Christopher tm Herdt (cherdt) wrote,
Christopher tm Herdt

Bike repair: replacing chainrings

I have no idea why the gears in the front of a bike are called chainrings. The set of gears on the back of a bike are often referred to as sprockets, and a collection of integrated sprockets as a cassette. They are all gears to me, but if you call them by special names the mechanics at the bike shop will take you more seriously.

The chain was slipping a lot on my 2000 Bianchi Volpe, particularly when I was trying to accelerate hard. This frequently happened when I was trying to cross an intersection. It wasn't safe. The front and rear derailleurs looked fine, so I suspected it needed a new chain and new chainrings.

I stopped by my LBS (Local Bike Shop) and bought a new chain. I explained the slipping I was experiencing, and the mechanic told me a new chain would probably make it worse. Still, a chain is cheaper and easier than chainrings, so I thought I'd try it.

The bike mechanic was right. Or, at least, the new chain didn't help. It was time to replace the chainrings. And this is where things got complicated: what do you replace them with?

When you try to buy chainrings, there are a lot of different factors that come into play. One was the number of teeth. That was pretty easy, I could just count them. My larger chainring had 52 teeth (52T), and my middle chainring had 42 teeth (42T). I decided to leave my smallest chainring, which had 30 teeth (30T). It rarely gets used and didn't look very worn.

My bike's drivetrain is a Shimano Tiagra, but I didn't know the exact model, which complicated things. I could easily see that the crank had 5 arms, so I needed chainrings with 5 bolt holes. Then there is another measure, called the BCD (Bolt Circle Diameter). This has nothing to do with the size of individual bolts, it's the diameter of the circle that would pass through all the bolt holes. The 2 best sites for explaining this, and how to measure it, were:

My chainrings had a BCD of 130mm.

Another wrinkle is that some drivetrains use 2 chainrings (doubles) and some use 3 chainrings (triples). I'm not certain this is totally critical, but it is important that the outer chainring is meant to be an outer chainring, and the middle chainring? Probably OK as long as it's not mean to be an outer chainring. Yet another wrinkle is that chains can vary based on the size of the cassette. By bike has a 9-speed cassette, so I looked for chainrings that were designed for something similar. It looks like there's some wiggle-room there, but I don't know how much.

It's possible I worried about this way too much. My friend Chuck has cobbled together bikes out of all sorts of incompatible parts. But this was my first attempt to replace chainrings, and I wanted to get it right, if possible.

Eventually I deduced, based on some of the measurements I'd determined, that my drivetrain is the Shimano 4403. That's an old drivetrain and Shimano doesn't make any new 4403-specific parts. But some bike forums suggested that the 4403 was pretty standard, didn't do anything particularly strange, and that parts should be relatively interchangeable. I ended up with FSA Pro chainrings, 52T/42T, 130mm BCD, 5-bolt pattern.

I watched a couple YouTube videos, both from English bike mechanics, before attempting the swap myself. Better to see it done a couple times than to wing it, right? Well, mostly right -- in both videos they removed the crankset from the frame. I was unable to get the crankset off my bike, but it turned out I was able to remove the old chainrings and replace them with the new chainrings without removing the crankset. I didn't even have to remove the pedals! I may have had to remove the crankset if I wanted to replace the smallest chainring, I'm not sure. You do, of course, need to remove the chain and put it back on when you're done.

After the repair, I took the bike out for a quick spin. Smooth ride. No chain slippage, even under load. Shifting wasn't great, but no worse than it was before the repair. It was a success!

I am leaving these notes here in case any other amateur bike mechanics are trying a first-time chainring swap. Good luck, it's not as bad as it first appears!
Tags: bicycle

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