Originally Posted by cyclintom

Think about this: if the filler is at the bottom is is pointed up so that the sealant can always drain out. At any other position it can get filled with sealant that drips from the top of the tire onto the inside of the rim and drips around the rim.

New wheels are high tension low spoke count rims. I paid less for a set of cyclocross wheels than I would for a single tubeless rim. Where do figure on buying correctly cut spokes these days? "Good 2nd hand hubs" mostly aren't. (https://www.ebay.com/itm/Campagnolo-...0AAOSwridaAKpj) Why would you build a wheel yourself using 105 hubs? Or Centaur?

I can build an old 36 or 32 spoke count wheel in a half hour but why should I when I can get a lower end Campy or Fulcrum wheel so cheaply?

If you want to run racing tires and not get flats you could always opt for sealant filled tubes and have exactly the same problems you would have with a tubeless tire.

If you get an effective flat resistant tire such as the Gatorskin you have fewer problems (you don't have to worry where the stem is) and you very seldom get flats. With Gatorskins in five years I've had less than a dozen flats. With the supposedly flat resistant Michelin Power Endurance I had worse riding characteristics and 3 flat in two months. And they were all with glass shards that I'm pretty sure wouldn't have given me a flat on the Continentals.