then why do axles break far more often in horizontal drop out arrangements? the quick release is made to purely hold the wheel in place, not to provide structure. i can bend a qr in my hand. we are talking about vertical loads here, not horzontal ones.

I've been riding and racing bikes since the early 70's and I have never seen any dropout break other then in an accident. So even though the strength thing makes sense, and yes it does make sense, I've never seen a dropout break even on heavy loaded touring bikes or when clydesdales were riding on horizontal dropouts. Maybe that problem may have happened to cheap WallyWorld or Sears bikes etc but not on the better bikes. In fact most bikes I ever saw fail was when strong clydesdale types put too much torque on the frame then the frame would seperate, but most of those breaks happened to older AL frames and the frame snapped usually in the BB area.

I would appreciate not being called names. Name calling and insults have no place here. (Thanks Mods for removing the offending words)

Please stop and think for just a moment about what I said. Please. I did NOT say that the quick release bears any of the weight. It does NOT. It simply holds the knurled nuts together, and THEY (the knurled nuts), by *pinching* the dropout opening, support the axle. The only time an axle will contact a dropout in any way traumatic enough to cause it to break, is if it is LOOSE. That is not the case under normal circumstances. The locknut and the quick release nut hold the axle in place, and the locknut, threaded onto the axle, is what supports it. Can you get that? The dropout opening, whether horizontal or vertical, does NOT support the weight of the bike on the axle in any meaningful way. It's a guide for the positioning of the axle, and it provides a structure for the locknuts to grip. That is what it does. Axles typically break at the point where the bearing cone is threaded on, because that is where the stress is. Not in the dropout.

Ridelugs,
I'll take a stab at answering your question (even though you didn't ask nicely)..

Axles break more often on older freewheel hubs, where the drive side bearings are further from the dropout than the non drive side. It just so happens that these hubs are usually in horizontal dropout frames.

and perhaps to a lesser extent, if a wheel is dished incorrectly, the wheel may be placed in a horizontal dropout crooked in order to centre the rim in the frame. This may put nasty force on the axle, making it more likely to fail.

- Joel

blah blah that if those same axles were placed in a vertical drop out, thier failure rate would drop exponentially.

I break about one Campag 7sp freewheel axle a year, whether riding a bike with vertical dropouts or one with horizontal. Even though it is a sample of one (and therefore statistically meaningless), wouldn't you expect to see a difference using that logic?

I have vertical dropouts on the 1988 Schwinn mountain bike, horizontal on my road bikes (1959 to 1981). When pushing a rear derailleur to its cog size and chain wrap accommodation limits, horizontal adjustability can be VERY handy.