If it is time to replace your fork there are some aspects you should consider before buying a new one.
A first advice we can give you refers to the use of your new fork. Where do you plan to bike? Will the terrain be varied or very uneven? According to the difficulty of the terrain you will have a range of different forks available. Producers differentiate them as follows: 80 – 100 mm – cross country and marathon, 120-150 mm – all-mountain, 160-170 mm – enduro, 180 mm – freeride and 200 mm – downhill. Forks for downhill are the ones that are built differently compared to the ones for easier terrain as they use a double crown. Along with this you can opt for the width of the arms: 28, 30, 32, 34, 35, 36 or 40 mm. The thicker the arms are the more rigid will the fork be (but also heavy). For example it is not a good idea to install a 160 mm fork on a Cross Country bike. The headtube angle will become extremely small, the handling of the bike will change radically and you also risk breaking the frame in the area of the headtube.
Do not forget to check if the fork you want to buy is compatible with V-Brakes or hydraulic brakes depending on the system you use. Along with this if you use hydraulic brakes check if the mounting system is I.S. or PostMount because you may need to separately buy an adaptor for the correct mounting of the brake system.
Last but not least check what kind of Quick Release your fork has: the classic Quick Release (9 mm), QR 15 or QR 20? This QR should be compatible with your wheel’s hubs.
Once you have decided what kind of fork you want verify the size of your headtube again. In most cases the headtube is 1 1/8 inch (44 mm). Old bikes (before 1995) or low cost ones have a 1 inch headtube. You may also have a conical headtube 1 1/8 inch upper and 1.5 inch lower size or the oversized headtube which is 1.5 inch in diameter.
On a conical or oversized headtube frame you can mount a 1 1/8 inch fork. In order to do this you will have to use special adaptors which you can find in bike stores at cheap prices.
If you have a frame with a 1 inch headtube it will be impossible to mount a fork with a 1.5 inch steerer. Lately sets of headsets which allow mounting a conical steerer on a frame with a 1 1/8 inch headtube.
This being said let us now show you the actual mounting operation.
Step 1: the new fork will have a long steerer which you need to cut. Here you have more possibilities. You can use the same length as that of your old fork or you can cut it longer or shorter according to the position in which you want the handlebar to be in. We do not recommend you to cut the steerer shorter since it will not fit all bikes afterwards in case you will, at some point, decide to sell your fork. Mark the place where you want to cut the fork.
Step 2: For more precision use a special tool for cutting pipes. You can also use a hacksaw but it will be much harder to cut the steerer perfectly straight. If you feel capable use the hacksaw.
Step 3: Fix the cutting tool while tightening the black handgrip.
Step 4: Rotate the tool around the steerer while tightening the handgrip of the tool bit by bit.
Step 5: Once cut use an abrasive brick tool to soften the edge of the steerer.
Step 6: In order to fix the fork you will need to introduce a nut inside the fork. It is a component that is fixed on the interior of the steerer and that has a thread in the middle in which the cap’s screw will come in after mounting the stem. It must be inserted in the interior by hitting it in case you do not have a special pressing tool for this. It comes with the cap or separately.
Step 7: In the end you should also mount the inferior sliding way for the bearings. There is a special tool for this as well because it needs to be introduced at a straight level on the base of the steerer. Alternatively you can use a pipe that has a similar size to the sliding way. Do not forget to lubricate the zone where the sliding way will meet the steerer: it will help it enter more gently.
All that is left is to introduce the fork in the headtube and mount the stem. After tightening the headset’s cap verify if there is any play in the fork or if the cap is too tightened and the fork does not move easily.