Horizontal dropouts probably date back to the 1880's with the introduction of Chain Drive Safety Bicycles - as opposed to "Penny Farthing" High Wheeler Ordinary bicycles.
A 1925 illustration was the earliest image of a modern style Horizontal Dropout that I could find.
There were several reasons for using Horizontal dropouts. The first and most obvious was for adjusting the chain to the proper length. Next was to adjust for chain wear. The early chains were probably made of lower quality steel than modern ones and wore faster. They were also probably a lot more expensive to replace by modern standards too.
Horizontal dropouts can reduce the necessity for accurate chain stay lengths. Many bikes that we assembled or worked on during the 70's frequently had differences in the lengths of the chain stays by up to 10mm, especially entry level Bike Boom models.
The problem wasn't limited to cheap bikes... One busy Saturday afternoon in 1977 someone brought a brand new all Campy Bob Jackson into the shop complaining that it pulled to the left. He'd just bought it from a mail order house in SoCal. It was black with ivory panels. The pin striping, lug lining as well as the decals were perfect! Out of curiosity and to quickly get rid of him I took it out for a quick test ride. After 50 feet I brought it back. The bike not only pulled but it wanted to go in a circle to the left.
I put it up on the stand an immediately saw that the left chain stay was almost 3/4" (20mm) shorter than the right. The fool started to give me a hard time about a bike he bought somewhere else so I quickly showed him which way the door opened!
Back on topic, I remember an early 70's ad in a bike magazine claiming the the head tube angle of their bikes could be adjusted by the position of the rear wheel in the dropouts thus adjusting the handling. Yes it's very possible if the slots in the dropouts are positioned at a significant angle relative to the ground...
Vertical dropouts have been in use for decades. They require a considerable more amount of attention to make sure that the chain stays are the same length an the dropouts are accurately positioned relative to each other. With the advent of Shimano's SIS system hub widths became more standardized and with it rear triangle positioning.
Most builders in the US used frame jigs so frame alignment issues were significantly reduced. Most UK builders and many on the continent took less caution in their construction. Just braze it and cold set it! The famous UK builder Jack Taylor bragged that he never measured anything... He just "eyeballed" the tubes then coldset them for alignment afterwards!
Dropout adjuster screws were a feature developed for quick wheel changes in competition. All of the replacement wheels were built to the same amount of dish and so on. Pull the old wheel out, put the new one in and move it back to the stop screws. 10 seconds and gone!
One other thing, I was always puzzled buy the cheap stamped steel dropouts used on some high quality British frames like Mercian, Holdsworth and so on ...
Finally the answer that I got from some esteemed folks in the UK... Those dropouts allowed an owner to use SS, Fixed Gear, and an Internal Gear Hub like a Sturmey Archer or... a Derailleur on a claw hanger ??? All of those could have been done with Campagnolo or Zeus forged dropouts. Penny Wise and Pound Foolish???
Simplified answers...
verktyg
