Hacking the Travel Agent
Sorry about doubling up posts on the same day, the blog has been quiet for a while. Bike nerds will enjoy this one, everyone else will probably prefer the one about Markworth Forest.
A little while ago I mentioned modifying a QBP Travel Agent (normally used to make linear pull brakes work with drop bar levers) to make a Shimano 9sp barend shifter work with a SRAM i9 internal gear hub. I finally got around to finishing that project this weekend. Here is the modified Travel Agent:
I made a new pulley which has a 10mm inner groove and a 36mm outer groove. That means that for every millimeter that the cable is moving as it enters the Travel Agent that it will be moving 3.6mm on the output. That should create the right ratio to increase my barend shifter “clicks” to work with the i9 hub. Sadly it didn’t work out that way. The3rd to 4th jump on the SRAM i9 shifter takes a bit more cable pull than the other gears, and the 3-4 jump on the Shimano shifter pulls a little less cable than the other gears. I checked two Shimano shifters just to be sure. I could get the hub to index well in all gears but that one. Hopefully J-Tek makes a barend shifter for the i9 that is as nice as the one that they make for the Nexus 8.
While I was making pulleys I decided to use the Travel Agent to fix the rear brake on our tandem. It has a disk brake which is designed to work with road levers, but the pads were always dragging on the disk. A regular Travel Agent would fix that problem, but make the brake have too little mechanical advantage (it doubles the cable pull which halves the mechanical advantage). I made a new pulley with a 3:2 ratio instead.
First I roughed out the two steps in a chunk of aluminum that will form the pulleys. One that I was done I drilled a hole between the two pulleys using the milling machine (I found it easier to do this before finishing the pulley). To get the angle correctly I just put one vise into another:
Then I made the pulley grooves on the lathe. My lathe was bought used and came with tons of cutting tools in various shapes. One happens to be the perfect size for a bicycle brake cable:
I cut the pulley off of the aluminum stock, made a brass bushing for the pulley to rotate on and did a little finish work. The outer groove is 30mm and the inner groove is 20mm, giving me the 3:2 ratio.
This is what it looks like on the tandem:
This setup feels a lot better than the stock setup and still seems to provide plenty of braking power. The return spring on the Avid BB7 Road is a little wimpy too, so there is a supplementalspring added to the cable. I’m looking forward to riding the tandem without having the disk brake rubbing and making lots of noise.
Sweet, that’’s really clean. I haven”t checked any tandem lists or forums, but once word gets out I bet you”ll get a few requests to make these.
Which shifters did you measure that cable pull difference on? Jtek told me there was a non-consistent cable pull to the Shimano shifters back when I first asked them about making a shiftmate for the Sturmey XRK-8. I looked at my bar-ends and STIs but never had a measuring rig as accurate as yours to confirm it.
I just measured Shimano 9sp shifters. It’’s funny that they and the SRAM i9 had the inconsistency at the same gear but in different directions.
I”ll email JTek and ask about i9 shifters and if they are coming.