Frame parts explained – Anatomy of a bicycle

Last Updated on May 1, 2022 by FAB Jim The Cyclist

The frame is the core of every bike and brings everything together.  While frames may have a different shape, they all have the same parts.

Head Tube

The head tube is the short tube at the front of the frame.  It is used to connect the handlebars/stem and the fork.  A headset is used inside the tube which is basically the bearings which allow free movement. 

Top Tube

You step over the top tube to get on the bike, sometimes it’s called the crossbar.  It connects the head tube to the seat tube.  This tube adds a lot of strength to the frame.  Depending on the type of bike it may be parallel to the ground, angled or very curved for some ladies’ bikes.  Most older bikes have a tube which is parallel to the ground, while newer bikes tend to be more angled.

Down Tube

Connects the head tube to the bottom bracket shell, where the crank/pedals are.  You will often see a water bottle cage mounted to the down tube.    

Bottom Bracket Shell

This is where the bottom bracket is installed.  The bottom bracket is used to install the crank arms.  There are many types of bottom brackets, but basically it holds bearings that allow your crank arms to move freely.  They are usually threaded in, but sometimes pressed.

Seat Tube

The seat post is inserted into this tube.  It is central to the bicycle frame.  At the top it connects with the top tube and the seat stays.  At the bottom it connects to the down tube and the chain stays.  The top of the seat tube will have a post clamp which hold the seat post in place. 

Seat Stays

These run from up near the top of the seat tube down to the dropouts.  There are two, one on each side of the wheel.  The brakes will be attached to the seat stays.  Disc brakes attach to one of them at the bottom, or rim brakes attach near the top above the tire.

Chain Stay

Run from the bottom bracket shell back to the dropouts, they follow the route of the bike chain.  There are two, one on each side of the wheel.

Dropouts

The dropouts are the slots in the frame where the rear tire axle is inserted into.  The come in different shapes depending on the type of bike.  Sometimes there is a derailleur hanger or a place to mount one.  Dropouts for derailleur bikes typically are more vertical to the ground, while single speed bikes have more horizontal dropouts. Some like my Surly are in-between so they can be used for either. 

Material

Early bikes were mostly made of steel.  Chromoly is one of the more common types of steel used.  Titanium has been used a little as an option that is lighter than steel, but with similar ride qualities.  In the 90s aluminum became popular for making lighter frames.  Most newer frames will likely be aluminum.  Now the lightest frames are mostly made from carbon, but they are more expensive.

Tube Length and Angles

The tubes on the frame come in all different lengths and angles.  The length mostly effects the size of the bike, while the angles effect how the bike rides.  A more angled head tube will generally make the front wheel easier to turn.  The seat tube angle will affect where you are sitting in relation to the pedals and handlebars.  Bikes with different angles can feel very different to ride.  You can have an upright comfort bike ride, or an aero race bike position.

Tube Shape

Early bikes had round tubes.  Modern tubes come in all different shapes for strength, aesthetics, and aerodynamic properties.   

Frame construction

Steel

Lugged – many vintage steel frames were lugged and brazed.  Lugs were used at the joints, and straight tubes would be brazed into them.

Electro-forged – Schwinn used a unique process called electro-forging.  They used a high electric current to turn the ends of tubes molten and pushed them together to weld it.  The process was much faster than conventional methods.

Welding – steel frames were also welded together.  You can sometimes see where the welds are.

Aluminum

Welded – aluminum frames are welded together.  Tubes are shaped using different methods including hydro and air forming. A nice aluminum frame can be mistaken for carbon due to all the shapes it can be formed into.

Titanium

Welded – titanium frames are welded together.

Carbon

Carbon is made into sheets which is then mixed with an uncured resin.  The frame pieces are then cut out from these sheets of carbon fiber.  The pieces are then assembled, frames often use hundreds of different pieces.  They are wrapped around bladders and pre-formed pieces.  Then they are put into molds, heat and pressure is applied.  Then they are baked to remove any moisture.  The surface then has to be prepped, pre-formers and bladders are removed.  Now the frame is machined with all the holes that are needed.  Finished parts are then combined using a carbon fiber glue.  Then the frame is prepped and painted.  So yeah, it’s all pretty complicated and time consuming.

Why did the picture go to jail?

Because it was framed…