1994 Gan Lemond Team bike with Zap
In it's stock form, the Zap Microprocessor unit has two switches/controller units attached to it. Mavic recommends that when installing the switch that resides on top of the handlebars near the stem, it be mounted with the shift buttons facing the front of the bike, as shown in the image to the left

Personally, I have found that if the shifter is turned 180 degrees so as to face the rider, it becomes much easier to operate the shifter, using the thumb on your right hand to effect the gear change. In this position, the left button will shift up the cluster (towards the bigger cogs), and the right button down the cluster (towards the smaller cogs). 

In stock form, I believe the shifter that is meant for positioning below the right side brake lever works very well. In a sprint situation, it is quite simple to grip the handlebars, and move your right hand index finger to effect the gear changes down the cluster, all while throwing the bike from side to side during the sprint. At those times when you're out of the saddle and your hands are gripping the brake hoods, it is relatively simple to get to the shift buttons with a spare finger on your right hand. Which finger you use is personal preference.
Zap Toggle Switch Shifter
Most of the pro teams who used the Zap system on their bikes, had a spring loaded toggle switch attached to the back of the right side brake lever, as shown in the image to the left.

Presumably, this switch was mounted by either drilling a small hole through the lever, or epoxying it to the back of the lever, and running a cable through the inside of the brake hood. I believe the idea of this was to prevent accidental shifts that are possible if the stock shifter is used.

Placement of the original shifter design on the handlebar is very important if the rider wishes to prevent accidental shifting. It is worth taking the time to ensure the cable installation is done correctly.

Although I have not used the toggle switch setup, it is my opinion that in a sprint situation, when your hands are on the handlebar drops, it would be nowhere near as as accessible as the stock shifter. Personally, I would find reaching for a toggle switch difficult. For me it would be as difficult as trying to shift gears in a sprint with a STI setup. Perhaps I just don't have the co-ordination for it!
Stock Zap Shifter
Adding additional cable length to increase the distance between the microprocessor and the shifter, or for adding additional shifters, is a relatively simple procedure to perform. Note however, that making modifications to the stock cabling setup is done at your own risk.

You will need the following:

(a) Soldering Iron and Solder.
(b) Cable Cutter.
(c) Length of 2-wire electrical cable, ideally similar in size to the stock shifter cabling.
(d) Electrical Wire Stripper.
(e) Heat Shrink Plastic Tubing.
(f) Heat source to shrink tubing.
(g) Optionally, a Circuit Tester to determine which shifter wire shifts up/down.
- The Steps -
1. As the shifter that is normally mounted on the top of the handlebar is furthest away from the microprocessor, this is the most logical point to perform a cable extension, or add an additional shifter. Using a wire cutter, cut the shifter cable approximately 4 - 5 inches away from the shifter itself.
2. Strip the wire ends of both the separated shifter, and the cable that it was previously joined to.
3. For a cable extension, perhaps for a Tri-bar mounted shifter, work out approximately how much extra cable you will need by holding the cable in place alongside the handlebar, and roughly determine it's intended route. Cut to the desired length, remembering to allow for slack, handlebar rotation, etc.

If adding an additional shifter, the new shifter's wires will need to be matched up with those of the separated shifter, and twisted together. i.e. match up the blue wires, and twist the bare wires together to form one. Repeat for the white wires.

4. For a cable extension, make sure the piece of cable you are adding has been stripped at both ends. Before soldering, remember to place two pieces of heat shrink tubing somewhere along this new cable. Simply solder the shifter's white Zap wire to one of the wires on the new cable, ensuring that this wire matches up with the white wire that leads to the microprocessor. Solder this together also. Repeat this process for the blue wires. Using your heat source of choice, shrink the heat shrink tubing over the joins. You now have an extended Zap shifter.

For an additional shifter, the cable extension part of this step will apply. Brieftly, take the new shifter and the separated one, making sure that their wiring is twisted together where they join. Solder this to the shift cable that was cut in step 1. Remember to match up the coloured wires, and add the heat shrink wrap for strength. You now have an additional Zap shifter. Due to the parallel circuit that the Zap controls use, it possible to add as many switches/controls as you desire.

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