My bass doesn't have tyres.
Seriously, though, there are several types of bass strings:
Nickel roundwound- these are probably the most popular. Generally made of steel with a nickel plating applied. They tend to have a warmer sound than stainless strings.
Stainless roundwound- These are the brightest sounding, although some, like DR Lo-Riders, can be pretty warm as well.
Nickel or Stainless flatwound- these are actually the coolest strings available.
Warm, deep and thumpy. Not so hot for slapping, although Larry Graham did use them some early on.
Half-round- usually a roundwound that has been ground down to a smooth or semi-smooth surface. Has some of the "ring" of rounds, but feels smooth.
Tapewound- Warm and dark. A metal core with a nylon wrap that attempts to approximate a string-bass type tone.
There are others, and variations of the ones listed, but I think these are the main types.
