nayr497

I was taking my winter project '07-'08 commuter for its first commuter ride yesterday when a few months of work evaporated via a tacoed chainring. I'm wondering if anyone has any insight.

I had ridden the bike in heavy rain two days prior. The chain was a bit slack and I was waiting on a half link to arrive at the LBS. By Monday the chain had tightened a bit after drying out. I put some chain lube on it about a half hour before my ride.

Heading down an extremely steep hill, not going that fast, feathering my front brake. All of a sudden I feel my feet spin free and some crunching. People in cars drive way too fast down this hill and sneak up on me so I had to come to a stop and move to the shoulder. When I did I looked down and the chainring was crunched in half.

I am thinking that lubing the chain loosened it a bit and the potholed winter road caused the chain to jump on the ring, then it catch out front, the crank turned a half revolution and pulled it in half.

1) I am happy nothing caught and I still have my front teeth.

2) Anyone have a similar experience and care to shed some insight? I'm at work getting it back together and would like to avoid this from happening again. I have ridden with a chain that loose before, but maybe the hill made it jump more than flat riding?

3) The chainring is gone. I'm trying to salvage the crankarm but one of the spiders (mainly confined to just the tab - this is an old road double CS) was bent a bit. It is pretty tough bending those back. I have done it carefully. Is it worth it to use a bent/rebent drive-side crank arm or should I just shell out for a new one? I like my teeth, but I also would like to go the safest, most inexpensive route available.

I'll have to take a photo today of the ring so this story is believable...

Yoshi

Sounds to me like you were missing some chainring bolts and didn't realize it - a loose chain isn't going to cause that kind of damage (and most people on this forum seem to run their chains way too tight anyway).

mattface 

In my opinion you should just get a new crank. If you're not too long legged, chucks has a Sugino XD2r in 160mm for $30. It's got steel rings. Not pretty maybe, but durable. I'd also change the chain at this point.

Re-Cycle

How *was* your chainline? Was it a fairly standard drivetrain or did you have like a skip tooth left drive two speed whizbang odd drivetrain?

zacked

Are all you chainring bolts still in and tight? Folded chainrings usually mean loose bolts.

nayr497

All the chainring bolts were in there. I had just assembled everything, so they were pretty tight. But, upon disassembling everything after the carnage some of the bolts felt loose. Now, maybe this is because they weren't tightened enough to begin with or maybe some loosened as the ring was pulled away from the arm spiders. Two of the bolts/ring spots were pulled away from the spider and the ring even cracked where the bolts secured it.

How would loose CR bolts make this happened? Might the CR slip, then the chain move, then fold over?

The chainline was nearly perfect.

So, I'm gathering the culprit was loose bolts, I'm just trying to picture the mechanics of this failure.

Thanks for the info so far.

Yoshi

The loose bolts would allow the chainring to to move laterally thus causing the force vector on the chainring to go in a direction the chainring is not designed to handle. Once that occurs, the chainring will start to pull on the loose bolts which aren't properly supporting the chainring, and you'll end up with catastrophic failure.

nayr497

Okay, thanks for that, Yoshi.

If one doesn't not have a torque wrench what type of tightening down are we talking with chainring bolts. Like...red in the face, as hard as you can turn them? Or, pretty darn snug, then check them after a short ride and snug down again?

Greasing the bolts is advisable, right?

jeac

i had this happen before

except the chain ring was one piece with the cranks, so it couldn't have been loose bolts

teiaperigosa

you need sugino grand supertighties with durable ace chainking....woulda never happned if you stepped your game up

Aeroplane

Those cranksets need the inner chainring in order to provide enough strength so that they don't fold up like a pancake (like yours probably does).

nayr497

I'd love to step my game up...but this is my winter commuter/rain bike/bar bike/not afraid to lock up bike...Sugino/Dura Ace goods are a little out of the budget for this ride

teiaperigosa

where is Nilbog?

I would just say write my sn on the tt or st ...and you'll have no prob's, but that might be outta my jurisdiction... the goonies only travel so far

mattface 

That Sugino crank at Chucks will cost you about the same as a new chainring.