2pedals5

Stop me if I'm wrong but dropouts are only for the multi gears and horizon is for single speed? If I am wrong, that we can still use horizon with multi gears, then what's the purpose of having either 1 of those 2? Why does it matter if one favor dropouts more and not horizon? Or the other way around. Thanks.

tagaproject6

The horizontal drop out is to allow for the wheel to be pulled back and keep the proper chain tension and allowing for when the owner want to change sprocket sizes. Read up: https://www.sheldonbrown.com/singlespeed.html

zze86

Dropouts is the term for the slot that the wheels axle slips into without having to first derail the chain. You can just unclamp the wheel and it should be able to drop out. Thus the name. This is in contrast to the track fork end, where you have to take the chain off the sprocket before you can slide the wheel out. There are two main flavors of dropouts, horizontal and vertical.

The horizontal dropouts used to be the dropout of choice and could be used with fixed/SS/IGH or a multi-gear setup. The advantage of the horizontal dropout is that it is more versatile, allowing the user to alter chain/chainstay length. They were however trickier to install a wheel on since you have to align the left and right sides to keep the wheel centered in the rear stays.

The vertical dropout's advantage is it's simplicity. Slide the wheel on and clamp. However, you lose the ability to adjust the chain/chainstay length and subsequently are rather hard to to do fixed/SS/IGH conversions on. There are tricks in playing with the gearing/tooth count to get a workable chain tension though.

Horizontal dropouts used to be the norm since they allowed for multiple setups and allowed for variability in frame tolerances. Back in the day when bikes were hand built, the horizontal dropout gave the frame builders some wiggle room if they didn't get everything aligned correctly. As well, racers back in the days used to have different sprockets on either side and would flip them around depending on conditions. The horizontal dropouts allowed them to adjust their chain tension as necessary.

The vertical dropout is dominant these days because it's simple and, IIRC, because racer boys and lawyers. There was a period in time when quick releases were made with aluminum clamp surfaces in an effort to further reduce weight. The aluminum clamps were not so good at holding the wheel in horizontal dropouts.

79pmooney

zze86 spells this out nicely, What he doesn't say is that it was common on better horizontal dropouts to have small adjusting screws at the rear. These screws were adjusted from behind the dropout and extended into the dropout. They served to limit how far back the axle could be slid. Normally they were used to properly locate the rear axle and freewheel relative to the rear derailleur although they are also very handy for locating the rear axle to set the correct chain slack for SS/fix gear riding. Either way, they make aligning the wheel nearly as quick and easy as vertical dropouts.

That said, it should be pointed out this was (and still is) all driven by professional racing where very large sums of money were often riding on fast wheel changes. On the track, the rules re: flat tires are completely different. Fast wheel changes during races aren't needed. (Also nobody cares if you have to touch the chain since dirt is not allowed on the track, quite unlike roads.)

I have one vertically dropped bike. They are a joy but they also mean that bike will never be anything other than a geared bike. By contrast, I requested horizontal dropouts on my first custom just so I could go fixed if I ever wanted to. Never thought I would. Then last year, I realized that bike could be set up fixed gear and ridden for Cycle Oregon's first ever week ride including significant gravel. Did it and loved it. That bike may well spend all of its remaining years as a fix gear.

Ben

Viich

Rear-opening is usually called track ends and is not properly a dropout. Those can be used with single speed, fixed gear or not, or internal geared hubs.

all of those can be used easily with horizontal dropouts as well, as can a rear derailleur either if there's a derailleur hanger or by using a claw adapter.

Vertical dropouts are derailleur or single speed (or IGH) with some sort of chain tensioner. They are easiest for derailleur bikes - as mentioned, it always lines up.

woodcraft

Horizontal dropouts depend on high clamping force to keep the wheel from shifting and hitting the chainstay.

I believe that the vertical dropout was developed to fix this issue.

JohnDThompson 

My recollection from working in the industry BITD is that vertical dropouts came to be preferred because they maintain precise alignment of the derailleur with the gear cluster, and indexed shifting systems suffer more than friction systems when this alignment is not maintained.

The weaker, but lighter "open cam" quick releases became popular only after vertical dropouts became common.

woodcraft

I recall the "stomp on the pedals- wheel slip- skid" phenomenon. Never got the point of horizontal dropouts, & those delicate screws.