SPOILER: So, Fernando and I set off to the city to buy the relay switch...In the process, I had to climb the tallest bridge in the country again. Which was pretty fun, because this time I verified that the modifications I made to my brakes were reliable on the descent.
Many people would be afraid to make a fast descent on brakes that are 33% lighter than they originally were. Moreover, the plastic (zylex/kevlar) cable acutators I used were degradable; they lose 50% of their yield strength after 2 years - down from 1500N (it's been 3.5 years since I've installed them). The ordinary bike uses braided steel cable...
But I can trust my life to the application of my knowledge and theory. It's easy to make a conservative estimate how much tension is in your cables with impulse-momentum theorem (you just need braking time, and the mass of yourself + bike). I also knew that the material properties of the aluminium alloy bolts and nylon washers etc. They would only fail under MANY cycles of braking (irrelevant issue), or if they are overtensioned. The shear stresses are just too weak to break them. The tension on the derailleurs is much larger than the tension on your brakes, by the way.
So I made a safe descent. Proof of concept.
Many people would be afraid to make a fast descent on brakes that are 33% lighter than they originally were. Moreover, the plastic (zylex/kevlar) cable acutators I used were degradable; they lose 50% of their yield strength after 2 years - down from 1500N (it's been 3.5 years since I've installed them). The ordinary bike uses braided steel cable...
But I can trust my life to the application of my knowledge and theory. It's easy to make a conservative estimate how much tension is in your cables with impulse-momentum theorem (you just need braking time, and the mass of yourself + bike). I also knew that the material properties of the aluminium alloy bolts and nylon washers etc. They would only fail under MANY cycles of braking (irrelevant issue), or if they are overtensioned. The shear stresses are just too weak to break them. The tension on the derailleurs is much larger than the tension on your brakes, by the way.
So I made a safe descent. Proof of concept.
What happened before that:
As I explained, we needed some lead solder to get the work done. The soldering iron pictured operates at 40W and has too thick a tip; we used another one eventually.
As I explained, we needed some lead solder to get the work done. The soldering iron pictured operates at 40W and has too thick a tip; we used another one eventually.
The first round of repairs was pretty miserable - we tried to squeeze the pins back into the plate, but they were too stiff. We removed as much solder off the pins as we could, but it wouldn't work.The second round I decided that we need Fernando's help. He explained that the thing is a relay switch; not a transformer, despite the coils. *facepalm* There are 8 connection pins underneath, some of which are shorted in pairs. A 12V DC input charges it up, and the attraction between the plates causes it to 'connect', switching the pairs which are shorted.
So, we got to work trying to repair it. Dismounted and disconnected the circuit board for the module this time. With a powerful soldering iron this time, and some leverage with a screwdriver, the stubborn pins dislodged from the circuit board, and the relay switch came off. A sadder story ensued... basically...
The thing is broken beyond repair. The pins inside were all shattered in pieces. And this thing is really budget and vintage, because the baseplate was brittle and, in Fernando's words, "Koko crunch." So, we nonchalantly broke it into pieces to vent our frustration. Then, at this point..."You know... why do you need to repair this? This thing is really cheap you know, I can buy it for $4."
We explained that we don't know the ratings etc. on it. So he surgically dissected the coil and tried to experiment with it. The multimeter didn't get an extremely good reading, but it seemed like 12V DC and the shorting/switching of the pairs of pins were all that we needed to know. The pairs were confirmed with a multimeter to the circuit board. The nominal ratings etc. on the plastic cover seemed to be correct. So we decided to set out on bicycle to buy a replacement.
"Yeah man, this is really cheap. I might even have a replacement at home."
My other team mate replied, "You know, you have no life if you can find a replacement for this at home."
I could only laugh.
Anyway, the problem with a replacement is that you can never find the exact same piece unless you sent the unit back to the factory. The pins wouldn't fit into the holes on the circuit board nicely, so you have to mount the new switch on the board while insulated from all the lead-tin connections, and route wires from the newly located plate into the original holes, rather than soldering the plate into the holes directly. Hard-wiring, I believe it's called. (There's quite a number of these configurations on the controller's boards by the way, suggesting that it has been repaired many times before.)
So, we packed up and Fernando put wrapped the circuit board around my (diminishing supply of) bubble wrap and put it in his bag. And my other team mate discovered another strange element about the controller as we were packing up:
A spring-type fuse. None of us knew it existed before this. Something new I learned of for the day. Anyway, we found some replacements after much difficulty. Got a spare just in case. Then we left the repairs for another day, once again.
High quality stuff. The yellow one’s smaller, and rated for 1A only. Weren’t too confident of using it. The black one’s OK. But anyway, Fernando found a replacement at home nonetheless (LOL), which explains the third one pictured below.
For the reasons explained, the relay switch (light blue) was mounted a new location, and wires were routed to the original locations of the pins to act as ‘proxy pins’. Professional job by Fernando, really. Looks pretty.
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