specialmonkey

On my commute home last night my drive train started feeling a bit off, then on every revolution, at one point in the circle, it would catch for a very short moment (perhaps only 1/8"), but I could pedal through it ... I stopped quickly to examine the bike and couldn't see anything, I figured it must be a freewheel gone bad or maybe my bottom bracket was in need of service. I could roll fine, so I think that gave the hubs / axles a pass. But oddly, it seemed I could pedal backwards OK too, it was just the forward revolutions ...






Any guesses?


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By the time I got home and turned the bike upside down, I could see one chainring bolt on the granny was loose and shearing off. The catching was less evident pedaling backwards perhaps because the side of the bolt that would come into contact with the chain stay was already sheared off? I'm not sure that makes sense ...



This is not a picture of the actual crank, but one where I exchanged the bad bolt for a good one.
On the bike in question (an '87 Trek 520 Cirrus), the clearance is still pretty slim, even with all my chainring bolts tight.

Andrew R Stewart 

Before I'd go much further I'd make sure I had the proper tools on hand to work on the crank. That photo shows the sad results of makeshift tooling.

If a ring bolt is rubbing against the frame , or something, then there's a good chance that the frame, or the something, will show the rubs/damage. I'd be more concerned about that in the long term as replacing a crank (or it's ring bolt) is simple compared to that of a frame...

The diagnosis should be straight forward. See what might be rubbing. A slip of paper, or short length of tape, placed between the crankset and the frame will show any rubbing. This rubbing might only be when there's pedaling forces being applied (and not so when not actually riding). Removing the chain from the area can help.

When looking for this type of problem I find it good advice to do a thorough cleaning of the general area and specific parts involved. I've seen kickstand mounting plates or other items be the contact points as example. Andy

specialmonkey

Diagnosis was loose and sheared chainring bolt. Replaced it and tightened all chainring bolts. Now cured.

The pictured crank was not the actual crank, it's one I received in a used bike purchase.

By design, I think chainring bolts are softer than cromo chainstay tubes. The loose bolt was rubbing the lug connecting the bottom bracket to the chainstay, and as far as I can tell, is undamaged. I only rode a few miles with the loose bolt.

TimothyH

Why is the crank all chewed up around the square taper hole?

It looks like someone tried to pry it off with a screwdriver.

With all due respect, this is an indication that whomever has been working on this bike doesn't have the right tools and/or know the right techniques.

dsbrantjr

OP has stated twice that the crank pictured is not the one being described.

79pmooney

Ah, but - a picture is worth a thousand words. It will take a few more such posts to catch up!