SunTour made an absolutely excellent set that was used by loads of Japanese made road bikes in the 80's. Yes, they are huge, but they were extremely well. Inside the right shift lever, there is a round pawl, similar to what is used inside a Sturmey-Archer 3 speed hub. There is a small metal piece , somewhat like the letter L, that sits above it. As you move the lever back, the L piece slides over the pawl. Hence the ratcheting sound. There is also, among the other 20 or so parts inside, a small spring above this too. Sure made shifting a cinch .