Bicycle Forks
Bike forks are either rigid or suspension. This article deals with issues common to both types. Suspension forks are complex and so varied that it is impossible to address specific issues, but we plan to publish a helpful article later.
Types of Forks
Forks are classified by how they connect to the stem--either a threaded or threadless system.
Threaded forks are found on older bikes, cheaper new models, kids' bikes, and cruisers. The top of the fork steering tube is is threaded. The top headset bearing race is threaded onto the fork, cupping the top ball bearings. Then a locknut is threaded onto the fork, usually with a lock washer in between. The fine adjustments to the tension on the headset bearings are made with the top bearing race, and then the locknut is screwed against the top bearing race to hold everything together. The stem is placed into the fork steering tube and held in place with a long chock nut.
Threadless forks are found on most newer good quality bikes. The fork steering tube is not threaded. The headset top bearing race is slipped onto the fork and held in place by a plastic compression ring. The stem and spacers are slipped on the fork. A top cap is placed on the stem, and a compression bolt is placed through it and into a star nut that has been forced into the interior of the fork steering tube. The fine adjustments to tension on the bearings are made with the compression bolt. Then the stem is tightened onto the fork steering tube.
Threadless systems are superior. They are lighter, easier to adjust, and hold their adjustment much, much better.
A threaded fork should not be used with a threadless headset, unless you're willing to assume some risk in return for saving the cost of a new fork. A threadless headset will not hold onto a threaded fork as securely as a threadless fork.
Fork Parts
Rigid fork parts include:
steering tube, that fits through the frame head tube
crown race, where the bottom part of the headset fits
crown
blades or legs
brake bosses
dropouts, where the wheel axles attach
Fork Sizes
There are 4 fork sizing issues:
Steering tube diameter. Steering tube diameter may be 1" (many road bikes), 1 1/8" (the most common size on all bikes), or 1 1/4" (used for a few years in the mid 1990s on mountain bikes).
Steering tube length. Steering tube length is measured in millimeters from the crown race to the top of the tube. When installing a fork, the steering tube is cut to length to fit the bike's head tube, stem, and headset. Threaded forks must be purchased in the right length, so the fork threads are exposed at the top of the bike's head tube, where the headset parts need to screw on.
Wheel size. Fork legs must match the wheel size, commonly 26", 27", 650C, or 700C on adult bikes.
Brake boss fittings. Forks must have brake fittings that are right for your bike. Cantilever and V brakes use the same fittings--bosses on the brake legs. Road brakes connect through a hole in the fork crown. Forks for the different wheel sizes space the brake connections for the specific wheel size, so the fork has to be correct for your brakes to fit.
Fork Damage
Riding a bike with a loose headset can result in damaged steering tube threads and a ruined headset. Many professional bike shops can repair damaged fork threads.
It is important to often check for a loose headset.
Fork blades can be bent, especially cheap carbon steel forks. Some professional bike shops can bend steel rigid forks reasonably straight and realign fork dropouts. Bent aluminum forks are dangerous and should be replaced. It is dangerous to ride on a bent fork.
Cracked carbon forks are very dangerous. Carbon forks can suddenly fail. Carbon forks should be closely examined often. If a carbon fork the smallest of crack, it should be replaced.
Most fork issues are really headset problems.
Install a Fork
To replace a fork, follow the procedure to replace a headset. For detailed headset photos see Headset Parts.
Additional tools you will need to cut the fork steering tube are:
hacksaw
preferably a steering tube cutting guide, to make a square cut
preferably a vise
star nut setter
hammer
Park Cutting Guide and Star Nut Setter
The procedure to replace a threadless fork is:
1. Follow the procedure to replace a headset.
2. Install the fork crown race on the new fork with a fork crown race setter (step 10).
3. Complete steps 11 through 15 for replacing a headset. You now have the fork in the bike's head tube and the headset installed.
4. Prepare to measure where to cut the fork steering tube. Slip all the spacers you intend to use on the fork, and then the stem. You can use as many spacers as you want to achieve an upright riding position. Once you cut the fork, you can never make it longer, but you can cut it shorter, so err on the long side with more spacers. Draw a line around the steering tube at the top of the stem.
While working on a fork on the bike that is not yet connected to the stem, hold it on the bike with a big rubber band run between the fork legs and over the bike's top tube.
5. Take the fork out of the bike. Think about something else for a minute. Stick it back in and repeat step 4--"Measure twice, cut once." Cutting a fork too short is a major screw-up.
Before the measurement, did you:
install all the headset parts, top and bottom, including bearings
install all the spacers you want to use
install the stem?
6. Put the fork in steering tube cutting guide, and hold the guide in a vise. Cut the steering tube 2-4 mm below your line. You want the top of the steering tube to be slightly below the top of the stem, but not far. The top of the steering tube should never be below the top stem bolt.
You can cut a fork without a cutting guide, you just won't do as good a job.
7. File off any burrs.
8. Install the star nut in the fork. (Note the star nut has to be the right size for the diameter of the steering tube.) Thread the star nut on the star nut setter, with the concave side pointing toward the setter. Put the star nut over the top of the steering tube, rest the fork legs on the work bench, and hammer the star nut in with the setter.
9. Continue the procedure to install a headset.
