Frame & Fork
The frame is the main and most important component of a bike as all other components are attached to it. The vast majority of frames follow the standard diamond frame design comprising:
- Head tube - This is at the front end of the frame and houses the headset, which contains the bearings for the fork.
- Top tube (cross-bar) - Traditionally, the top tube is horizontal and parallel to the ground. In a compact-geometry frame and mountain bike frames, however, the top tube is sloped downward towards the seat tube for greater comfort. Step-through frames have a top tube that slopes steeply allowing the rider to mount and dismount the bike more easily.
- Down tube - This connects the head tube to the bottom bracket shell.
- Seat tube - This contains the seatpost and saddle whose height can be adjusted by altering how far the seatpost is inserted down into the seat tube.
- Chain stays - These connect the bottom bracket shell to the rear fork ends and run parallel to the chain.
- Seat stays - These connect the seat tube to the rear fork dropouts. A dual seat stay meets the seat tube at two separate points, side by side, whereas a wishbone or single seat stay joins the stays together into a monotube which in turn attaches to the seat tube.
- Bottom bracket shell - This holds the bottom bracket and connects the chain stays, seat tube and down tube.
The fork consists of two fork ends which hold the front wheel in place, two blades which join to the fork crown which, in turn, attaches to the steerer tube, held in place in the frame by the headset. Handlebars are attached to the steerer tube which allows you to steer your bike.
Suspension forks are now commonplace on mountain bikes and available on some hybrid bikes. Here are three important key terms to consider:
(1) Travel - this is the amount of movement in the suspension fork when pressure is applied. Travel can be externally adjusted by turning a knob on top of one of the legs on more expensive forks. In mountain biking greater travel is beneficial for riding downhill, but less for climbing.
(2) Rebound - this is the rate at which the suspension fork bounces back after it has been depressed. This can be adjusted on most forks to alter the feel of the bike.
(3) Lockout - this turns off the suspension so there is no travel whatsoever. Some mountain bikes do this for road use or when climbing as it can make the bike more efficient.
The shock absorber or rear suspension shock has two main functions:
it enables the rear wheel to move up when it hits something, therefore reducing the impact felt through the bike. It achieves this by use of a coiled spring (for big hits on downhill mountain bikes) or compressed air (XC mountain bikes and hybrids).
it can dampen the ride by controlling the speed of axle movement of the rear wheel at the stage when it takes the hit and when it rebounds to its original position.
Many shock absorbers come with rebound control and lockout, as with some suspension forks.