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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.
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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. |
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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. |
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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.
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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. |
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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.
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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.
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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.
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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.) |
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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. |
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