The forks are two dropouts in the frame of the bike that hold the front tire, connected at the top by a crown. Above that, the steerer tube goes up through the headset and attaches to the handlebars enabling the rider to steer the bike. An axle (called a skewer) is threaded through the fork dropouts and the wheel, and clamps the two together.
Forks have many different profiles depending on the type of bike. Road bikes generally have narrow and rigid forks, while mountain bikes often have suspension. Forks with suspension create stable traction by keeping the bike in more frequent contact with the ground. They also help protect the bike from being damaged by providing a cushion while riding, which is why they're mostly used on the rugged terrain.
Forks come in many styles, and are a built-for-purpose component critical to performance in any riding conditions.
The most complex forks go on mountain bikes. Quality of each component of the fork (spring or dampening) effects the final price, and depending on company preference and suspension details, they can get quite pricey. Assorted parts such as o-rings and boots can be purchased online for pennies. Whole suspension forks come in under $60, but high quality forks will be greater than $200, with some special performance suspension going for more than $1,000.
Let's not leave out road cyclists. Road forks (often called 'rigid forks') from the finest manufacturers can sometimes be made of 100% carbon composite and alloys. On average they're around $350, but some nice performance models can exceed $1,300. The forks are an important part to the structure of the bike, with qualities like the rake that have a small margin of error to make the difference.
To swap your forks, you will need these tools:
Let's get started:
Put the bike securely on a repair stand.
Remove the front wheel and disconnect any brake lines
Remove the top cap on the headset and loosen the stem bolts.
The old steerer tube should slide out, but it is very important to maintain the order of the headset components. The old forks should be free and clear of the frame.
Take the crown race from the old set of forks using the Crown Race Puller. It will need to be inserted on the new forks to give bearing to the headset when they are installed . Misuse of the crown race tool can damage the forks or the crown race itself.
Steerer tubes are manufactured long enough to fit any custom build imaginable. They are expected to be cut to the right length.
With the bike still on the stand, assemble the headset again by putting the new forks though the head tube. Put on the crown race, the spacers and the stem. With everything connected, mark with a pencil or sharpie where the excess steerer tube begins above the stem. The cut will be 4mm below this mark to make sure the steerer tube fits within the stem reasonably. Remove the forks and prepare to cut.
Secure the forks on a workbench, and measure and make a new mark 4mm below the current line on steerer tube.
Using the hacksaw and sawguide, cut the forks 4mm lower than the mark made. File the rough edges after the cut for a professional finish.
With the forks now cut to fit your bike, the final piece is to install the star nut (on threadless headsets). This is what the top cap screws into to keep the forks from falling out the bottom. Secure the forks a repair stand. Park Tool makes a good installation tool, but for those of us on a budget, you can run a screwdriver through the center of the star nut, and lightly hammer it down into the steerer tube about an inch. There isn't a standard distance the star nut needs to be, just shallow enough for the the top screw to grab on to it.
Install the steerer tube through the headset, apply the spacers and the stem, and screw down the top cap to secure everything in place.
Replace the front wheel and the brake lines, and the job is complete.
Obviously the fork dropouts need to be deep and wide enough to accommodate the right wheel. The arms should have the right amount of rake for the frame.
Bike forks are designed with something called rake. Basically the forks are curved forward to counteract the drag received while riding. This aids the bike in keeping sharp steering as the forks keep the wheel pushing ahead. It is measured as the inline difference between the center of the steerer tube, and the center of the skewer hole in the dropout.
Most road racing bikes have a rake offset of 40-45mm. Large wheels often require a larger rake offset.
Steerer Tube dimensions
Steerer tubes can be different lengths to meet desired handlebar heights on both threaded and threadless headsets. They can't stand higher than the headset stack, and are therefore cut to the proper height upon installation. There are numerous adapter kits for smaller diameter steerer tubes fitting into different headsets.
A major aspect in mountain bike forks is the suspension. Telescopic, shock absorbing forks emerged in the early 90s, and technology has improved to today's space-age, ultra-sensitive suspensions systems we know and love. Suspension forks usually consist of a spring and a damper. The spring can be any mechanism (metal coil or even compressed air) that provides a varying level of stability to the fork. The damper could be a series of valves which oil passage is limited, providing the suspension with a softer bottom-end during high stress impacts.
Some relevant suspension terms: