Presta to Schrader Conversion I've been procrastinating for several years about converting from Presta to Schrader on my Trek 4200 with Bontrager AT650 (erto 559x19) wheels. What finally convinced me was getting three flats on a recent ride. I had only one spare tube and getting a repair shop here in Thailand to deal with the Presta valves was a challenge; even though I had an adapter.

What still concerns me is assertions that doing so will weaken the rim. 

I also have a Trek 7.2 FX which uses Bontrager AT750 wheels which have much narrower rims but use Schrader valves.

Does Bontrager do something special with the 750s that enables them to use the larger valve stem hole without weakening the rim?

Or, will I be OK drilling the 650s out to 8.5mm?
Regarding original question: I have personally drilled out approximately 6 wheelsetsto convert them from prestato schrader valves. I have not yet had any of these rims break even though I use them to drive down flights of stairs and such. Mind you I am a lighter rider (approx 80kg), these (aluminum) rims were used on hardtail mountainbikes. Make sure to debur(remove sharp edges from ) the drilled hole properly.

To try and decrease the amount of flats you get you could do a few things:

-make sure the rim tape is in good condition (no tears/holes, make sure it's properly aligned in the rim)

-remove rim tape and check if any spokes protrude more than approx 1-2mm past the top of the spoke nipples, if they do consider shortening these spokes with a file, dremel or the like or adjusting the spokes such that they are no longer protruding (if possible whilst keeping the wheel true/round)

-check rim bed/edges (where the tire contacts the rim) for damage (sharp/pointy spots which could potentially damage the tire), if you find any sand/file them down so they're smooth

-check your tire for pieces of metal/glass/other objects embedded in/under the surface (by rotating the tire whilst removed from the wheel and pinching it every 2cm approx to reveal any splinters, do this on both the inside and outside of the tire

-check if valve hole has any sharp edges

-whilst installing inner tube make sure it is not pinched in between rim and outer tube (inflate inner tube just a bit, check if it is not pinched anywhere and if all is good inflate the rest of the way)

For fixing flats I personally find it useful to clamp the inner tube with patch in between a bench vice (potentially with some pieces of wood in between the vice and the tube/patch for wider inner tubes), you could also use screw clamps for this purpose.

If you're a heavier rider you might want to refrain from drilling the hole larger but as mentioned I have had no issues with it so far and have been using the 'converted' wheels for considerable time now.