Chainrings need to be replaced if they start to look like those on the previous page or if your chain skips across the teeth under pressure. The teeth that wear fastest are those under the most pressure – the ones at the top and bottom of the chainring when the cranks are horizontal. If your chainring teeth are worn sharp or the faces of the teeth are splayed outwards, replace the chainring.
Bent teeth will catch on your chain as you pedal, preventing the chain from dropping off the chainring at the bottom of each pedal stroke. This doesn’t necessarily mean a new chainring though. Remove the chainring, as below, and clamp it flat in a vice. Support the chainring as near as possible to the bent tooth and ease it gently straight with pliers. Try to do this in one movement – sawing the bent tooth back and forward will weaken it.
Step 1: The smallest chainring must be removed first. Rest the chainset on a workbench or the floor, protecting the teeth of the largest sprocket with cardboard. Use an Allen key to undo all the bolts on the smallest chainring a half-turn, then go round removing them completely. Look at the orientation of the chainring. Note whether they face inward or outward. If any chainrings have spacing washers, note their location so you can refit them afterwards.
Step 2: Undo each of the Allen keys holding the middle and outer rings a half-turn each, and then go back around and remove them completely. Be careful undoing these bolts; they are hard to shift, then give suddenly, so mind your knuckles. Mostly, an Allen key does the job, but sometimes the nut on the back of the chainring moves around. A special tool, called a chainring bolt tool, is made for holding this and is available from your bike shop.
Step 3: As you remove the chainrings, note their orientation. There will often be a tab on the middle chainring and a peg on the outer chainring – both of these will have to line up with the crank when you refit the chainrings. Check which way the chainrings face – the middle chainring may have plates riveted to the inside face. Check also if there are any washers between the chainrings, as you will need to replace these correctly.
Step 4: Clean the chainrings and check them for wear. You put most force on your pedals when your cranks are horizontal, so the two areas of chainring at 90˚ to the crank will wear fastest. Hooked or pointed teeth, like the ones in this picture, will not mesh properly with your sprocket. This chainring will need replacing. Take worn chainrings with you when you go to buy new ones, so that you can match up the bolt pattern.
Step 5: Clean the crank, especially the arms that the chainrings bolt onto – clean out the bolt holes. Clean any of the old rings that you reuse. Check them carefully for bent teeth or teeth that have splayed under pressure. Bent teeth can be carefully bent back; the alignment is critical, so take your time about it. Splayed teeth are a sign that the ring needs to be replaced.
Step 6: Reassemble middle and outer chainrings. Be sure to get the orientation and position right – line up chainring tabs with your cranks. If your crank has a chainring bolt underneath it, orientate the chainring, so the tab is directly opposite the crank. Refit any washers. Grease the bolt threads and refit them one at a time. Tighten each one firmly home, but don’t go mad – the bolts have a fine thread and snap if you overtighten. Refit the smallest chainring.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Chainrings: removal, sizes, orientation, fitting and wear”