The top of the pad still has some material left on it, so should I have let the pads go longer? No. Look at the second picture, below. This focal plane shows more clearly that the pad has not worn evenly, to the point that the copper "panhandle" that sticks out of the caliper has obviously been hitting the rotor during braking. Oops. So yeah, it was time to change the brake pads. (Both of these images are from a single picture from a Lytro camera.)
Measuring with a caliper, the top had about 1200µm of pad left, and the bottom had at most 300µm left.
Interestingly, the inner pad is worn completely differently. The inner pad wore evenly top-to-bottom, but the front edge had had 950µm and the trailing edge 1100µm. So the inner pad wore almost flat by comparison, except the leading edge wore a bit faster.
Why did the pads wear unevenly? Well, I'm not sure.
Maybe I didn't set the brakes up correctly? BB7s mount with spherical fixing bolts, so that you can align the caliper in two axes with the rotor. Avid's old instructions and Park's current instructions for this are laughably incomprehensible, so I figured out a simple way: put in new pads, set the pad distance. Squeeze the caliper shut on a trued rotor. Tighten the fixing bolts. So maybe that's wrong? I checked the Avid site. Avid's new instructions are pretty much exactly what I did. So that's not it: AFAICT, the caliper was set up with the caliper coplanar with rotor. There isn't much play in the caliper inboard and outboard, but even if there were, the caliper is designed to compensate for that by adjusting the pad positions. This is necessary, since hubs vary in exactly how outboard from centerline they mount the rotor.
Maybe I had let the pad positions get out of adjustment? The BB7 has two knobs to adjust the pads inwards as they wear. The outer pad is on a piston that pushes it towards the fixed inner pad. Obviously, the rotor makes an angle as it's bent, and obviously the force of the rotor is going to be higher on the top of the outside pad and the bottom of the inside pad. But that's the opposite of the wear pattern on the outside pad, and the inside pad wore evenly. So that seems unlikely.
Maybe the pads aren't coplanar? I doubt the caliper body is bent, since it's so stout that I find it hard to believe a blow hard enough to bend it wouldn't throw it completely out of alignment. The piston that pushes the outer pad in obviously has more play in it than the fixed holder on the inner side, so maybe this is just as good as it gets. One way to look at it is, I got 3 winters off of a set of brake pads that cost me less than $20. (The rear pads will probably last a couple more winters yet.)
Hmm. If you have any experiences or ideas, or interesting things to measure, I'm all ears.
I bought the school district's math book, Everyday Math and it's, how do I put this politely? A steaming pile of crap.
So we've pretty much had to write our own math book. The first unit to cover was probability and statistics. Everyday Math had a table of actual US immigration data and a series of measurements of an (imaginary) girl's head. The reader was expected to understand that one set of measurements was dominated by noise, whereas the other had inconsequential measurement error. I'm not sure why 8th graders are supposed to intuit this, given that many scientists have trouble with these concepts. But more importantly, all of the problems were boring.
So we decided to make some measurements of our own, so we'd have some data that we had a real feel for. What to measure? I figured nuts from the store, but a pine nut weighs around 3 grams, and our scale reports only to the whole gram. So we would have been overwhelmed by measurement error, which isn't normally all that much fun.
At right is our combined data from the first two harvests, conveniently of exactly 12 tomatoes each. X is tomato weight in 5g buckets. Y is count per bucket. Sven is predicting the third harvest will drive up our average weight and standard deviation. We've been looking at the data to see if it has an approximately Gaussian shape. We'll get more formal about that in a little while.
AA battery packs are cool, but check for corrosion.
If your bike is transportation, or if you like endurance bike sports, you need bike lights. Hikers, climbers, spelunkers, and other outdoor sportsters also often need lights or other battery-powered devices. AA NiMH batteries are a cheap, effective solution. Damp weather and vibration can corrode contacts and degrade performance. Damp combined with vibration degrades performance faster.
The picture at right shows a corroded spring contact in a battery holder. This contact is at the bottom of the holder, and the spring at the top of the holder was not noticeably corroded, so moisture was the main problem here. (Leaking batteries cause corrosion too, although not in this case.)
GPS units that take AA batteries are notorious for vibration problems causing arcing, resulting a sort of electro-fretting that damages contacts on both the battery and the GPS. This is noticable because the GPS tends to reset whenever it loses power. The same problem happens with battery-powered lights, it's just hard to tell when it is happening. But the damage is serious. You can sand the corrosion off the contacts, but spring contacts are usually plated to combine long fatigue life with good electrical performance. Once the plating is corroded, the contact will never work as well again. Many GPS units now build Li-ion batteries into the device, eliminating this problem. Garmin makes a kit for their older GPS units with an extra tension spring to increase the spring force on the batteries. If you have this kit, I strongly recommend using the tension spring. It looks like an extra bit of hassle, but it is actually very useful.DiNotte make nice lights that provide several hours of operation from a pack of four NiMH AA batteries in a standard "BH343" battery holder, shown at right. The holder has a connector like a 9v battery and cost $1-$2. Good NiMH batteries like Eneloops cost $3 each and a 4-pack will hold 10-12Wh.
I tape the batteries into the holders with electrical tape and use a battery pack charger (popular with RC enthusiasts) as I explain here. Taping the batteries seems to eliminate vibration problems. (Otherwise, the batteries will sometimes slip all the way out of the holder, a serious vibration problem!) But you then have to use a pack charger, which is very convenient anyway.
A battery pack is only as good as its weakest battery, so it's even more important to use good quality NiMH batteries when pack charging. I notice that after about two winters, battery life seems to get much shorter. So I tear the battery packs apart and recondition them with a La Crosse BC-700 battery charger.
Actually, a battery pack is only as good as its weakest link. I started scrutinizing the battery holders when I noticed that they would eventually crack, which reduces spring tension, and the connector to the light would also loosen, and would need to be (carefully, without shorting!) re-crimped once in a while. Then I started noticing corrosion. Once I started looking for corrosion, I started to find a lot of it.
I think 2 Seattle winters is probably all one of these battery holders is good for. After that, it's time to break down the battery pack, and recondition the batteries. The BC-700 tells me what the reconditioned capacity of each battery is. If it's over 90% of new, I put them into a new battery holder and give them another 2 years. Otherwise they get relegated to less demanding applications and eventually recycled. Either way, I now only use the battery holders once.