18t hollow ratchets first started when they came out with 190 hubs, I think. An early one I opened a long time ago, long before there were 36t ratchets, had hollow 18t ones.

I installed the 36 tooth ratchet in my gf's 190 hub. Thought it would make the sound a little more buzz like instead of clack like if that makes any sense, and a little more "light" sounding, as the DT hubs can be quite loud. Well, it's still loud, and indeed more buzz like, but I think we both might prefer the 18 tooth for the road bike. I've heard that the "instant" engagement thing is more apt to be sought after by the mountain bike crowd, but for the road it really doesn't make much of a difference. Before I switch it back I may reinstall it with a little more grease on the star ratchet to see if it quiets the sound a bit.

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My Colnago C59
My Special Colnago EPQ

Applying a light coat of grease on the ratchet "teeth" will definitely quiet down the hub. I just wiped off the old grease and reapplied using DT Swiss grease and it certainly silenced my 190 hub. Just make sure you don't apply too much -- i've read somewhere that this may have caused incidents where the ratchets were "sticking."