Travel Agent Installation and Adjustment
This article will review the installation and adjustment of the Travel Agent™ brake cable system (figure 1). The Travel Agent™ is a brake cable pulley system that increases the amount of cable pull. The cable is routed around an inner pulley, and then is routed to a larger outer pulley. As the inner pulley rotates, the outer puller will travel around a greater circumference and this increases the linear distance of cable pull. While this system increases the amount (distance) of cable pull, it does not increase the total amount of work done on the cable to stop the bike.
Figure 1. Note: Travel Agent need not come apart for normal installation. This "exploded view" is included above only to show parts.
We use our hands and fingers pull the brake lever. The cable then moves some distance and with some amount of force according to the design of the lever. A lever designed to pull relatively large amounts of cable does so by sacrificing pulling force. This is because there is a trade-off between cable pulling force and cable travel.
Consider older brake levers used for cantilever brakes. The distance between the lever pivot and the cable end determine the amount of cable that is pulled. From the lever pivot to the brake end fitting is typically around 21mm (figure 2). This in effect creates a circle with a circumference of about 132mm. If the lever is pulled so it moves 20-degrees, it will pull about 7mm of cable through the housing, and this in turn pulls the arms and brake pads to the rim.
Figure 2. Flat bar lever designed for cantilever brakes
The modern linear pull levers have a pivot to brake-end radius of about 42mm (figure 3). The circumference of the circle of rotation is about 263mm. If the lever is pulled so it moves 20-degrees, it will pull about 15mm of cable. However, this hand lever is pulling the brake cable with less force. However, the lever arms of linear pull caliper brakes are longer compared to the lever arms of older cantilevers. The long caliper lever arms increase the force and decrease the distance traveled by the brake pads, restoring the force available in the system, and applied to the brake pads and the rim.
Figure 3. Long travel brake lever
The modern drop-bar shift/brake levers have a radius around 18mm, which is a circumference of about 113mm (figure 4). Pulling the lever about 20-degrees will pull 6mm of cable through the housing. The dual-pivot brakes used for these levers are designed to work with this amount of cable travel.
Figure 4. Typical shift/brake lever for drop bars
The Travel Agent™ is useful when an incompatible lever is matched with a brake caliper requiring a long cable pull. As an example consider the road lever above used to with a pair of linear pull caliper arms and/or mechanical disc brakes. The long arms of linear pull arms require a fair amount of cable be pulled. Using the road lever, the brake pads would need to be set practically touching the rim in order to stop the bike. The Travel Agent™ mechanism was designed as a solution for just this problem. The Travel Agent™ increases the amount of cable travel and allows the brake pads to be set a normal distance from the rim or rotor and effectively stop the wheel.
The Travel Agent™ can also be used simply as a "noodle" replacement on the linear pull brake. There are two hole options in the top of the Travel Agent™ body. Route the cable so it travels directly around only the larger outer pulley (figure 5). This will not increase the cable pull of the system (figure 6).
Figure 5. Routing cable for a direct pulley with no increase in travel
Figure 6. Travel Agent™ used as a noodle-replacement without any multiplying effect
Cable travel will double when the cable is routed to the inner pulley and then fed to the outer puller. The inner pulley will rotate the same angular amount as the outer pulley, but the outer pulley will pull more cable (figure 7).
Figure 7. Routing for doubling travel of cable travel at lever
When installing the units, install new cables. The old cables will be too short and new cables are always a good idea. To install the Travel Agent™ to increase cable pull, route the adjusting barrel and cable through the hole that is directed at the smaller or inner pulley (Figure 8).
Figure 8. Route barrel adjuster (if any) an cable through hole aligned with inner pulley
Pull out cable slack and route the cable back and through the small access hole in the pulley (figure 9). The access hole should be aligned so hole is counter clockwise near the barrel adjuster.
Figure 9. Route cable through bridging hole
Feed the cable out the back side of the Travel Agent™ body and along the outer pulley grove (figure 10).
Figure 10. Cable is routed to outer pulley
The cable continues around the Travel Agent™ and out the hole (figure 11). It is now ready to fit into the caliper brake carrier in place of the "noodle".
Figure 11. Fully routed cable
It is critical the cable at the bridging hole be adjusted so that transition never contacts the brake linkage during operation of the brake. If the cable routed at the bridging hole strikes the brake-noodle carrier or the Travel Agent body, it will jam and not allow further pulling or braking. Imagine a line through the pulley center that is parallel to the barrel adjuster and cable entrance. This line defines a 12:00 and 6:00 axis of a clock face (figure 12). The bridging hole should not be allowed to rotate past the 5:00 position (figure 13).
Figure 12. Cable bridging hole sitting at the 4:00 position
Figure 13. Cable transition at the 5:00 position will result in braking failure
The brake cable must make a sharp transition through the bridging hole from the inner pulley to the outer pulley. This will kink and flatten the cable wires to a small extent, and this is normal for this system. Inspect this transiton anytime the bike is serviced and replace cable as necessary.
Figure 14. Bend of cable at transitional bridging hole
The Travel Agent™ can also be set up in an "in-line" configuration. It is placed in the middle of the housing route. This is useful for mechanical disc brake calipers or linear pull brakes when used with common drop bar road levers. The amount of cable pull is nearly doubled (figure 15).
Figure 15. In-line option for the Travel Agent™