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Shimano CT-90 Crankset Slowly Loosening

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Shimano CT-90 Crankset Slowly Loosening

Old 07-10-21, 08:07 PM
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Papa Tom
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Shimano CT-90 Crankset Slowly Loosening

My dear old GT outpost has a Shimano CT-90 crank that has been recalled. Shimano has not been able to supply the replacement crank (and accompanying parts) for a year and a half, and they now say we're looking at at least September before the warranty kit is available. In the interim, I replaced the crank with an FC-M361 Acera Crank, but that caused a bunch of problems and I quickly went back to the CT-90, the lesser of two evils. Now, all of a sudden, my CT-90 crank is slowly coming loose -- not something it did before I swapped it for that brief time. After about twenty miles of riding, it is loose enough to start clicking a bit. I have not pushed it any further than that before tightening both sides with a wrench I've started carrying around.

What could be causing this? Could it be jealous that I replaced it for four days? PS: I replaced the bottom bracket about a month before I swapped the crankset. This included replacing the bolts that secure the crankset. Now I am back to the original bolts. Threading issue? Is the BB toast?
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Old 07-10-21, 08:28 PM
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To be sure- it's the crank arm's coming loose from the BB axle?? If so then retightening the crank arm retaining bolts will redraw the arm up the axle's taper and re snug it. If so then it means that either the bolts were/are not fully torqued (about 30 foot pounds for most tapered square units) or the arm's tapered hole has become distorted and will never stay tight again. Andy
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Old 07-11-21, 06:43 AM
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Originally Posted by Andrew R Stewart View Post
To be sure- it's the crank arm's coming loose from the BB axle?? If so then retightening the crank arm retaining bolts will redraw the arm up the axle's taper and re snug it. If so then it means that either the bolts were/are not fully torqued (about 30 foot pounds for most tapered square units) or the arm's tapered hole has become distorted and will never stay tight again. Andy
Yes, it's the cranks arms coming loose from the BB axle. I've tightened both sides several times and they keep loosening. I don't have a torque wrench, but I've replaced enough BB's and cranks to have a feel for how tight they should be. My guess is that the bolts for the temporary replacement crankset were threaded slightly differently and distorted the BB. Others, like you, have suggested that maybe it's the arm that is damaged.
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Old 07-11-21, 07:11 AM
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Since you'll need to replace both the bb and crank, I'd suggest you look for a current HT2 setup that will be compatible with your gearing. You'll appreciate the lack of drama, so easy.
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Old 07-11-21, 08:25 AM
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Originally Posted by Papa Tom View Post
Yes, it's the cranks arms coming loose from the BB axle. I've tightened both sides several times and they keep loosening. I don't have a torque wrench, but I've replaced enough BB's and cranks to have a feel for how tight they should be. My guess is that the bolts for the temporary replacement crankset were threaded slightly differently and distorted the BB. Others, like you, have suggested that maybe it's the arm that is damaged.
I can't say I have seen loosening crank arms (tapered square, cottered are rather different) that have been from damaged bolt threading THAT ALSO didn't have obvious and visual thread damage. What I have seen way too many of is incorrect bolts, welded nuts (to the studs on Maxi versions) lock tightened bolts with arms flopping about. So do the current bolts show thread damage? Have you tried different bolts with different results? Have you examined the arms' tapered holes for even slight distortion, generally first at the corners? Once an arm comes loose and is ridden even a little bit the soft AL arm will be augered out by the harder steel axle. BTW the axle is also harder then the bolts are so it's the bolts that usually will show thread damage first. Andy
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Old 07-11-21, 08:36 AM
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Originally Posted by Andrew R Stewart View Post
I can't say I have seen loosening crank arms (tapered square, cottered are rather different) that have been from damaged bolt threading THAT ALSO didn't have obvious and visual thread damage. What I have seen way too many of is incorrect bolts, welded nuts (to the studs on Maxi versions) lock tightened bolts with arms flopping about. So do the current bolts show thread damage? Have you tried different bolts with different results? Have you examined the arms' tapered holes for even slight distortion, generally first at the corners? Once an arm comes loose and is ridden even a little bit the soft AL arm will be augered out by the harder steel axle. BTW the axle is also harder then the bolts are so it's the bolts that usually will show thread damage first. Andy
Thanks, Andrew and Shelby. I will check all these things out today. First, I promised my wife we'd take a little ride so she can practice on her new e-bike.
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Old 07-11-21, 10:21 AM
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If it loosened in 20 miles, you didn't install it correctly. Not tight enough, and you didn't re-torque it after riding a couple of miles. The crank is toast now.
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Old 07-11-21, 10:25 AM
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You're waiting on a warranty replacement crank. Cut a couple beer can shims, crank it down, hope for the best until the replacement is available.
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Old 07-11-21, 11:41 AM
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Checking on eBay just now, I see someone offering an old-but-apparently-unused CT-91 crankset with 170 mm arms.

I think that's what directly followed the (recalled) CT-90.
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Old 07-11-21, 02:25 PM
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OK, I put the "new" crankset (the Acera that replaced the CT-90) back on, tweaked some derailleur settings, and tomorrow, I will take a long ride to see if this solves the problem. I had decided to eat this crankset after I installed it and started experiencing problems with chain rubbing and bad shifting. I think re-positioning the derailleur and re-setting the limit screws may have resolved these issues. I will keep you informed, as I know the status of my fifty-dollar crankset and quarter-century-year-old bike are going to weigh heavily on you all for the next 24 hours.
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Old 07-12-21, 06:49 PM
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So far, so good. I only took the bike out for seven miles tonight (rain!), but everything seems to be solid with the replacement crank installed. I guess I must have deformed the original with improper torque.

Speaking of which, where can I buy my first torque wrench, and how much should I spend on one that will only be used for my bikes?
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Old 07-12-21, 08:02 PM
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Today i served a customer that had 3 different crank arms loosen and require replacement, the third one was still on the bike. I did the usual explanation about steel axles augering out Al arms and not keeping the retain bolt tight. He was seeking a who new (and different arm/axle design) crank. His Mom gave funding for replacement arm #4... After I installed it I had the kid ride the bike outside (around the shop and lot) and I retightened the retaining bolts. I showed the kid how tight they needed to be and he said he never got them that tight. Interesting that he understood the terms and relationships (newton meters, torque and material strengths) but had no clue as to how much effort equated with anywhere near enough torque.

This is not an uncommon challenge for many. Andy
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Old 07-12-21, 08:10 PM
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Make sure when you get the replacement that you get everything. Should come with a crank, BB, front der, and a chain. Last two of these I dealt with one shop lost the BB and the other shop kept the chain and front der. I fix bikes for people who can't afford them and these kinds of losses suck. Shimano has begun shipping them out though. I got both sets in May though the two cranks were dropped off 6 and 15 months prior.
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