bass Double Bass Strings have a braided steel thread core. For most players, the Superflexible are a "smoother" sounding steel, rope core string than the very popular Spirocores.
Recently, some rockabilly-slap styled players have found that they like the solo gauge Superflexibles. For many, they're just a little bit harder to slap than non-metal strings, but for steel they're pretty easy on the fingers. They have a bit more "click" - but also more actual note - than plain gut or nylon, especially on the E and A; they will probably sound a little "twangy" until they break in (especially if you're used to gut or similar). But they're reported to have a long life, and they work quite well for "regular" pizz sound as well. And of course, these strings will work with a magnetic pickup like the Schaller (while gut or nylon won't).
From the Manufacturer:
The solid core has been replaced by a cable-type structure made up of braided steel threads. This reduces the string's stiffness, improving its elasticity.
Superflexible is incomparably responsive and its tone seems to literally radiate from the instrument, without the least harshness. Durability, tuning stability and purity of open fifths are unsurpassed.
Superflexible is a dependable all-round string for the discerning musician, at home with every type of bowing and in every musical situation.
Medium Superflexibles have blue thread at the bottom and top; the solo gauge has a yellow top with a blue bottom.
designed to be tuned one step up from standard tuning,
commonly tuned to EADG for a "light" gauge playability