Aircraft and automobile tyres can be divided into two main types, based on the orientation of the tyres’ body plies (the layers of tough fabric that form the main structural body of the tyre); radial tyres are built with the body plies laid perpendicular to the bead and tread (so that the body plies extend straight out radially - hence the name - from the tyre bead, straight across from one side to the other at a 90º angle beneath the tread, and straight in radially to the bead on the other side of the tyre), while bias-ply (or simply bias) tyres have the body plies laid at an oblique angle.
Radial tyres are the more common type by far, for very good reason; they are stronger than bias-ply tyres and wear much more slowly.
I was, thus, rather surprised to learn that the tyres used on the space shuttle’s main landing gear were of the older, weaker bias-ply type:
Wheels and Tires
The main landing gear tires are 44.5 by 21 inches and have 16 cord layers in a bias-ply design. They are normally inflated with nitrogen to a pressure of 370 pounds per square inch (psi). The maximum allowable load per main landing gear tire is 132,000 pounds. With a 60/40 percent tire load distribution, the maximum tire load on a strut is 220,000 pounds. The main gear tires are rated at 225 knots maximum ground speed and have a life of one landing. [Shuttle Crew Operations Manual; my emphasis.]
The space shuttle’s main landing gear would seem to be one of the worst possible places to use bias-plies instead of radials; with each tyre carrying up to 59.9 tonnes (66 short tons) and having to cope with touchdown speeds in excess of 200 knots, it would seem imperative to use the strongest tyres available. The magnitude of the stresses placed on these tyres was so great that, while most aircraft tyres can make it through dozens to hundreds of flights before having to be changed, the shuttle MLG tyres were rated for a grand total of one use before replacement. Additionally, with only four MLG wheels per shuttle orbiter, even a single MLG tyre failure would significantly impair the vehicle’s braking capability, which is most emphatically not a good thing for something going as fast as a landing shuttle orbiter. Given all this, one would think that radial tyres would be the obvious choice, hands down, for the shuttle’s main landing gear, since their greater strength and slower wear would considerably decrease the chance of a tyre failure upon landing - yet the shuttles instead used bias-ply tyres. Why?