Smallest front sprocket?

[INTP_ty]

I know someone has dug into this. What’s the smallest sprocket available for a bicycle?

Calculating gear wrenches isn’t a problem. Making sure I can get a coaster with whatever size sprocket I want will.

Cliffs:

Out of the ~6 bikes I’ve motorized, not a single one fit as every bike has a different frame geometry. Some of these old bikes have a Low drop So you end up hitting the crank

 

[The_Aleman]

If you're using traditional 1-piece cranks, the smallest I've seen is 25T on kiddie bikes. You can only go so small on those type of cranks because of the index hole. I'd measure the distance between center of bottom bracket to engine case to figure out how much room you'll actually have.

(Teeth * Pitch) / 3.14 = approximate sprocket diameter. Divide that number by 2 and if it's smaller than the BB-to-case measurement, it'll fit.

Classic Lucky 7 style: https://www.amazon.com/Alta-Bicycle-Chainring-Sprocket-Multiple/dp/B07MDHP74L/
36T = ~5.75" diameter

Sunlite Generic: https://www.amazon.com/Sunlite-Single-1-Piece-Chainring-28t/dp/
28T = ~4.45" diameter