Bike repairs

Last Wednesday, my Twitter friend @bemyax mentioned that I hadn’t written much about bicycling this year. I replied

@bemyax Since too many of my previous “adventures” have been falls, collisions, and near-collisions, the lack of reports is a blessing.

2:03 PM Wed Jun 1, 2011

The blessing didn’t even last the rest of Wednesday. On my way home from work that evening, about a block from my house, I had a small “incident.”

I’d just crossed a busy road and was riding on the sidewalk so I wouldn’t make the drivers nervous. A woman was walking her dog up ahead of me, and I went off the sidewalk onto the parkway grass to give her a wide berth.

Almost as soon as I had both tires off the concrete, I hit a Buffalo box hidden in the grass. It was low enough that I didn’t notice until I was on top of it, but high enough to rattle my teeth when I hit it.

Buffalo box

(Photo by Pam Broviak.)

My steering was so exquisitely precise that both tires went over the box. My front tire came away unhurt, but the rear was flat before I’d even gotten back onto the sidewalk. I’ve never had a tire go flat that fast before. It was like someone had hooked up a shop vac and sucked all the air out of it.

I couldn’t bear to examine the bike that night—I was sure the rim had been dented. Luckily, I was wrong. When I looked things over Thursday morning, the only damage were the classic “snakebite” holes of a pinch flat in the rear innertube.

Pinch flat holes in innertube

I’m pretty sure I’ve never had a pinch flat before—my flats come from glass and other debris poking through the tread and tube.1

I have another bike adventure coming up, as I undertake a repair I’ve never done before—replacing a chainring.

My chain has been skipping and jumping off the middle chainring lately. Not surprising when you see how much its teeth have worn.

Worn middle chainring

See how pointy the middle teeth are compared to the teeth on the inner and outer sprockets? And how the valley between the teeth is unsymmetrical? The teeth are wearing on one side because that’s where all the bearing pressure is. The new tooth shape is allowing the chain to slip off when I’m pushing hard. The chain itself is also worn at the link pins, so it’s effectively longer than normal and doesn’t fit on the sprocket as well as it used to.

The wear is almost certainly due to the grit kicked up from the crushed limestone trails I do much of my riding on. It gets between the chain and the sprocket and grinds away at the aluminum. When the tooth geometry is messed up, that puts more stress on the link pins and they (or the holes in the link side plates) wear faster.

As you can probably guess from the photo, I do virtually all my riding on the center chainring. I have a new chainring on order and will be replacing it and the chain in a week or so. Expect a short post if the repair goes well and a long one if it goes poorly.

  1. Which reminds me: I have over 3,200 miles on these tires and they’re getting thin. I need to order a new pair before I’m down to the cords.