Your bicycle's fork corresponds to the geometry of the frame, the wheel size of the bike and its intended purpose. Some bikes, like a downhill mountain bike, are designed with a longer fork to absorb larger impacts during this style of mountain biking. Forks can also come with different rakes, which describes the way the fork bends near the axle of the wheel. Choosing the right fork size is critical to preserving the handling and comfort of the bike.
Fork in the Road
Your fork can come with many different characteristics. Before choosing a fork of a given size, consider what options you'll want in your new fork. Suspension forks use two hydraulic pumps to absorb the impact and vibration from rough pavement and trails, and are common on mountain bikes. Downhill mountain bikes use an extra-long travel "rail" style suspension, which absorbs the shock of impact with much larger obstacles. Traditional forks can also come with mounts for bottle cages and racks. For racers, carbon fiber forks can reduce your bike's weight for better performance. Finally, forks can come with several different mounts for brakes. Cantilever, V-brakes and disc brakes all use different mounting points.
The length of a fork is measured from the center of the axle to the base of the crown race seat, where the fork and headset meet. This measurement can vary by a few millimeters across different brands. A fork of a longer length will decrease the head tube angle, and a fork of a shorter length will increase it. According to Damon Rinard for Harris Cyclery, a difference of less than half a degree in angle is negligible, but if your head tube angle changes by more than half a degree, you'll want to pay attention to how this affects the handling of the bike.
Rocking and Rolling
Different bicycles often come with different wheel sizes. This critical characteristic of fork length needs to be accounted for, since a fork that is too large for your bike's wheel size will negatively impact the handling of the bike. Most road bikes come with a 700c wheel, but road bikes for much smaller riders can use a smaller size like a 26" wheel. Mountain bikes use 29" and 26" wheels, and the 29" wheel size is identical to a road bike's 700c, but with a wider rim. If that's not enough, your front hub comes in different widths as well to correspond to the width of the wheel. Make sure you confirm the compatibility of your new fork and your front wheel with a manufacturer or bike professional, or test out the fit before making a purchase at a bike shop.
Getting a Handle On Things
The rake and stiffness of your new fork can affect the way the bike performs, and this can have a positive or negative affect on any length differences between your old and new fork. A fork with a longer rake will increase the handling of the bike, so you'll be able to get away with a longer fork that decreases the head tube angle without inhibiting your ability to steer. Similarly, a stiffer fork will also positively affect the steering. If your bike already handles well, you can decrease your fork length to smooth out the steering and find a happy medium.
Steer a Course
A fork nests in the headset of the bike, which in turn nests inside the head tube of the bike. This allows the front wheel to turn freely for proper handling. The length of the steering tube of the fork is intentionally longer than it needs to be on new forks. Once you find the optimal height for your handlebars using spacers and a stem, you can use a metal saw to cut the excess steering tube off. Bike shops perform this modification routinely, and can guide you in the process or trim the tube for you.