TT bikes have both horizontal and vertical dropouts but most of these bikes are manufactured in horizontal dropouts. horizontal dropouts of TT bikes have major advantages, as you know these dropouts are straight horizontal 1 inch in their length and their major advantage is that you can easily move and bring the rear wheel of your TT bike back and forth while in vertical dropouts, it is not possible to bring the rear wheel front and back, so moving of the rear heel front and back would help you a lot in maintaining and adjusting the tensioning of the chain, if your chain is loose and you want to stretch your chain, this horizontal dropouts would help a lot and you can easily bring your rear wheel in the back direction, these are the min advantages of horizontal dropouts on TT bikes.
Besides this, these horizontal dropouts are also beneficial by choosing and installing different sizes and different widths of tires, when you want to replace your rear tire or wheel, then you have easy access to choose different brands of tires, while the vertical dropouts are limited to a certain region, and it is not possible to choose different sizes and width of tires for vertical dropouts, so you can easily install and replace your rear wheelsets because there is a certain distance between the seat tube and the horizontal dropouts, this distance also plays an important role in maintaining body weight of the rider and the center of gravity is not affected. if your bike frame has horizontal dropouts then you have extra chances of installing different sizes of tires without any issues.
Horizontal dropouts contain adjustment screws which are used to adjust the placement of the rear wheels and you can easily maintain the distance of the rear wheel from the seat tube and from the crankset and also helps a lot in maintaining the chain lining.these are old traditional discovery , nowadays new bikes are manufactured with the vertical dropouts, and these horizontal dropouts are not widely available , because the dropouts also have disadvantages for modern bikes and new hard terrains, their are many mountain bikers and road bikers which have their normal life routine of increasing bike speed and also increases pedal striking, due to which these are continuously pushing the pedals with a larger force and mostly on uphill and other hard trails, their are more chances of removing and pulling of the rear wheel from the horizontal dropouts , because the shape and designing of these dropouts are not able to adjust the rear wheel within the dropouts , as a result the rear wheel will come off from bike frame which is dangerous regarding accidents and crashes.
Horizontal dropouts are not made for QR screwers but nowadays almost every bike wheel is installed and uninstalled with the help of QR screwers these horizontal dropouts contain thru-axles that are tightened by nut bolts on the left and right side of the wheel, so using these QR screwers in bike frames also decreases the value of the horizontal dropouts.
Horizontal Dropouts Are Aerodynamic:
Horizontal dropouts are well known and are famous for their aerodynamic properties, the distance between the seat tube angle and the rear wheel is smaller than the vertical dropout wheels, and the experiments have proved that the dropouts are specially made more aero and the drag force of the air does not affect much on these bikes, the main examples of horizontal dropout bikes are TT bikes and Triflow bikes.
How do Horizontal Dropouts Improve Handling?
The direction and facing of these dropouts are in the backward direction and these dropouts have extra space and room to bring your rear wheel in the forward direction when you bring your rear wheel near the set tube and frame of your bike, it will improve your handling on both hard trails and road trails, it is the reason mostly riders install their rear wheels near the seat tube for better control and grip on their bikes.
Rear Cogs And Cassettes Of Wheel:
Horizontal dropouts would also play a major role in adjusting and maintaining the bike chain on the teeth and cogs of the cassette, if the chain is not shifting properly from lower to higher and higher to lower teeth of the cassette then you can easily move and change the position of the rear wheel by bringing back and forth within the horizontal dropouts. it will improve the shifting of the chain and also you can easily change the position of the cassette of the rear wheel, but these things are not possible on vertical dropouts.
Horizontal Vs Vertical Dropouts?
Vertical dropouts are much better as compared to horizontal dropouts, the reason is that vertical dropouts have a downward direction and a s single place of room for thru-axles and QR skewers to hold the wheel in a single place firmly, due to which rear wheels are not able to move and change their direction and dimensions along with the hub axles, but horizontal dropouts have backward direction and the axle is free to move back and forth if you are pedaling hardly or you are climbing through hard terrains, it is the reason that bike industry has upgraded and improved from horizontal to vertical dropouts.