With one-magnet cyclecomputers, you usually have a choice of several places on the wheel to attach the spoke magnet, depending on the spoke pattern of the wheel. Generally, it is best to mount the magnet as close in toward the hub as possible. The closer in you mount it, the more slowly it will pass by the sensor, giving the sensor's magnetic switch more time to respond. If the magnet is too far out, the computer may give erratic readings at higher speeds.
Once the magnet is installed, attach the sensor to the fork or stay. Test the computer and make any needed adjustments to sensor or magnet position before securing the wire or attaching the mounting shoe to the handlebar.
Most cyclecomputers come with plastic tie-wraps to secure the wire to the frame. These work O.K., but they are very unsightly unless they match the color of the frame. You can do a better-looking, more professional job if you secure the wire with transparent plastic tape. The best I have found is clear mylar package-sealing tape. It is commonly available in 2-inch wide rolls, usually with a handy dispenser/cutter. Make sure the relevant parts of the frame and fork are clean, and wash your hands before taping the wire down.
Once the magnet and sensor magnet are installed, turn the bike upside down in the repair stand and remove the front wheel. This will make it easier to secure the wire.
Front-Mount Cable Routing:
The wire should run up the back side of the fork blade, slightly to the inside. This is not only more aerodynamic, but it helps keep the wire inconspicuous. If the sensor is mounted in front of the fork blade, make sure that the wire crosses back on the inside of the blade. Leave a very small amount of slack were the wire leaves the sensor to where you begin to secure it to the fork, just in case slight sensor adjustments become necessary later.
The most common mistake in wire routing is to attach the wire to the head tube of the frame. This should never be done with a front-mount cyclometer, because you then have to allow two large loops of slack where the wire enters and leaves the head tube so that it won't get tugged on when the handlebars turn. The wire should always follow the front brake cable. Since the fork, brake and handlebars always move together, the wire can be secured along its full length.
Usually, I prefer to run the wire up along the back side of the brake cable, securing it to the cable with clear tape. If the wire is longer than it needs to be, the excess can be bundled up and tucked into the bottom of the steerer tube.
For bikes that are likely to be overhauled often, it is more convenient to wrap the wire around the brake cable in a spiral. This makes it easier to disconnect the wire for headset service, handlebar swaps, etc. The downside of the spiral wrap approach is that is can be unattractive, especially on bikes with bright-colored brake cable housing.
When you install a front-mount cyclometer on a bike with cantilever brakes, install the magnet and sensor on the left side if possible. This allows you to run the wire out along the back of the cantilever and in along the left side of the transverse cable to the main cable. Since most front cantilevers have the transverse cable anchored on the left side, this will still allow the transverse cable to be unhooked to release the brake for wheel changes.
Suspension forks actually are easier to install cyclometers on. Run the cable up the slider to the brake bridge, then follow the cable housing to the handlebar.
The sensors from some cyclecomputers won't fit the large-diameter legs of suspension forks. If you have a box of old junked front derailers, you were right not to throw them away. Many older model front derailers were secured to the frame by two bolts running through a "pillow-block" type clamp, as opposed to the hinged clamp used on modern units. Two of these clamps back-to-back can make a very solid, elegant- looking clamp to hold the sensor to the fork blade. If the fork blade is less than 1 1/8" in diameter, you will have to use a small shim.
If that is all too much trouble for you, the easy way to install a cyclecomputer on a suspended bike is with a rear-mount kit.
Rear-Mount Cable Routing:
Wires from rear-mount cyclecomputers or cadence pickups should be run under the chainstay, under the bottom bracket and under the down tube to keep them out of sight.
For mountain bikes or hybrids, secure the wire to one of the gear-shift cables running from the down tube to the handlebar-mounted shift lever.
For bikes that don't have handlebar-mounted shift levers, it is usually best to run the wire from the down tube directly to the bottom of the front brake cable, then follow up as with front mounts. You must allow sufficient slack to allow the handlebars to turn as far as they can possibly go in both directions without tugging on the wire, or the wire will surely break.
The slack loop must be kept clear of the tire. If the wire can possibly touch the tire, it will, and it will wear away to nothing in no time at all.
The slack loop should be kept where it will not get in the way of normal operation of down-tube mounted shift levers, or the rider may accidentally yank on the wire while trying to shift.