The Rear Dropout — Not Discussed Much, But Essential

In our previous blog post, we discussed why replaceable rear derailleur hangers exist. In this post, we are going to discuss the rear dropout. Wow…another suspenseful and thrilling topic! OK…not so much. However, for all things related to replaceable rear derailleur hangers, the rear dropout is essential.

So…what is a rear dropout? The rear dropout is the portion of the frame where the rear derailleur hanger is attached. The rear dropout is the joining point (the confluence) of the chain stay and the seat stay on the rear portion of the bike frame (see picture below). There is a rear dropout for both the drive side and non-drive side of the bike.

rear dropout

The Essential Rear Dropout

On bikes that use a replaceable rear derailleur hanger (versus a frame-integrated hanger; see our previous blog post), the rear dropout on the drive side of the frame (i.e., the right side of the frame while looking forward on your bike) has a unique shape that was purposely designed to match the shape of the attachment area of the rear derailleur hanger. Below is a picture of the drive-side rear dropout (and replaceable hanger) before it has been joined to the chain stay and the seat stay (picture courtesy of Bear Frame Supplies). Notice how the rear dropout is purposely shaped to conform to the three-dimensional shape of the replaceable rear derailleur hanger.

Drive-side rear dropout and hanger before attaching to chain stay and seat stay

Once the drive-side rear dropout is attached to frame, it creates a strong and seamless unit. See the picture below that highlights the unique shape of the rear dropout as attached to the chain stay and seat stay (the white frame color) relative to the shape of the rear derailleur hanger (black in color) where it attaches to the rear dropout.

rear dropout and hanger

Therefore, one can obviously conclude that the frame manufacturer purposely selected a specific rear dropout and rear derailleur hanger combination when designing and developing the bike frame. For example, a frame manufacturer will most likely use a beefier replaceable rear derailleur hanger on a mountain bike than might be used on a road bike; mountain bikes typically come into contact with the ground more often than do road bikes, and hence the need for a beefier rear derailleur hanger. Therefore, the rear dropout must have the requisite three-dimensional shape and geometry to accommodate the desired replaceable rear derailleur hanger for that mountain bike.

As we stated in our previous blog post, a replaceable rear derailleur hanger is meant to bend or break when exposed to excessive force, and thereby ensure that the frame does not bend or break (which would be a much, much more costly repair). Hence the advent of replaceable rear derailleur hangers, and hanger-specific rear dropouts used to attach the replaceable rear derailleur hanger to the bike frame.

Hopefully, you will find this post as a useful bit of information (or trivia) about your bike, and the importance of rear dropouts and replaceable rear derailleur hangers.

As always…please let us at know if we can assist you in any way. Happy riding!

Derailleur hangers for avid mountain bikers, road cyclists and bike shop owners