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Issues with FSA modular crankset adjustment, diagnosis

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Issues with FSA modular crankset adjustment, diagnosis

Old 06-23-22, 03:36 PM
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eddiearniwhatev
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Issues with FSA modular crankset adjustment, diagnosis

Hi, my drive side fsa comet modular crank gets chronically loose. Problem started when I over tightened the drivetrain on my Kona unit X (single speed).

I was riding with the chain tension too high for too long, and one day the chain just became loose again - I suspect something "gave" in the crankset b/b.

So I just tightened up the crank bolt and kept riding. Now, no matter how much I adjust the crank bolt and retaining cup, it won't hit that sweet spot between too much play and too much resistance.

I am told I need to replace the b/b and the crankset. Seems wasteful to me. I need more information.

What is likely broken or worn down here? What upon close examination should I look for? Thanks

Last edited by eddiearniwhatev; 06-23-22 at 03:38 PM. Reason: Clarity
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Old 06-27-22, 09:29 AM
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With your chain too tight, you very well may have worn the teeth of the chainring and/or rear cog. As well as worn the chain so there is too much spacing between the links which will only wear out faster any new ring or cog you put on it. And it's not unreasonable to expect that your BB bearings might be worn too.

After you let a bike shop fix everything. Then get them to show you what a properly tensioned chain is for your single speed bike. A tight chain is a bad thing.
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Old 06-27-22, 11:28 AM
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Originally Posted by eddiearniwhatev View Post
I was riding with the chain tension too high for too long, and one day the chain just became loose again - I suspect something "gave" in the crankset b/b.
Did you check the sliding dropouts to see if the axle moved forward?

Originally Posted by eddiearniwhatev View Post
Now, no matter how much I adjust the crank bolt and retaining cup, it won't hit that sweet spot between too much play and too much resistance.
There should be neither play nor resistance.

Originally Posted by eddiearniwhatev View Post
I am told I need to replace the b/b and the crankset. Seems wasteful to me. I need more information.
Perhaps those who told you this can provide the information.
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Old 06-27-22, 11:42 AM
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I took a close look at the situation and saw that the inside of the crank arm is worn unevenly, and the external bottom bracket seems more recessed on one side than the other.

So I could replace just the bottom bracket and just the parts of the crank that are worn unevenly, but ordering such parts is way harder than just getting a new b/b and set of cranks.

The place I went to for advice was the shop where I bought the bike. They simply said that I need new parts and offered to install them. I'm not going to bother them for more information, that's what the internet is for.

Insofar as this forum is concerned, I am interested in how I might have specifically damaged the bottom bracket and or the crank by over tightening a single speed drive train then riding on it for several months. I'm curious if anyone else has seen this set of symptoms, or if anyone has perspective on the situation, things I might look for. I'm new to modular cranks and external bottom brackets as part of a single speed drive train.

My theory is that by tightening the chain then cycling on steep terrain I ended up directing the force of the power stroke to the bottom bracket shell, compressing it on one side. Therefore when I tighten the crank bolt on the drive side either it bottoms out on the bottom bracket, or it has too much play because it doesn't have a parallel surface to spin on.

Presently, when I take the cranks off the bike and attach them to one another, there is no play whatsoever - the problem lies entirely in the area where the two surfaces meet.

Last edited by eddiearniwhatev; 06-27-22 at 11:48 AM.
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Old 06-27-22, 11:51 AM
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Originally Posted by eddiearniwhatev View Post

The place I went to for advice was the shop where I bought the bike. They simply said that I need new parts and offered to install them. I'm not going to bother them for more information, that's what the internet is for.
The professional mechanics looking at your bike in person should actually be a much better source of information than strangers on the internet.
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Old 06-27-22, 11:59 AM
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Originally Posted by eddiearniwhatev View Post
The place I went to for advice was the shop where I bought the bike. They simply said that I need new parts and offered to install them. I'm not going to bother them for more information, that's what the internet is for.
So instead of just asking the person at the shop who actually looked at it and made the diagnosis, you're soliciting information from internet strangers who have never seen the bike, the cranks, or the bottom bracket? You must like taking the scenic route.
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Old 06-27-22, 12:01 PM
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They've never seen the bike since I bought it from them. I'm the only one who has looked at it since it was damaged. I'd rather do all the work myself, including diagnostics, hence use of this forum.

This isn't a "things you should ask a professional mechanic" forum, rather one in which people pool and share information so they can solve mechanical issues themselves.
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Old 06-27-22, 12:06 PM
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No one has seen the bike but me. I'm on this forum to access information so I can diagnose the problem and discuss it. As I have been doing on this forum for the past decade, and as I do in all sorts of fora for my job. It is a widely accepted use of the internet. Gathering and crowd sourcing information.

If you don't want to offer constructive advice or observations, please stop trolling. It's annoying.
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Old 06-27-22, 08:37 PM
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Always such a pleasure to help out a friendly member. 🙄

I think your sliding dropouts slipped forward under the climbing load, and then for whatever reason, you torqued down on the crankarm until it bottomed out, possibly wallowing out the crankarm splines and/or side-loading the bearings. With no photos of the cranks and no response regarding the state of the sliders, this is all just conjecture on my part. But since you're so easily angered by the suggestion that you get an actual hands-on diagnosis, I guess you and your "crowd source" will eventually figure it out. Or not.
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