Your one-stop shop for everything you need to know about dropouts 

By Oscar Huckle

Published: Thursday, 18 July 2024 at 11:00 AM


The dropouts are a fundamental part of a bicycle frame, enabling the wheel axles to be secured. 

A bike has two pairs of dropouts – one at the fork for the front wheel to attach and another at the rear, connecting the frame’s seatstays and chainstays. 

There are different styles of dropouts to suit different types of bike, each of which has pros and cons, and there are reasons why you a frame designer may choose one over another. 

In this guide, we’ll take you through the common bicycle dropout types, as well as their features. 

Bicycle dropout types 

The axle needs to be slotted into an open dropout. – Scott Windsor / Our Media

While there are four dropout types, the most simplistic way of splitting them is between open and closed. 

An open dropout relies on a quick-release or bolted axle, enabling the wheel to slide under or outside of the dropout. 

SRAM UDH hanger removal
This is a closed dropout. – Gary Walker / Our Media

A closed dropout houses a specific insert that the axle threads into. The axle needs to be fully removed for the rear wheel to slide out of the frameset. 

There are some rare exceptions, such as Mavic Speed Release, where a brand will spec an open dropout on the opposite side of the thru-axle insert to speed up wheel changes. 

Vertical dropouts

Vertical dropout on old commuter bike
The wheel installs on a vertical plane. – Scott Windsor / Our Media

On a vertical dropout, the wheels are installed by securing the axle in the dropout, which faces downward.

Unlike a horizontal dropout, the wheel can only be secured in a single position.

Compared to a horizontal dropout, a vertical dropout relies on the gears being set up properly with the wheel’s positioning. If the wheel is installed wonkily, the gears are unlikely to shift smoothly. 

Semi-vertical dropouts 

This style is found on most bikes that use a quick-release skewer. – Oscar Huckle / Our Media

Similar to a vertical dropout, a semi-vertical dropout has more of a cant to it and is the most common type found on many modern quick-release bikes. 

They function in exactly the same fashion as a vertical dropout. 

Horizontal dropouts 

Horizontal dropout on old commuter bike
The axle needs to be adjusted correctly on a horizontal dropout to ensure optimal chain tension. – Scott Windsor / Our Media

Horizontal dropouts tend to be found on older bikes, but also on time trial bikes, fixies and singlespeed bikes – as well as kids’ bikes

On a horizontal dropout, the wheel is installed on a horizontal plane into or away from the frame, rather than on a vertical plane. 

You can position the axle at different points in the dropout to adjust chain tension. This is important for fixed-gear bikes, where the chain isn’t tensioned by a rear derailleur.  The fact you can make adjustments to the chain’s tension means a horizontal dropout enables you to convert your bike to singlespeed or allow for chainstay length adjustment. 

You can also adjust the position of the axle in horizontal dropouts to tweak your bike’s wheelbase. A longer wheelbase will make a bike more stable, whereas a shorter wheelbase quickens reactions. 

Trek’s Stranglehold dropouts enable you to adjust the effective chainstay length or run a bike as a singlespeed. – James Huang / Immediate Media

Horizontal dropouts can also be closed, such as Trek’s Stranglehold system, which allows for 15mm of adjustment. 

The disadvantages of a horizontal dropout are that it can slip under heavy load and they’re slightly more finicky to remove and install a wheel into. 

Thru-axle dropouts 

Thru-axle dropouts on the 2022 Vitus Venon prototype road bike
Thru-axle dropouts are the new kids on the block. – Russell Burton / Our Media

A thru-axle dropout is a fully closed structure with a threaded insert bonded into the dropout. The insert is what the thru-axle threads into. 

Thru-axles were first introduced on mountain bikes to enable a much more stable and rigid connection between the wheel and dropout, with a significantly reduced chance of it loosening and falling out on a ride compared to a quick-release skewer. 

It also means the wheel is always installed straight and there’s less chance of your disc brake rotor rubbing on the brake pads

Not only have they become one of the main mountain bike axle standards, over time they’ve become the de facto road bike axle standard too. 

Bicycle dropout features

There’s more to a dropout than meets the eye – here are some of the common features you may find on them. 

Lawyer lips 

Lawyers lips on fork dropout
Note the tab on the inside. – Scott Windsor / Our Media

Lawyer lips (also known as lawyer tabs) are tabs on the inside face of both dropouts that protrude slightly. 

These are designed to stop the wheel from falling out if it isn’t secured properly and catch the end of the skewer. 

Eyelets 

Niner RLT 9 RDO dropout with Campagnolo Ekar components
Note the eyelet above the dropout. – Oscar Huckle / Our Media

Some dropouts feature eyelets, so you can mount panniers, mudguards or even bike trailers

These are often found on bikes designed around versatility, such as touring bikes or gravel bikes

Derailleur hangers

Jack Luke holding a derailleur hanger
This little part plays a crucial role in the quality of your gear shifting. – Jack Luke / Our Media

If your bike has gears, there will be some form of derailleur hanger for the rear derailleur to attach to. This is what enables the gears to shift optimally. 

On some bikes (primarily older ones), the hanger forms part of the dropout. 

The problem with this is that if the dropout bends, the frame may be compromised. You can try to gently bend it back using heat, but the dropout will be weaker in structure than it was before. You have to be very careful with how much heat you apply to avoid misshaping the hanger, so there’s an element of risk. 

Horizontal dropout on old commuter bike
The axle needs to be adjusted correctly on a horizontal dropout to ensure optimal chain tension. – Scott Windsor / Our Media

On the vast majority of bikes now, though, the derailleur hanger is a separate piece that installs into specific holes fashioned into the dropout. The derailleur hanger is specific to your frame, and the number and/or positioning of the holes differs between brands. 

With this design, the derailleur hanger is a replaceable and sacrificial part that’s designed to bend or break in an impact. The rear derailleur simply attaches to it. 

SRAM tried to unify derailleur hanger standards when it introduced its Universal Derailleur Hanger. Many mountain bikes and some gravel bikes now use this standard. 

Chain tugs

Chain tugs are sometimes used to prevent the rear wheel slipping and keep the chain tight on horizontal dropouts. Examples include the Surly Hurdy Gurdy chain tensioner. 

Adjustment screws

Note the adjustment wheel on this BMC Timemachine. – James Huang/BikeRadar

Some dropouts feature adjustment screws, mainly on horizontal dropouts, and this is to set the wheelbase (the distance between the wheel axles) longer or shorter. They can also be used to install wider or narrower tyres.

You set them to a certain position so you don’t have to reset the wheel’s position each time. 

Flip chip dropouts 

FiftyOne Assassin against a pink background
There are three positions on this dropout to choose from. – Oscar Huckle / Our Media

Some bikes feature flip chip dropouts, which are common on mountain bikes and even some gravel bikes.

These enable you to alter the bike’s geometry, simply by turning the dropout the opposite way to increase or decrease the bike’s wheelbase. 

Brands go about this in different ways, so there can be some subtleties between manufacturers. 

On some designs, the brake caliper has a sliding mount so you don’t need to realign your brake each time you change the flip chip’s position. 

Dropout adaptors 

A dropout adaptor enables you to convert between different designs. For example, you may have a bike with a vertical dropout but want to convert it to a fixed gear, so you’d require an adaptor with a horizontal design.