One fine day my speedometer stopped working. Some research showed that the plastic gear in my front axle had gone. So I got me a new one and put it in. And it work like before. For two weeks that is. Same problem, same remedy. But it started me thinking. Twice in two weeks seems a little odd. My first Transalp did 50.000km without a hitch. This was happening to me at 28.000km.
But we were planning for France and I was busy with other things. It worked again didn't it ? Yes, and it broke down 50km south of Paris. No dealer in site, so we went on without speedometer. This also means that you're without information on speed, distance done and distance to go. our first tank was emptied amazingly fast, but with two people, a lot (!) of luggage and headwind explain a lot. Using my watch and the one-hundred-meter-signs I calculated 120kmph at 6000revs. And 100kmph at 5000 revs. But that aside. We went home after two weeks, one week early because of the rain (1997).
The cost for that stupid plastic gear really went up this way. So I asked my dealer. What's happening ? Well, the plastic gear is driven by a steel gear which is driven by the front axle. The plastic gear drives the speedo cable. My steel gear showed damage and it ate the plastic gear. What can I do ? Replace the steel gear ! What does it cost ? Whaaat !! Briliant idea: use a bicycle computer.
So I went to some bicycle shops, talked to fellow Transalpers and searched Internet. I settled for a Sigma BC800. Waterproof, shows current speed and one of total distance, trip distance, time, trip time or average speed. That's more than the steel gear does.
Then it goes like this: (1) find a place for the computer, (2) bring the cable down to your front wheel, (3) mount the sensor, (4) mount the magnet, (5) configure the circumference of your front wheel. Every time the magnet passed the sensor an electric puls is transmitted to the computer. Based on circumference and time it calculates speed and all the other things.
First I put the BC800 to the right of my left mirror. The cable turned out to be to short. For a bicycle it;s ok, but a Transalp is a lttle higher. You can order a BC800 for 'rear wheel mounting' which comes with a longer cable. But almost no bicyclist does that so they don't stock those. So I extended the cable myself. Extra length of similar cable, cut, strip, solder and insulate.
I mounted the sensor to the speedo cable using two tie-wraps. The magnet is on a spoke. But only after widening the hole in it. Transalp spokes are thicker than bicycle ones. Last thing to do is configure the circumference of my tire. Mark on your front tire and on the street the 'down center position'. Then roll the bike prcisely two revolutions forward and mark again on the street. Measure between the two marks and divide by two. My Michelin Sirac measures 2130mm / 83.86inch. And then ..... test drive !!
To my suprise it worked. I thought. I read '-20kmph'. Minus ?? Yes, minus. My first thought was that was in reverse. But I clearly wasn't. Speeding up it showed '+30kmph'. That was better. Later I understood that the '-' and '+' show if you are uunder or over your average speed up to now. Quite handy, these manuals ....
After some fine-tuning, it quit. Logically of course. I found that the distance between magnet and sensor is very important. Just a little to far and the sensor misses the magnet. The magnet should also pass the sensor right in the middle. A little up or down and it misses.
I soon found the position next to my left mirror unsatisfying. Looking at the BC800 I had to turn my head away from the road. And away from the road spells problems. So I looked for and found a better spot. Directly above the warnig lights, on the dashboard edge. I now only have to lower my eyes a little to see it (my 'tripmaster' I call it).