Condensed answer: Some time trial (TT) bikes use rear-facing horizontal dropouts so that the rear wheel can be as close as possible to the seat tube for better aerodynamics.
Modern TT bikes have a cutout in the seat tube allowing the rear wheel to be maximumly close to the bottom bracket. The purpose of this engineering is to make the bike more aerodynamic, stiffen the frame and maximize the power transfer to the rear wheel.
If the classic vertical dropouts are kept, the rear wheel can’t be as close to the frame because it’s removed and installed vertically – a procedure demanding more clearance.
Horizontal dropouts, on the other hand, allow minimal distance between the tire and the frame because the wheel is removed by pulling it back and installed by pushing it forward. Both motions are horizontal and thus do not require as much clearance as a vertical system.
Another advantage of horizontal dropouts is that the bike can have maximally short chainstays.
Shorter chainstays keep the frame stiffer and increase the power transfer to the rear wheel.
Chain Tension Adjustment
Horizontal dropouts allow the user to move the wheel forward and back to increase or decrease the chain tension.
This option matters only to people who want to run the bike in a one-speed mode which is pretty rare for a TT bike.
The Downsides Of Horizontal Dropouts
- Wheel Security
During hard braking, there’s a chance for the rear wheel to come out if it’s not fully secured.
When the rider presses the brakes, the arms grab the rim at the chainstay bridge and act as a pivot point around which the tire wants to rotate due to the forward inertia.
During extreme braking, the force may be strong enough to overcome the clamping force of the quick-release, and the wheel may come loose or even escape from the dropouts.
Hence why it’s very important to make sure that the wheel is secure.
Track bikes and fixed-gear models do not experience this problem because they use track ends (deeper rear-facing horizontal dropouts) coupled with bolt-on axles rather than quick-release skewers.
The nuts on the bolt-on axles bind a lot stronger than a quick-release and stop the wheel from getting out of the dropouts.
The chain on track and fixed-gear bikes is also holding the wheel in place. If the chain is tensioned properly, it won’t be long enough to allow the wheel to completely come out.
- Tricky Rear Wheel Installation and Removal
Many people say that it’s more difficult to remove and re-mount a rear wheel on a bike with horizontal dropouts because the wheel has to be pulled back, and the chain and the quick-release skewer are often getting in the way.
With practice and a few technique tweaks, the procedure becomes almost as simple as it is with vertical dropouts.
The removal process is as follows:
Step 1: Put the chain on the smallest sprocket at the back and on the smallest chainring at the front to create as much chain slack as possible.
Step 2: Open the quick-release of the brake.
Step 3: Remove the quick release altogether.
Step 4: With one arm, rotate the derailleur’s cage forward towards the bike to create even more chain slack.
Step 5: Pull the wheel back with the other arm to clear it of the dropouts.
Step 6: Move the wheel forward and under the dropouts to get it free from the chain.
Step 7: Move the wheel away from the bike.
Note: Another problem that may occur is a non-centered wheel that rubs against the frame or the brakes. This happens because horizontal dropouts do not have a fixed place for the wheel, unlike vertical dropouts.
To avoid this scenario, push the quick-release forward towards the bike until it touches the pre-set screws of the dropouts which are there to ensure that each side of the axle is at an equal distance from the frame.
FAQ: Do all TT bikes have horizontal dropouts?
No. There are many models with vertical dropouts for easier wheel changes.