Previously I wrote on how front shifting happens between chainrings, and I noted that mine were particularly worn. I got my Shimano 105 FC-5750 cranks second-hand to begin with, so adding thousands of kilometers on that starting point eventually meant Hyro’s front shifting was going to deterioriate sooner rather than later.
Unlike with a chain, measuring wear on chainrings isn’t straightforward. I found though that one giveaway is deteriorating quality and ability of upshifts to the big chainring. If you’ve ruled out the front shift cable and front derailleur adjustment, and you still have upshift failure in larger cogs, your chainrings may well be the culprit.
Unfortunately, many Pinoy cyclists would rather just replace an entirely fine cranket instead of just the worn rings, partly because they may not know any better, and partly because new chainrings aren’t easy to find locally in bike shops. Add to that the recent proliferation of proprietary asymmetrical BCDs (bolt circle diameters = the diameter of an imaginary circle that runs through all a crankset’s bolts) from Shimano, Campagnolo, and FSA, and you have one bike maintenance job that is losing popularity pretty quickly.
Today I’ll be walking you through chainring replacement on a crankset with a five-bolt spider of 110 mm BCD. This is pretty common on 50/34T and 46/36T chainring combinations. For reference, a 53/39T crank uses a 130 mm BCD, and many cranks for fixed gear use have an even bigger 144 mm BCD.
The process uses mostly standard tools, with the exception of a chainring nut wrench. This is best described as a fork with its tines shortened and thickened, then bent over at a 90-degree angle. Its purpose is to prevent chainring nuts from spinning as you loosen the chainring bolts on the other side.
On Shimano Hollowtech II two-piece cranks, I find it a little easier to remove the non-drive side crank arm first, then tap the crank spindle end with a rubber mallet just enough to free up room behind the chainrings. This way, the bottom bracket shell will act as a sort of “third hand.”
Insert the chainring nut wrench into the grooves of the chainring nuts on the back side of the chainring. On the front side, use the relevant tool to break the chainring bolt free. On my Shimano 105 FC-5750 crank, you will need a T30 Torx key and quite a bit of force, as Shimano recommends a torque spec of 12-15 Nm.
Once all five pairs of chainring nuts and bolts are loosened, the chainrings should come apart from the crank spider. Pay attention to how they come apart, as most of the time chainrings are meant to mount in only one correct orientation. This is to ensure correct “clocking” of the various shifting ramps and pins and ultimately smoother front shifts.
One feature to watch for on the big chainring is the chain drop prevention pin. On a Shimano crank, this is intended to live underneath the drive side crank arm. In the event of an overly eager upshift, or a sloppy high limit on a front derailleur, this pin is the last line of defense protecting your crank arms against the surprisingly savage cutting friction of a dropped chain.
My outgoing big chainring lost this pin for some reason…hence the battle scars all over my crank.
Re-installation is the exact reverse of removal, although some prep needs to be done. As chainring nuts and bolts are a common cause of creaks, first you’ll want to clean them well with degreaser and a rag or paper towel, then smear some fresh grease on.
Align your new chainrings correctly onto the spider. On these Shimano chainrings, the chain drop prevention pin on the big chainring and an engraved little triangle on the small chainring have to line up with the drive side crank arm. You can see these on the inside.
Put the chainring nuts and bolts on the spider and turn until finger-tight. Then break out your torque wrench, attach its T30 driver bit, and tighten them to 12-15 Nm in a star pattern.
You’re done! Reinstall the crankset and you should be in good riding condition again.