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Chainwheel Repair

The chainwheels, also called chainrings, are the front gears that attach to your crankset. The teeth on these rings can be damaged by flying rocks, by dragging the big chainring over a log or rock ledge, or by sloppy shifting. They can also wear out. If you let your chain wear and stretch, it will wear down the teeth on your chainrings.

Note that several teeth are slightly chipped on these rings. This occurs from rough chain transfers -- forcing a shift while fighting uphill.

The metal ring at the top is a "tooth fairy." This aluminum ring keeps the large chainwheel from landing on rocks and logs.

Bent tooth:

A single bent tooth can sometimes be straightened by clamping the tooth in an adjustable open-end wrench, then gently tweaking it back. Often, there is invisible cracking in the metal at the base of the tooth -- and your attempt to straighten the tooth will break it.

Grasp a single tooth, with the tip of the wrench where the ring widens at the base of the tooth. Tweak gently, stopping often to inspect the tooth's position. 

When straightening a bent tooth, note the position of the tips of the nearby teeth. The tooth tips DO NOT line up in a perfectly straight line around the chainring. The teeth meander from one side of the ring to another, to help the chainring pick up the chain when shifting. You should tweak the tooth over to its former position.
Dolphin-fin teeth:

Softer chainrings will eventually wear into a dolphin-fin shape. The side of the tooth that faces upward on the rear-wheel side of the ring forms a cup, with the top of the tooth overhanging the cup a little. This is especially a problem in the small chainring, and creates easy chain-suck! You can buy a little extra wear by smoothing out the "leaning" edge with a file or grinding wheel.

Here a Dremmel tool is trimming back overhanging edges on the side of the tooth that faces rear when viewed from underneath the bottom bracket.

While you can buy a few extra rides from your little chainring by reshaping the teeth with a file or Dremmel tool, you should plan on replacing the ring once you see that the teeth are changing shape.

Here's a severely-worn chainring. This is a 32-tooth middle chainring with 5-bolt attachment. The top ring is new, the bottom shows thinned teeth, with erosion on the power-side of each tooth. See how the teeth have a "smoother" less-steep slope on the left-hand side? The chain has worn down the surface of the tooth where the ring is pushing against it.

Even more severe damage. The teeth have been worn down to nubbins. At this point, the chain may "break loose" and slip around the ring suddenly. This happens when you're attacking a steep slope or trying to accelerate rapidly.

When you see chainring or cassette tooth wear, you should replace the chain at the same time you replace the rings. As the chain wears, it gets longer, so it doesn't fit precisely on the teeth of the ring. This leads to wearing-down of the teeth.

Replacing chainrings:

To replace a chainring, you need to pick a replacement that matches the old one. Obviously, you need to match the number of bolts that hold the ring on the crankset. The chainring may attach with 4 bolts or 5 bolts. And, you need to match the position of the bolt holes. This is determined by the distance between the bolt-holes (not the distance from the center of the ring to the hole). Then you select a ring with the right number of teeth. For example, you might order a small chainwheel in 22-tooth or 24 tooth. The 22-tooth gives you more climbing power (lower gearing).

Let's run through an example. I've broken 4 teeth out of my big chain ring, and it won't take the chain any more. I need to order a new one. Inspecting the ring, I see four mounting bolts. I get out my metric measuring stick. It's exactly 130 mm between the centers of two adjacent bolts. So I'm going to order a 4-bolt/130-mm ring. I count the teeth -- 44. In the catalog, I see that the ring I need comes in 44- and 46-tooth. Sometimes I'm cranking on the flat in the highest gear, and I'm maxed out -- can't get the bike to move faster. So I order 4-bolt/130-mm/46-tooth.

For the small chainring or middle chainring, you'll need to pull the crankset off the bottom bracket, so you can get to the ring-mounting bolts. (You need to expose the bolts to measure the distance between the bolts.) If you're not absolutely certain how to order the correct chainring, it's best to take the crankset or old chainring into the bike shop. Chances are, they'll have a replacement you can take home with you. 

Large chainring only:

If you're replacing a large chainring only, you can often (depending on your crank and pedal) pass the ring over the crank.

Remove the bolts from the outside of the large chainring. 

Slide the chainring over the crank and pedal.

Now slip the new wheel on. Apply Loctite to the bolts and fasten it on. Easy.

Taking it apart:

If you're replacing the small chainring (or the entire set) you'll definitely need to remove the crank. You need a crank extractor tool. See the section on cranks.

Move the chain off the chainrings, and onto the metal edge of the bottom bracket.

Using your crank-puller, take the whole thing off the bottom bracket.

If you're replacing only the small chainring, just flip the chainring set over and remove the bolts from the inside. (You won't need to disturb the two bigger rings, nor the outside bolts.)

The outer rings are connected with a bolt and treaded spacer. Remove the bolts with a hex wrench.

You may need to use a wide-blade standard screwdriver to keep the spacers from turning.
Reassemble the chainring set, inserting your new chainring(s). Be sure that, if there are collars for spacers, you get the spacer into the collar.

Rings go flat-side out. (The side of the ring that faces the bike frame may have surface features that assist in transferring the chain.)

Apply Loctite to the threads of the bolts.
Fasten the bolts securely. Double-check the fit of bolts and spacers within the guide collars on the rings.

Go around and give each bolt an extra twist.

Be sure the chain is around the bottom bracket, then put the crank back on.