"if you put presta valves into shrader holes you'll eventually get unrepairable tube failures at the valve hole. At 90 psi, the tube will creep up into any little space - and "POW""
What's interesting, Mark, is that doublewall rims come drilled both ways! Older MA-2's, MA40's and Campy Omega have 6mm holes thru both walls, while newer Reflex, OpenPro, Fir and even an old Nisi doublewall all have big (9-10mm) holes in the inner wall. All for Presta valves using the highest clincher pressures.
"Cut yourself a little "pillow" about ½" wide by 1" long and put it on the valve before you install it in the rim. This little thing will compress and support the valve"
I note that the valves in these dbl-wall rims are to a degree supported from tilting by their 2nd (outer) wall, but how does your "pillow" support the valve (or does it support the adjacent wall of the tube?)?
I run my Shraeder-drilled rims using presta valve tubes at 70-80psi using Chrolux and Mod58 rims with 1-1/8" tires. I have test-stretched new tires with this setup at 110psi out on the hot balcony for a day, also with no failures.
Did your failures show any of the tube bulging out from an enlarged (Shraeder-sized) valve hole (assuming singlewall here)? ...or did the tear occur internal to the rim? I might assume that if the valve were off-center to the hole (and this assumes that it must not have been pulled out far enough to fully seat the bulge at the base of the Presta stem into the Shraeder-sized hole) that the thickened tube wall surrounding the stem might bulge out under pressure. Is that what happened in your experience? Might some failures be from hand-pump motions where no 2nd wall is present to stabilize the valve? I try to brace the head of my mini-pump against a solid object to prevent excessive motion and valve tilting, but I can see where this could cause problems for some users and/or in some situations.
Another Failure I have seen many times (and many more that looked like imminent failure), is the severe stretching of the tube just outside of the thickened area surrounding the valve. I have noted that many times a valve stem does not come far enough out of the rim during inflation unless it is tugged out early in the inflation process, if not before. The tube then gets overstretched as air pressure forces it against the inside rim wall, potentially tearing it away from the raised base of the valve stem. I note that this is more likely when a larger tire's sidewalls splay outward from a much narrower rim, as in this case the tube is getting dragged around a corner by air pressure forcing it into the depths of the rim cavity, while the thickened rubber around the stem is also resisting being bent sharply between opposite beads as it must in order to reach the rim. I doubt that there is a real problem with 6mm presta drillings causing this from the stem bulge preventing the tube wall from laying fully flat against the rim wall, as the rubber bulge is only a few mm high along the stem, but I also have yet to see a failure from the bulged base of the Presta stem being forced (by air pressure) into larger drillings, manywhich exceed 10mm diameter!
A lot of possibilities to discuss on this subject as there are many variations to internal and external hole sizes, pressures, distance between beads and rim configurations where the valve hole is flanked by vertical walls of "dual-hollow" protrusions used for pinning singlewall rims. Tube size vs. tire size is yet another variable.
One added note, about switching a canti-braked bike from 27" to 700c rims: Using a wider rim will serve to reduce an overly-sharp pad-dive angle, while also maintaining a leverage ratio more similar to that which resulted using the previous 27" rims. This can be more critical on bikes with more widely-spaced canti-bosses. My early-'80's Trek 520 begs for a wheel swap as the original fork gives no fender clearance atop the 27" tire, while my Trek 720 now has wide Mod4 rims to effect a more normal braking action.
Auburn, CA usa
David - my own experience runs counter to yours - if you put presta valves into shrader holes you'll eventually get unrepairable tube failures at the valve hole. At 90 psi, the tube will creep up into any little space - and "POW".
Years ago I turned down some neoprene plumbing washers to take care of rims that were sized for Schrader valves. I think they were Wobblers or something.
Now for the promised little trick. Find some leftover handlebar tape (not cloth, the modern foam stuff) or some wet suit closed cell neoprene material. Cut yourself a little "pillow" about ½" wide by 1" long and put it on the valve before you install it in the rim. This little thing will compress and support the valve.
See pic at http://tinyurl.com/