Moving parts wear out, particularly moving parts that are in constant contact with other moving parts. And even more so if there’s mud and grit involved. So it is with chainrings. All that spinning away, right down low near the goop, chain shuttling this way and that between them… It’s no wonder they get a bit weary. Worn chainrings manifest themselves in a number of ways. Increased occurrence of chainsuck (where the chain refuses to leave the ring it’s on when you shift, preferring to be carried up into the chainstay and create unpleasant gouging) is a common symptom, along with other related shifting maladies, noise and, if left long enough, slippage. And you really don’t want the chain to slip on one of those steep grunty climbs. If the rings are looking shaky, replace them.
It almost goes without saying that you should clean your bike first. Almost, but obviously not quite.
1. Not many tools needed here. Just a suitable crank extractor and 8mm and 5mm Allen keys. You might also need a flat-bladed screwdriver. Oh, and some grease which clearly got camera shy here.
2. Here’s a middle ring ripe for replacement. The teeth are all thin and pointy (although they haven’t quite got to the “surf’s up” stage where they’re so worn that they actually become undercut on one side like a shark’s tooth) and one of them has given up the ghost altogether and fallen off. No wonder the shifting’s not all it might be.
3. To get at the rings we need to take the drive-side crank off. First undo the bolt that holds it on to the bottom bracket axle. Usually this is an 8mm Allen bolt, although they always used to be 15mm hex bolts for which you’ll need a suitable socket. The bolts can be hard to shift, but pointing the crankarm backwards, the tool forwards and pushing down on the pedal with one hand and the tool with the other usually shifts them. If you can arrange the tool suitably, grabbing the crank and the tool in one hand and squeezing them together can be effective too.
4. With the bolts out, make sure any washers under the bolt heads have come out too and use a suitable crank extractor to pull the cranks off. These have two parts, an outer part that threads into the extractor threads in the crankarm and an inner part that winds in and pushes the crank off against the end of the bottom bracket axle. Note that square taper and splined cranks need different tools – one for a square taper just goes inside the axle on a splined setup. There are various bodges to get around this, but it’s a lot cleverer just to get the right tool. With the tool firmly fitted in the extractor threads, turn the inner part clockwise until the cranks come off.
5. Once the cranks are off, look around the back and you’ll see the Allen bolts holding the chainrings on. The inner ring has bolts all to itself. Undo them and the ring just lifts off. If it’s only the inner that needs replacement, you’re laughing – they’re usually the same all the way around so there’s no alignment issues to worry about. Just make sure the side with shoulders on the mounting holes faces inwards (the bolt heads sit on the shoulders), grease the bolts and put them back. Tighten the bolts a bit at a time, working from one bolt to the one opposite rather than around in a circle.
6. If it’s the middle or outer rings (or all of them) that need replacing, you’ve got more bolts to deal with. The middle and outer rings are held either side of the crank spider with a threaded sleeve on the inside and a bolt that threads in to it from the outside. With any luck the sleeves will be sufficiently gunged up that you can simply undo the bolts with a 5mm Allen key. If the whole lot rotates, you’ll need to find a flat bladed tool to hold the sleeve still on the back.
7. With all the chainrings off, get your new ones out and boggle at what a different shape the teeth are. You’ll need to get replacement chainrings of the right dimensions – these are five-arm compact chainrings with a bolt circle diameter (the diameter of the circle that the mounting bolts sit on) of 94mm for the outer and middle and 58mm for the inner. Most Shimano cranks are now four-bolt while older MTBs may have 110/74mm five-bolt chainrings and road bikes are different again. They’re all also available with different numbers of teeth – if you’re happy with your gearing just get ones the same. You safest bet is to take the old ones or the crank arms to the shop to make sure you’re getting the right rings to suit.
8. Modern chainrings have all sorts of pins, pegs, ramps and cunning tooth profiling for smoother shifting. This means that they rely on being positioned in a certain orientation to put all the cleverness in the right place. Outer rings usually have a short peg on the outside – this should be positioned behind the crankarm where it stops the chain getting wedged should it fall off the top. Middle rings will have some sort of mark that, if it’s next to a mounting hole like this, should be positioned opposite the crank arm. If the mark’s between two holes, it goes behind the crank.
9. The next bit needs a bit of juggling. Both the outer and middle rings have shoulders in the mounting holes where the bolt heads or ends of the sleeves sit. The shoulders need to face away from each other. Line up both rings and the crank spider, push a sleeve in from the back and thread a bolt in from the front. Grease the bolt threads first. Once one’s in things get a lot easier and you can fit the rest. Again, tighten from one bolt to its opposite neighbour. You may need something to stop the sleeves from turning, or you may get lucky and have them stick on their own.
10. Grease the end of the bottom bracket axle and put the driveside crank back on. Put the chain on to a chainring first, otherwise you end up with the cranks on but the chain not running around them which can prove mildly irritating. Also make sure the cranks are level – this is very obvious on square tapers, but splined tapers will allow you to put cranks on a few degrees out if you’re not paying attention. With the crank on, put any washers that came out under the head of the crank bolt, grease the threads and do the bolt up proper tight. On ISIS Drive splined cranks, just tighten them until the crank hits the stop on the axle. On others, tighten to the recommended torque which is usually about as much as you can get on with one hand and a decent-length Allen key.
There’s hundreds of top maintenance tips in the BM archive.