In the 1960s, you levered a set of Avons or, after 1964, Dunlop Triangulars, onto your race bike's wheels and you were set for any weather because all tires had water drainage patterns molded into them. When, in dry use, the edges of the tread blocks deformed in a few miles to make feathered edges, we clamped a matt knife blade in a pair of Vise Grips, dipped it in water, and neatly cropped away the edges. Presto! More tire life for free. If it rained, the drainage was there to cope.
With the coming of slicks in 1974, even a light drizzle became a problem. So, for this not-rain/not-dry condition, hand-cut slicks—later called intermediates or molded intermediates—were the medicine. Also made were full rains, which had extensive deep-water drainage grooves and were also made from especially soft rubber.
In the rainy races I've seen, full rains have almost never been the right choice because, if the rain diminishes, such tires quickly heat up, lose grip, and become as useless as a qualifying tire with two laps on it. Those who chose intermediates or gambled on slicks usually won those races.
Full rains in full rain are an amazing sight, as the big rear tire throws a plume of spray high into the air, making the bike pretty much invisible from behind. Such tires were soft and their tread blocks weak, causing heat-generating distortion in use. If heavy rain turned to a weak drizzle, such tires quickly overheated and turned to grease.
I was surprised when MotoGP banned intermediates, limiting the choice to just slicks or full rains. When I asked why no intermediates, I was told nobody used them anyway.
In steady, heavy rain there is no choice –an intermediate, with just a few shallow drainage grooves, cannot prevent the build-up of a water-wedge ahead of the tire, onto which it can all too easily climb. But so often there is just enough rain to keep everything moist, with only the occasional puddle. This is the common intermediate condition, and an intermediate tire, with its more durable rubber, is able to keep up a good laptime as the track dries.
Therefore, I suspect that in this era of spec tires, the tire manufacturers wanted an end to teams being allowed to hand-cut their own intermediates, as was so very often done in 500cc Grand Prix racing. Gotta keep those corporate liability lawyers sleeping through the nights.
