Wool uniforms were quite heavy prior to World War II, and there were a few attempts to alter the traditional flannel suits before double knit polyester uniforms arrived with a bang (or a thud, depending on your viewpoint) in the early 1970s. Another issue was mobility, particularly for running and throwing. One experiment can possibly be tied to Branch Rickey, as several Brooklyn-affiliated clubs tried flannel shorts and a rayon pullover shirt in the early 1950s. This experiment was most famously associated with the Hollywood Stars, but the uniform style was also worn by the Ft. Worth Cats, Miami Beach Flamingos, Bisbee-Douglas Copper Kings, and others. The shirts were made of the same “durene” rayon/cotton blend that football and hockey had adopted (although the shorts were made of the standard wool flannel material).
In 1956 the Los Angeles Angels made a big splash when they announced their “waffle weave” uniforms. Former Angel star Max West had gone into the sporting goods business, and this innovation was his idea. These were regular flannel suits, but the shirts and pants featured a knit three-color knit “waffle weave” panel that was designed to give the player more freedom of movement. Gene Mauch was enlisted to model the uniform at a press conference, which was covered by the L.A. Times’ fashion writer Jeanne Hoffman. The success of the uniform is not known, but the following season (the Angels’ last in the PCL) saw a return to the traditional all-flannel suit. More about the Angels waffle-weave unis from our friend Gaylon White here. (Ebbets Field made this jersey for a few years until our supply of waffle weave material ran out).
It is unknown why these uniform experiments did not remain in favor. Likely the shorts were not amenable to sliding, and it is possible the rayon fabric for the shirts did not launder well. Ultimately the flannel era was extended by the fabric manufacturers blending synthetics like acrylic into the wool, producing a much more comfortable fabric that also allowed the uniform to be less baggy and more “tailored”. At the same time varieties in the traditional pattern were developed that had a much shorter sleeve, or no sleeves altogether, giving us such classic styles as the Mantle-era Yankee uniforms and the vest styles employed by Kansas City (later Oakland), Cleveland, Cincinnati, and other clubs.
In the end of course, the flannel era passed, and by 1973 all professional baseball teams were wearing the new poly knits – for better or worse.