The top tube is the highest piece of tubing on the frame that runs from the headtube near the stem to the top of the seat tube.
The head tube is the tube at the front of the bike where the down tube and top tube intersect. It holds the headset bearings that allow the steerer tube to move.
Seat post collar
The seat post collar is a piece of metal, usually aluminum, which wraps around the outside of the top of the seat tube and torques down to hold the seat post in place.
Bottom bracket shell
The bottom bracket shell is the connecting point for the seat tube, down tube, and both chain stays. It houses the bottom bracket and crankset.
- Seat Stays: The seat stays are two diagonal pieces of the frame that run down from the seat tube to the back of the frame. The seat stays provide rigidity to the frame and some compliance for rougher roads/terrain.
- Chain Stays: The chain stays connect the bottom bracket shell to the rear dropouts of the frame. The shorter the chain stays, the more responsive the bike will feel. The longer the chain stays the more stable the bike will feel.
- Drop Outs: Drop outs are the slots on the fork and the chain stays where the wheels fit into the frame.
- Derailleur Hanger: The derailleur hanger is a piece that connects to or is a part of the drive side chain stay. It allows the rear derailleur to connect to the frame and will shear off if the bike is put under major stress, instead of the chain stay breaking and ruining the bike.
- Fender/Rack Braze Ons: Some frames will have mounting points on the fork and/or on the rear seat stays for mounting fenders or bike racks on.
- Disc Tabs: Disc brake frames will have mounting posts on the non-drive side chain stay and on the non-drive side of the fork. These are the areas where you would mount disc brakes.
- Canti Posts: Older cyclocross or mountain bikes, or new canti-specific frames might have posts on the rear seat stays and the fork for cantilever brakes, which are like V-brakes but with larger tire clearance and better braking modulation.
A steel frame is sturdy and absorbs some vibrations when riding but is the heaviest of the three.
Aluminum frames are the most popular, they are both light and stiff, and are widely used across many bike styles. The trade-off is that you feel a bit more vibration from the terrain.
A carbon frame is the lightest of the four materials and is also the strongest. Carbon fiber is a weave of carbon that is held together with a special glue. Carbon fiber is extremely durable and light but does tend to be more expensive than aluminum or steel framed bikes.
Titanium is a frame material usually used by custom frame makers. It combines the strength and durability of aluminum with the lightweight construction found with Carbon Fiber. Titanium frames usually tend to be the most expensive.